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ENVIRONMENTAL SCIENCE, ENGINEERING AND TECHNOLOGY

EUTROPHICATION CAUSES, ECONOMIC IMPLICATIONS AND FUTURE CHALLENGES

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ENVIRONMENTAL SCIENCE, ENGINEERING AND TECHNOLOGY

EUTROPHICATION CAUSES, ECONOMIC IMPLICATIONS AND FUTURE CHALLENGES

ALAIN LAMBERT AND

CAMILA ROUX EDITORS

New York


Copyright © 2014 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com

NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data Eutrophication : causes, economic implications and future challenges / editors, Alain Lambert and Camila Roux. pages cm Includes bibliographical references and index.

ISBN: (eBook)

1. Eutrophication. 2. Eutrophication--Economic aspects. 3. Eutrophication--Control. I. Lambert, Alain. II. Roux, Camila. QH96.8.E9E9624 2013 577.63'158--dc23 2013026968

Published by Nova Science Publishers, Inc. † New York


CONTENTS Preface Chapter 1

Chapter 2

Chapter 3

Chapter 4

Chapter 5

vii The Performance of Mechanical Aeration Systems in the Control of Eutrophication in Stagnant Waters Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed, Benmamar Saadia and Gotteicha Khadidja Using Cyanobacteria as a Biosorbent for Heavy Metals in Waste Waters: Feasibility and Challenges Chaoyang Wei, Di Geng and Hongbing Ji Old Wine in New Skins - Eutrophication Reloaded: Global Perspectives of Potential Amplification by Climate Warming, Altered Hydrological Cycle and Human Interference Martin T. Dokulil

1

59

95

Assessing Daphnia Population Dynamics and Recovery Patterns after Exposure to Multiple Environmental Stressors in a Eutrophic Lake Paul Oberholster, Jackie Dabrowski and Anna-Maria Botha

127

Blue-Green Algae Blooms: Environmental and Health Consequences Piotr Rzymski and Barbara Poniedziałek

155


vi Chapter 6

Chapter 7

Chapter 8

Chapter 9

Chapter 10

Contents Eutrophication and Recovery of the Large and Deep Subalpine Lake Maggiore: Patterns, Trends and Interactions of Planktonic Organisms between Trophic and Climatic Forcings Giuseppe Morabito and Marina Manca

183

Current and Future Eutrophication of Nearshore Marine Environments: Causes, Consequences and Potential Management Strategies Laura J. Falkenberg, Sean D. Connell and Bayden D. Russell

215

Promoting Mechanism of Rare Earth on Water Eutrophication Jiang Yu

239

Lifecycle Eutrophication Impact of Future Energy Systems Diego Iribarren, Ana Susmozas and Abel Sanz Study of the Particle Size of the Natural Coagulant Tanfloc SG to Obtain Drinking Water by Coagulation/Flocculation Joseane Debora Peruco Theodoro and Rosangela Bergamasco

Index

261

277

287


PREFACE In this book, the authors present topical research in the study of the causes, economic implications and future challenges of eutrophication. Topics discussed include the performance of mechanical aeration systems in the control of eutrophication in stagnant waters; using cyanobacteria as a biosorbent for heavy metals in waste waters; a state-of-the-art review on eutrophication research in the context of climate warming; assessing Daphnia population dynamics and recovery patterns after exposure to multiple environmental stressors in a eutrophic lake; the environmental and health consequences of blue-green algae blooms; eutrophication and recovery of the large and deep subalpine Lake maggiore in Italy; causes and potential management strategies of current and future eutrophication of nearshore marine environments; promoting mechanisms of the rare earth industry on water eutrophication; the life-cycle eutrophic impact of future energy systems; and the study of particle size of the natural coagulant tanfloc SG to obtain drinking water by coagulation/flocculation. Chapter 1 – The techniques used for the restoration of lakes or to prevent eutrophication are numerous (chemical, biologic, mechanical, etc.). Due to their excessive costs and the relatively insignificant outcomes of some of these techniques, the process of artificial aeration is one of the most promising methods. Several strategic techniques for the control of nutrients are selected for this study: artificial destratification by the bubble plume, partial (or total) lift hypolimnetic aerator, bubble plume oxygenate and Speece Cone oxygenation, and others. Each of these methods has both advantages and disadvantages. Technical and economic analyses established by different researchers reveal that hypolimnetic oxygenation is the most favorable for nutrient control. In hypolimnetic aeration systems, the bubble plume appears


viii

Alain Lambert and Camila Roux

to be the most economical and perhaps the most simple among the systems proposed for Standley Lake (Colorado, U.S.A), even as other researches selected the Speece Cone aeration system in other applications. Based on existing hypolimnetic aeration research, this study provides a synthesis of a number of issues related to the aeration in lakes and reservoirs, including the efficiencies and the advantages and disadvantages of these aeration systems. This study also concentrates on the economic and technical aspects associated with these aeration systems. The authors found that the use of oxygen limits the nitrogen saturation and in contrast with using air. They demonstrate that the most efficient hypolimnetic aeration system is the bubble plume diffuser; although accidental destratification may occur. The authors show as well that the destratification can be used in winter because the temperature of the lake is not modified. However, the hypolimnetic aeration is used in summer in order to avoid the homogenization of the lake temperature during this period. Chapter 2 – Biosorption has become a promising approach for the treatment of wastewater containing heavy metals. The identification of organisms with high heavy metal adsorption capacities is a topic of increasing interest in biosorption research. Cyanobacteria are widely distributed worldwide, and numerous studies have indicated that these organisms have great potential for the adsorption of heavy metals in wastewater. Algal blooms are rich in Cyanobacteria, and lake eutrophication can produce large amounts of low-cost, collectable blooms. Algal resources in the natural environment, such as substances from algal blooms, may have potential for use as commercial biosorbents. This chapter will provide a comprehensive introduction to the properties of these cyanobacterial substances, the mechanisms of their adsorption of heavy metals, the factors influencing the adsorption and pretreatment processes, immobilization techniques, and the adsorption properties of cyanobacterial biosorbents. The feasibility and challenges of using cyanobacterial substances as biosorbents for heavy metals in wastewater will be fully discussed, along with the prospects for future studies in these areas. Chapter 3 – Natural or anthropogenic enrichment of surface waters through input of nutrients, commonly referred to as Eutrophication, is essentially a catchment related process. The relative importance of different hydrological pathways in the water shed are therefore of crucial significance. Although eutrophication has a rather long history, the problem and its implications became particularly apparent in the mid-20th century as a consequence of population density, urban development, tourism, industry and agricultural practices. To maintain sustainable human societies profound water


Preface

ix

management was and is required including concepts to restore or rehabilitate surface waters and to prevent further deterioration. Mitigation of nutrient input was successful in many regions but failed or responded slowly in others, often as a result of in-lake processes. The growing water demand and the lack of clean water in large parts of the world necessitate elaborate models in the near future particularly under warmer climate scenarios. In a warmer world many consequences of eutrophication will potentially be amplified. Interaction of climate change with eutrophication will proliferate harmful algal blooms (HABS), spread infectious diseases, changes pathogen communities and favours microparasites among several other abiotic and biotic components affecting ecosystems. Persistent eutrophication may exceed ecological thresholds and lead to regime shifts. The symptoms of cultural eutrophication will certainly worsen when global temperatures increase and human impact intensifies further. Concepts and models are needed for future mitigation specifically for developing countries of the inter-tropical zone because initial attempts at applying temperate zone control measures in these regions have been largely unsuccessful. Chapter 4 – Cultural eutrophication in South Africa is recognized as one of the most serious water problems affecting aquatic food webs of inland waterbodies, largely because of anthropogenic pollution from waste water treatment plants and agriculture activities. In previous studies the decline of daphnids during midsummer has been attributed to a range of different single factors. However, very little is known about the exposure of daphnids to a combination of multiple environmental stressors including eutrophication in relationship with different population growth phases. This study examined population dynamics (impact on different population growth phases) of Daphnia pulex in Lake Loskop, South Africa during spring and midsummer, in comparison to a laboratory study imitating fish predation, as well as exposure to synthetic extracellular cyanotoxin (microcystin-LR) and an increases in surface water pH due to algal blooms caused by eutrophic conditions. Monthly sampling from October 2009 to March 2010 in Lake Loskop showed that D. pulex was abundant during the peak of the cyanobacteria Microcystis spp., but began to decline after the start of degradation of Microcystis spp. population and concurrent release of the cyanotoxin, microcystin-LR, in the water column during December 2010. Analysis of the fish stomach contents of Oreochromis mossambicus and Micropterus salmoides indicated that adult daphnids were predated upon during late summer. Elevated pH values (mean of 9.6 – 10.5) were measured during midsummer at sites dominated by phytoplankton blooms of Ceratium


x


hirundinella (Müller) and Microcystis spp. It appeared from this study that the reduction in D. pulex populations in the field study was concurrent with laboratory studies that produced population declines similar to those observed in the lake. Laboratory studies were done by simulated individual and combined effects on D. pulex with treatments that included the addition of synthetic microcystin-LR, artificial increase of pH (10.1 - 10.7), and removal of adult D. pulex to simulate fish predation. Results from the laboratory study indicated that D. pulex populations were negatively impacted by each of the individual and combined treatments. From the authors‘ results it was evident that the combination of synthetic microcystin-LR, simulated fish predation and elevated pH applied in the laboratory studies during the exponential peak growth phase of the daphnids had the most drastic effect, causing a major population decline with no sign of recovery which was very similar to the authors‘ field observations. These findings have clear implications for eutrophication management of Lake Loskop, since D. pulex is key filter feeders within the community of herbivorous zooplankton. Chapter 5 – Blue-green algae (Cyanobacteria) are a wide group of photosynthetic organisms associated with marine and freshwater. Due to the climate changes as well as cultural eutrophication the expansion of these microorganisms in surface water is observed worldwide and has been associated with various environmental consequences and human health threats. Under specific environmental conditions several species can form massive dense blooms. Furthermore, cyanobacteria can synthesize and secrete organic metabolites which vary in a degree of toxicity and mechanism of action including hepato-, dermato-, and neurotoxicity. Bioaccumulation of cyanotoxins has also been demonstrated. Therefore, water reservoirs and aquatic livestock should be subject to regular quality control monitoring in order to protect human and animals from exposure. Sustainable land use in water catchment areas and prevention against eutrophication are essential in controlling blue-green algae blooms. Chapter 6 – Planktonic organisms are considered good indicators of environmental changes, even more sensitive than abiotic variables per se. In relatively recent years, the importance of pluriannual plankton series has become increasingly important, also for management purposes and in the process of bridging the gap between environmental science and management policy. Plankton studies in Lake Maggiore date back to mid-1900s, although regular monitoring started in the late 1970s, in the framework of an agreement between the Swiss and Italian Governments. The full long-term data series (1981-2011) entirely covers the lake‘s recovery, since full mesotrophy of the


Preface

xi

mid-1970s, to present oligotrophy. Response of plankton communities to eutrophication reversal after lake restoration included a gradual increase in the number of phytoplankton taxonomic units and in cell density along with a decrease of average cell size. Changes in taxonomic composition, population density and mean body size were also tracked in the zooplankton. Data the authors obtained through these studies, however, pertaining to each single level of biological organization, do not allow for highlighting quantitative changes in trophic relationships per se and in ecosystem functioning driven by changes in trophy. Environmental changes, such as those attributable to eutrophication/oligotrophication processes, as well as to climate, are expected to affect not only taxonomic composition of planktonic assemblages, but also the trophic relationships and the ecosystem processes. Moreover, during the lake‘s oligotrophication the role of climatic constraints became increasingly important in controlling plankton dynamics, affecting phytoplankton nutrient supply, resource ratio, population phenology and the whole life cycles of the organisms involved. The authors‘ aim is to track the ecosystem response by analysing the phytoplankton-zooplankton relationship from a functional point of view, trying to find the key driver across different steps of the lake's trophic history, in an attempt to disentangle climate- from trophy-related responses of lake ecosystems. Chapter 7 – In all ecosystems, including near shore marine environments, resource availability regulates the productivity of individuals, species, and, ultimately, communities. Where human activities enrich limiting resources in these systems, rates of primary production and accumulation of organic matter are modified, a process widely referred to as eutrophication. Over the past few decades eutrophication in marine environments has been driven by an unprecedented increase in the rate at which they receive nutrient inputs from human-derived sources. Typically, such nutrient enrichment alters the availability of resources required for growth to favour opportunistic, or ‗weedy‘, species at the expense of those which are longer-lived such as canopy-forming algae and coral. These changes have important ecosystem consequences due to the subsequent shift in dominant habitat, productivity and energy flows. While the link between nutrient inputs and eutrophication appears obvious, ‗surprising‘ or ‗unexpected‘ ecological outcomes have revealed the importance of considering the mediating role played by features of the regional environment, such as historical nutrient loads as well as patterns of nutrient retention and accumulation. In the future, the process of eutrophication is anticipated to become further complicated, its effects exacerbated and ‗surprising‘ effects more common as human-driven change


xii


manifests at a global scale. While coastal eutrophication in the 20th and 21st centuries has most often been related to the excessive loading of nutrients, human impacts resulting from increasing populations and industrialisation are placing additional pressure on natural systems. A key change which will influence eutrophication in marine systems is the increasing emission of CO2 to the Earth‘s atmosphere, as 30 – 50 % of this gas is absorbed by oceanic waters where it alters carbonate chemistry and increases carbon availability. Therefore, the future responses of coastal systems to human-derived nutrient inputs will be modified by this overarching addition of a second resource, carbon. As primary producers such as algae use carbon for photosynthesis, it is anticipated that enrichment of this additional resource will further enhance the productivity in species which grow rapidly and intensify changes to ecosystem structure and function. Given the potential negative consequences of phaseshifts from long- to short-lived species, such as canopy forming algae to mats of algal turf, management strategies to prevent further eutrophication (i.e., enhance resistance) and promote recovery (i.e., improve resilience) are being developed. Strategies to achieve these aims will focus on preventing, or interrupting, the feedbacks which promote ‗weedy‘ species and will likely centre on managing nutrient inputs and biotic controls. Further challenges are, however, still ahead of us in the development of a refined understanding of the dynamics of coastal eutrophication under simultaneous changes to nutrient inputs, CO2 enrichment and other environmental forcing factors. Chapter 8 – With the extensive development of the Rare Earth (RE) industry, a mass of RE migrates from lithosphere into hydrosphere and biosphere. Based on the suitable amount of rare earth elements (REEs) effecting efficiently on plants (including microalgae and floating aquatic plants) growth, Duckweed Spirodela was selected as the experimental material which belongs to typical floating plants in the upper reaches of the Yangtze River, the promoting mechanism of REE on water eutrophication in the upper reaches of the Yangtze River was explored through the experiment of cerium (Ce) influencing on Duckweed Spirodela‘s growth and photosynthesis. The results showed that appropriate concentration range of REE could promote Duckweed Spirodela absorbing N, P effectively (the maximum value of respectively), improve the carbonic anhydrase activity (a maximum value of 0.684 EU/µg) and improve the efficiency of light energy utilization and transformation which made photosynthesis improved, and thus it could promote plant growth, with the highest rate of growth of Duckweed Spirodela reaching 63.85%, and maximum value of chlorophyll up to 9.594 mg/L, and further, with the increase of Duckweed Spirodela breeding density, dissolved


Preface

xiii

oxygen (DO) reduced but chemical oxygen demand (COD) raised (a maximum value of 6.448 mg/L) in water, which eventually led to the water quality worsen. The research results demonstrate that the suitable contents of REE can promote small aquatic plants such as Duckweed Spirodela growing, thereby contributing a positive catalytic effect to the development process of eutrophication. This no doubt provides basic theory for sustainable development and utilization of rare earth, and provides a scientific basis for the study of rare earth influencing on the promoting regularity of water eutrophication and integrated control of water pollution. Chapter 9 – Future energy systems are expected to mitigate the environmental impact of the present energy sector through the modification of current fossil-based systems and the progressive implementation of renewable energy systems. However, to date, efforts are mainly focused on the reduction of the lifecycle greenhouse gas emissions associated with the energy sector. Thus, unlike global warming, other impact categories such as acidification and eutrophication are often disregarded when evaluating the environmental performance of energy systems. This chapter uses the life cycle assessment (LCA) methodology to evaluate the potential eutrophication impact of a set of energy products derived from systems which are expected to play a role in a future, sustainable energy sector: (i) electricity from coal power plants with CO2 capture, (ii) electricity produced according to a biomass integrated gasification combined cycle scheme, (iii) biodiesel from the esterificationtransesterification of free fatty acid-rich wastes, (iv) synthetic fuels from the fast pyrolysis of biomass followed by bio-oil upgrading, and (v) hydrogen from indirect biomass gasification. The main sources of the lifecycle eutrophication impact of these products are identified. Furthermore, in order to give insights on the suitability of these alternative energy products, their eutrophication impact is compared with that of conventional ones (viz., electricity for the grid, fossil diesel, first-generation biodiesel, fossil gasoline, and hydrogen produced via steam methane reforming). Chapter 10 – In this work the authors performed the study on the use of natural coagulant tanfloc SG in the process of coagulation/ flocculation/sedimentation for water treatment. Studies were developed and evaluated on the size of the flakes formed. It was employed in this study using the Olympus Microscope Uplan Fl 10x / 0.30 (Olympus UPM-TUC) and counter-particles HACH HIAC 9703. The dimensions of the flakes trainees were characterized by the diameter and area of this particle. The studies conducted in determining particle size, was applied to water treatment turbidity of water and 50 uT treatment with 150 uT turbidity (turbidity units).


xiv


This work has been proposed in the following time coagulation/ flocculation/decantation (1, 2, 4, 6, 8, 10, 20, 40 and 60 min). The results showed that for 60 minutes, a greater aggregation of the flakes, in which succeeded the formation of flakes with a diameter of 444 μm and 116,919 μm2 area.


In: Eutrophication Editors: A. Lambert and C. Roux

ISBN: 978-1-62808-498-6 © 2014 Nova Science Publishers, Inc.

Chapter 1

THE PERFORMANCE OF MECHANICAL AERATION SYSTEMS IN THE CONTROL OF EUTROPHICATION IN STAGNANT WATERS Gafsi Mostefa1,, Djehiche Abdelkader1,†, Kettab Ahmed2,‡, Benmamar Saadia2,§ and Gotteicha Khadidja1, 1

Research Laboratory of Civil Engineering: RLCE, Research Team, Water Resources, University of Ammar Telidji Laghouat, Laghouat, Algeria 2 Laboratory Research in Water Science:LRW-Water/ENP National Polytechnic School El Harrach, Avenue Hassen Badi, Algiers, Algeria

ABSTRACT The techniques used for the restoration of lakes or to prevent eutrophication are numerous (chemical, biologic, mechanical, etc). Due to their excessive costs and the relatively insignificant outcomes of some of 

E-mail: [email protected]; [email protected]. E-mail: [email protected]; [email protected]. ‡ E-mail: [email protected]. § E-mail: [email protected].  E-mail: [email protected]. †


2

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al. these techniques, the process of artificial aeration is one of the most promising methods. Several strategic techniques for the control of nutrients are selected for this study: artificial destratification by the bubble plume, partial (or total) lift hypolimnetic aerator, bubble plume oxygenate and Speece Cone oxygenation, and others. Each of these methods has both advantages and disadvantages. Technical and economic analyses established by different researchers reveal that hypolimnetic oxygenation is the most favorable for nutrient control. In hypolimnetic aeration systems, the bubble plume appears to be the most economical and perhaps the most simple among the systems proposed for Standley Lake (Colorado, U.S.A), even as other researches selected the Speece Cone aeration system in other applications. Based on existing hypolimnetic aeration research, this study provides a synthesis of a number of issues related to the aeration in lakes and reservoirs, including the efficiencies and the advantages and disadvantages of these aeration systems. This study also concentrates on the economic and technical aspects associated with these aeration systems. We found that the use of oxygen limits the nitrogen saturation and in contrast with using air. We demonstrate that the most efficient hypolimnetic aeration system is the bubble plume diffuser; although accidental destratification may occur. We show as well that the destratification can be used in winter because the temperature of the lake is not modified. However, the hypolimnetic aeration is used in summer in order to avoid the homogenization of the lake temperature during this period.

Keywords: Aeration, eutrophication, destratification, hyolimnetic aeration, temperature, dissolved oxygen, efficiency of aerators, thermal stratification, techniques of restoration, lake

1. INTRODUCTION The problems of water quality are related to the decrease in the dissolved oxygen content, particularly in the lower layers (Davis, 1980). These lower layers may deteriorate significantly if the dissolved oxygen consummated in biochemical processes is not renewed by surface aeration or photosynthesis (Zic et al., 1992). The content of the dissolved oxygen (DO) is one measure of the water quality (Davis, 1980; Steinberger et al., 1999). The amount of DO in a water body is an indication of level of microbiological activity, the amount of decaying organic matter present, and level of reaeration. In addition, DO is


The Performance of Mechanical Aeration Systems …

3

probably the most significant parameter relating to the sustainability of fish habitat (Steinberger et al., 1999). In many reservoirs, solar heating creates stable temperature stratification during the summer months when warm surface water floats over the colder deep water referred to as the hypolimnion (Mobley, 1997). This Thermal stratification of lakes and reservoirs can result in substantial hypolimnetic oxygen depletion (Kyung Soo and Subhash, 1993; Goloka Behari et al., 2005), which may have a negative impact on the cold-water fisheries, the drinkingwater treatment process, and water quality downstream of hydropower reservoirs (McGinnis et al., 2000; Gafsi et al., 2012b). When the duration is sufficiently long, oxygen depletion resulting from biochemical and biological demand can occur in the hypolimnetic water that became isolated from the water surface. The immediate consequences are varied and can include the formation of iron and manganese solution and suspension compounds, methane, hydrogen sulfide, ammonia, phosphorus, and which cause to fish killing (Schladow, 1993; Gafsi et al., 2012a), accelerated internal recycling of nutrients, solubilization of metals, and provoke also taste and odor problems that are undesirable in water supplies (Speece, 1994; Gafsi et al., 2012a). These conditions eradicate fish populations because the eggs deposited in anoxic sediments may not develop (McGinnis et al., 2004). As well, the oxygen deficit is caused by the ice layer during cold time. This ice covering prevents the oxygen transfer to the air-water interface (Prepas and Burke, 1997; Stefan et al., 2000), and consequently provokes the die-off of fish. Generally, for a cold water fishery, the hypolimnion DO should exceed 5-7 mg/l. The hypolimnion oxygenation system must operate for much of the stratification to maintain the fishery (Speece, 1994). The acceptable minimum dissolved oxygen (DO) level to prevent fish mortality is 2-2.5 mg/l. Nonetheless, in practice the minimum desired level is 3 mg/l (Stefan et al., 2000)

2. PURPOSE OF RESTORATION OF RESERVES Lake restorations are designated to improve the hypolimnetic oxygen levels in order (Wuest et al., 1992): 

To support a biological fermentation as in effluent or sewage treatment, or to replace oxygen in river or drinking water supply (Motarjemi and Jameson, 1978);


4

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al. 

 

To limit the recycling of phosphorus from the sediments into the lake water (Wuest et al., 1992)-Create habitat adequate for fisheries (Wuest et al., 1992; Gafsi et al., 2005); Hydropower discharges (Wuest et al., 1992; Gafsi et al., 2005); Recluse water treatment cost/chemicals (Wuest et al., 1992; Gafsi et al., 2005).

The artificial mixing of stratified lakes with aerators may cause numerous changes, for example: 



 



Prevent thermal stratification from becoming established and to increase the dissolved oxygen throughout the water column (Davis, 1980; Bernhardt et al., 1985); Prevent surface river or lake parts from freezing over (Kobus, 1968; Wen et al., 1987), and make barriers against saltwater intrusion in rivers and lakes ( Ditmars and Cederwall, 1974), and delay ice formation in harbors and inland waterways (Wilkinson, 1979); Production of surface currents to protect harbor areas against high amplitude waves (Baines and Leitch, 1992; Brevik et al., 2002); Prevention of oil slicks from spreading after oil tanker accident, or protection of coastal habitats against damage from oil (Hussain et al., 1984; Chen et al., 2000); In the composition of the algal population, in the total possible number of algae, and in the algal growth rate (Schladow, 1992; Brevik et al., 2002), as well as bring about an almost complete compensation of the oxygen deficit resulting from metabolic activity (Bernhardt et al., 1985).

3. ARTIFICIAL AERATION The artificial aeration of oxygen-depleted lake waters is one of many restoration methods (Janczac et al., 2001). It is undoubtedly the most often used technique (Janczac et al., 2001, Brzozowska et al., 2001), and the reasons for its popularity are relatively low costs and few technological difficulties (Janczac et al., 2001). The obtained results reveal that the decrease in



5

the trophic level resulted not only from the limitation of internal loading (due to the improvement of aerobic conditions) but caused also persistent changes in the bottom sediments (Brzozowska et al., 2001). Aeration of reservoir release can be achieved by various methods (Ruane et al., 1977; Kyung Soo and Subhash, 1993); however, the techniques currently used are often expensive and inefficient (Kyung Soo and Subhash, 1993). According Kyung Soo and collaborator (1993), the development of an autoventing hydroturbine appears to be promising as a costeffective long term solution to enhance the oxygen dissolved downstream of many reservoirs. Of the various artificial mixing methods used, air bubblers, where compressed air is continuously injected through a diffuser set at or near the bottom of the reservoir, are commonly used because of their simplicity (Wuest et al., 1992; McGinnis and Little, 1998) and have been employed efficiently in practice as a reservoir/lake restoration technique since the early 1950s (Schladow, 1992; Zic et al., 1992). There are two large categories of artificial aeration of lakes: destratification and hypolimnetic aeration. In the first case, the entire lake is mixed, usually by release of compressed air from a perforated air line laid along the bottom of the lake (Fast, 1978; Roberston et al., 1991), and in the second case, the objective is to maintain thermal stratification, while oxygenating the hypolimnion (Fast et al., 1973). These restoration techniques can be used separately or in combination. In the case of separately used systems (Wuest et al., 1992; Gafsi et al., 2006):  

Artificial mixing of the water column during the cold season; Input of oxygen into the hypolimnion during summer in such a way as to preserve the stratification.

While, for combined used systems, which can be switched between artificial mixing mode using coarse air bubbles, and hypolimnetic oxygenation or aeration mode using fine oxygen or air bubbles, respectively. Each diffuser is operated using air or oxygen during the summer, and air for artificial mixing in the winter (McGinnis et al., 2004). Each of these methods has its advantages and inconveniences.


6

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al.

Fast, 1978, Gafsi et al., 2009. Figure 1. A simple lake destratification system.

4. AERATION SYSTEMS 4.1. Aeration by the Destratification System The problems of water quality are frequently managed by the use of artificial destratification devices, among which the most popular is the air bubbler system (Paterson et al., 1989; Roberston et al., 1991; Lemckert et al., 1993). Destratification using aeration was first reported by Scott and Foley (1919). Using this method, destratification is commonly achieved by injecting air through a single air diffuser (figure 1) (Fast, 1978, Gafsi et al., 2009). A single air line leads from the shore-based compressor to the deepest point in the lake. The distal end of the air line is perforated to allow the escape of air (figure 1). The rising air bubbles cause hypolimnetic water to upwell and mixing warmer surface water. In addition to the air injection described, other techniques include mechanically pumping bottom water to the surface, and mechanically pumping surface water to the bottom (Fast, 1978).

4.2. Hypolimnetic Aeration Hypolimnetic aeration is widely accepted method for maintaining or restoring oxygenated water in lakes. Although they are designed to minimize



7

the mixing between the hypolimnion and the epilimnion, by the nature of their operation it is nearly impossible to avoid some mixing between these layers which has the unwanted consequence of reducing or eliminating the stratification (Lindenschmidt and Hamblin, 1997). Hypolimnetic aeration and oxygenation are commonly used to add dissolved oxygen to water bodies while preserving stratification. The main of hypolimnetic aeration is to aerate the isothermal hypolimnion to avoid perturbation of the thermal-density structure of the metalimnion (Kortmann et al., 1994).The practice of this method is aimed to introduce oxygen only into the hypolimnion of a standing water body without disturbance of the temperature gradient (Bernhardt et al., 1985; McGinnis et al., 2002; Gafsi et al., 2009). It is particularly suitable for improving the dissolved oxygen (DO) in drinking water reservoirs when the hypolimnion extends for more than 10 m and the ratio (Asseda and Imberger, 1993):

Vepi Vhyp

£2

where Vepi and Vhyp are the epilimnion and the hypolimnion volumes respectively. Efficiency, operational cost, the necessary investment, or all the three may vary considerably and impose some limitations on the use of most of aerators (Raney et al., 1973). In 1973, Fast and Coll, used in their study in the Hemlock lake. This one is located in the Pigeon River State Forest about 85 km south of the Straits of Mackinaw, Michigan (in USA). This lake it is eutrophic, and had its hypolimnion aerated but thermal stratification maintained. For this, and for the restoration of this lake, these authors used in their study the Hemlock Lake Hypolimnion aerator. This system floated freely in the center of the lake (figure 2). Hemolock Lake has a surface area of 2.4 ha and maximum depth of 18.6 meters and has highly stained. Hemlock Lake stratifies normally during 1969; by early June the metalimnion extended from 3 to 8 meters, and the monimolimnion began at 14 meters. Temperatures taken during early June 1969 ranged from 18°C at the surface to 4.5°C at the bottom of the lake. This lake was aerated during 1970.


8


Figure 2. Cross-sectional view of Hemlock Lake aerator (Fast and Coll, 1973).

As results, these authors have found that: 



   

The artificial hypolimnetic aeration greatly altered the limnology of Hemlock Lake in general, and its oxygen and temperature regimes in particular; The monimolimnion was eliminated by mixing with the hypolimnion and the concentrations were greatly increased throughout the hypolimnion; The hypolimnetic oxygen values often exceeded the surface values; Profundal sediments were gelatinous and adhesive before aeration, and after aeration, these sediments readily fell apart when handled; The rise in average alkalinity and decrease in average pH; The growth of algal has been increased.



9

Another hypolimnetic oxygenation technique used by Fast and Coll in 1975. This technique, involves drawing water to the shore, injecting it with pure oxygen gas under high pressure, and then returning it to the hypolimnion. This is known as side-stream pumping (SSP) or side-stream supersaturation and is one of the (figure 3). The SSP system uses liquid oxygen and conventional water pump, whereas most other hypolimnetic oxygenation systems compressed air and a special aeration chamber. Oxygen concentrations were increased from 0.0 mg/l to more than 21 mg/l by the system while strong thermal stratification was maintained. The SSP system was tested in Ottoville Quarry, Ottoville, Ohio, during the summer of 1973. The Ottoville Quarry is a moderately eutrophic quarry located in Ottoville, Ohio, about 30 km northwest of Lima (in Peru). It measures 0.73 ha surface area, 63.4x103 m3 total volume, about 35.0x103 m3 hypolimnetic volume, and 18 m maximum depth. By his side, Fast A.W (1978) proposed a comparative study for the three systems of hypolimnetic aeration (Fast, 1978; Gafsi et al., 2009):   

Side-Stream Pumping (S.S.P) (Figure 3); Partial airlift Hypolimnetic aerator; Full airlift hypolimnetic aerator: This system is typically operated at pressures high than 3 atm and water temperatures of 10°C or less. The efficiency of this system is based on the great solubility of pure oxygen and its pressure injection in the water (Fast et al., 1975; Gafsi et al., 2005).

Fast, 1975; Gafsi et al., 2009. Figure 3. Side-Stream Pumping (S.S.P).


Table 1. Applicability of various aeration methods for Patrick Henry dam

High DO increase

++

+

Fluctuating reservoir pool Minimal effect on power production Minimal increase in dissolved nitrogen Low Capital Cost

TailWater Diffusers 1. Oxygen 2. Compressed Air

??

Oxygen Injection Upstream 1. Penstock 2. Diffuser 3. Downflow Bubble Contact

--

Tailrace deeper than 10 ft (3.05 m)

Side-Stream Supersaturation

-

+++

+?

Weir Aeration

--

Mechanical Aerators

++

U-Tube Aeration 1. Compressed Air 2. Oxygen Gas

Multi-Level Venting

--

++

+

+

-

++

?

+

+

+++

+?

Turbine Venting

Submerged Weir 1. Inflexible 2. Flexible

Coldwater tailwater

Group III Aeration of Reservoir Releases

Hypolimnion Aeration 1. Diffused Air 2. Diffused Oxygen

Conditions Under Which Applicable Aeration Technique is to be Applied

Group II Selective Withdrawal

Destratification 1. Mechanical Pumping 2. Diffused Air

Group I Reservoir Aeration

? +

+ +

++

-+ ++

++

++

+

?

--

-

-

-

?++

+-

??

-+

++

+

?

-+

+

?

+

+++

+?

++

--

--

-

+

--

-

-

-

+++

+-

Note:+ indicates a relatively positive effect of the particular aeration method. - indicates a relatively adverse effect of the particular aeration method. ? Indicates a unknown effect on the condition indicated. Ruane et al., 1977.



11

Ruane et al., 1977. Figure 4. Definition Sketch for Small-Bubble Oxygen Injection Method.

Many authors compared the fees of three types of hypolimnion aerators, and found out that the conception of full airlift hypolimnetic aerator has the exploitation fee the much less and the higher efficiency. The S.S.P has a much smaller capital cost (Fast, 1978; Gafsi et al., 2009). Ruane and Coll (1977), have conducted a technical economic study on several technical methods of aeration, and determined the best diffuser, and the best location for injecting the oxygen for injecting in Patrick Henry Dam (in USA). Reservoir aeration was not considered feasible at Fort Patrick Henry Dam. The stream reach below the dam is classified as a cold-water fishery, and destratification would increase water temperatures in the release. In addition, there was concern that aeration of the hypolimnion with air would


12


introduce a problem of nitrogen supersaturation. Aeration with oxygen might be feasible, but was considered unattractive because of reservoir hydraulics and capital cost. The method chosen for development for use at the dam was diffusion of oxygen into the reservoir upstream from the turbine intakes. Methods of correcting low DO concentrations in reservoirs releases may broadly classified into three major groups:  Control of DO concentrations within the reservoir;  Selective withdrawal of reservoir water with acceptable quality;  Aeration of reservoir releases. The comparison of the various available methods at Fort Patrick Henry dam is shown in table 1 follow. The volume of water in the well-aerated epilimnion and the probable rate of replenishment of this volume were too limited to allow selective withdrawal of water with a high concentration of DO. Even if selective withdrawal were possible, capital cost would be high and water temperatures would increase downstream. As result, these authors, concluded for the installation at Fort Patrick Henry Dam (figure 4), that the methods considered most feasible were those for reaerating releases. Also, of the possible methods, it determined that (Ruane et al., 1977):  

  

The turbine venting would not increase DO concentrations to desired levels; U-tube aeration was determined to be uneconomical because of high capital cost and reduced efficiency of power generation resulting from loss of head on the turbines, the methods considered most feasible were those for rearing releases. In addition, nitrogen supersaturation was a concern; Weir aeration was, determined uneconomical because of high capital cost and reduced efficiency of power generation; Mechanical surface aerators were uneconomical because of the large number (about 90) of units required; The most feasible methods of aeration available were diffused-aid aeration in the tailrace and injection of molecular oxygen into turbine releases;


The Performance of Mechanical Aeration Systems … 











  

13

Of the various techniques of injecting oxygen into turbine release, injection through diffusers either upstream or downstream from the dam appeared most feasible; Upstream injection could have several disadvantages if all the oxygen were not absorbed and bubbles were allowed to pass through the turbine system. Possible problems include a reduction in turbine efficiency, corrosion of the turbine and the associate discharge system, and adverse effects on pressure taps used to measure turbine discharge; Downstream injection would not affect the turbine discharge system and that is conceivable that the turbulence downstream from the dam could result in a high oxygen transfer efficiency. The investigation concludes that injection of oxygen at the upstream appeared to be promising; The disadvantages of diffused-air aeration as compared to oxygen injection were a high power requirement for air compressors (about 5 % of the power generating capacity of the plant), a lower capability for adding DO at concentrations greater that about 5 ml/l, and higher capital cost; The advantages of oxygen injection are that high DO levels can be obtained, the initial investment is relatively low for some the technique, and the effects on production of power during peak demands should be negligible for most methods; The most economical source of oxygen would probably be liquid oxygen from the storage thank rather than generation of gaseous oxygen at the site, because the oxygen requirement is seasonal, highly variable, and relatively low; The method selected was oxygen injecting using small pore diffusers located upstream from the turbine intake; The diffuser with a smaller pore size was then evaluated and found to yield more promising results; The effect of spacing between the diffusers was found to affect significantly the oxygen transfer efficiency of the larger pore diffuser.

In 1977, Fast invented an aeration system for fishes, this system consists either in floating park or a channel constructed with a cheap plastic material (Figure 5-a, and 5-b). It can be used in conjunction with aeration or with hypolimnetic oxygenation. The water is ascended either by air compression or


14


by mechanical means. This system could cause a decrease in nutritional elements from the lake; the thermal stratification is maintained. The natural stripping by this method restores not only the lake, by supplies also fresh lowpriced fishes (Fast, 1978). These systems present a big potential for their use in eutrophic stratified lakes or reservoirs in temperature. However, they can be used in marine water and in the isothermal conditions (Fast, 1978). Bernhardt and Coll (1985), reported results of 15 years hypolimnetic aeration in the Wahnbach Dam (Germany) (Volepi = 20.106 m3, Volhyp = 16.106 m3, depth = 45 m), using a hypolimnetic aerator developed by the Wahnbach Reservoir Association. He proved that, despite the huge development of occasional algae, the consumption of DO at the sediment-water interface is compensated during the stratification. Always, according these authors, no anaerobic condition has been produced on the bottom of the lake (DO was maintained at > 4 mg/l). In addition, the iron and manganese concentrations are suppressed in the hypolimnion, and treatment to remove the manganese is not necessary.

(a)

(b)

Fast, 1978. Figure 5. Loating systems for fish re-aeration.



15

The release of orthophosphate from sediments remains insignificant (internal load), and consequently prevents a rapid renewal of the eutrophication. During the stratification, the hypolimnion conserves a temperature less than or equal to 10°C for the month of October, and stays within acceptable limits for drinking water (Bernhardt et al., 1985). In 1994, Richard E. Speece, compare oxygen absoption efficiencies for the case of the bubbles plumes and Spêece cone, and shows that show that in impoundment and shallow reservoirs with depths below 30 m the fine bubble diffusers require a rise height of about 30 m within the hypolimnion to ensure efficient oxygen transfer to the hypolimnion. Moreover, it has been found that the bubbles must be maintained in contact with the water for approximately 100 seconds to achieve oxygen absorption efficiencies in excess of 80%. One such oxygen transfer device is the Downflow Bubble Contact Oxygenator Speece Cone developed by the author. Water is introduced downward at the tip of the cone with a velocity sufficient such that oxygen bubbles cannot escape out the top and that the bubble swarm is also prevented from collapsing. As the cone cross-section increases, the downward water velocity component decrease until it is less than the buoyant velocity of the bubbles. Thus, the bubbles are trapped indefinitely within the cone and efficient oxygen can be achieved. Finally the author concludes that for the benefit of maintaining stratification and also for efficient oxygen transfer to the hypolimnion in the shallow reservoir, free rising bubble plumes should be avoided. Also this author mentions that the key to successful hypolmnion oxygenation is horizontally induced flow from of the highly oxygenated sidestream. Vertically induced flow from a bubble plume has horizontal recycle cell zone of influence of about 4 times the bubble rise height within the hypolmnion. Thus, for a hypolimnion depth of 30 m, the oxygenated water would distribute itself within a cell having the relatively small distance of 120 m diameter around the bubble source axis. In his conclusion, the author mentions vertical circulation is hindered by density gradients due to temperature, while horizontal momentum of cold water inflow encounters negligible density differences and has been observed to travel entirely through run-of-the –river impoudments of 80 km length, but this may not be sufficient for maintaining oxic conditions in the entire hypolimnion in very large impoundment, it still may be adequate to provide a favourable habitat for the cold water fishery of an entire impoundment in a 3 km zone (Speece, 1994; Gafsi et al., 2012a).


16

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al. Table 2. Hypolimnetic and Layer Aeration system. Aerators Features

Total layer aerator capacities (3 aerators) Total hypolimnetic aerator capacities (2 aerators) Porex (TM) diffuser elements with air distribution insert sleeves Two Sullair rotary screw compressors Manifold regulators and valving to direct airflow to each aerator independently in any proportion of total compressor capacity. Destratification mode (integrated into layer and hypolimnetic aerators) Maximum airflow capacity of each aerator exceed Kortmann et al., 1994.

0.45x106 m3 /day water flow 5662 l/min operational airflow 0.30x106 m3/day water flow 1132 l/min operational inflow

60 HP, 6792 l/min (total both compressors) 6792 l/min airflow and 1.5xx106 m3 /day water flow. 4246 l/min

In an effort to alleviate in-lake nutrient loading and unsatisfactory raw water quality, Kortmann and Coll (1994 ) utilized in their study an aeration system, that designed specifically for Lake Shenipsit, to take advantage of its natural properties (oxygen availability and demands, heat distribution, stratification structure, compensation depth, and vertical intake depth). Three layer aerators (Figure 6), with selective to aerate and circulate waters from 4.7 to 10.7 m. The main design features of these aerators are showed in the table 2. The transfer efficiency as function of influent oxygen concentration and temperature was showed respectively in the figure 7 and 8. As influent DO decreased, efficiency increased (Figure7), and approached zero as influent DO approached saturation at ambient temperature. The relationship between transfer efficiency (Figure 8 and water temperature was similar if one data point is considered aberrant. In the figure 7 and 8, the dashed line represents the linear regression line with all data include; solid line deletes the aberrant observations (circled). True solute phase transfer efficiency and apparent transfer efficiency were functions of influent DO concentration (Figure 9, 10, and 11). Apparent efficiency appears to increase as temperature increases (Figure 11). Although a weak relationship, this suggests that increasing the fraction total aerator flow



17

from shallower aerator intakes increases oxygen delivery to the bottom of the design layer and decreases dependence on compressed airflow as a DO source.

Kortmann et al., 1994. Figure 6. Process schematic of multiple layer aeration approach implemented at Lake Shenipsit in 1987.


18


Kortmann et al., 1994. Figure 7. Hypolimnetic aerator efficiency reported as a function of influent oxygen concentration.

Kortmann et al., 1994. Figure 8. Hypolimnetic aerator efficiency reported as a function of temperature.



Kortmann et al., 1994. Figure 9. Layer aerator efficiency as a function of flow-Weighted influent oxygen concentration.

Kortmann et al., 1994. Figure 10. Layer aerator efficiency as a function of minimum influent oxygen concentration.


19

20


Layer Aerators Efficiency (%/m) 20 R2 = N = DF = SIGN

15

Apparent Efficiency 10 (%/m)

.115 42 40 < 90%

5 0 8

10

12

14

16

18

20

22

Mean Effluent Temperature

Kortmann et al., 1994. Figure 11. Layer aerator efficiency as a function of mean effluent temperature.

According these authors, the layers aeration depth range was selected based on iterative computer simulation modeling of heat and available dissolved oxygen (DO) mass, volume sediment contact area ratios thermal resistance to mixing to mixing as function of temperature, and supply intake depth ranges. Layer aeration combines the attributes of artificial circulation and hypolimnetic aeration methods and can be described as a depth-discrete artificial circulation technique. Like destratification, it uses both photosynthetic and diffusion sources of oxygen to overcome oxygen demand. Like, hypolmnetic aeration, it maintains necessary vertical temperature structure for coldwater fish and zooplankton refugia. Layer aeration alters thermal structure, creating several functional thermoclines, while retaining stratification stability. The Lake Shenispsit covers 212 ha to mean and maximum depth of 9.9 m and 20.7 m respectively. Lake shenips exhibits a limited littoral zone. Only 8 % of lake area is less than two meters deep. The volume sediment contact area ratio between 6 and 12 m is disportionately large (metalimnetic depth range). This was one reason for selection of layer aeration strategy. As result, the author and Coll (1988), report that: 

Oxygen transfert efficiency for hypolimnetic aerators averaged 4.7 % m-1, almost double the 2.5 % m-1 cited for hypolimnetic aerators in a



  









21

review by Bernhardt (1967). Mean ambient DO concentration during the eight tests was 4.85 mg/l; Layer aeration has a greater influence on metalimnic thermal structure than hypolimnetic aeration. The straticication remain stable; Layer aeration perturbs naturals heat distribution, and applies best in lakes with metalimnetic anoxia; The layer aeration system at Lake Shenipsit required 3.25 m3 /d km2 needed for complete circulation (Lorenzen and Fast, 1977), and the 41 m3 /d km2 mean airflow of 15 hypolimnetic aeration case studies (Cooke et al., 1986). The Lake Shenipsit aeration system cost $943/ha (1987 dollards) which compares favorably to an estimated $5404/ha (1986 dollards) for hypolimnetic aeration installation cost (Cooke et al., 1986); Both implementation and operation of layer aeration appear to be cost-effective for restoring middepth habitat refugia continuous to epilmnetic forage resources (for zooplankton and coldwater fisheries), and for creating high quality water layers at supply intake depths (for phytoplankton and metals avoidance); Layer aeration provided a cost-effective aeration alternative, especially in stratified eutrophic lakes with anoxia metalimnia. It provides a mechanical means for reversing the ascent of compensation depth which occurred during eutrophication; Layer aeration uses available ambient DO sources (as in artificial circulation), while maintaining desirable temperatures and stratification stability; Layer aeration takes advantage of the airlift function for depthdiscrete mixing (like destratification) as well as the oxygen transfert from bubble to water (like hypolmnetic aeration).

In 1997, Mark H.Mobley, studied a diffuser aeration system (Figure 12). In this report, the Tennesse Valley Authority (TVA) has developed an efficient and economical aeration diffuser design that has been installed and operated successfully at six TVA hydropower projects, one TVA nuclear plant and two non-power reservoirs. The line diffuser design transfers oxygen efficiently, and minimizes temperature destratification and sediment disruption by spreading the gas bubbles over a very large area in the reservoir. This system was also installed in Spring Hollow Reservoir (in Virginia), for the studied of Vickie Singleton and Coll (2007).


22


Mobley, 1997; Vickie et al., 2007. Figure 12. Line Diffuser Design.

As results, the author shows that reservoir water quality profiles have display dramatic increases of dissolved oxygen in the hypolimnion with no significant disruption of thermal stratification. Representative reservoir dissolved oxygen ant temperature profiles upstream and downstream of the oxygen diffuser installation at Cherokee dam are shown in Figure 13. Also, the author, mentioned, that, in five years of operating experience, TVA has obtained satisfactory results in the operation of the line diffuser systems. The porous hoses have maintained their bubble pattern and have proven to be resistant to clogging and damage. Constant tailwater monitoring and frequent oxygen flow adjustments have been used by TVA to control oxygen usage.



23

In conclusion, the author had mentioned in this paper that TVA line diffuser is economical solution for meeting difficult dissolved oxygen requirements at hydropower projects. The line diffusers are designed to be installed and maintained without the use of divers, greatly reducing installation and maintenance costs. Operational costs are minimized due to high oxygen transfer efficiency and operational flexibility. Also, the author had shown that TVA has developed a high level of expertise from the solution of a variety of aeration problems with these systems at six of its hydropower installation.

Mobley, 1997. Figure 13. Reservoir profiles at Cherokee Dam, August 14, 1995.

Table 3. Financial comparison of TVA line diffuser system and existing system Existing System Line Diffuser Installation Cost Current Operation and Maintenance Cost (Avg. Year) Annual Savings (Avg. Year)

$1.234,200

Mobley, 1997.


TVA Line Diffuser $2.277,000 $785,300 $448,900

24


Always concerning TVA line diffuser installation, Mark H. Mobley and William D. Proctor (1997), Compare these last with the existing system at Richard B. Russell Dam (this one is located on the upper Savannah River near Elberton, Georgia, on the Georgia-South Carolina border), and estimated a capital cost and annual operation and maintenance costs for these two diffuser systems in the table 3. In addition to a substantial annual cost saving, these authors mentions that, the TVA line diffuser design would provide the advantages of 12 diffuser lines in the reservoir that can be operated independently, fitted with actuators for remote control, and economically refurbished with new porous hose if necessary (Mobley and William Proctor,1997). Based on a study of Standley lake (Colorado: USA), McGinnis and Little (1997), described a technical and economic analysis of these three systems (Figure 15): Airlift hypolimnetic aerator, Bubble-plume oxygenator, and Speece Cone oxygenator, in order to select the most appropriate aeration mechanism for a specific lake, thus optimizing both the design and operation to ensure the greatest oxygen transfer efficiency (see tables 4, 5 and 6).

Airlift Hypolimnetic Aerator (Figure 14-a) Full-lift hypolimnetic aerators typically consist of a vertical riser tube, a diffuser inside the bottom of the tube, an air air-water separation chamber at the top of the riser, and one or two return pipes, called downcomers (McGinnis et al., 1997; Vickie et al., 2006). Bubble-plume Oxygenator (Figure 14-b) Many researches on bubble plumes has emphasized the potential of destratification systems (Peterson and Imberger, 1989; Schladow, 1993) and artificial oxygenation of the hypolimnion (Wuest et al., 1992) to improve water quality of inland water. One solution of the restoration of lake is to install bubble-plume diffusers that replenish hypolimnetic oxygen without destratifying the reservoir (Wuest et al., 1992; McGinnis et al., 2001). Bubbleplume diffusers are generally linear or circular (Figure 18 and 25) and inject either air or oxygen at relatively low gas flow rate. These systems are most suitable for deep lakes where the bulk of the bubbles dissolve in the hypolimion and the momentum generated by the plume is low enough to prevent significant erosion of the thermocline (McGinnis et al., 1997; Vickie et al., 2006). For example, partial erosion of the thermocline and warming of the hypolimnion may cause premature destratification of the reservoir. Higher temperatures and plumes induced mixing may also be responsible for an



25

increase in hypolimnetic oxygen demand (Little and McGinnis, 2000; McGinnis et al., 2001).

Speece Cone Oxygenator (Figure 14-c) The Speece Cône was invented by Dr. Richard Speece, who originally termed it a downflow bubble conctor (McGinnis et al., 1998). The system consists of a source of oxygen gas, a conical bubble contact chamber, a submersible pump, and a diffuser that disperses highly oxygenated water into the hypolimnion (McGinnis et al., 1997; Vickie et al., 2006). Typically, pure oxygen is used in Speece Cône, air is used in Partial-Lift hypolimnetic (McGinnis et al., 1997; McGinnis et al., 2002), and bubbleplumes use oxygen or air (McGinnis et al., 1997; Little and McGinnis, 2000). Pure oxygen is used for hypolimnetic oxygenation to prevent the accumulation of molecular nitrogen which can be toxic to the fishes (Wuest et al., 1992; Gafsi et al., 2005). Thus, air aeration leads to elevated levels of turbulence within the hypolimnion which may increase sediment oxygen demand or results for an accidental destratification. Air aeration also leads to elevated levels of turbulence within the hypolimnion which may increase sediment oxygen demand or results in an accidental destratification (Beutel, 2002; Gafsi et al., 2012a).

a

b

McGinnis et al., 1997; Gafsi et al., 2009.

c

Figure 14. Schematic Representation of Three Oxygen Input Devices.


26

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al. Table 4. Partial-lift hypolimnetic aerator

Variable and predicted performances: -Air flow (Nm3/s) -Height of ascending tube (m) -Diameter of ascending tube (m) -Flow of drained water (m3/s) -Increase in oxygen concentration (g/m3) -Efficiency in oxygen transfer (%) -Total oxygen transfer (16 aerators) (kg/jour) - Oxygen transfer by aerator (kg/day) McGinnis et al., 1997; Gafsi et al., 2012a.

Values 0.12 12.2 3.10 1.17 4.60 16 464 7400

Table 5. Bubble plume Variable and predicted performances: -Oxygen flow (Nm3/s) -Initial diameter of bubbles (mm) -Length of the diffuser (m) -Initial speed of the plume (m/s) -Height of the plume rise -Efficiency in oxygen transfer (%) -Total oxygen transfer (kg/day) McGinnis et al., 1997; Gafsi et al., 2012a.

Values 0.069 2.5 2.500 0.038 1.5 93 7400

Table 6. Speece Cone Variable and predicted performances: -Oxygen flow (Nm3/s) -Initial diameter of bubbles (mm) -Imposed water flow (m3/s) -Detention time of bubble (min) -Increase in oxygen concentration (g/m3) -Total oxygen transfer (kg/day) -Efficiency in oxygen transfer (%) McGinnis et al., 1997; Gafsi et al., 2012a.

Values 0.068 2.0 1.3 2.0 66 7400 94



27

As results, McGinnis and Little (1997) showed that the bubble plume diffuser is the most economic system as well as being the most simple of these three systems. Their conclusions were based on the following:  

In the partial-lift hypolimnetic aerator, the efficiency of oxygen transfer is the lowest (16%); For the Speece Cone, and the bubble plume, the efficiency of oxygen transfer is very similar (94% and 93% respectively). In addition they found that a high value of the water velocity must be maintained in the Speece Cone in order to ensure that the bubbles would not reach equilibrium with the water. This may lead to a huge accumulation of bubbles and coalescence in the cone leading to a decrease in the total efficiency.

Also, these same authors, have conducted another study on Speece Cône, where developed a model that predicted gas-bubble dynamic and oxygen transfer. As shown in figure 15, the device generally consist of a source of oxygen gas, a conical downflow bubble contact chamber, a submersible pump, and a diffuser that disperses oxygenated water into the hypolimnion. The predicted performances of Speece Cône at different depths are shown in the table 4 (McGinnis and Little, 1998).

McGinnis and Little, 1998. Figure 15. Diagram of Speece Cone.


28

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al. Table 7. Speece Cône performance at varying depths

Depth (m)

Qgas (L/s)

0 20 10 40 20 60 30 80 40 100 50 120 McGinnis and Little, 1998.

△DO (g/m3 ) 17 33 50 66 83 101

Total Oxygen Transfer (kg/day) 2.200 4.300 6.400 8.600 10.700 12.800

Bubble residence Time (s) 107 75 69 66 64

Lindenschmidt et al., 1997. Figure 16. Schematic of the hypolimnetic aerator.



29

Table 8. Characteristics of Lake Tegel Surface area Volume Maximum depth Mean depth Lindenschmidt et al., 1997.

400 ha 24.6 million m3 16 m 6m

McGinnis and Little (1998), mentions in their conclusion, that the performance of the cone may compromised if too large bubble is produced. The water and oxygen gas flow rates have a substantial impact on the performance of the cone. They add that this process model, when coupled with a suitable cost model, should prove useful in the preliminary design and economic optimization of Speece Cone oxygenators. Lindenschmidt and Coll (1997) have realized their study in the Lake Tegel, in Berlin, which studies the hypolimnetic aeration in this lake. For this, they utilized the aerator presented in the figure 16. The aerator are of Limnox hypolimnetic type which supply air to the hypolimnion and are indented to not upset the thermal stratification of the lake. The characteristics of the lake are given in Table 8. In their conclusion, Lindenschmidt and Coll (1997), found that:  



For aerator design in Lake Tegel its mixing may be well represented as a conventional bubble plume and not as a hypolimnetic aerator; The finding that model errors were not reduced by treating the effect of aeration as a hypolmnetic mixed layer implies that the aerators do not mix the hypolimnion as expected but rather transport hypolimnetic water upwards to thermocline; The action of the aerators interacts with natural mixing processes. The buoyant plumes lift more dense water into the metalimnion thereby thinning the eplimnion. A thinner eplimnion enhances the vertical shear across the thermocline resulting in a higher rate of entrainment of hypolimnetic water in to the eplimnion thereby reducing the strength and duration of the stratified periods.

In their papers appeared in 1992, and 2004 respectevely, Wuest and Coll (1992), McGinnis and Coll (2004) describes a diffuser system «Tanytarus» (Figure17). This system was designed by the two Swiss engineers, E. Jungo and U. Schaffner, is presently in operation in several deep Swiss lakes. This


30


system of diffusers for both modes of operation, have been successfully designed and employed in several medium-sized Swiss lakes (Baldeggersee, Sempacherseee, Hallwilersee). Thus, according to McGinnis and Coll (2004), the system of the diffuser «Tanytarsus», is installed in 1986, as a final ultimate technique of restoration to fight the anaerobic middle found in the lake of Hallwil. This system is alternated between two modes of artificial aeration, the first is the aeration by the system of déstratification using big air bubbles; and the second is the hypolimnetic oxygenation, using respectively tiny bulls of air or oxygen. These systems are more suitable for deep lakes where the charge of dissolved bubbles in the hypolimnion and the momentum generated by the feathers are sufficiently weak to avoid a significant erosion of the thermocline. The six (06) diffusers of figure 17, have a diameter of 6.5 m each, and are located in a circular configuration of 300 m diameter near the middle of the lake (Figure 17 and table 9). Every diffuser uses air or oxygen during summer season for the hypolimnetic aeration mode and air during the cold season for the destratification aeration mode. The Halwill Lake is eutrophic lake, phosphorous limited that has been experimented in the anaerobic middle during the summer season for the last century (McGinnis et al., 2004). Table 6 shows the characteristics of the lake as well as the system of the diffuser that has been installed in the Hallwil Lake, when the destratification and the aeration or the hypolimnetic oxygenation is set.

Wuest, et al., 1992; McGinnis et al., 2004. Figure 17. One of the six diffusers of Tanytarsus that has a diameter 6.5 meters.


The Performance of Mechanical Aeration Systems … Table 9. Special characteristics of the Halwill Lake and the system of the diffuser (MciGnnis et al., 2004): The gas pressure is 1 bar; temperature is 0°C Parameters Maximum depth (m) Average depth (m) Surface (106 m2) Total Volume of water (106 m3) Shape of the des diffusers Number of the diffusers Diameter of the diffuser (m) Average depth of the diffusers (m) Gas flow of all diffusers (Nm3. h-1) Maximum depth (m)

Value 46.5 28.9 9.9 285 Circular 6 6.5 46 46-148(O2) 180 (air)

Camacho et al., 2000. Figure 18. Schematic diagram of the split-cylinder airlift reactor.


31

32


Sanchez Miron et al., 2002. Figure 19. The configuration of photobioreactors and air spargers, all dimensions in mm.



33

Another type of aerator as it happens airlift bioreactors (figure 18), has been studied by Fernando Camacho and Coll (2000), in order to developed models for prediction and interpretation of the observed steady-state axial dissolved oxygen concentration profiles in tall airlift bioreactors. The airlift bioreactors are used in advanced activated sludge processes for treating wastewater and in the bioprocess industry. The aspect ratio of airlift reactors typically exceeds 6, but much greater aspect ratios are seen in wastewater treatment operation that rely on high hydrostatic pressure in a deep airlift column to enhance oxygen transfer. Compared with conventional activated sludge processes, oxygen transfer rates in airlift devices are up to 10fold greater (4). In addition to providing oxygen, sparged air provides the motive force for circulating wastewater and suspending microbial flocs (Camacho et al., 2000). These authors, showed in their comments, that the reactor geometry parameters such as, the static heigh of liquid in the vessel, the cross-sectional areas of the riser and the downcomer channels, and the cross-sectional area available for flow under the baffle or the draft-tube, affect the oxygen concentration profiles. The design and scale of the reactor may significantly affect performance during biogical treatment of wastewater (Camacho et al., 2000). Sanchez Miron and Coll (2002), showed in their paper, one study, which compares biomass production in three compact, large-diameter vertical reactors (a bubble column, a split-cylinder airlift device, and a draft-tube airlift bioreactor) of the same overall configuration (figure 19). These systems of aeration are simple devices that have gained wide acceptance in gas-liquid contacting application in bioprocessing, the chemical process industry, and treatment of wastewater. According these authors, until the year 2000, the bubble columns and airlift reactors are not used as photobioreators, except for investigational purposes. Bubble columns and airlift photobioreactors can be useful for culturing phototropic organisms requiring light as a nutrient. Light availability in bubble coumns and airlift devices is influenced by aeration rate, gas holdup, and the liquid velocity (Sanchez Miron et al., 2000). In their conclusion, these authors, have found, that bubble column and airlift photobioreactors of up to 0.19m in diameter can attain a final biomass concentration and specific growth rate that is comparable to values typically reported for narrow tubes (e.g., 0.03m in diameter) of conventional horizontal tube reactors. The good performance of the large-diameter vertical reactors is explained partly by an absence of severe photoinhibition and photo-oxidation


34


of the biomass in them. Also, because of a good capacity for removing oxygen, the biomass in vertical reactors does not experience oxygen inhibition of photosynthesis (Sanchez Miron et al., 2002). A study on oxygen transfer and mixing in mechanically agitated airlift bioreactors, has realized by Yusuf Chisti and coll (2002). Their work aims on hydrodynamic and mass transfer characterization of a large (>1m3) impellerassisted airlift bioreactor. The Measurements realized by these authors, were made in a concentric draft-tube bioreactor (Figure 20) that was agitated with two identical downward pumping Prochem Maxflo T hydrofoil impellers. The 5-bladed impellers, 0.32m in diameter, were mounted on a 0.039m diameter shaft placed at the centerline of the bioreactor vessel. The vertical distance between the impellers was 0.68 mand the lower impeller was located 1.02 m from the bottom of the tank.

Yusuf Chisti and Ulises Jauregui-Haza, 2002. Figure 20. The hydrofoil impeller-agitated airlift bioreactor.



35

Vickie et al., 2002. Figure 21. Schematic representation of airlift aerator LPA 1.

In Their conclusion, these authors have produced the following results (Yusuf chisti and Ulises Jauregui-Haza, 2002): 



Use of low-power axial flow impellers in the downcomer of an airlift bioreactor can substantially enhance the rate of liquid circulation, mixing and gas–liquid mass transfer relative to operation without the agitator; however, the performance enhancements occur at the expense of a disproportionate increase in the power consumption. Increasing concentration of the relatively light fibrous solids greatly reduces the volumetric gas–liquid mass transfer coefficient.


36

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al.  



Surface aeration contributes but little to the total gas–liquid mass transfer in large bioreactors. In mechanically agitated draft-tube reactors, air sparging of the riser zone may or may not improve the mixing performance, depending on the intensity of the mechanical agitation. Mechanically stirred hybrid airlift reactors are well-suited for use with shear-sensitive fermentations that require good oxygen transfer and bulk mixing than can be provided by a conventional airlift reactor.

In their study, Vickie and Coll (2002) used ten airlift aerators in Lake Prince and 17 in Lake Western Branch. The aerator (figure 21) used in this study is part of the city of Norfolk‘s aeration system installed in two of its water supply reservoirs, Lake Prince and Lake Western Branch. The lakes are located in Suffolk Country, VA and have a total capacity of approximately 49x106 m3. In the past, anoxic conditions have developed in both lakes during the stratified period. Aerators were installed to increase dissolved oxygen and, hence, to improve water quality. The table 10 gives a summary of the relevant dimensions of this aerator during testing. In their report, these authors (2002) concluded that results represent a useful advance in the understanding of oxygen transfer in airlift aerators and, because of the fundamental nature of the model, suggest that this approach may be applied to other types of oxygenation devices. Also, they show that operation of the downcomer diffusers had almost no impact on the measured water flow rate in the riser. This allowed an abbreviated energy-balance model to be applied to the airlift aerator. By varying a single parameter (the frictional loss coefficient of the air–water separator) the model was found to provide results similar to those obtained in external-loop airlift bioreactors. These results support the use of an energy balance approach to determine water flow rate in airlift aerators (Vickie et al., 2002). Table. 10. Dimensions of Airlift aerator (LPA1) Parameter Riser length Downcomer length Riser diameter Downcomer diameter Vickie et al., 2002.

Value (m) 10 5 1.1 1.1



Beutel, 2002. Figure 22. Schematic of submerged contact chamber oxygenation system used in Camanche Reservoir, California and Newman Lake, Washington.

Vickie et al., 2006. Figure 23. Photograph of full-lift aerator prior to installation.


37

38


Bubble plume oxygenation

operational cost $ 850 $ 1.000

$1 million Not reported

Disadvantages

Very high oxygen transfer efficiency. Oxygen discharged horizontally over sediment-water interface. System efficiency independent of lake depth

Need for a submerged pump and chamber.

Low operating cost compared to on shore chamber. System efficiency independent of lake depth.

Need to construct 175-foot deep U-tube. Pumping involved.

$ 1.000 $ 1.200

Shallow pure oxygen Utube

Advantages

No pumping. Good horizontal distribution of oxygen.

Not reported

Diffuse deepwater oxygenation

$1 million

Deep pure oxygen U -tube

Not reported

Pure oxygen submerged chamber

Not reported

System (reference)

Capital cost

Table 11. Oxygenation systems

Tube only 20-30 feet deep. System efficiency independent of lake depth.

By pumping air through the diffusers, it can also be as destratification system.

Oxygen release above and away from sedimentwater interface. System efficiency decreases with lake depth. May impact thermal stratification. Pumping involved. Compared to deep U-tube, less oxygen delivered per unit flow through the system. System efficiency decreases with lake depth. Oxygen released above and away from sediment-water interface. System can impact thermal stratification.


operational cost $ 3.000

Pure oxygen on shore pressurized chamber

Not reported

System (reference)

Capital cost


Advantages

Disadvantages

Most facilities on shore. System efficiency independent of lake depth.

High pumping cost.

39

Beutel, 2002, Gafsi et al., 2008; Gafsi et al., 2009.

Vickie et al., 2006. Figure 24. Photographs of Speece Cone and discharge diffuser prior to installation at Camanche Reservoir, CA.

In 2002, Marc Beutel discusses a number of issues related to hypolimnetic oxygenation including advantages of hypolimnetic oxygenation systems over traditional aeration methods. Also, he discusses facility requirements and costs associated with various types of oxygenation systems, and the effects of oxygenation on water quality. The author conducted his study in Lake Baldegg, (in Switzerland), wherein it utilized, four main types of oxygenation systems: side stream


40


oxygenation, bubble plume oxygenation, diffuse submerged contact chamber oxygenation (Figure 22), and deep-water oxygenation (Table 11). The table 11 shows the costs as well as the advantages and the disadvantages of the different oxygenation systems. Vickie and Little in 2006 have showed in their paper, supporting information on selected hypolimnetic aeration and oxygenation installation (table 12), and the clear schematic for three hypolimnetic and oxygenation systems: Airlift aerator (Figure 23), Speece Cone (Figure 24), and bubble diffuser (Figure 25). Their work is based on the use of a simple discretebubble model to predict oxygen transfer in the various hypolimnetic aeration and oxygenation systems.

Vickie et al., 2006. Figure 25. Photographs of linear (top) and circular (bottom) bubble-plume diffusers.



41

Table 12. Summary of selected hypolimnetic aeration and oxygenation installations documented in the literature Maximum Depth (m)

Volume (106 m3)

Oxygenator type

Year installed

Oxygen addition (kg/d)

Wahnbach Reservoir, Germany Mirror Lake, Wisconsin Silver Lake, Wisconsin Larson Lake, Wisconsin Lake Waccabuc, New York

23

24

Fill-lift aerator

6744

1,560

61

0420

Fill-lift aerator

6754

666

Fill-lift aerator

6754

63

Waterbody

64 64

0467

Fill-lift aerator

6751

44

61

246

Partial-lift aerator

6751

130

Ottoville Quarry, Ohio

66

04041

Side stream pumping

6751

62

Spruce Knob Lake, West Virginia Clark Hill Reservoir Georgia

4

Full-lift aerator

6752

27

6753

54,40

Lake Ghirla, Italy

62

6754

0,13

Lake Nantua, France

24

Bubble-plume diffuser Submerged pumping oxygenation system Side stream oxygen injection

6754

200-250

Black Lake, British Columbia

7

0466

Full-lift aerator

6756

Tory Lake, Ontario Lake Sarkinen, Finland Lake St, George, Ontario

60

04033

Full-lift aerator

6756

65

443

Mixoxaerator

6760

Full-lift aerator

6760

Lake Tegal, Germany

64

4244

6760

4,500

Weblinger See, Germany Lake Baldegg, Switzerland Lake Pyhajarvi, Finland Lake Sempach, Switzerland

64

640

6766

120

44

654

Limnox partial-lift aerator Limno full-lift aerator Bubble –plume diffuser

6764

3,000-4,500

mixox

6761

1,300

65

444

6762

3,000

Lake Hald, Denmark

16

22

Richard B, Russell Reservoir, Georgia

25

64450

14074

440

64

24

Bubble –plume diffuser Bubble-plume diffuser Bubble –plume diffuser

575 6763


200,000

42

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al. Table 12. (Continued) Maximum Depth (m)

Waterbody Glen Lake, British Columbia Lake Hallwil, Switzerland Lake Kallvesi, Finland Midical Lake, Washington St.Mary Lake, British Columbia Lake Shenipsit, Connecticut Lake Muggesfelde, Germany Amisk Lake, Alberta Lake Huruslahti, Finland Lake Krupunder, Germany Medical Lake, Washington Lake Prince, Virginia Newman Lake , Washington Camanche Reservoir, California Douglas Dam, Tennessee Lake Western Branch, Virginia Lake Stevens, Washington Tombigbee River, Alabama Heart Lake, Ontario Whittaker Lake, Ontario Spring Hollow Reservoir, Virginia Upper San Leandro Reservoir, California

Volume (106 m3)

61 25

463

16 66

444

746 (mean)

Oxygenator type

Year installed

Oxygen addition (kg/d)

Full-lift aerator

6764

40

6764

1,000-7,100

Bubble –plume diffuser Mixox aerator Limno partial-lift aerator

6764 6764

225

Full-lift aerator

6764

311

46

6441

Layer aeration

6765

512 (after retrofit)

46

46

Tibean full-lift aerator

6765

500

34 (north basin)

25 (north basin)

Bubble-plume diffuser

1988

750-1,000

Mixox aerator

1990

26 10,5

0,28

Tibean full-lift aerator

1990

80

18

6,2

Full-lift aerator

1990

500

10

13,9

Full-lift aerator

1991

4,100

10

28,6

Speece cone

1992

2,000

41

545

Speece cone

1993

9,000

38

1700

Bubble-plume diffuser

1993

100,000

11

24,4

Full-lift aerator

1993

6,600

Full-lift aerator

1994

2,900

44 11

n/a

u-tube

10,9

0,78

Full-lift oxygenator

1995

140-200

11

0,39

Full-lift oxygenator

1995

140-200

55

7,2

1998

250

2002

9,000

51

Bubble-plume diffuser Bubble-plume diffuser

23,600

Both experimental and permanent units are included (Vickie et al., 2006).



43

Vickie and Coll (2006) concluded that:  



The airlift model should also be extended to other full- and partial-lift aerator designs; The Speece Cone model should be verified against field data for a range of applied gas flow rates. Also, a method should be developed to predict the effect of plume operation on near-field boundary conditions, short circuiting of plume detrainment, and plume fallback beyond the equilibrium depth; The operation of hypolimnetic aeration and oxygenation devices usually alters the DO concentration profiles and thermal structure of a waterbody. Oxygen transfer efficiency is a function of the surrounding water column properties, establishing a feedback loop that continually changes system performance. This effect is most pronounced during operation of bubble-plume diffusers because plume performance depends strongly on the vertical density gradient. Also, they add, that, the interaction of the aerator/oxygenator with the water column should be accounted for in the design and operation of bubble-plume diffusers as well as the other aeration and oxygenation devices.

5. COMPARED EFFECTS OF AERATION SYSTEMS 5.1. Compared Results of Destratification Intermittent and Destratification Permanent In 1988, Christian Steinberg and Coll, compared two methods of detratification, intermittent and permanent destratification in Lake Fischkaltersee, so as to demonstrate that certain physical actions (i.e., mixing) can be applied successfully to lake remediation. Futhermore, they wanted to show that Lake Remediation measures, which include these physical actions, can serve as a rather effective and rapid manipulation tool in managing cyanobacterial blooms in lakes, mostly independent of the nutrient pool. These two remediations started in April 1980 and April 1985. They demonstrated that intermittent destratification has some advantages over permanent destratification. The authors concluded that intermittent destratification provides very rapid remediation and also appears to be applicable in relatively


44


shallow lakes (‹15 m), whereas permanent destratification methods appear risky in shallower waters (Christian Steinberg et al., 1988; Gafsi et al., 2009).

5.2. Compared Results of Destratification and Hypolimnetic Aeration Systems Richard J. Ruane and his co-workers (1977) have compared the two methods of aeration (Table 13) applied to the Patrick Henry Dam (south of Holston river, USA). As remark, Ruane and his co-workers (1977) reported that destratification and hypolimnetic aeration are not applicable to the Patrick Henry dam. They explained that the first method could increase the water temperature and consequently lead to ominous effects on the pisciculture, since the dam is classified as a coldwater fishery. In addition, the hypolimnetic aeration with air causes nitrogen supersaturation, and the aeration with oxygen is not promising due the high cost of oxygen (Ruane et al., 1977; Gafsi et al., 2009]. In order to increase the oxygen content in the hypolimnion, oxygen gas can be used instead of air. The advantage of using oxygen gas is that a compressor is not necessary, and that oversaturation with nitrogen is avoided (Ruane et al., 1977). Table 13. Application domain of the aeration methods for the Patrick Henry dam Hypolimnetic Aeration with diffusion

Conditions under which the aeration technique must be applied

Destratification Air Diffusion

Air

Oxygen

Cold water Strong increase in DO Minimal effect on the production of energy Minimal increase in the dissolved nitrogen Small cost

-

+ ?

+ ?

+

+

+

+

?

?

?

+

+

+

-

-

Mechanic Pumping -

+ Indicates a positive effect on the particular aeration method. - Indicates a negative effect on the particular aeration method. ? Indicates an unknown effect for the shown condition. Ruane et al., 1977; Gafsi et al., 2008; Gafsi et al., 2009.



45

The published results of the effect of oxygenation on the nitrogen concentration are not consistent. The total nitrogen and total ammonium diminish in some surveys, but increase in others (Vickie et al., 2002; Gafsi et al., 2009). In 1988, Christian Steinberg and Coll, studied the hypolimnetic aeration method during the summer months and tried to ensure continual thermal gradients. To allow dead algae to settle and minimize internal mixing, whole lake mixing in spring and autumn was proposed to provide maximum oxygen transfer and optimal nitrification at all depths. In their study, these authors emphasized that hypolimnetic aeration is 10 times more costly than intermittent destratification. Hypolimnetic aeration possibly will amplify the production by enhancing eddy diffusion (Steinberg et al., 1988; Gafsi et al., 2009).

5.3. Compared Positive Results of Hypolimnetic Aeration and Destratification Aeration 5.3.1. Destratification Aeration Systems 



 



Destratification is generally effective, especially when the presence of hydrogen sulphide, iron, manganese and other conditions associated with anaerobic water are problematic (Fast, 1978, Gafs et al., 2009). Destratification may limit the proliferation of algae, if the mixing is total, and if the lake has sufficient depth in its euphotic region (Fast, 1978, Gafs et al., 2009). Many studies showed a substantial increase in the distribution of fish depth associated with destratification (Fast, 1978, Gafsi et al., 2009). In winter, the destratification system can prevent the fish killing by ice covering the lakes. Sometimes, the destratification can increase the fish production, by bringing to the surface the nutritional elements regenerated from the hypolimnion, which are not precipitated by the increase of the reduction-oxidation potential or by the CaCO3 (Fast et al., 1973, Gafs et al., 2009). Destratification using the bubble plume system may have some effect on the water quality, because these plumes can occupy the whole water column or compartmentalize this water column by a plume cascade. This has a large impact on the water quality. For instance, a


46

Gafsi Mostefa, Djehiche Abdelkader, Kettab Ahmed et al. high concentration of nutritients from the lake sediments can be transferred rapidly to the photic region by a bubble plume that occupies the whole water column. In contrast, a plume cascade should have a small transfer capacity of nutritional elements. Also, in some circumstances, a plume cascade may be undesirable, although the mixing efficiency is small (Shladow, 1992; Gafsi et al., 2009).

5.3.2. Hypolimnetic Aeration Systems The hypolimnetic aeration has many advantages over the destratification system. The nutrients (nitrogen and phosphorus) are not transported to the epilimnion where they can stimulate algae growth, and the process can preserve a cold water habitat for fishes such as salmon and trout (Vickie et al., 2002; Gafsi et al., 2009). The advantage of hypolimnetic aeration is the potential to re-supply dissolved oxygen while preserving the thermal stratification (McGinnis et al., 2004; Gafsi et al., 2009). McQueen and Lean conclude that (McGinnis et al., 2004; Gafsi et al., 2009): 

    

A well conceived oxygenation system can maintain the stratification and need not significantly increase the temperature of the hypolimnion water (McGinnis et al., 2004; Gafsi et al., 2009); The level of hypolimnion oxygen increases (Little et al., 2000; Gafsi et al., 2009); The concentrations of iron, manganese, hydrogen sulphide and methane decrease (Little et al., 2000; Gafsi et al., 2009); The population of zooplankton is not affected in general (McGinnis et al., 2004; Gafsi et al., 2009); The concentrations in chlorophyll A are typically not changed (McGinnis et al., 2004; Gafsi et al., 2009); The depth distribution of cold water fish populations is increased (McGinnis et al., 2004; Gafsi et al., 2009);

Hypolimnetic aeration has no effect on the depth distribution of algae. The most promising means by which the hypolimnetic aeration may affect the algae density are (Fast, 1978):  

The modification of the nutrient cycle; Creation of a change in the composition of species and the zooplankton density, the benthic fauna and other trophic levels.


The Performance of Mechanical Aeration Systems … SUMMER WITHOUT HYPOLIMNETIC AERATION

SUMMER WITH DESTRATIFICATION

47

SUMMER WITH HYPOLIMNETIC AERATION

TEMPERATURE ( OC) 0

30

40

50

60

70

30

2

4

6

8

0

40

50

60

70

30

40

50

60

70

4

6

8

0

2

4

6

8

80

DEPTH (FEET)

10

20

30

40

0

2

OXYGEN (ppm)

10

Fast, 1978; Gafsi et al., 2006. Figure 26. Influence of artificial aeration on fishes in an eutrophic lake during summer time.

Hypolimnetic aeration can increase the diversity of species by the creation of an adequate habitat for coldwater fishes (Fast, 1978; Gafsi et al., 2009). Sometimes, hypolimnetic aeration is preferred over destratification in the management of fisheries and in the supply of domestic and industrial water, since the total mixing may promote an increase of algae (Fast, 1973; Gafsi et al., 2009). Fast (1978) reported that the hypolimnetic aeration creates an adequate habitat for cold water fishes in different lakes where no previous aeration has been carried out (Figure 26) (Fast, 1978; Gafsi et al., 2006). The hypolimnetic aeration can be also used to prevent winter fish death. The application of this system during the summer yiels to the oxidation of organic materials and reduce then the oxygen demand in winter (Fast, 1978). Fast (1978) reported that hypolimnetic aeration creates an adequate habitat for coldwater fishes in different lakes where no previous aeration has been carried out. Hypolimnetic aeration can also be used to prevent winter fish


48


death. The application of this system during the summer yields the oxidation of organic materials and subsequently reduces the oxygen demand in winter. Other advantages of hypolimnetic oxygenation (Beutel, 2002): 

 

Weak rate of water recycling minimizing the turbulence in the hypolimnion thus reducing SOD and limiting accidental destratification; It maintains a high level of DO during the whole period of stratification; Low energy cost.

5.4. Compared Negative Results of Aeration Systems It has been reported that the aeration using air compression raises the nitrogen gas concentration and consequently may cause fish death (McGinnis et al., 2002; Vickie et al., 2002); however McQueen and Lean found no unfavorable effect on the fish population (Vickie et al., 2002). The concentration of hypolimnetic nitrogen in Waccabuc Lake increased to 150% of saturation for 80 days during continuous hypolimnetic aeration (figure 27) (Fast, 1978; Gafsi et al., 2009). The use of oxygen injection helps to avoid problems related to nitrogen oversaturation (Beutel, 2002). Also, the destratification system could increase the water temperature, while the air hypolimnetic aeration may cause the problem of nitrogen oversaturation (Ruane et al., 1977; Speece et al., 1994). The destratification system is less effective in the reduction of algae (Fast, 1978; Gafsi et al., 2009). Destratification increases the sediment temperature as well as the water flow on the sediments, which would increase the phosphorus exchange rate with the sediments (Fast, 1978). Destratification can increase the growth of nutritional elements in the eutrophic region, and hence stimulate algae growth (Fast, 1978; Gafsi et al., 2009). When the lake is strongly eutrophied and contains an active mud layer, hypolimnetic aeration can not by itself maintain a production of respiratory equilibrium, and cannot even limit this equilibrium. Many studies found that the circulation of the hypolimnion in small lakes causes the movement of nutrients (phosphates) by eddy diffusion from the hypolimnion to the metalimnion, where the phytoplankton biomass (principally the blue green algae) is significantly increased (Bernhardt et al., 1985).



49

The artificial destratification in Casistas Calif Lake (San Diego, USA), caused excess oversaturations of nitrogen to about 140% relative to the surface pressure (figure 28) (Fast, 1978; Gafsi et al., 2009). Fast (1978), mentioned that all the parameters affecting the nitrogen concentration during the destratification with air compression are unknown. He concluded that they include probably the mixing level, the air bubble density, the plume‘s vertical speeds, the depth of the air injection, the quotient of the water total volume to the total injected air volume and finally the water oxygen content (Fast, 1978; Gafsi et al., 2009).

Fast, 1978; Gafsi et al., 2009. Figure 27. Temperature values and concentrations of oxygen and nitrogen vs the depth during the hypolimnetic aeration in Waccabuc lake.


50


Fast, 1978; Gafsi et al., 2009. Figure 28. Concentrations of oxygen, nitrogen and temperature values vs the depth during the destratification aeration in Casistas Calif Lake.

Aeration by destratification may result in numerous harmful impacts on the lake. Some of these include: 



These systems give rise to a microthermal stratification near to the surface, which may result in an increase in algal growth (Fast, 1978; Gafsi et al., 2009); They provoke a temperature rise in the lake during the summer, eradicating cold water species such as trout and salmon (Fast, 1978; McGinnis et al., 2001) ;


The Performance of Mechanical Aeration Systems … 

51

The change in some chemical properties as the concentration of nitrogen and phosphorous could present a problem for the distribution of fishes, zooplankton, benthic fauna, and other biota ([Fast, 1978; Gafsi et al., 2009).

According to Brian Kirke and Ahmed El Gezawy (1997), the method of destratification has three problems (Kirke et al., 1997):  



It consumes a lot of energy: The compressors use power range of 37 to 100 kW, and they can be ineffectual for large reservoirs; A plume thrust is not able to penetrate a thick thermocline and the destratification is achieved only by the friction process of air bubbles, with ineffective mixing; Introduction near the lake floor of high energy can increase the turbidity and hence disturb the sediments and release of nutrients.

CONCLUSION It has been shown in the studies described above that all the mechanical aeration techniques have both advantages and disadvantages. For the restoration of a lake or a reservoir, a system must satisfy both technical and economic aspects. Aeration by destratification is a simple technique which needs less means compared to hypolimnetic aeration, but at the same time remains inefficient in the conservation of the thermal equilibrium of lakes. Technically, hypolimnetic oxygenation is most efficient but is more costly. This study allows us to conclude that:  The aeration by oxygen limits the nitrogen saturation; in contrast the aeration by air creates it;  The most efficient hypolimnetic aeration system is the bubble plume diffuser; although an accidental destratification may occur. In shallow reservoirs, these systems should be avoided, because it can entrain the colder, hypolimnion water and carry it through the thermocline into the epilimnion and to the surface by the momentum induced by the bubble plume;  The line diffusers are designed to be installed and maintained without the use of divers, greatly reducing installation and maintenance costs.


52




  

    



Operational costs are minimized due to high oxygen transfer efficiency and operational flexibility; The TVA line diffuser installation was successful in reducing total dissolved iron in the reservoir to minimize water supply treatment costs; The full airlift hypolimnetic aerator has the exploitation fee the much less and the higher efficiency; The free rising bubble plumes are to be avoided in order to maintain stratification; Destratification can be used in winter because the temperature of the lake is not modified. However, the hypolimnetic aeration is used in summer in order to avoid the homogenization of the lake temperature during this period; For reservoirs and dams, summer destratification is not profitable due to the warming-up of the reservoir water which can be a problem; Nevertheless, some experiments showed that the mechanical aeration is not lasting for the following reasons; Duct pipes are frequently plugged by algae, and consequently become difficult to use; When the mechanical aeration is stopped, the lake rapidly becomes eutrophic; Mechanical aeration equipment is generally placed about 7 cm above the lake floor. This does not allow restoring the region below and therefore leads to the hypoxia and disappearance of fishes living near the lake floor; Consequently, and according to the depth and dimension of the exploitations, we can distinguish two kinds of restorations: The restoration by destratification appears to be the most adaptable during the cold months and to shallow lakes or reservoirs whose use is exclusively agricultural. Hypolimnetic aeration suits deep lakes or reservoirs that have different uses (feeding, irrigation, leisure, etc.), allowing the amortization of the invested fees for the system.

However, it should be noted that in the case of lakes or reservoirs used for fish farming, the hypolimnetic oxygenation limits the amount of nitrogen introduced as compared with systems that aerate using air, and guarantees the preservation of the fish.



53

REFERENCES Aseada, T., and Imberger, J. 1993. Structure of Bubble Plumes in Linearly Stratified Environments. J. Fluid Mech., 249, 35-57. Baines, W. D., and Leitch, A. M. 1992. Destruction of Stratification by a Bubble Plume. J. Hydraul. Engng ASCE 118, 559-577. Bernhardt, H. 1967. Aeration of Wahnbach Reservoir without changing the temperature profile. J. Am. Wat. Work Assoc. 63;943-964. Bernhardt, H., and Clasen, J. 1985. Recent developments and perspectives of restoration for artificial basin used for water supply. Intern. Congr. on Lake Pollution and Recovery, 213-227. Brevik, I., and Ø. Kristiansen 2002. The flow in and around air-bubble plumes, Int. J. Multiphase Flow, 28(4), 617-634. Brzozowska Renata, Helena Gawronska, Jolanta Grochowska 2001. Nutrient release from the bottom sediments of artificially aerated lake Dlugie. Limnogical Review, 1, 25-32. Camacho, F. R., Garcia, J. L., Molina, E., Chisti., Y, 2000. Axial Inhomogeneities in Steady-State Dissolved in Airlift Bioreactors: Predictive Models. Chemical Engineering Journal, pp 1-13. Chen, M. H, and S.S.S. Cardoso 2000. The mixing of liquids by a plume of low-Reynolds number bubble. Chemical Engineering Sciences 55, 25852594. Cooke, G. d Welch, E. B., Peterson, S. A., and Newroth, P. R, 1986. Lake and Reservoir Restoration. Butteworth. Publ., New YorkPrecursor Control. Amer. Wat. Works Asso. Research Foundation, Denver, CO. Davis, J. M. 1980. Destratification of reservoirs- A design approach for perforated-pipe compressed-air systems. Water Serv., 84, 497-505. Ditmars, D., and Klas Cederwall, 1974. Analysis of Air- Bubble Plumes. In Proc. 14th Conf. Coastal Engng, Copenhagen, Ch. 128, pp. 2209-2226. ASCE. Fast, AW., Brian Moss, et Robert G. Wetzel. 1973. Effets of Artificial Aeration on the Chemistrie and Algae of Two Michigan Lakes. Water Ressources Research, 9, 624-647. Fast, AW., William J. Overholtz, et Richard A. Tubb. 1975. Hypolimnetic Oxygenation Using Liquid Oxygen. Water Ressources Research, II, 29429. Fast, AW. 1978. Arificial Aeration as a lake restoration technique. Proceeding of National Conf on lake restoration, 121-131.


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Gafsi, M., Kettab, A., Benmamar, S., and Benziada, S., 2005. Etat de Connaissance sur les Différents Systèmes Mécaniques Impliqués dans la restauration des Lacs et réservoirs. Congrès International sous le thème: De l‘Eau pour le développement Durable dans le Bassin méditerranéen, LRS-EAU/RMEI, le 21-22-23 Mai, 2005, Alger (Algérie). ALgerian Journal of Technologie (AJOT), special issue, 363-373. Gafsi, M., Kettab, A., Benmamar, S., et Benziada, S. 2006. L‘eutrophisation dans les Eaux de Surface; Causes, Effects et Luttes. Third International Conference on Water Resources in Meditererranean Basin. Watmed3 Tripoli-Lebanon 1-3. Gafsi, M., Kettab, A., and S. Benmamar, S, 2008. The Strategy of the Control of Nutrional Elements in the Water Reserve. Twelfth International Water Technology Conference 27-30 March 2008 Alexandria – Egypt. Proceeding: pp. 1699-1710 (2008). Gafsi, M., Kettab, A., Benmamar, S., and Benziada, S, 2009. Comparative Studies of the Different Mechanical Oxygenation Systems Used in the Restoration of Lakes and reservoirs. International journal of Food, Agriculture and Environment-JFAE Vol 7. (2)-2009. Gafsi, M., Kettab, A., Djehiche, A, 2012a. Study of the Oxygen Transfer Efficiencies in the Different Methods Used in the Technique of Hypolimnetic Aeration. Advanced Materials Research Vols. 452-453 (2012) pp 1014-1019 © (2012) Trans Tech Publications, Switzerland. Gafsi, M., Kettab, A. 2012b. Treatment of Water Supplies by the Technique of Dynamic Aeration. Procedia Engineering 33 (2012) 209 – 214. www.elsevier.com/located/procedia. Goloka Behari sahoo and David Luketina (2005). Gas Transfer During Bubbler Destratification of Reservoirs. Journal of Environmental Enginering, Vol.131, No.5, May 1, 2005, pp.702-714. Hussain, N. A and Narang, B. S. 1984. Simplified analysis of air-bubble plumes in moderately stratified environments. Journal of Heat Transfert, Vol.106. Paper N°. 83-HT-69. Trans. ASME C: 106, 543-551. Janczak Jerzy and Andrzej Kowalik 2001. Assessment of the Efficiency of Artificial Aeration in the Restoration of Lake Goplo. Limnogical Review, 1, 151-158. Kirke, B., and El Gezawy, A, 1997. Design and Model Tests for an Efficient Mechanical Circulator/Aerator for lakes and Reservoirs. Water Research, 31, N°.6., 1283-1290. Kobus, H. E. 1968. Analysis of the Flow Induced by an Air-Bubble system. In Proc. 11th Conf. Coastal Engng, London ASCE, 1016-1031.



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Kortmann, Robert W., George W. Knoecklein, and Charles H. Bonnell, 1994. Aeration of Stratified Lakes: Theory and Practice. Lake and Reserv. Manage. 8(2): 99-120. Kyung Soo Jun and Subhash C. Jain. (1993). Oxygen Transfer in Bubbly Turbulent Shear Flow. Journal of Hydraulic Engineering, Vol. 119, N0. 1, January, 1993. Lemckert, C. J., and J. Imberger (1993). Energetic bubble plumes in arbitrary stratification, J. Hydraul. Eng., 119(6), 680-703. Lindenschmidt, K. E., and Hamblin, P. F, 1997. Hypolimnetic Aearation in Lake Tegel, Berlin. Wat. Res. Vol. 31, N°. 7, pp. 1619-1628, 1997. Little, J. C., and McGinnis, D. F., 2000. Hypolimnetic Oxygenation: Predicting Performance Using a Discrete-Bubble Model. Proceeding of 1st World Water Congress, International Water Association (IWA), Paris, France. Lorenzen, M. W., and Fast, A. W, 1977. A Guide to Aeration/Ciculation Techniques for lake magement. Ecol. Res. Ser. EPA-600/3-77-004. U.S. Environ. Prot. Agency, Washinton, DC. Marc Beutel. 2002. Improving Raw Water Quality with Hypolimnetic Oxygenation. AWWA 2002 Annual Conference Marc Beutel, Brown and Caldwell Environmental and Consulting 201 North Civic Drive, Walnut Creek, CA 94596 925-210-2844. McGinnis, D. F et Little, J. C. 1997. Nutrient Control in Standley lake: Evaluation of Three Oxygen Transfer Devices. Proceeding of the IAWQ/IWSA joint Specialist Conference Reservoir Management and Water Supply-an Integrated System Prague, Czech Republic. McGinnis, D. F et Little, J. C., 1998. Bubble dynamics and oxygen transfer in Speece Cone. In Proceeding of the IAWQ/IWSA joint Specialist Conference, Reservoir Management and water Supply- an Integrated System, Prague, Czech Republic. McGinnis, D. F, Little J. C and Wuest A. 2001. Hypolimnetic Oxygenation: Coupling Bubble-Plume and Reservoir Models. Proceedings of Asian WATERQUAL 2001, IWA Regional Conference, Fukuoka, Japan. McGinnis, D. F et Little, J. C. 2002. Predicting Diffused- Bubble Oxygen Transfer Rate Using the Discete-Bubble Model. Water Research 36, 46274635. McGinnis, D. F, A.Lorke, A.Wuest, A.Stockli, and J. C. Little. 2004. Interaction between a bulle plume and the near field in a stratified lake. Water Resources Research, 40, W10206, doi:10.1029/2004WR003038.


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Miron, A. S., Camacho, F. G., Gomez, A. C., Molina, E. G., and Chisti, Y, 2000. Bubble-Column and Airlift Photobioreactors for Algal Culture. AIChE Journal, Vol. 46, N°. 9, September 2000. Mobley, Mark H. 1997. TVA Reservoir Aeration Diffuser System. Technical Paper 97-3 Presented at WaterPower 97 August 5-8, 1997, Allanta, Georgia. Mobley, Mark H., and William D. Proctor, 1997. Richard B. Russell Forebay Aeration Using Line Diffusers: Cost Comparison with exixting System. Tenesse Valley Authority Resource Group, Engeneering Services.Engeneering Laboratory. Norris, Tenesse, May 1997. Mobley, Mark H, Gary E. Hauser, Dan F. McGinnis, R. Jim Ruane, 2000. Diffuser System Modeling and Design for Dissolved Oxygen Enhancement of Reservoirs and Releases. International Association of Hydraulic Research Symposium 2000, Charlotte, North Carolina. Motarjemi, M., and Jameson, G. J., 1978. Mass Transfer from very Small Bubbles: The Optimum Bubble Size for Aeration. Chemical Engineering Science Vol. 33, pp. 1415-1423. Paterson, J. C., and Imberger. J., 1989. Simulation of bubble plume destratification systems in reservoirs. Aquatic Sciences. 51(1).3-18. Prepas, E. E, and Burke, J. M., 1997. Effects of Hypolimnetic Oxygenation on Water Quality in Amisk, Alberta, a Deep, Eutrophic Lake with High Internal phosphorus Loading Rates. Can. J. Fish. Aqua. Sci. 54:21112120(1997). Richard J. Ruane, Svein Vigander, et William R. Nicholas., 1977. Aearation of Hydro Releases at Ft. Patrick Henry Dam. Proceeding of American Society of Civil Engeneers, 103, N°. HY10, 1135-1145. Raney Donald C., and Terry G. Arnold, 1973. Dissolved Oxygen Improvement by Hydroelectric Turbine Aspiration. Journal of the Power Division. Proceedings of the American Society of Civil Engineers, Vol. 99, N°.P01, May, 1973, 139Roberston, D. M., Schladow, S. G., and Patterson, J. C. (1991). Interacting Bubble Plumes: The Effect on Aerator Design. Environmental hydraulics, 1991., pp. 167-172. Schladow, S. G., 1992. Bubble Plume Dynamics in Stratified Medium and the Implications for Water Quality Amelioration in Lakes. Water Resources Research, 28, N°.2, 313-321. Schladow, S. G., 1993. Lake Destratification by Bubble-Plume Systeme: Design Methodologie. Journal of Hydraulic Engeneering, Vol.119, N°3, March, 1993, 350-367.



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Scott, W. and Foley, A. L. 1921. A method of direct aeration of stored water. Proc. Ind. Acad. Sci. 1919:71-73. Speece, Richard E., 1994. Lateral Thinking Solves Stratification problems. Water Quality. WQI 3, 12-15. Stefan A. McCord, P. E., S. Geoffrey Schladow, and Theron, 2000. Modeling Artificial Aeration Kinetics in Ice-Covered Lakes. Journal of Environmental Engineering, 126, N°.1, 21-31. Steinberger Christian, and Gitta M. Zimmermann 1988. Intermittent Destratification: A Therapy Measure Against Cyanobacteria in Lakes. Environmental Technologie Letters, 9,337-350. Steinberger Nancy and Midhat Hondzon 1999. Diffusional Mass Transfer at Sediment-Water Interface. Journal of Environmental Engineering, 125 No.2, Paper No.15976, 192-200. Vickie L. Burris, Daniel F. McGinnis and John C. Little., 2002. Predicting oxygen transfer and water flow rate in airlift aerators. Water Research 36, 4605-4615. Vickie L. Singleton and John C. little 2006. Designing Hypolimnetic Aeration and Oxygenation Systems. Environmental Sciences and Technology, 40, No.20, 7512-7520. Vickie L. Singleton, Gantzer, P., and Little, J. C, 2007. Linear Bubble Plume Model For Hypolimnetic Oxygenation: Full Scale Validation and Sensitivity Analysis. Water Resources Reserach, Vol. 43, W02405, doi: 10.1029/2005WR004836, 2007. Wen, J., and Torrest, R. S, 1987. Aeration-induced circulation from line sources. I: Channel flows, J. Environ. Eng., 113(1), 82-98. Wilkinson, D. L.1979. Two-dimensional bubble plumes. Journal of Hydraulic Division ASCE, 105(2), 139-154. Wuest, A., Brooks, N. H.et Imboden, D. M, 1992. Bubble plume modelling for lake restoration. Water Resources Research, 28, 12, 3235-3250. Yusuf, C., and Ulises, J. J. H, 2002. Oxygen transfer and mixing in mechanically agitated airlift bioreactors. Biochemical Engineering Journal 10 (2002) 143–153 Zic, K., Stefan, H. G., and Ellis, C, 1992. Laboratory study of water destratification by a bubble plume. J. Hydraul. Res, 30, N° 1. 7-27.





Chapter 2

USING CYANOBACTERIA AS A BIOSORBENT FOR HEAVY METALS IN WASTE WATERS: FEASIBILITY AND CHALLENGES Chaoyang Wei1,, Di Geng1,2 and Hongbing Ji2 1

Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, China 2 Civil & Environment Engineering School, University of Science and Technology, Beijing, China

ABSTRACT Biosorption has become a promising approach for the treatment of wastewater containing heavy metals. The identification of organisms with high heavy metal adsorption capacities is a topic of increasing interest in biosorption research. Cyanobacteria are widely distributed worldwide, and numerous studies have indicated that these organisms have great potential for the adsorption of heavy metals in wastewater. Algal blooms are rich in Cyanobacteria, and lake eutrophication can produce large amounts of low-cost, collectable blooms. Algal resources in the natural environment, such as substances from algal blooms, may have potential for use as commercial biosorbents. This chapter will provide a 

Corresponding author: [email protected] (C. Wei).


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Chaoyang Wei, Di Geng and Hongbing Ji comprehensive introduction to the properties of these cyanobacterial substances, the mechanisms of their adsorption of heavy metals, the factors influencing the adsorption and pretreatment processes, immobilization techniques, and the adsorption properties of cyanobacterial biosorbents. The feasibility and challenges of using cyanobacterial substances as biosorbents for heavy metals in wastewater will be fully discussed, along with the prospects for future studies in these areas.

Keywords: Cyanobacterial blooms; Adsorption; Heavy metals

1. INTRODUCTION The large amounts of industrial effluents and domestic sewage produced by industrialization and urbanization have accelerated the deterioration of aquatic environments. [1] Heavy metals are discharged into natural water in the industrial production process, the impact of these toxic heavy metals on the environment and on human health have been thoroughly documented [2] Various industries produce and discharge wastes containing different heavy metals, including the mining and smelting, surface finishing, energy and fuel, fertilizer and pesticide, metallurgy, iron and steel, electroplating, electrolysis, electro-osmosis, leatherworking, photography, electrical appliance manufacturing, metal surface treating, aerospace, and atomic energy installation industries. [3] Untreated sewage from these plants has adverse impacts on the environment. [4] The metals of most immediate concern are Cr, Mn, Fe, Cu, Zn, Hg, Pb, and Cd. These heavy metal elements are mined from the Earth‘s crust. [1] Thus, there are concerns about shortages of these metal resources, as well as their potential to cause serious environmental pollution, threatening ecosystems and human health. [3] Heavy metals have serious negative effects on multiple organs in humans, particularly the kidneys, reproductive system, liver, brain, and central nervous system. [1] Table 1 lists the adverse effects of certain heavy metals on human health. Therefore, the effective reduction and remediation of heavy metal pollution to limit its harm to humans and the environment is one of the greatest problems currently faced by environmental researchers. [5-7]


Using Cyanobacteria as a Biosorbent for Heavy Metals …

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Table 1. The adverse effects of heavy metals on mammals Element Cu Mn Mo Zn Co Cr Ni V As Cd Hg Pb

Accumulating organ Muscle Skeleton Liver Prostate, eye Skeleton Skin Skin Fat Hair, nails Bone, kidney, liver Kidney, hair Liver

Adverse effect Jaundice, Wilson‘s disease Ataxia Stunted growth Anemia Heart failure, polycythemia Inhalation causes lung cancer Dermatitis and absorption cause lung cancer Weakened growth Onychomycosis, blackfoot disease, liver cancer Pain Encephalitis, neuritis Brain damage, neuritis, kidney damage

1.1. Treatment of Heavy Metal Wastewater Heavy metal wastewater is the major type of wastewater. Because the presence of heavy metals may cause serious harm to human health, heavy metal wastewater treatment is imperative. [8] Environmental engineers and scientists are faced with several challenging tasks, including the development of low-cost wastewater treatment technology. [9] The methods for removing metal ions from aqueous solutions may generally be categorized as physical, chemical, or biological. [3] Physical methods include membrane separation, adsorption, solvent extraction, ion exchange, evaporation, and concentration; chemical methods include chemical precipitation and electrochemical methods. Biological methods include bioremediation, biological flocculation, and biological adsorption. In general, it is expensive and potentially risky to remove heavy metals from the environment using physical and chemical methods, which can generate hazardous by-products. Most of these methods are also commercially impractical because of their high operating costs or the difficulty of treating solid wastes, although physical and chemical methods have been proposed and applied to remove metal ions from effluents. The use of conventional technologies, such as ion exchange, chemical precipitation, reverse osmosis,


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Chaoyang Wei, Di Geng and Hongbing Ji

and evaporative recovery, is often inefficient and/or very expensive. Therefore, new heavy metal removal and recovery methods are urgently needed, particularly for wastewater containing low concentrations of heavy metals (1 to 100 mg/L). [4].

1.2. Biosorption In recent years, the application of biotechnology to control and remove heavy metals has received increasing attention as its potential applications in the field have expanded. [3] ―Biosorption‖ describes the removal of heavy metals by their passive binding to non-living biomass in an aqueous solution. [10, 11] This relatively new technology can be used to remove heavy metals from industrial wastewater through the use of biological materials. [12] The advantages of biosorption processes include low operating costs, minimal volumes of chemical and/or biological sludge for disposal, high detoxification efficiency of very dilute effluents, and no nutrient requirements. These advantages have been the main motivations for developing full-scale biosorption processes to eliminate heavy metal pollution. [13]

1.3. Biosorbent To determine the suitability of a material as a biosorbent, several factors must generally be considered: the mechanical stability of the adsorbent, the selective adsorption properties of the target compound, the equilibrium adsorption capacity, the adsorption rate, and the cost of the application. [14] Many materials have been extensively studied as biological adsorbents for the removal of heavy metals or organic matter from wastewater. The tested biosorbents can be classified into several categories: bacteria (e.g., Bacillus subtilis), fungi (e.g., Rhizopus arrhizus), yeast (e.g., Saccharomyces cerevisiae), algae, industrial wastes (e.g., S. cerevisiae waste biomass from fermentation and the food industry), agricultural wastes (e.g., corn cobs), and other polysaccharide materials. [15] The functional properties of some groups of microorganisms, such as bacteria, fungi, yeast, and algae, have been well reviewed. [3] Within the last decade, the use of biosorbents has emerged as one of the most promising alternatives to traditional heavy metal management strategies. Recently, attention has turned to the use of dead algae and other



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microorganisms as biological adsorbents to remove heavy metals. Algae are ideal candidate biosorbents for the removal and concentration of heavy metals [11] due to their high tolerance to heavy metals, ability to grow both autotrophically and heterotrophically, large surface area/volume ratios, phototaxis, phytochelatin expression, and potential for genetic manipulation. [16] Algae do not require much added nutrients and can produce a large amount of biomass as autotrophs, and unlike other organisms and microorganisms such as bacteria and fungi, they generally do not produce toxic substances. The binding of metal ions to the algal surface depends on different conditions, such as the ionic charge of the metal ion, the species of algae, and the chemical composition of the metal ion solution. [17] Due to the variety of structural characteristics, adsorption mechanisms, adsorption capacities, and adsorption efficiencies of algae, algal adsorption technology has a wide range of potential applications in the treatment of heavy metal wastewater. However, due to many limiting factors, this method remains in the research stage, and the practical applications of algal adsorption in wastewater treatment remain limited. Many questions remain for further investigation. [18]

2. CYANOBACTERIAL BLOOMS 2.1. Current Eutrophication Situation A United Nations Environment Programme (UNEP) survey has revealed that 30% to 40% of lakes and reservoirs worldwide are affected by varying degrees of pollution. [19] Outbreaks of toxic blue-green algae are a global phenomenon, although the formation and duration of the bloom varies with location. In Europe alone, toxicity bioassays or high performance liquid chromatography analysis yielded an unexpected result: toxic blooms occur with high frequency (42%-90%) in 11 European countries. [20] Surveys in Europe using toxicity bioassays or high performance liquid chromatography have revealed an unexpectedly high frequency of toxic blooms. In China, there are 2,759 lakes with a total area of 91,019 km2, or 0.95% of the area of the country. Approximately one third of these lakes are freshwater lakes, which are mainly distributed in the middle and lower reaches of the Changjiang (Yangtze River); all are shallow, and most have been eutrophic or are in the process of eutrophication. Lake Taihu is a typical large, shallow eutrophic lake. Due to eutrophication, cyanobacterial algal blooms


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occur frequently in this lake. [21, 22] In May 2007, the accumulation of an algal bloom around the intake pipe of a water treatment facility clogged its filtration system and left millions of people without drinking water. [23, 24]

2.2. Water Eutrophication Hazards The eutrophication of lakes and reservoirs is a consequence of the degradation of nutrients from agricultural run-off and untreated industrial and municipal wastewater. In both developed and developing countries, the accelerated eutrophication of lakes and reservoirs in many locations has resulted in a serious deterioration of water quality in the last century. [25, 26] The destruction of water quality caused by eutrophication has a number of implications. [23] For example, increases in the amounts of cyanobacteria, algae and plants present in the water will reduce the quality of water for human consumption; increased turbidity and particulate matter may clog water filters and generate compounds that give the water a bad taste or odor. Eutrophication can also lead to a reduction in species diversity in bodies of water at all trophic levels. The frequent dominance of eutrophic waters by cyanobacteria is of additional concern with respect to water quality because these organisms are commonly able to produce toxins. [27]

2.3 Characteristics of Cyanobacteria 2.3.1. The Concept of the Bloom Eutrophication refers to the influx of nitrogen, phosphorus, and other nutrients into lakes, reservoirs, or gulfs with slow flow, causing the multiplication of algae and other plankton and the subsequent deterioration of water quality due to human activities. When such multiplication occurs in a lake, it is called a ―bloom‖. [9] ―Bloom‖ refers to the phenomenon in which the number of phytoplankton cells becomes higher than the average number in a specific area or water body. [28] Blooms (waterblooms) may be caused by algae, diatoms, or green algae, which are the main natural algae in bodies of water, but upon eutrophication, the blooms eventually involve mainly algae. When a bloom occurs, the water turns blue or green. The bloom phenomenon occurs worldwide, highlighting the effects of water pollution; at the same time, it is also further exacerbated by air and soil pollution. Blooms have been called ―the cancer of water‖, which indicates the seriousness of the problem. [29]



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2.3.2. Bloom Formation The ability of cyanobacteria to adapt their growth and persistence to environmental conditions is superior to that of other algae. For example, many cyanobacterial species remain dormant or maintain some of their cells in a dormant state until favorable conditions arise. Some species have specialized cells that transform nitrogen gas into forms that are required for nutrition (nitrogen fixation). Unlike algae, which require carbon dioxide gas for photosynthesis, most cyanobacteria can utilize other carbon sources, such as bicarbonate, which is more common in alkaline or high pH environments (a common characteristic of many lakes in Alberta). The success of cyanobacteria and its ability to adapt and form blooms has been widely attributed to its ability to regulate its buoyancy. Cyanobacteria have the ability to form gas-filled cavities that reduce cell/colony density, allowing the cells to float to the surface. This ability to migrate vertically through the water column affords access to optimal levels of light and nutrients. Once such an environment has been located, cyanobacteria can further change the water environment to facilitate their own growth and slow the growth of other algae. For instance, cyanobacteria may reduce light availability to algae by creating shade and may adjust the water pH by reducing carbon dioxide levels. As a result, many cyanobacteria become prominent and increase greatly in number near the surface of nutrient-rich (eutrophic) lakes, reservoirs, and ponds. During windy periods, cyanobacteria can produce a large number of gas-filled cavities to counter the downward drag of water currents. When calm conditions return, over-buoyant groups rise to the surface and form more severe algal blooms. When wind blows waste into a harbor or near the coastline and beaches, the blooms are even more severe. The use of fertilizers in agricultural applications has been associated with massive nutrient inputs to lakes and estuaries that have caused blooms in the second half of the last century. The rapid development of tourism and aquaculture has resulted in a corresponding increase in toxic blooms in coastal waters. Thus, these blooms can be regarded as terrestrial weeds, which by definition are plants that interfere with human usage. As human activities increase and diversify, the number of toxic blooms will increase. [28] 2.3.3. Characteristics of Algal Blooms The term ―algae‖ is used to refer to a large group of diverse organisms, all of which contain chlorophyll and perform photosynthesis. [10] There are 7 divisions of algae, 4 of which contain true algae. Algae can be broadly divided


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into the following: Cyanophyta (blue-green algae), Chlorophyta (green algae), Rhodophyta (red algae), and Phaeophyta (brown algae). These divisions are subdivided into orders, which are subsequently divided into families, genera, and species. [30] Blue-green algae are one of the most primitive single-celled algae (the scientific name, Cyanophyta, was initially recommended by Sachs (1874)). These organisms do not have a nucleus; rather, the cells contain the nuclear material, usually in a granular or mesh organization, and the chromosome and pigment are evenly distributed throughout the cytoplasm. The general color of Cyanophyta algae is blue-green, but a few appear red. The most important species of Chlorophyta, or green algae, include the microcapsule Chara, the recessive Chara, the implicit pole Chara, and the glued rod Chara. Most of these algae express colloids on the exteriors of their cell walls and may also be referred to as ―sticky‖. [31] More than 30,000 types of algae have been identified, which are widely distributed in various freshwater and marine environments, but not all algae can form algal blooms. Water blooms of algae often involve Cyanophyta, most commonly Microcystis, Anabaena, Oscillatoria algae, flat-crack algae, Aphanizomenon algae, the A‘s Anabaena, Spirulina, etc. The common water algae also include Euglena, Euglenophyta, the green algae Chlamydomonas reinhardtii, and Bacillariophyta (Cyclotella). Spirogyra have also been observed floating in the water, and turn board algae are regarded as filamentous water bloom algae. [32] Cyanobacteria are highly capable of using light energy. Bloom-forming cyanobacteria such as the fishy algae Anabaena, Aphanizomenon flos-aquae, and tube cell algae (Cylindrospermopsis) use their characteristic heterocysts to fix free ammonia in the atmosphere into biologically available nitrogen sources that can be used by other vegetative cells of the organism. Due to the strong reproductive capacity of cyanobacteria, under suitable conditions of light, temperature, and pH, these species usually undergo geometric growth, resulting in a thick layer at the lake surface known as an indigo bloom. [31] Cyanobacteria occupy the most diverse ecological habitats of all photosynthetic organisms. They are found in cold and hot, alkaline and acidic, marine, freshwater, saline, terrestrial, and symbiotic environments. This broad habitat range is facilitated by the presence of a PSII reaction center; these organisms can extract electrons from water and are therefore not limited to environments with other, scarcer reduced electron donors, as are other nonoxygenic photosynthetic prokaryotes. In fact, cyanobacteria are able to outcompete other species in any environment with water and sunlight.



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Cyanobacteria that grow successfully in a diverse range of environments benefit from the temporal and spatial variations that have occurred during its evolution, with the acquisition of many genes and physiological properties. [33, 34] Cyanobacteria are excellent candidates for bioengineering applications, particularly in agriculture, pharmaceuticals, health care products and biofuels, demonstrating the diversity of their morphological attributes and ecological distribution. Cyanobacteria possess a unique combination of plant-like photosynthetic capacity and microbe-like ability to yield high-value products under controlled conditions in a short period. [35]

2.4. Resource Utilization in Cyanobacteria The treatment of algal blooms in large lakes is a global problem. Researchers in many countries have attempted to control blooms in a variety of ways, including pre-oxidation, [36] coagulation and flocculation, [37] and clarification by either dissolved air flotation [38] or sedimentation. [39] However, these methods inevitably cause a number of environmental issues, and the problem cannot be solved rapidly for a large body of water. The use of physical methods to collect the algae is advantageous because physical collection is rapid and causes no environmental pollution. Algal collection was important for controlling the algal bloom in Lake Taihu in 2007. However, thousands of tons of algae can be collected daily during a severe bloom, and the development of methods for addressing these toxic algae has become a pressing problem. [40] Several cyanobacterial resource utilization methods are described in the following sections.

2.4.1. Extraction of Useful Substances 1. Natural pigment extraction Cyanobacteria are rich in natural pigments such as chlorophyll, carotene, phycocyanin, and lutein. Fat-soluble pigments and watersoluble pigments are extracted using different methods. The phycocyanin and carotenoids in Spirulina have commercial applications as natural pigments for food and cosmetics. 2. Phycobiliprotein extraction Cyanobacteria can perform photosynthesis and contain a special class of photosynthetic accessory pigments called phycobiliproteins. These


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Chaoyang Wei, Di Geng and Hongbing Ji pigments comprise up to 15% of the cell dry weight of the cyanobacteria. The three categories of phycobiliproteins are phycoerythrin, phycocyanin, and allophycocyanin. They are primarily used as natural pigments in the food and cosmetics industries. 3. Exopolysaccharide extraction Many cyanobacteria secrete large amounts of mucilaginous polysaccharide substances, which are typically referred to as extracellular polysaccharides. These biological polymers represent approximately 5% of the cell dry weight. Cyanobacterial extracellular polysaccharides have certain physiological activities; their main functions include the prevention of dehydration and poisoning by other bio-phagocytic and antibacterial agents, the chelation of cations that are necessary for cellular activities, such as Ca2+ and Fe2 +, and flocculation. 4. Physiologically active substance extraction Cyanobacterial extracts have been found to have antibacterial effects, but the structure of these active substances remain unclear. Most of them may be toxic, such that the possibility of pharmaceutical use still requires more study. Many blue-green algae can be used to extract enzyme inhibitors, and cyanobacteria-produced pigment can strongly absorb ultraviolet light, which may be a useful property for an ingredient in cosmetics. [41] 5. Vitamin extraction Blue-green algae are rich sources of many vitamins. For example, Spirulina (Arthrospira) is rich in vitamin B12 (2 to 6 times richer than raw beef liver) and vitamin E, and 20 g of Spirulina contains 100% of the recommended daily allowance of Vitamin B12, 70% of B1 (thiamine), 50% of Vitamin B2 (riboﬂavin), and 12% of Vitamin B3 (niacin). It also contains nutrients, including other B complex vitamins, beta-carotene, vitamin E, manganese, Zn, Cu, Fe, Se, and γ linolenic acid (an essential fatty acid). [35]

2.4.2. Production of Organic Fertilizers The use of cyanobacteria as a fertilizer was first proposed by a Japanese, who mainly use algae as an organic fertilizer. The efficiency of algal fertilizer is greater than that of general fertilizers; the nitrogen, phosphorus, and potassium contents of algal fertilizer are higher than those of vegetable-based organic fertilizers such as soybean meal or milk vetch. In addition,



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cyanobacteria do not contain heavy metals that are harmful to crops and humans, and these fertilizers do not contaminate the soil after their use.

2.4.3. Production of Biogas Fresh cyanobacteria have the potential to produce 487.3 mL/g TS (total solid) biogas or 491.0 mL/g VS (volatile solid) biogas, with a methane content of 64.91%. An annual production capacity of 5000 t (the amount of dry algae) is sufficient to produce more than 200 million m3 of biogas per year, supplying 10,000 urban households. 2.4.4. Production of Biodiesel Per hectare, blue-green algae can produce 90,000 liters of biodiesel, similar to the yield of crops planted in the ground, but the cyanobacterial biodiesel conversion ratio is 50% by weight. However, only 0.26% of these lipids could be recovered from the cyanobacteria in Lake Taihu, indicating that it is not feasible to extract oil from cyanobacteria-produced biodiesel. [41]

3. BIOLOGICAL ADSORPTION PROCESS 3.1. The Mechanism of Adsorption The removal of toxic metal ions from polluted waters using microbial biomass has been studied extensively. Microorganisms can accumulate heavy metal ions through two processes: (1) biosorption, ―an energy-independent binding of metals to cell wall‖, and (2), ―an energy-dependent process of metal uptake into the cells‖. [2] Biosorption refers to the adsorption reaction between the metal ions on the algal cell surface and functional groups in the cell wall. The basic mechanism of adsorption is a surface complexation between the bioaccumulated metal cations and the functional groups on the algal cells. The alginate in the algal cells contains a hydroxyl group, an amino group, a carboxyl group, and so on, and plays an important role in the adsorption. Bioaccumulation refers to the cell surface binding and deposition or combination of metal ions with the plasma membrane, followed by the active transfer of the ions to the intracellular space via the activities of hydrolytic transfer enzymes. This is related to metabolic activity and is a slow and energy-intensive process. After they are absorbed by the algal cells, some heavy metal ions in various forms


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can combine with intracellular organic molecules or may be stored in the cytoplasm and organelles (Figure 1). [42] Many bacteria and algae are capable of passively adsorbing high levels of dissolved metals in their cell wall or envelope, usually via a charge-mediated attraction. Cyanobacteria produce a substance called mucilage, which is composed mainly of polysaccharides. The biological adsorption of heavy metals is believed to occur through these polysaccharides. It has been widely documented that microbial biomass has the ability to accumulate and remove heavy metals from water. The true mechanisms through which microbes adsorb heavy metals are not yet clear. However, either living or dead biomass rapidly reaches equilibrium with the surrounding medium through accumulation; thus, this mechanism can be described by the Freundlich adsorption isotherm and is assumed to represent physical adsorption or biosorption. Functional groups that can capture metals, such as hydroxyl, phosphate, amino, and carboxyl groups, are present on the surface of the algal biomass and play a key role in the ability of this material to bind metals. Special attention must be paid to the fact that the cyanobacterial envelope consists mainly of polysaccharides, which are negatively charged and rich in uronic acids; thus, they exhibit high metal-binding capacity. Recently, for the marine cyanobacterium Phormidium valderianum BDU 30501, it was suggested that carboxyl groups are the primary sites for metal binding based on infrared spectra of the biosorbent preparation. [2]

Figure 1. The mechanism of biosorption and bioaccumulation.(Chojnacka K. Biosorption and bioaccumulation – the prospects for practical applications. Environment International 2010,36:299-307, reproduced with permission).



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3.2. Pretreatment Technology Because cell surface sequestration is the main biosorption process, the modification of the cell wall can greatly change the binding of metal ions. A number of methods have been employed to modify the cell walls of microbial cells to enhance the metal binding capacity of the biomass and to elucidate the mechanism of biosorption. These pretreatments may involve the removal or masking of specific groups, the exposure of more metal binding sites, or other modifications to the surface characteristics/groups. [3] The algae used to adsorb heavy metal ions are typically pretreated by simple physical processing, chemical processing, or chemical modification. Physical processing may include drying or crushing. Drying at different temperatures can change the composition of the algae and destroy its structure. Thus, the drying temperature should not be too high, generally lower than 100°C; sun-drying and freeze-drying are also effective. Simple chemical treatments may include HCl or organic immersion. HCl or organic solvents can be used to wash off Ca2+, Mg2+, and Na+ ions as well as other soluble substances on the seaweed, thereby increasing the number of adsorption sites on the algae and favoring adsorption. To generate an algal adsorbent, Yang et al., pulverized Sargasso algae to 1.0 to 1.4 mm, soaked it in 0.1 mol/L HCl for 3 h, washed it with deionized water, and finally oven-dried the material at 40 ~ 60°C. A variety of chemical modification methods may be used. The main purposes of these methods are to enhance the adsorption properties of the adsorbent, to improve its mechanical properties, and to increase its chemical stability. [44] The main objective of the chemical conditioning of the biomass is to facilitate contact with functional groups responsible for metal uptake and to create new functional groups or to cross-link biomass chains. Several of the most common pre-treatment reagents are listed below: CaCl2: promotes the cross-linking of the alginate polymeric chains. Formaldehyde and glutaraldehyde: facilitate chemical cross-linking between adjacent functional groups, mainly hydroxyl groups. NaOH: Alkali treatment is usually used to reduce protonated groups. This process substitutes the sodium ions on functional groups, increasing the electrostatic attraction to positive metal cations and facilitating ion exchange. HCl: acid washing can replace the light metals on the biomass surface with protons. In addition, acid treatment can create additional bonding sites


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(amino groups) by dissolving polysaccharide compounds of the external cell wall. [17]

3.3. The Difference between Living Algae and Dead Algae Biosorption refers to the removal of heavy metals from an aqueous solution through passive binding to non-living biomass. This means that the removal mechanism is not controlled by metabolism. By contrast, bioaccumulation refers to the removal of heavy metals in an active process, which requires an active metabolism and thus living organisms. Recently, studies of biological adsorption mechanisms have been strengthened by demonstrating that biomass can be employed to eliminate heavy metals from industrial effluents (e.g., from the mining or electroplating industry) or to recover precious metals from contaminated solutions (Table 2). [10] Living cells have certain advantages in terms of scale and the potential for continuous application; large-area applications can be achieved by directly adding a nutritional source subculture. However, there are some limitations to the use of living cells for wastewater treatment: l.

Live algae can be very sensitive to heavy metal concentrations (toxic effects) and other conditions (such as pH and temperature). In some cases, the pH value of the wastewater fluctuates, and the content of harmful metals and other biologically toxic materials is high, often beyond the tolerance of the algae, thus making the biological activity of the algae difficult to maintain over time. 2. Living algae need to continue to supplement their diets to maintain their survival, and this will lead to increases in the biological oxygen demand (BOD) and chemical oxygen demand (COD) of the wastewater. 3. Live algae absorb a percentage of the heavy metals intracellularly, making heavy metal recovery more difficult. 4. The elution reagents used after adsorption are often extremely acid or alkaline, and they may not be tolerated by living algal cells. [45] Toxicology studies have indicated that some living microorganisms have the ability to accumulate metallic elements. However, subsequent studies revealed that inactive/dead microbial biomass can passively bind to metal ions



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through various physicochemical mechanisms. Therefore, the study of biosorption, i.e., the removal of metal ions or organic compounds by inactive cells, has become an active field. The capacity of a biosorbent for metallic ions is a function of the chemical makeup of the microbial cells in the material. Although our understanding of the mechanisms of biosorption is currently limited, these mechanisms may include ion exchange, complexation, coordination, adsorption, electrostatic interaction, chelation, and/or microprecipitation. [3, 15, 46] Crushing the cell walls of dead algae exposes more carboxyl, amino, aldehyde, hydroxyl, mercapto, phosphoryl, and carbonyl groups. Other internal functional groups are also exposed and become available for metal ion binding while cell membranes lose their selectivity, thus facilitating the entry of metal ions. Biosorption by dead algae does not involve any metabolic processes, so there are no active transport-enriched pathways, but proteins and polysaccharides play important roles in the accumulation of heavy metals. Therefore, the adsorption capacity is significantly increased by crushing. [18] Table 2. A comparison of biosorption and bioaccumulation[43] Biosorption Passive process Dead biomass Metals bound to cell surface Adsorption Reversible process Nutrients not required Single-stage process Rapid rate Not controlled by metabolism No toxic effect

Bioaccumulation Active process Live biomass Metals bound to cell surface and interior Absorption Partially reversible process Nutrients required Double-stage process Slow rate Controlled by metabolism Danger of toxic effects caused by contaminants No cellular growth Cellular growth occurs Intermediate equilibrium Very low equilibrium concentration of concentration of metal ions metal ions *(Chojnacka K. Biosorption and bioaccumulation – the prospects for practical applications. Environment International 2010, 36:299-307, reproduced with permission)


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Dead algae are able to adsorb Pb, Cu, Zn, Ni, Cd, Ag, Hg, U, Au, and Co. The advantages of algae include good selectivity, high adsorption, adsorption speed, wide pH range, applicability to a wide range of concentrations, low cost, and low pollution. Therefore, algal biosorption has become an important area of research. However, there are some drawbacks in the use of dead algae, such as its poor mechanical strength and chemical stability, which limit its practical application. [47]

3.4. Immobilization Techniques Biological cell immobilization is a basic technology in which free cells are retained in a finite region of space by physical or chemical means, where they are maintained in an active state and can be used repeatedly. [48] Microbial biomass consists of many small particles, resulting in the disadvantages of low density, poor mechanical strength, and little rigidity. By contrast, biomass immobilized on solid structures exhibits the size, mechanical strength, rigidity, and porosity necessary for metal accumulation. Immobilization can also be used to produce beads and granules that can be stripped of metals, reactivated, and recycled, in a manner similar to ion exchange resins or activated carbon. Various applications are available for biomass immobilization. The principal techniques of biosorption are based on adsorption on inert supports, entrapment in a polymeric matrix, covalent bonding in vector compounds, or cell cross-linking, the applications of which have been previously described in the literature. [49] 



Adsorption on inert supports: After disinfection, support materials are inoculated with a fermentation agent. After continuous culture for some time, an obvious outer membrane forms on the microbial surface. Zhou and Kiff used this technique in 1991 to immobilize Rhizopus arrhizus fungal biomass in reticulated foam biomass support particles; Macaskie et al., [50] immobilized Citrobacter sp. using this technique. Scott and Karanjakar [51] immobilized an Enterobacter aerogenes biofilm on activated carbon. Bai and Abraham [52] immobilized Rhizopus nigricans on polyurethane foam cubes and coconut fibers. Entrapment in polymeric matrices: A variety of reagents can be used as polymers for this purpose, including calcium alginate,







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polyacrylamide, polysulfone, and polyethylenimine. The materials immobilized by calcium alginate and polyacrylamide exist in the form of gel particles. The materials obtained by immobilization in polysulfone and polyethyleneimine have the highest mechanical strength. Covalent bonds to vector compounds: The most common vector compound (carrier) is silica gel. The resulting material is generally presented in the form of gel particles. This technique is mainly used for immobilized algae. Cross-linking: Cross-linking agents can be used to form stable cell aggregates. This technique is equally applicable to the immobilization of algae. The most common cross linkers are formaldehyde, glutaric dialdehyde, divinyl sulfone, and formaldehyde–urea mixtures. [49]

The immobilization technique plays a key role in the practical application of biosorption, particularly by dead biomass. S. cerevisiae has been immobilized using different support materials for practical biosorption. Important immobilization matrices used in biosorbent immobilization include sodium or calcium alginate, polysulfone, polyacrylamide, polyurethane, and silica [15]. The choice of immobilization matrix is very important because the polymeric matrix determines the mechanical strength and chemical resistance of the final biosorbent particle, which is utilized for successive sorption–desorption cycles. The cost of the biosorbent preparation must also be considered. To date, most studies have used a particulate biosorbent mounted in an adsorption column, similar to ion exchange resins. The costs of the immobilizing agent should not be ignored either, although cell entrapment imparts mechanical strength and resistance to chemical and microbial degradation of the biosorbents. The low density and size of free cells render them unsuitable for use in a column, and they tend to plug the bed, resulting in large drops in pressure. Support matrices suitable for biomass immobilization include alginate, polyacrylamide, polyvinyl alcohol, polysulfone, silica gel, cellulose, and glutaraldehyde. [3]

3.5. Influencing Factors The factors that influence the biosorption process include the following:


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Chaoyang Wei, Di Geng and Hongbing Ji 1. Within the range of 20-30°C, temperature generally does not affect the adsorption performance. 2. The pH could be the most important parameter in the biosorptive process; it influences the chemistry of the metals in the solution, the activity of the functional groups in the biomass, and the competition of metal ions. 3. The specific uptake may be influenced by the biomass concentration in solution. An increase in specific uptake is observed at lower concentrations of biomass. In 1988, Gadd et al., observed that an increase in biomass concentration leads to interference between binding sites. However, Fourest and Roux have argued, in lieu of the above hypothesis, that the decreases in specific uptake were probably due to the metal concentration shortage in solution. Thus, biomass concentration should be considered in any application employing microbial biomass as a biosorbent. 4. In wastewater treatment by biosorption, more than one type of metal ion is present, and the removal of one metal ion may be influenced by the presence of other metal ions. For instance, while the uptake of U by bacterial, fungal, and yeast biomass was not affected by the presence of Mg, Co, Cu, Cd, Hg, or Pb in solution, the presence of Fe2+and Zn2+ influenced U uptake by Rhizopus arrhizus, and the uptake of Co by different microorganisms seemed to be completely inhibited by the presence of U, Pb, Hg, or Cu. [49]

3.6. Desorption If biosorption is to become a practical alternative wastewater treatment method, the biological regeneration of the adsorbent must be achieved to maintain the low cost of the adsorption process and metal recovery. For this purpose, the desorption of the adsorbed metals and the regeneration of the biosorbent material for another cycle of application are desirable. The desorption process should have the following characteristics:   

The metals generated should be in a concentrated form; For effective reuse with undiminished metal uptake, the biosorbent should be restored to its original condition as closely as possible; and There should be no physical changes or damage to the biosorbent.



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One desorption method involves the washing of the metal-laden biosorbent with an appropriate solution to regenerate the biosorbent. The type and strength of this solution depend on the extent of binding of the deposited metal. Dilute solutions of mineral acids, such as hydrochloric acid, sulfuric acid, acetic acid, and nitric acid, can be used for metal desorption from biomass. [49]

3.7. Modeling of Biosorption: Isotherm and Kinetic Models 3.7.1. Equilibrium Modeling The analysis of equilibrium data is important for the development of an equation that can be used for designing purposes. To describe the equilibrium established between adsorbed metal ions on the biomass (qe) and metal ions remaining in solution (Ce) at a constant temperature, the Langmuir and Freundlich models, as well as other classical adsorption models, have been studied extensively. The Langmuir equation describes the sorption of a monolayer onto a surface containing a finite number of accessible sites:

qe 

qmax bCe 1  bCe

where qmax represents the maximum quantity of metal ions per unit weight of biomass able to form a complete monolayer on the surface (mg/g) and, b represents a constant related to the affinity of the binding sites for the metal ions (sorbate, L/mg). It should be noted that when the adsorbent surface is completely covered by metal ions, the adsorption capacity reaches a limit; the value of this limit is qmax. To assess adsorption performance, this quantity is very useful, particularly in cases where the sorbent does not reach its full saturation, because it enables an indirect comparison between different sorbents. The empirical Freundlich equation describes sorption on heterogeneous surfaces macroscopically:

qe  K F Ce

1/ n


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where KF indicates adsorption capacity (L g−1) and n is an indicator of the effect of concentration on the adsorption capacity that represents the adsorption intensity (dimensionless). [53]

3.7.2. Kinetic Modeling To investigate the mechanism of heavy metal biosorption onto algal biomass, it is important to test pseudo-first order and pseudo-second order kinetics models to determine which best fits the experimental kinetics data. The linear forms of the pseudo-first order (Eq. (1)) and pseudo-second order (Eq. (2)) kinetics models can be described as follows: [54, 55]

log( qe  qt )  log qt 

k1 t 2.303

t 1 t   2 qt k2 qe qt

(1)

(2)

where qe and qt describe the amounts of heavy metals adsorbed per unit weight of adsorbent at equilibrium and at time t, respectively (mmol/g), k1 is the rate constant of the pseudo-first order kinetics equation (min-1), and k2 is the rate constant of the pseudo-second order kinetics equation (g/mmol.min). [56]

3.8. Technology and Commercial Applications Biosorbent particles packed in sorption columns are perhaps the most effective devices for the continuous removal of heavy metals. Biosorption columns undergo cycles of loading, regeneration, and rinsing. At the beginning of the operation, the column is fitted with absorbent material, and the wastewater is then flowed through the packed bed to allow the heavy metal ions to be absorbed by the biological adsorbent. After the metal-sorption capacity of the biosorbent is saturated, the column is removed from operation. Acid and/or hydroxide solutions are used to regenerate the packed bed; this regeneration produces small volumes of heavy metal concentrates suitable for conventional metal-recovery processes. To remove the remains of the regenerants and suspended solids captured in the column, the cycle ends with rinsing and/or backwashing of the bed with water. To enable a truly



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continuous adsorption process, a pair of adsorption columns may be run simultaneously. Thus, while one column is undergoing regeneration and rinsing, the other is being loaded with heavy metals. [13] An example of the practical application of a biosorbent in the literature is described below. [30] A schematic diagram of a biosorption unit is shown in Figure 2. The unit consists of the following parts:  





Tank: The simulated wastewater solution (250 L) containing 10 mg/L nickel ions (Ni2+). Pump: To pump the simulated solution at constant volumetric flow rate of 1.5 cm3/sec, power consumption should be 1.5 kW/(220 – 240 V). Glass column: The column is 100 cm in height, 5 cm in diameter, and 2.5 mm in wall thickness. Two glass discs of 5-cm thickness are installed at distances of 20 cm from the upper and lower ends of the glass column. The discs are perforated (0.5 mm holes) to maintain a uniform downward flow of the simulated solution. Biosorbent bed: The bed is 50 cm in height and 5 cm in diameter and is created by sandwiching 500 g of dry biosorbent material (0.75 mm particle diameter of Laminaria saccharina) between the 2 circular glass discs described above.

Figure 2. Schematic diagram of the biosorption Unit. [30] (open access).


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Table 3. Selected properties of the biosorbent material and the bed [30] No. Properties Value 1 Moisture 9% 2 Ash content 0.5% 3 Porosity 0.49 4 Bulk density 0.5 g/cm3 AL-Hamadani FHK. Removal of Nickel Ions Using A Biosorbent Bed (Laminaria saccharina) Algae. Iraqi Journal of Chemical and Petroleum Engineering 2012, 13:47-55, open access.

In accordance with a reference method, the moisture and ash contents of the biosorbent material were determined, along with its porosity and bulk density. [57] Some of the technological parameters of the biosorbent material and the bed are presented in Table 3. [30] The biosorption process was recently commercialized and approved by the EPA (EPA/540/S5-90/005). Several commercial biosorbents are available, namely AlgaSORB® and AMT-BIO-CLAIM®. AlgaSORB® is produced by Biorecovery Systems. This algal sorbent is sold as a powder, 1-3 mm particle diameter, at a price of 28 €. The sorbent is composed of a biofilm of the filamentous multi-cellular green alga Spirogyra immobilized in a silica gel. The commercial portable systems for biological sorption offered by the company Resource Management & Recovery consist of 2 columns, each containing 7 L of biosorbent. The company reports a wastewater treatment efficiency of 0.5 L/min and declares that it is possible to construct larger systems with 100 times greater treatment efficiency by producing larger columns or by adding additional columns to the system (http://www.cluin.org/ products/site/complete/resource.htm). [43]

4. FEASIBILITY OF CYANOBACTERIA AS AN ADSORBENT 4.1. Large Biomasses of Cyanobacteria The eutrophication of water bodies by agricultural non-point source pollution and sewage has become an increasingly serious problem. [58] In wastewater treatment plants, cyanobacteria achieve maximum values of 99.8% of the total phytoplankton [59]. Furtado et al., [60] reported that the abundance of cyanobacteria (91.7% in summer and 96.4% in autumn) was always higher



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than the abundance of other algae (8.3% in summer and 3.6% in autumn), indicating their major contribution to the phytoplankton communities in these systems. [35] Bodies of waters such as the Swan River in Western Australia, western Lake Erie and the San Francisco Bay in the United States, and Lake Taihu and Lake Dianchi in China, contain huge amounts of phytoplankton organisms. The concentration of Microcystis aeruginosa in the Swan River of Western Australia has reached 100,000 cell/ mL. In Lake Erie, these organisms cover a water area greater than 20 km2, and the Microcystis aeruginosa concentration reaches 400 million cells/L; San Francisco Bay has a cyanobacterial band of up to 180 km. In Lake Dianchi, the Microcystis aeruginosa concentration reached a maximum value of 3.24 billion cells/L, and the Meiliang Bay (of Lake Taihu) blooms can last for 8 months. [58] In Lake Chaohu, the gross cyanobacteria biomass (dry weight) reached 500,000 to 700,000 t. In Lake Dianchi, the potential resource of cyanobacteria is approximately 5,000 t (dry weight) per year. [61] During an algae bloom in Lake Taihu in 2007, 1,000 t to 2,000 t of fresh cyanobacteria biomass were removed from the lake each day, with a total removal of 200,000 t of cyanobacteria in the lake. [62] In 2008, more than 2,800 t of cyanobacteria (dry weight) were collected during a bloom in Lake Taihu. [61] These data indicate that water blooms may serve as widely distributed and abundant sources of algae. Fully automatic and semi-automatic algal harvesters have been used for water bloom collection and drying, which guarantees the collection of cyanobacterial bloom biomass feedstock. Therefore, it would be very practical to convert these cyanobacterial blooms into biological adsorbents. [58] The global eutrophication of lakes and water bodies, i.e., the frequent outbreaks of large-scale, long-term Microcystis blooms, provides extremely rich biological resources for exploitation. [63]

4.2. Adsorption Effects of Cyanobacteria In recent decades, harmful blooms have occurred frequently worldwide. [64] In 2007, Lake Taihu underwent a severe algal bloom on a large scale, and collecting algae was a critical step in bloom control. With thousands of tons of algal residue (AR, Cyanobacteria) collected each day, the disposal of these toxic algae became an urgent issue. [65] Without further management, large amounts of refloated blue algae would lead to serious further environmental pollution. Several resource recovery approaches have been proposed to


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reclaim the blue algae in Lake Taihu, such as incineration, composting, and the retrieval of bioenergy (hydrogen/methane) via anaerobic digestion. [66] However, none of these methods have yet been implemented. [67] Cyanobacteria have been identified as good candidates for metal biosorption. [68] The biotechnological potential of Microcystis for metal biosorption has been particularly intensively studied. [69-71] Microcystis may be a promising biosorbent candidate for metal removal due to its strong interaction with cations and its availability as a reliable and inexpensive biomass source, and the abundant occurrence of cyanobacterial blooms worldwide. However, all available data on the biosorptive capacity of Microcystis have been obtained with laboratory-grown biomass and laboratory-cultured cells, which are isolated from field-grown blooms. To date, there have been no reports on the biosorption of metals by biomass harvested directly from naturally occurring Microcystis blooms. [72] Li et al., were the first to suggest the use of cyanobacterial blooms as adsorbents, and these authors investigated some basic aspects of U biosorption by powdered biomass from a lake-harvested Microcystis bloom. This is the first attempt to investigate metal sorption by lake-harvested water-bloom cyanobacteria for biotechnological (metal-removing) applications. U biosorption by the cyanobacterial bloom powder was strongly affected by pH. The optimum pH range for U uptake was 4.0 to 8.0. The biosorption data fit the Freundlich model, and they also fit the Langmuir model at pH 3.0, 5.0, and 7.0. The use of Cyanobacteria bloom powder for U biosorption has the advantages of rapid adsorption, high capacity, and simple desorption, suggesting that the biomass from this naturally abundant and nuisance cyanobacteria bloom is a promising biosorbent. In addition, because the cyanobacteria bloom powder can be produced on a large scale and is easily transported, the dried powder is safer and more convenient than viable cells for future applications. [72] Wang et al., performed an in-depth study in which they processed algae from the Lake Taihu, Lake Dianchi, and Lake Guan Bridge blue-green algae blooms into 3 algal powders to study their heavy metal adsorption capacities. The Lake Dianchi algal powder had an adsorption capacity of 8.31 mg/g and 7.21 mg/g for Cr and Cu, respectively, whereas the Lake Taihu algal powder had adsorption values of 6.19 mg/g and 5.83 mg/g and the Lake Guan Bridge algal powder had adsorption values of 6.71 mg/g and 6.26 mg/g. The Cr and Cu adsorption capacities of the Dianchi cyanobacterial bloom were higher than the adsorption capabilities of the Lake Taihu algae and the Lake Guan Bridge algae. All 3 types of materials adsorbed more Cr than Cu. Hydroxyl, amino,



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and carbonyl functional groups play key roles in the process of biomass adsorption of heavy metals by cyanobacterial blooms. Cyanobacterial blooms are thus worthy of further study due to their abundance and their potential to remove heavy metal pollution, turning waste into usable material. [63] In addition, Wang et al., performed another set of experiments in solutions with different pH values (3 to 6) to compare the adsorption kinetics of copper ions among lake-harvested cyanobacteria. The authors used Freundlich equation fitting to explore how the cyanobacterial bloom biomass efficiently removed the heavy metals. The experimental results revealed that, under different pH conditions, the cyanobacterial biomass in a dialysis bag used for the biosorption of Cu2+ reached equilibrium within 2.5 h. The Freundlich model can be used to simulate the biosorption process of Cu2+ by cyanobacterial biomass. There are a large number of studies of cyanobacteria biomass, and immobilized adsorption and elution merits further study to evaluate the biological potential of the adsorption-based removal of heavy metals. [58] Zhang et al., were the first to propose the production of activated carbon from algal bloom residue. They evaluated the adsorption properties of this residue under varied operational conditions. The adsorption kinetics were investigated both experimentally and via modeling approaches. Moreover, a series of analyses, including scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR), were conducted to elucidate the adsorption mechanism. The results revealed that ARAC (algal bloom residue derived activated carbon) is a good candidate adsorbent material for the effective removal of Cr(VI) in contaminated water. We infer from the results of the experiment that this novel approach not only eliminates unwanted algal residue but also produces high-quality activated carbon. Moreover, the remarkable Cr(VI) removal capability of ARAC is based on its adjustable adsorption properties. The pH significantly influenced the adsorption of Cr: when the proton concentration increased, the efficiency of Cr(VI) removal also increased, with the optimal pH of 1.0. The percent removal of Cr(VI) was inversely proportional to the initial Cr(VI) concentration, but proportional to the adsorbent dose. The equilibrium of the adsorption process was consistent with Langmuir‘s model, indicating that the adsorption process is monolayer adsorption, and the kinetics of the adsorption process were in good agreement with the pseudo-second-order equation, suggesting that the reaction rate was proportional to the concentration of the two reactants. The SEM-EDS and FTIR data demonstrated that the adsorption of Cr(VI) on ARAC leads to the


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formation of carboxylic and hydroxyl moieties, which can be attributed to the oxidation of the ARAC when Cr(VI) is reduced to Cr(III). [67] Wu et al., studied algal bloom cyanobacteria from the eutrophic lake and investigated its Sb(V) adsorption characteristics and influencing factors. The experiments used cyanobacteria collected from Lake Taihu during the summer cyanobacterial bloom outbreak period using a 25 mm plankton net. After the removal of aquatic plants and other visible impurities, the sample was taken back to the laboratory for separation and purification and was stored in airtight freezers. Their experimental results indicated that the cyanobacteria exhibited good performance in the treatment of wastewater containing Sb(V), reaching an adsorption equilibrium in approximately 1 h. The adsorption capacity gradually weakened as the pH increased, and the Sb(V) adsorption isotherms fit the Freundlich equation. [73]

5. RESEARCH PROSPECTS 5.1. Issues and Challenges 5.1.1. Problems in Cyanobacteria Salvage and Concentration Before it can be used as a heavy metal adsorbent, blue-green algae must be efficiently harvested from lakes and concentrated. Cyanobacterial blooms typically occur between April and October; therefore, these biomass resources are subject to seasonal restrictions, and industrial production may not occur continuously, constraining the industrialization and economic benefits. The efficient harvest of blue-green algae remains a technical problem. [61] Because of the microscopic dimensions of the harvested biomass, there are serious restrictions on the applications of single-celled cyanobacteria. [35] Cyanobacteria are single-celled prokaryotes, and they exist as small cells floating in the water. Certain harvesting methods may cause bubbles of algal cells to burst when collected. Most of the existing harvesting equipment consists of simple tools, but traditional fishing methods are labor intensive, with low efficiency. Manual or simple mechanical harvesting operations retain the high water content of blue-green algae, which may be greater than 97%. Conventional dehydration methods are difficult to achieve with isolated cyanobacteria, and removing moisture by spray-drying is costly. These technical limitations in harvesting require the development of efficient cyanobacteria separation and enrichment equipment. In addition, cyanobacterial resource utilization or disposal is limited due to their high



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transportation and pre-treatment costs. The key to algae utilization may be weight loss, i.e., dehydration to reduce the cyanobacterial water content. [61]

5.1.2. Problems in Cyanobacterial Detoxification One topic of great concern during harmful algal blooms is the toxins produced by cyanobacteria. The toxins produced within the cells are not released until the cells break open, leading to unpredictable toxicity of the blooms. Although normal water treatment processes can remove cyanobacteria and their toxins, additional steps may be necessary when cell or toxin concentrations are high. Extracellular toxins are more difficult to remove than intracellular toxins. In aquatic environments, heterotrophic bacteria will degrade these toxins, but the toxins may accumulate because the bacteria cannot reach the toxins rapidly enough when the water is stirred slowly. The World Health Organization has set a limit of 1.0 µg/L for Microcystin-LR in drinking water, but no standards for recreational waters are available. The most commonly occurring toxins are microcystins. At least 60 microcystins have been identified to date, and their toxicities are vastly different. Nodularin is very similar to microcystins in many respects and has similar effects. Anatoxins are very poisonous and can cause paralytic shellfish poisoning. Lyngbyatoxin and aplysiatoxins can both cause cancer, and lyngbyatoxin causes seaweed dermatitis. Cylindrospermopsis can damage many organs, particularly the liver. The toxins produced by cyanobacteria can also damage other aquatic organisms. The toxins produced by Scytonema hofmanni and Fischerella muscicola can inhibit photosynthetic electron transport. The toxin produced by Anabaena flos-aquae paralyzes the green algae Chlamydomonas, and microcystins inhibit carbon fixation in other phytoplankton. 5.1.3. Gaps in the Relevant Basic Studies Since the 1970s, many studies have been conducted to investigate the heavy metal enrichment capability of live algae and their tolerance mechanisms. In addition, a large number of studies have focused on optimizing the conditions for heavy metal adsorption by dead algae, investigating the mechanism of adsorption, adsorbent modification, and immobilization technology. A great deal of research has focused on the use of purebred frond, such as Chlorella, crescent algae, Spirulina, Chlamydomonas, Anabaena, or other species, as an adsorption material. [63] However, little


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research has been performed regarding algae in the eutrophication of water bodies as a biological adsorbent. [58]

5.1.4. Gaps in Practical Application Cyanobacteria can remove heavy metals from wastewaters containing single or multiple types of heavy metals, and thus is a promising tool for biosorption. However, most studies have remained at the laboratory stage, using different immobilizing agents and different cyanobacterial genera (Table 4). [35] The application of biosorption remains challenging. Substantial effort must be made to improve the biosorption process, including improvements in the immobilization of biomaterials and their regeneration and re-use and the optimization of biosorption processes, among other issues. [3] The technology of wastewater treatment, whether by cyanobacteria alone or in combination with other biosorbents, is established only in laboratories, to date, no treatment technology has been scaled up to the commercial level. This technology can be fully exploited with in-depth research in the selection of cyanobacterial genera and their tolerance to pollutant(s). [35] Few investigations have been conducted to determine the compatibility of biosorbents with real industrial efﬂuents. However, there have been several unsuccessful attempts to scale up the biosorption process or to commercialize the process based on experience with conventional sorption operations. Because biosorption technology remains uncompetitive with other types of large-scale environmental metal removal applications, it has not yet achieved practical application. [3]

5.2. Outlook The development of biosorption technology has followed two trends. One is to use living cells to remove pollutants in hybrid technology; another is the development of good commercial biosorbents that are similar to ion exchange resins, which will require significant effort to exploit the market. [74] The latter also requires in-depth study of the immobilized biological material, as well as the optimization of the parameters of the biosorption process and physicochemical conditions for reuse and recycling. Significant efforts should be made to further study the mechanisms involved in biosorption or metal–microbe interactions. [75] Market factors will play a key role in the successful application of biosorption. Volesky suggested



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[74] that the applications of certain types of biosorption are forthcoming, inviting ―new technology‖ enterprise ventures and presenting new and very different challenges that will arise when the knowledge of biosorption is adequate. Table 4. Bioremediation of different types of wastewater by cyanobacteria [35] Type of wastewater Synthetic heavy metal solution Synthetic heavy metal solution Synthetic heavy metal solution Industrial sewage water Industrial effluent (salt and soda company) and sewage water Industrial waste water (Cr(VI) plating industry) Synthetic heavy metal solution Synthetic wastewater Synthetic wastewater Domestic sewage effluent Ground water Synthetic heavy metal solution Urban wastewater Synthetic wastewater Synthetic wastewater Swine-wastewater Synthetic polyphosphonate water Refinery wastewater

Cyanobacteria Aphanothece halophytica Lyngbya taylorii

Compound Zn Cd, Pb, Ni, Zn

Phormidium laminosum Nostoc linckia Anabaena subcylindrica

Cu, Ni, Zn

Nostoc PCC7936

Cr(VI)

Oscillatoria anguistissima Anabaena doliolium Spirulina platensis Oscillatoria sp. Synechococcus sp. strain PCC 7942 Phormidium laminosum Phormidium sp. Phormidium laminosum Anabaena CH3 Spirulina maxima Spirulina spp.

Zn, Cu, Co

Aphanothece microscopica

Zn, Cd Cu, Co, Pb, Mn

Nutrients, Cu and Fe NO3−, NH3, PO4− NO3− and orthophosphate NO3− N and P NO3− and PO4− NO3− and PO4− NO3− and NH3 NH3–N and total phosphorus Hexamethylenediamine-N,N,N,Ntetrakis(methylphosphonic acid) CO2 biofixation

* Gupta V, Ratha SK, Sood A, Chaudhary V, Prasanna R. New insights into the biodiversity and applications of cyanobacteria (blue-green algae)—Prospects and challenges. Algal Research 2013,2:79-97, reproduced with permission.


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Several investigations have demonstrated that biosorption can replace conventional systems for the removal of heavy metal ions from aqueous solutions. The development of biosorption processes should be studied further, with a particular focus on models, the regeneration and immobilization of biosorbents, and treating of real industrial wastewater. [3] Because of the extensive research and significant economic benefits of biosorption, some new biosorbent materials are poised for commercial exploitation. [49] Cyanobacteria are good replacement adsorbents among algal biosorbents because they are widespread in nature and exhibit good metal sorption properties. Microcystis spp. are the dominant species in eutrophic freshwater lakes during cyanobacterial blooms. Several lines of evidence have indicated that Microcystis has great potential as an effective biosorbent for the removal of heavy metals from contaminated waters. Microcystis cells can selectively remove heavy metals. For metal cycling in eutrophic aquatic environments, the bioaccumulation of different metals in bloom-forming Microcystis may be particularly significant. Parker et al., [69] suggested that microcystin algae recovered from natural lakes exhibited substantial Cu, Cd, and Ni sorption capacities. Luoma et al., [76] observed that a spring phytoplankton bloom was accompanied by remarkable reductions (more than 50%) in the dissolved Cd and Ni concentrations in South San Francisco Bay, USA, due to the accumulation of metal ions in the phytoplankton. The results of that study indicated that the bloom-forming Microcystis has a high Cd adsorption capacity. Therefore, Microcystis biomass, which is naturally abundant and otherwise a nuisance in cyanobacterial blooms, may be an excellent candidate for application in the removal of Cd from natural waters. [29]

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[69] Parker, DL; Mihalick, JE; Plude, JL; Plude, MJ; Clark, TP; Egan, L; et al., Sorption of metals by extracellular polymers from the cyanobacterium Microcystis aeruginosa fo. flos-aquae strain C3-40. Journal of Applied Phycology, 2000, 12, 219-224. [70] Pradhan, S; Rai, LC. Biotechnological potential of Microcystis sp. in Cu, Zn and Cd biosorption from single and multimetallic systems. BioMetals, 2001, 14, 67-74. [71] Pradhan, S; Rai, LC. Copper removal by immobilized Microcystis aeruginosa in continuous flow columns at different bed heights, study of the adsorption/desorption cycle. World J. Microbiol. Biotechnol. 2001, 17, 829-832. [72] Li, P-F; Mao, Z-Y; Rao, X-J; Wang, X-M; Min, M-Z; Qiu, L-W; et al., Biosorption of uranium by lake-harvested biomass from a cyanobacterium bloom. Bioresource Technology, 2004, 94, 193-195. [73] Shan, WU; Fu-hong, SUN; Yuan-bo, YAN; Ming, CHANG; Fengchang, WU. Biosorption of Sb(Ⅴ) by Cyanobacteria from Taihu Lake. Research of Environmental Sciences, 2012, 764-769. [74] B. V. Biosorption and me. Water Res, 2007, 41, 4017-4029. [75] JL W; C. C. Biosorption of heavy metals by Saccharomyces cerevisiae, A review. Biotechnology Advances, 2006, 24, 427-451. [76] N LS; A vG; Lee B G ea. Metal uptake by phytoplankton during a bloom in South San Francisco Bay, Implications for metal cycling in estuaries. Limnol Oceanogr, 1998, 43, 1007-1016.




Chapter 3

OLD WINE IN NEW SKINS EUTROPHICATION RELOADED: GLOBAL PERSPECTIVES OF POTENTIAL AMPLIFICATION BY CLIMATE WARMING, ALTERED HYDROLOGICAL CYCLE AND HUMAN INTERFERENCE Martin T. Dokulil EX Institute for Limnology, Mondsee, Austria Assoc. University Vienna, Dept. Limnology, Vienna, Austria

ABSTRACT Natural or anthropogenic enrichment of surface waters through input of nutrients, commonly referred to as Eutrophication, is essentially a catchment related process. The relative importance of different hydrological pathways in the water shed are therefore of crucial significance. Although eutrophication has a rather long history, the problem and its implications became particularly apparent in the mid-20th century as a consequence of population density, urban development, tourism, industry and agricultural practices. To maintain sustainable human societies profound water management was and is required including concepts to restore or rehabilitate surface waters and to prevent further deterioration. Mitigation of nutrient input was successful in many


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Martin T. Dokulil regions but failed or responded slowly in others, often as a result of inlake processes. The growing water demand and the lack of clean water in large parts of the world necessitate elaborate models in the near future particularly under warmer climate scenarios. In a warmer world many consequences of eutrophication will potentially be amplified. Interaction of climate change with eutrophication will proliferate harmful algal blooms (HABS), spread infectious diseases, changes pathogen communities and favours microparasites among several other abiotic and biotic components affecting ecosystems. Persistent eutrophication may exceed ecological thresholds and lead to regime shifts. The symptoms of cultural eutrophication will certainly worsen when global temperatures increase and human impact intensifies further. Concepts and models are needed for future mitigation specifically for developing countries of the inter-tropical zone because initial attempts at applying temperate zone control measures in these regions have been largely unsuccessful.

Keywords: Nutrient input, lakes, rivers, algal blooms, catchment

INTRODUCTION Eutrophication is perhaps the greatest threat to water quality worldwide particularly in developing countries. It affects all types of inland waters, rivers and streams. More recently a new paradigm on coastal eutrophication emerged (Duarte 2009). This new paradigm emphasizes its global dimension and the connections with other global environmental pressures. Population growth, economic development and lifestyle changes have added to the problem (Ansari 2011). The massive impact of cultural eutrophication on natural waters in general, and on water availability, water quality and water usage has generated an enormous volume of literature on causes, consequences, monitoring and management. It is the aim of the present review to summarize the state-of-the-art in eutrophication research in the context of climate warming.

DESCRIBING EUTROPHICATION Eutrophication is defined as the enrichment of surface waters by inorganic plant nutrients, mainly phosphorus and nitrogen, as a result of slow natural or human induced accelerated processes. This nutrient load originating from the


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water shed increases productivity of the receiving waters (Dokulil 2013a, b). Hence, eutrophication essentially is a catchment orientated process. Phosphorus has generally been identified as the most important nutrient limiting productivity in inland waters (e.g., Vollenweider 1968, Likens 1972, Schindler 1974, 1977, OECD 1982). Nitrogen limitation has recently attained more attention evolving into a controversial discussion (Howarth, & Marino 2006, Schindler et al., 2008, 2012, Sterner 2008, Lewis & Wurtsbaugh 2008, Dolman et al., 2012). Nitrogen limitation seems responsible for macrophyte decline justifying the control of both phosphorus and nitrogen, at least in shallow lakes and estuaries (Moss et al., 2013). Phytoplankton biomass is affected by an increase in inorganic nitrogen due to atmospheric deposition in unproductive lakes in the northern hemisphere (Bergström & Jansson 2006). The authors conclude that these systems are limited by N in their natural state. Dai et al., (2012) tested the hypothesis that ammonia regulates the succession of cyanobacterial blooms. They could show that ammonia can be an important factor to determine the distribution of common algal species and cyanobacterial bloom in freshwater systems. In addition, it must be emphasised that some authors see eutrophication also as an increase in the supply of organic matter (e.g., Nixon 2009). This view might be rectified when dealing with coastal or marine eutrophication which certainly needs viewing ecosystem changes on a larger scale. In general and in almost all textbooks however, the term eutrophication is used to describe the increase in concentrations of plant nutrients in aquatic ecosystems (e.g., Harper 1992, Mason 2002). Besides the already above mentioned phosphorus and nitrogen, silicon, iron or manganese are sometimes cited as potential limiting substances. The nutrient status and/or productivity of a body of water is commonly quantified by a range of variables characterising water quality (Figure 1) The continuum of trophic levels is divided into several levels ranging from very poor in nutrients (ultra-oligotrophic) to over-saturated with nutrients (hypereutrophic). For more details refer to e.g., OECD 1982; Scholten et al., 2005; Andersen & Conley 2009; Ansari et al., 2011 among the vast literature in the field. Nutrient enrichment enhances primary productivity resulting in growth and finally excessive development of phytoplankton, sessile algae and macrophytes (Figure 2). When nutrient loading is small, phytoplankton, submersed macrophytes and sessile algae on submersed vegetation prevail. As loading increases, macrophytes decline and phytoplankton gaines


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more and more importance. At further increased nutrient input, phytoplankton dominates the ecosystem after submersed macrophytes have vanished. At very high nutrient concentrations, emergent or free floating vegetation competes with phytoplankton (Figure 2). This sequence is often associated with a regime shift (Figure 3) when ecological thresholds are exceeded leading to the existence of multiple stable states (Scheffer 1998, Dokulil & Teubner 2003, Dokulil et al., 2006, 2011).

Figure 1. Conceptual diagram showing the gradients of nutrients, productivity, algal biomass and transparency in the trophic continuum from ultra-oligotrophic to hypereutrophic. Nutrients, productivity and biomass increase while transparency decreases at higher trophic status.

Figure 2 Conceptual illustration of the contribution of various vegetation forms to primary productivity as nutrient load increases. Submersed vegetation, algae on sediments dominate at low nutrient loading. As loading increases, phytoplankton contribution becomes dominant until at very high loading emergent or free-floating vegetation co-dominates.


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TP / Biomass / Turbidity

Turbid regime

Threshold Resilience Resilience Threshold

Clear regime Time

Figure 3. Eutrophication as a regime shift. The fluctuating line indicates the concentration of certain variables such as total phosphorus (TP), algal biomass, turbidity etc. Dotted lines show the means for the clear-water regime (oligotrophic) and turbid regime (eutrophic). The thresholds for both are shown by dashed lines. Distance of the mean to the thresholds is interpreted as resilience. Modified from Carpenter (2003).

HISTORY OF CULTURAL EUTROPHICATION Anthropogenic eutrophication is perhaps the oldest environmental impact imposed on the biosphere by man. During the pre-agricultural hunting and picking stage total human population was small and eutrophication insignificant. As a consequence of agriculture, settlement and increasing population growth cultural eutrophication became progressively apparent. Eutrophication is thus not a recent phenomenon. It has continuously accompanied human activities in a variable but steadily increasing degree. Locally cultural eutrophication can have been far more significant in the past than today. Cultural eutrophication has fundamentally changed the cycling of carbon, nutrients and water. The circulation within small regions characteristic in earlier days was changed and the circles were first opened when natural space and times scales were exceeded, and subsequently widened to global scales. Biogeochemical cycles are significantly changed nowadays. Stored carbon particularly from fossil fuels is reassigned with the consequence that the


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atmospheric CO2 concentration has increased from 240 to 380 ppm since the beginning of the industrial period. Phosphorus is used in large amounts. Nitrogen gas is fixed from the atmosphere in similar amounts as nitrogen fixers and the hydrological cycle is altered in many ways. This fracturing of the original cycling, introduces a new, global cycling pattern that changes the overall functioning of the globe. Human population growth and increased eutrophication are therefore two aspects of the same cause (Wassmann & Kalle 2004).

SOURCES, SYMPTOMS AND RESTORATION The causes of enhanced nutrient input to surface waters are manifold but can be divided into natural and anthropogenic components. Natural eutrophication usually is a slow geochemical process affecting lakes over long periods of time. A lake or reservoir can be naturally eutrophied for instance when situated in a fertile area with naturally nutrient enriched soils. Human influences are commonly categorised into two classes, point and non-point sources (Table 1). Table 1. Point and nonpoint sources of inputs to receiving waters  Point Sources  Wastewater effluent (municipal and industrial)  Runoff and leachate from waste disposal sites  Runoff and infiltration from animal feedlots  Runoff from mines, oil fields, unsewered industrial sites  Storm sewer outfalls from cities with a population >100,000  Overflows of combined storm and sanitary sewers  Runoff from construction sites >2 ha  Nonpoint Sources  Runoff from agriculture (including return flow from irrigated agriculture)  Runoff from pasture and range  Urban runoff from unsewered and sewered areas with a population