Algal biomass production and wastewater treatment ...

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Algal biomass production and wastewater treatment in high rate algal ponds receiving disinfected effluent a

a

a

Aníbal Fonseca Santiago , Maria Lucia Calijuri , Paula Peixoto Assemany , Maria do b

Carmo Calijuri & Alberto José Delgado dos Reis

c

a

Department of Civil Engineering , Federal University of Viçosa , Av. P.H. Rolfs – Centro de Ciências Exatas e Tecnológicas – Campus da Universidade Federal de Viçosa, Viçosa , MG , Brazil , 36570000 E-mail: b

School of Engineering of São Carlos - University of São Paulo , Av. Trabalhador São Carlense, 400 CP 359 , São Carlos , SP , Brazil , 13566590 E-mail: c

National Laboratory of Energy and Geology , Laboratório Nacional de Energia e Geologia, I.P. - Unidade de Bioenergia - Estrada do Paço do Lumiar, 22- Edifício F R/C, Lisbon , PORTUGAL , 1649-038 E-mail: Accepted author version posted online: 12 Jun 2013.

To cite this article: Aníbal Fonseca Santiago , Maria Lucia Calijuri , Paula Peixoto Assemany , Maria do Carmo Calijuri & Alberto José Delgado dos Reis (2013): Algal biomass production and wastewater treatment in high rate algal ponds receiving disinfected effluent, Environmental Technology, DOI:10.1080/09593330.2013.812670 To link to this article: http://dx.doi.org/10.1080/09593330.2013.812670

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Algal biomass production and wastewater treatment in high rate algal ponds receiving disinfected effluent Aníbal Fonseca Santiagoa 1, Maria Lucia Calijuria 2, Paula Peixoto Assemanya 3, Maria do

Downloaded by [INAOE], [Aníbal Santiago] at 04:41 13 June 2013

Carmo Calijurib 4, Alberto José Delgado dos Reisc 5 a

Department of Civil Engineering, Federal University of Viçosa, Viçosa, Brazil

b

School of Engineering of São Carlos - University of São Paulo, São Carlos, Brazil

c

National Laboratory of Energy and Geology, Lisbon, Portugal

Corresponding Author 1

email: [email protected]; postal address: Av. P.H. Rolfs – Centro de Ciências Exatas e

Tecnológicas – Campus da Universidade Federal de Viçosa – Viçosa, MG – Brazil – Postal Code: 36570000 PHONE/FAX: +55 31 38993098 Others Authors 2


Tecnológicas – Campus da Universidade Federal de Viçosa – Viçosa, MG – Brazil – Postal Code: 36570000 3


Tecnológicas – Campus da Universidade Federal de Viçosa – Viçosa, MG – Brazil – Postal Code: 36570000 4

email: [email protected]; postal address: Av. Trabalhador São Carlense, 400 CP 359 São Carlos, SP –

Brazil – Postal Code:13566590 5

email: [email protected]; postal address: Laboratório Nacional de Energia e Geologia, I.P. - Unidade

de Bioenergia - Estrada do Paço do Lumiar, 22- Edifício F R/C – Lisbon – PORTUGAL – Postal Code: 1649-038

Abstract Algal biomass production associated with wastewater is usually carried out in high rate algal ponds (HRAPs), which are concomitantly used in the treatment of such effluent. However, most types of wastewater have high levels of bacteria that can inhibit the growth of algal biomass by competing for space and nutrients. The objective of this study was to assess the influence of ultraviolet (UV) predisinfection on the performance of HRAPs used for wastewater treatment and algal biomass production. Two HRAPs were tested: one received effluent from an Upflow Anaerobic Sludge Blanket (UASB) reactor – HRAP – and the second received UASB effluent pre-disinfected by UV Downloaded by [INAOE], [Aníbal Santiago] at 04:41 13 June 2013

radiation – UVHRAP. Physical, chemical and microbiological parameters were monitored, as well as algal biomass productivity and daily pH and DO variation. The UVHRAP presented highest DO and pH values, as well as greater percentage of chlorophyll a in the biomass, which indicates greater algal biomass productivity. The average percentages of chlorophyll a found in the biomass obtained from the HRAP and the UVHRAP were 0.95±0.65% and 1.58±0.65%, respectively. However, total biomass productivity was greater in the HRAP (11.4 gSSV m² day-1) compared with the UVHRAP (9.3 gSSV m² day-1). Mean pH values were 7.7±0.7 in the HRAP and 8.1±1.0 in the UVHRAP, and mean values of DO percent saturation were 87±26% and 112±31% for the HRAP and the UVHRAP, respectively. Despite these differences, removal efficiencies of organic carbon, chemical oxygen demand, ammoniacal nitrogen and soluble phosphorus were statistically equal at the 5% significance level. Keywords: high rate algal ponds, wastewater, ultraviolet disinfection, algal biomass production, algae/bacteria systems

Introduction Reducing input costs (water, nutrients, etc.) is one of the main challenges in making algal biomass production economically feasible for its several purposes. According to Wijffels and Barbosa [1], the production of biofuel from microalgae, for instance, requires approximately 1.5 L of water per kg of biofuel produced. Water use can be much larger if losses by evaporation in open systems and water use for cooling closed systems are taken into account. In open systems, the annual water consumption in ponds for microalgae production is in the range of 11-13 million of L per ha [2]. Thus we highlight the importance of reusing wastewater, which also enables nutrient recycling.

Algal biomass can be grown as a by-product of high rate algal ponds (HRAPs) operated for wastewater treatment [3]. HRAPs are raceway-type ponds with depths in the range of 0.2-0.5 m, hydraulic retention times (HRT) from 3-10 days, and paddlewheels to provide mixing [4, 5, 6]. Algal photosynthesis produces the oxygen required for degradation of organic matter by heterotrophic bacteria. Nutrients and the CO2 resulting from oxidation are assimilated by the algae. The gentle mixing in HRAPs serves several purposes, including prevention of cell settling, elimination of thermal stratification, and promotion of growth of algae that form colonies which can be more easily removed by gravity settling. Additionally,


mixing promotes better nutrient distribution, improves light utilization efficiency, and removes the photosynthetically produced oxygen, which improves the air-liquid transfer and avoids inhibition of photosynthesis by excess of this element [7]. Fallowfield et al. [8] state that the adaptation of HRAP shapes and paddlewheel systems aim to improve efficiency in wastewater treatment and reduce land area requirements by optimizing algal photosynthetic oxygen production. According to Craggs et al. [6], despite some differences when compared to other stabilization ponds, HRAPs retain the advantages of simplicity and economy, and overcome disadvantages such as poor and highly variable effluent quality and limited nutrient and pathogen removal. Craggs et al. [6] presented a concept for using HRAPs for wastewater treatment and algal biomass cultivation for purposes of energy production (biofuel). The options presented in the conceptual schematic diagram are defined according to the requirements for effluent reuse or discharge into watercourses. Given that the needs and capacities of each unit process are known, a combination of such processes can be defined in order to achieve the desired global performance. Assuming concentrations of 10-103 MPN (100 mL)-1 of Escherichia coli are desired for effluent water quality, influent wastewaters with E. coli values of 107 MPN (100 mL)-1 would present a removal of only 2 log units, thus an additional 2 log removal would be necessary. In that case, a UV disinfection step could be added for the effluent to achieve the desired E. coli concentration. However, a hypothesis based

on the study conducted by Cho et al. [9] is that the UV disinfection as a pre-treatment prior to the HRAP can ensure that the microbiological quality of the effluent will be achieved, and also increase microalgae productivity. After a pre-disinfection step, the loads of bacteria and protozoa which negatively affect microalgae growth are reduced. According to Cho et al. [9], a large number of bacteria present in wastewater can inhibit microalgae growth by competing for space and nutrients, and bacteria grow faster than microalgae. The authors presented studies at laboratory scales and concluded that an adequate pre-treatment method


to remove competing microorganisms can be used for the effective production of algal biomass. Therefore, given the scarcity of information on pilot-scale wastewater treatment systems which include pre-treatment methods, the objective of this study was to assess the influence of ultraviolet predisinfection on wastewater treatment performance and algal productivity in HRAPs.

Material and methods The experiments were carried out in the municipality of Viçosa, State of Minas Gerais, Brazil (lat. 20°45´14”S, long. 42°52´54”W), at the Integrated Experimental Unit for Wastewater Treatment and Reuse, maintained and operated by the Federal University of Viçosa and the city’s Water and Wastewater Services (SAAE – Viçosa). In Viçosa, the annual average precipitation is 1221 mm and the annual average temperature ranges from 19°C to 20°C. The annual average relative humidity is 81%. The climate is Cwa (humid subtropical climate) according to the Köppen classification, characterized by dry winters and rainy summers [10].

Experimental unit description The experimental unit was installed near a full-scale Wastewater Treatment Plant consisted of a prefabricated steel UASB reactor, with an average effluent flow of 115 m3 day-1, volume of 48 m3, height of 5.7 m and HRT of 7 h. A portion of the UASB effluent was directed to a pilot-scale HRAP

system. The study of the UASB-HRAP combination was performed given the widespread use of such reactor in developing countries, since it is considered a low-cost and easy-operation option. The HRAP influent wastewater was primary effluent from the UASB reactor, and the UVHRAP received UASB effluent pre-disinfected by UV radiation. The experimental HRAPs were made from fiberglass and had the following dimensions: width = 1.28 m, length = 2.86 m, total depth = 0.5 m, useful depth = 0.3 m, surface area = 3.3 m², useful volume = 1 m³ and HRT = 4 days. Such units were embedded in the soil at 0.20 m. The paddlewheels were made out of 2


PVC paddles driven by a 1hp electric motor. Rotation was reduced by a reduction gear coupled to the motor and controlled by a frequency inverter (WEG, series CFW-10) to give a mean horizontal water velocity of approximately 0.10-0.15 m s-1, similar to values used in different studies [11,12] to assure the necessary agitation. For Oswald [4], velocities of 0.12-0.15 m s-1, depth of 0.3 m and HRT of 4 days are advantages of this type of system, aimed to provide the maximum biomass productivity at a minimum cost. To control the HRT at each pond, the flow was periodically regulated to 0.25 m³ day-1, 5 times a week, and the level of the supply tanks were maintained constant in order to guarantee the constant flow, The disinfection system was designed to achieve a final concentration of 10³ MPN (100 mL)-1 of E. coli, with an adopted effective dose of 21 mJ cm-² and absorbance of 42%, as suggested by Gonçalves et al. [13], who studied E. coli removal from UASB effluent by UV disinfection. Thus an applied dose of 203.1 mJ cm-² and applied dose per volume of 5.64 Wh m-3 were used in the disinfection unit. The characteristics of the disinfection reactor were: width = 0.16 m, length = 0.76 m, water depth = 0.10 m and HRT = 8.4 s. On the longitudinal axis, we installed three low pressure UVC lamps (0.050) (Table 1).

Productivity Park and Craggs [17] have used chlorophyll a to estimate the proportion of algae in the algal/bacterial biomass from HRAPs. The average percentages of chlorophyll a of 0.95% and 1.58%, found in the biomass obtained from the HRAP and the UVHRAP, respectively, were considered statistically different (p0.050). Craggs et al. [6] observed

N-NH4 removals of 64-67% for HRAPs with an HRT of 4 days; García et al. [22] reported removal efficiencies of 57% and 73% in HRAPs with HRTs of 3 and 7 days, respectively. Such results are in the same magnitude order as those presented in this study.

Effluent ammoniacal nitrogen concentrations of 11±8 mg N-NH4 L-1 and 10±9 mg N-NH4 L-1 were found in the HRAP and the UVHRAP, respectively. Considering that the diurnal pH values were below 8.2 in the HRAP and 8.7 in the UVHRAP, and the increase in nitrate concentration in the treated effluent of both ponds (Table 1), the N-NH4 removal can be mostly attributed to the nitrification process. An increase in Norg concentrations was also observed, which showed that the nitrification and biomass assimilation were the main processes of nitrogen transformation, given the conditions assessed in this study. Unlike these results, Craggs et al. [20] and el Hamouri et al. [21] obtained removal efficiencies of up to 91% and 62%,


respectively, and low nitrate concentrations in treated effluent, which showed that the main processes of N-NH4 removal observed in their studies were assimilation or volatilization. Because nitrogen assimilation by the biomass can be verified by the increment in Norg concentration, TKN removal is not effective without a process for biomass separation. For García et al. [22], this is the most important mechanism for the effective removal of nitrogen, given that the transformation of nitrogen into nitrate does not represent removal. Park and Craggs [17] evaluated total nitrogen removal in a system consisted of a settler (HRT = 6h) installed after an HRAP (HRT of 4 days and CO2 addition). The authors verified a removal efficiency of ~57% for the settler and 74% for the HRAP-settler system. Soluble phosphorus removals were 19% and 14% for the UVHRAP and the HRAP, respectively, with no statistical differences (p=0.11). Craggs et al. [6] found higher removal efficiencies of 14-24%. The main mechanisms for phosphorus removal in HRAPs are assimilation into biomass and chemical precipitation at high pH values [12]. Such processes can explain the slightly greater removal observed in the UVHRAP, which presented greater algal biomass productivity and higher pH values that allowed for the chemical precipitation of this element. E. coli removal of 2.1 log units was observed for the HRAP. This removal efficiency is similar to those obtained by Craggs et al. [6], who assessed hectare-scale HRAPs, but different from results obtained in research conducted in small-scale ponds, which presented removals of approximately 1.0 log unit. Despite the short length of the pilot-units, which does not allow for an efficient mixing of influent within

the pond volume before completing a circuit, short circuiting problems reported by Craggs et al. [6] did not occur in this study. In the UVHRAP, the E. coli removal of 1.1 log units was probably due to its lower influent concentration. Considering the first-order removal kinetics, or even that the organisms in this pond are more resistant (the least resistant were presumably removed by pre-disinfection), we can infer that UV disinfection interfered in the UVHRAP performance, and previously removed 2.0 log units of E. coli. The overall


removal for the pre-disinfection + HRAP combination was 3.1 log units. Similar or even higher removal efficiencies could be achieved by UV disinfection after the HRAPs. In that case, lower UV doses would be required to achieve the same efficiency, because of the lower effluent solids concentrations due to the removal of algal biomass. Figure 5 shows the abundance of the main algae genus found in the ponds during the monitoring period. Chlorophyceae was the most abundant class in the ponds. In the HRAP, Chlorella sp. and Desmodesmus sp. were present during the entire monitoring period, with average abundances of 34 and 36%, respectively. In June (low temperatures), Coelastrum sp. and Micractinium sp. were the most abundant and after that, the dominant species Chlorella sp. and Desmodesmus sp. reappeared. Lower abundances of Scenedesmus sp., Chlorococcum sp., Coelastrum sp. and Pinnularia sp. were observed in the beginning and in the end of the monitoring period. “Other genus” refers to those which presented abundance below 5%. Chlorella sp. and Desmodesmus sp. were also present in the UVHRAP practically during the entire monitoring period. Until June, their average abundances were 40% and 49%, respectively. After June, the abundances were inverted, 68% of Chlorella sp. and 21% of Desmodesmus sp. In June, we also noted the presence of Micractinium sp., although less abundant (12%). Peridinium sp. and Coelastrum sp. abundances were over 5%. The algal consortia observed in both ponds are typically found in such pond systems [3, 5, 17],

Pediastrum sp. was present in many studies involving ponds, although it was not found here. de Godos et al. [23] studied HRAPs for treating swine wastewater and found a greater diversity of genus such as Clhamydomonnas sp., Microspora sp., Clhorella sp., Nitzschia sp., Achananthes sp., Protoderma sp., Senelastrum sp., Oocystis sp., Ankistrodesmus sp., and Chlorella sp., this last one being the only genus also found in our study, despite the different type of effluent and geographical location. Special attention must be given to Desmodesmus sp., Coelastrum sp., Micractinium sp., which are colonial organisms with diameters usually greater than 200 µm, and are interesting from the view point of settleability, a desirable


characteristic for pond systems [11]. The greater abundance of Chlorella sp. observed after June in the UVHRAP indicates that pre-disinfection may favor the dominance of certain species. Although in case of dominance of unicellular Chlorella sp. this may not be interesting since other species may have greater diameters and better settleability characteristics. The process of recycling of settled biomass assessed by Park et al. [11], however, seems more effective for that purpose, leaving to pre-disinfection the role of increasing algae/bacteria ratios.

Conclusions Greater Chlorella sp. abundance after June in the UVHRAP indicates that pre-disinfection may be responsible for the dominance of certain species; however, the recycling of settled biomass seems to be more effective for that purpose. Pre-disinfection is then responsible for maintaining high algae/bacteria ratios. Pre-disinfection by UV radiation increased algal biomass productivity. The percentage of chlorophyll a in relation to total biomass (VSS) was greater in the UVHRAP; however, if total biomass productivity is considered, the system without pre-disinfection (HRAP) was more efficient. Treatment efficiencies were similar for both ponds, despite the greater photosynthetical activity in the UVHRAP,

as shown by the higher DO and pH values and greater percentage of chlorophyll a.

The wastewater treatment performance results were similar to those reported by other authors, and

demonstrate the replicability of the systems proposed in this study (UASB-HRAP or UASB-UVHRAP). Considering the widespread use of UASB reactors, mostly in countries with hot climate, such systems are clearly applicable.

Acknowledgments The authors acknowledge the financial assistance provided by the National Council for Scientific and Technological Development, CNPq, the Research Support Foundation of Minas Gerais, FAPEMIG and the Minas Downloaded by [INAOE], [Aníbal Santiago] at 04:41 13 June 2013

Gerais State/SECTES (Secretaria de Estado de Ciência, Tecnologia e Ensino Superior).

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Figure 1. Climatic conditions during the monitoring period: air temperature, total incident solar irradiation and PAR(a); Effluent temperatures and incident PAR (measured at the site) (b). Figure 2. Percentage of chlorophyll a in the biomass. Figure 3. Influent and effluent VSS variation in the ponds over the monitoring period. Figure 4. Diurnal behavior (mean values) of DO (a); and pH (b). Figure 5. Relative abundance of the main algae present in the HRAP (a) and in the UVHRAP (b) during


the monitoring period.






Table 1. Concentrations (mean±standard deviation) and removal values of water quality variables analyzed for pond influent and effluent samples

Temp. (o C) (36) pH (36) DO (% sat.) (33) Cond (mS cm-1) (35) Alk. (mg CaCO3 L-1) (34) TOCf (mg L-1) (34) CODf (mg L-1) (35) NTK (mg L-1) (36) N-NH4 (mg L-1) (36) Norg (mg L-1) (36) N-NO3 (mg L-1) (34) Ps (mg L-1) (35) Turbidity (UT) (33) TSS (mg L-1) (36) VSS (mg L-1) (36) Chlorophyll a (mg L-1) (31) E. coli MPN (100mL)-1 (30)

Pond influent mean±standard deviation

HRAP effluent mean±standard deviation

UVHRAP

24 ±1.7 7.1±0.4 23±4.4 799±31 221±71

24±2.2 7.7±0.7 87±26 655±367 60±54

41±10 99±25 48±18 40±13 8±9 2±1 4.1±1.1 57±26 96±149 75±98 -

20±7 73±29 28±25 11±8 17±10 17±9 3.5±1.3 95±62 200±79 152±57 1.5±1.2

52% 26% 42% 71% -113% -564% 14% -68% -108% -102%

19±6 69±25 23±13 10±9 13±7 16 ±14 3.3±1.3 73±44 145±54 124±46 2.1±1.0

55% 30% 52% 74% -62% -556% 19% -27% -51% -65%

>0.050 >0.050 0.069 >0.050 0.060 >0.050 0.11 0.052