Ecological Engineering 81 (2015) 62–69
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A comparative study of macrophytes influence on wastewater treatment through subsurface flow hybrid constructed wetland Shama Sehar, Sumera, Sana Naeem, Irum Perveen, Naeem Ali, Safia Ahmed * Department of Microbiology, Quaid-i-Azam University, Islamabad 45320, Pakistan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 November 2014 Received in revised form 5 February 2015 Accepted 5 April 2015 Available online xxx
Constructed wetlands (CWs) are engineered systems that attained much attention as a feasible wastewater treatment technology particularly for small communities. It has been widely considered that the type of vegetation and hydraulic retention time (HRT) are two key intervening ingredients which directly influence the performance of CW. This study aims to compare the efficiency of a laboratory scale sub-surface hybrid constructed wetland (SS-HCW) for domestic wastewater treatment planted with different plants species (Brachiaria reptans and Trianthema portulacastrum) at different hydraulic retention times. Our findings revealed that the CW planted with T. portulacastrum showed higher total suspended solids (TSS), total dissolved solids (TDS), SO42 , PO43 , NO3 and NO2 (70.03, 74, 76.59, 73.69, 80.40 and 81.46%) removal than B. reptans (58, 63.42, 53.98, 58.83, 70.34 and 62.64%) at 20 days HRT. Similarly a higher reduction in bacterial counts (5.10 103 CFU/mL) as well as fecal pathogens (265.5 MPN index/100 mL) was observed in T. portulacastrum than B. reptans at 20 days HRT. Moreover, the vegetated CWs demonstrated superior performance over unplanted control CWs. Therefore, the present study emphasizes not only the role of vegetation but also the selection of appropriate plant species in CWs for better performances. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Hybrid constructed wetland Organic contaminants Wastewater treatment Fecal coliforms
1. Introduction Despite of continuing depletion of existing water reservoirs throughout the world, the research on wastewater treatment and its utilization for human consumption have been receiving increasing attention. Therefore, the quest for green, cost effective and energy sustainable technologies is a subject of debate today. Among, current wastewater treatment technologies, the constructed wetland technology is considered as an eco-friendly, low cost technology with some distinct advantages such as; low operation and maintenance cost, as well as provide aesthetic value, generate usable plant biomass, and help support wildlife habitat (Poe et al., 2003; Solano et al., 2004; Carty et al., 2008; Shalaby et al., 2008). Constructed wetlands are effectively utilized worldwide for the treatment of various wastewater types such as; domestic wastewater (Vymazal, 2005; Song et al., 2009), food processing (Comino et al., 2011), fertilizer and chemical manufacturing (Domingos et al., 2007), tannery wastewater (Saeed et al., 2012), refinery effluent (Wallace and Kadlec, 2005), as well as
* Corresponding author. Tel.: +92 51 90643009. E-mail address: safi
[email protected] (S. Ahmed). http://dx.doi.org/10.1016/j.ecoleng.2015.04.009 0925-8574/ ã 2015 Elsevier B.V. All rights reserved.
for the elimination of emerging organic contaminants (HijosaValsero et al., 2010; Ávila et al., 2013). The constructed wetlands are formed by various beds loaded with inadequately sapped graded medium such as soil or gravel planted with vegetation coupled with microbial inhabitants that are essential for contaminants removal in surface water; groundwater or waste streams (EPA, 2000). Depending on the nature of flow, wetland systems are generally categorized into two major groups viz: surface flow and sub-surface flow. Further subsurface flow systems are categorized into horizontal and vertical subsurface flow wetland. Although, all of the mentioned systems are efficient in removing contaminants and pathogens from wastewater, however being possessing an elevated evaporation rate as compared to lagoons and ponds, the potential of reusable water in these systems is very limited. To overcome this concern, a configuration named as “hybrid CWs (combination of vertical and horizontal flow)” could be an appropriate alternate by having minimum water loss (Masi and Martinuzzi, 2007) and improved effluent quality with less total-N concentrations (Molle et al., 2008; Sayadi et al., 2012). Rawal Lake is the largest source of water supply for the twin cities of Islamabad and Rawalpindi (population over 2 million) for human consumption and is getting polluted rapidly by the addition of domestic waste/sewerage from associated streams and
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communities (residential colony and vicinity of Quaid-i-Azam University (QAU) Islamabad, Pakistan). Therefore, it is of great interest to treat the wastewater near their origin prior to its fraternization with lake water. Herein, in this work, we report labscale sub-surface flow hybrid constructed wetlands (SS-HCW) planted with B. reptans and T. portulacastrum and their treatment efficiencies were compared with varying hydraulic retention time. It was found that the presence of vegetations and their appropriate selection is essential to attain high treatment efficiency in CWs. Therefore, the similar strategy can be effectively utilized on larger scale (pilot scale) which could be a milestone towards preserving the largest water reservoir. 2. Materials and methods 2.1. Development and operational set up of SS-HCW Two parallel laboratory scale SS-HCW systems were constructed comprised of a septic tank with loading capacity of 500 L and three rectangular operational units (length width height = 4 1.5 1.5 ft). The first two treatment units with subsurface vertical and horizontal flow respectively were composed of three layers lying one over the other (organic soil, 12.5 cm, sand, 15 cm, and gravel, 7.5 cm) and planted with B.reptans and T. portulacastrum in two separate experimental set up. Selection of plant was made on the fact that they were found growing naturally in sewage contaminated natural stream and tolerant enough for contaminants and saturated soil conditions. The third unit consisted of a sand bed that served the final polishing step from the water that received treatment from the first two units. Polyvinylchloride pipes (length, 125 in.; inner diameter, 2 cm) were used to interconnect four units that were sequentially placed at decreasing levels (1 ft) to enable natural flow of water under gravity. Flow rate of water among different units was regulated by valves and nozzles. Before subjected into working conditions, the middle two units were kept soaked with fresh water for 3–4 weeks in order to acquire saturated growth of plants and associated microbial community in the rhizosphere, sand and gravel bed. This helped in the establishment of a compact bed suitable for wastewater treatment. SS-HCW was operated with different hydraulic retention times of 4, 8, 12, 16, 20, 24 and 28 days (from April 2012 to July 2013). Temperature was continuously monitored during the entire study by using thermometer and was found to be in the range of 30–45 C. Similar system was developed without any vegetation that serves as a control. A schematic representation of the overall treatment process is shown in Fig. 1. 2.2. System operation: Treatment of wastewater Domestic wastewater samples were collected in pre-washed (with detergent dilute nitric acid and doubly deionized water
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respectively) polyethylene bottles from residential colony and nearby areas and given a retention time of almost 3–4 h in the septic tank to allow settling of suspended particles and particulate material. After sedimentation, this partly treated wastewater was then subjected to treatment in the subsequent units of SS-HCW. During the course of the functional phase of wetland system, temperature was regularly monitored. Water samples were collected from each processing unit at different HRTs (4, 8, 12, 16, 20, 24 and 28 days) and analysed for various physico-chemical and microbiological parameters. 2.3. Physico-chemical analysis Physico-chemical parameters were determined by following American Public Health Association (APHA) Standard Methods (APHA, 2005). Organic contaminants including biological oxygen demand (BOD) was measured by 5-day BOD test (following 5210 B standard methods) and chemical oxygen demand (COD) by kit method; (high range 14541 and low range 14560 CSB/COD kits, (Merck, Germany)). Total dissolved solids (TDS) and total suspended solids (TSS) were determined by standard methods 1540 C and 2540 D respectively. Nitrates, nitrites, phosphates and sulfates were determined by standards methods; 4500 NO3–N, 4500 NO2–N, standard method 4500-P and 0375 Barium chromate respectively. 2.4. Microbiological analysis Domestic wastewater was subjected to microbiological analysis through colony forming unit (CFU/mL) and most probable number technique (MPN index/100 mL) of pathogenic indicators i.e. fecal coliforms and Enterococcus faecalis as per guideline of Bergey’s Manual of Determinative Bacteriology (Holt et al., 1994). Conventional serial dilution method was employed for CFU/mL of bacterial colonies in the influent and effluent samples. Wastewater samples were serially diluted in sterile water up to 10 10 and these dilutions were spread plated on nutrient agar plates and incubated at 37 C. After 24 h of incubation, the colonies appeared were enumerated by colony counter, and CFU of each colony was then calculated as follows: CFU/mL = number of colonies dilution factor/inoculum size. For determining the MPN index of pathogen indicators ((fecal coliforms, E. coli, Salmonella, Shigella, Klebsiella sp., Enterobacter and Citrobacter), untreated (influent) and treated (effluent) wastewater samples were incubated at 42.2 C for 24–48 h in MacConkey’s broth using multiple tube technique having inverted Durham tubes. Positive tubes were subcultured on MacConkey’s agar (MacA), Nutrient agar (NA) and Mannitol salt agar (MSA) plates and incubated at 37 2 C for 24–48 h. Positive isolates showing growth of bacterial colonies were confirmed by microscopy and checked for total count. 2.5. Microbial profiling of rhizosphere of B. reptans and T. portulacastrum
Fig. 1. Schematic illustration of subsurface hybrid constructed wetland (SS-HCW).
For determination of bacterial diversity colonizing the rhizosphere of B. reptans and T. portulacastrum planted in SS-HCW, 1 g soil sample was collected. Different dilutions of the order 10 3, 10 5 and 10 7 were prepared. From each dilution, 0.1 mL inoculum was pipetted and inoculated onto nutrient agar plates by spread plate technique and plates were incubated for 24 h at 37 C. After incubation, different types of colonies on nutrient agar plates were distinguished by morphological characteristics. For the purpose of obtaining pure cultures, different colonies were further sub-
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a
400
80 60 40
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20 0
3. Results and discussion
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Efficiency (%)
TSS (mg/l)
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0 4
8 12 16 20 24 28
HRT (Days)
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100 80 60 40
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Efficiency (%)
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HRT (Days) Concentraon Efficiency (%) Control B. reptans T. portulacastrum
Fig. 2. Comparative variation in the TSS (a) and TDS (b) of wastewater after treatment through SS-HCW planted with B. reptans and T. portulacastrum and without vegetation (control) at different hydraulic retention time.
cultured on a range of media including Eosin-methylene blue agar (EMB), mannitol salt agar (MSA), Salmonella Shigella agar (SSA), blood agar (BA), Pseudomonas citrimide agar (PCA) and MacConkey’s agar (MacA), and incubated at 37 C for 24–36 h. Finally, subcultured organisms were identified through Gram staining, microscopic and morphological characteristics, and biochemical analysis (Holt et al., 1994). 2.6. Statistical analysis Obtained results were subjected to statistical analysis and the treated sewage samples were compared by t-test, and P < 0.05 was considered as minimum value for statistical significance.
The variations in physical, chemical, and bacteriological parameters and their details prior to and after treatment through SS-HCW are discussed below. 3.1. Odor and pH Odor is an important physical parameter to determine the water quality. As mentioned before the domestic waste water used in this study had nasty and pungent odor due to large amount of household waste, nutrients along with excessive algal and microbial growth. After passing through vegetated units and sand bed of SS-HCW planted with B. reptans and T. portulacastrum, this foul odor was completely removed. This elimination of odor may be attributed to mineralization of the organic substances, removal of microbes during treatments in the hybrid system. Root hairs and rootlets provide an aerobic environment by developing internal air spaces (aerenchyma) which transport O2 (approximately 90%) to the root zone. This aerobic environment supports the activities of aerobic microorganisms that in turn facilitate the decomposition of organic matter and inorganic substances in water through degradation and nutrient uptake resulting in removal of pungent odor from wastewater (Zhang et al., 2007; Brix, 1997; Vymazal, 2011). Thus in this way, vegetated system facilitates better conditions for odor removal as compared to non-vegetated control systems. Also the increased HRT improved the contact time between microbes and wastewater which results in mass transfer and increased dissolved oxygen concentration which ultimately leads to eliminate nutrients and odor (NSDWG, 2008). Another vital parameter to characterize water quality is its pH value. According to WHO, 2004 standards, the pH of water should lie in between 6.5 and 8.5. The pH of collected waste water under study was 7.9. After subjected to wastewater treatment, pH of the planted as well as unplanted SS-HCW lied in a narrow acceptable range of 7.4–7.81. This slight decrease in pH might be due to the metabolism of sulphate, phosphate and nitrogenous compounds (Sakuma et al., 2008). 3.2. Removal of total solids Total solids in wastewater comprises of both total suspended solids (TSS) and total dissolved solids (TDS) including all particles suspended and dissolved in water such as silt, decaying plant and animal matter, industrial wastes, different organic and inorganic ions etc. The concentration of TSS and TDS in the untreated wastewater was 480.75 and 580.5 mg/L respectively much higher to the prescribed values by WHO, 2004, (25–80 and
