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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 , 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 Accepted author version posted online: 12 Jun 2013.Published online: 04 Jul 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, 34:13-14, 1877-1885, DOI: 10.1080/09593330.2013.812670 To link to this article: http://dx.doi.org/10.1080/09593330.2013.812670
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 1877–1885, http://dx.doi.org/10.1080/09593330.2013.812670
Algal biomass production and wastewater treatment in high rate algal ponds receiving disinfected effluent Aníbal Fonseca Santiagoa∗ , Maria Lucia Calijuria , Paula Peixoto Assemanya , Maria do Carmo Calijurib and Alberto José Delgado dos Reisc 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
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(Received 28 April 2013; final version received 31 May 2013) 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) pre-disinfection 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 radiation – UV HRAP. Physical, chemical and microbiological parameters were monitored, as well as algal biomass productivity and daily pH and dissolved oxygen (DO) variation. The UV HRAP 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 UV HRAP were 0.95 ± 0.65% and 1.58 ± 0.65%, respectively. However, total biomass productivity was greater in the HRAP (11.4 gVSS m−2 day−1 ) compared with the UV HRAP (9.3 gVSS m−2 day−1 ). Mean pH values were 7.7 ± 0.7 in the HRAP and 8.1 ± 1.0 in the UV HRAP, and mean values of DO percent saturation were 87 ± 26% and 112 ± 31% for the HRAP and the UV HRAP, 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 kilogram 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 to 10 days, and paddlewheels to provide mixing.[4–6] Algal photosynthesis produces the oxygen required for degradation of organic matter by heterotrophic bacteria. Nutrients and the CO2 resulting from oxidation are assimilated by
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
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 aims 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 with 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
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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, an ultraviolet (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 pilotscale wastewater treatment systems which include pretreatment methods, the objective of this study was to assess the influence of UV pre-disinfection 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 upflow anaerobic sludge blanket (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 as a low-cost and easy-operation option. The HRAP influent wastewater was primary effluent from the UASB reactor, and the UV HRAP received UASB effluent pre-disinfected by UV radiation. The experimental HRAPs were made from fibreglass 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 = 3.3m2 , useful volume = 1 m3 and HRT = 4 days. Such units were embedded in the soil at 0.20 m. The paddle wheels 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 ms−1 , similar to values used in different studies [11,12] to assure the necessary agitation. For Oswald,[4] velocities of 0.12–0.15 ms−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 was 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 UV lamps with wavelengths below 290 nm (known as UVC radiation) encased in a quartz tube, 15 W each, with 436 mm of length and 26 mm of diameter. Monitoring Monitoring was carried out from 31 January to 23 November 2012 and comprised periods of hot and rainy climate (Feb–Mar), intermediate (Apr), cold and dry climate (May– Sep) and the beginning of the rainy season (Oct–Nov). Effluent samples were collected at weekly intervals. Composite samples were collected every 2h (from 8:00 to 18:00h) for physical–chemical analysis. Simple random samples for the analyses of chlorophyll a and E. coli were collected at 10:00 and 12:00h, respectively. The variables pH, dissolved oxygen (DO), electrical conductivity (Cond) and temperature (Temp) were measured at the site, every 2h, using the Hach HQ40d portable meter (Luminescent Dissolved Oxygen – for DO). Photosynthetically active radiation (PAR) (400–700 nm) was measured on the water
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Environmental Technology surface using the LI-193 Underwater Spherical Quantum Sensor available from LI-COR. Flow regulation was performed every week by direct flow measurement (volumetric method). The physical–chemical analyses of influent and effluent wastewaters were carried out according to the Standard Methods for the Examination of Water and Wastewater.[14] The methods used for analysis of each variable are between parentheses: filtered chemical oxygen demand (CODf) (5220D – sample filtered with 0.45 μm membrane), total alkalinity (Alk) (2320B), turbidity (TUR) (2130A), total suspended solids (TSS) (2540D), volatile suspended solids (VSS) (2540E), ammoniacal nitrogen (N-NH4 ) (4500 – NH3 C), total Kjeldahl nitrogen (TKN) (4500-NorgB), organic nitrogen (Norg ) (calculated as the difference between TKN and N-NH4 ), nitrate (N-NO3 ) (4500-NO3 A), soluble phosphorus (Ps ) (4500 P C – samples filtered with 0.45μm membrane) and E. coli abundance (Colilert® ). Filtered total organic carbon (TOCf) was determined using the Shimatzu TOC 5000 analyser (samples filtered with 0.45 μm membrane). Chlorophyll a analysis was performed using hot ethanol (80%) as the extractant according to the Dutch standard norm NEN 6520/1981,[15] and based on Nush.[16] The variables DO and pH were measured every 2h along the nyctemeral cycles on 25 May, 22 July and 23 November 2012. Quantitative and qualitative phytoplankton analyses The samples for phytoplankton analyses were collected at 2 weeks interval. For qualitative analysis, samples were collected using a plankton net with 20 μm mesh size and preserved with formalin solution (4%). The centre of the round opening of the plankton net was lowered at middepth of the pond and dragged horizontally through the entire length of the pond. Identification keys, appropriate bibliography and specialists were consulted for the proper identification of the species present in the experimental units. For the quantitative analysis, 1L samples of the effluent were collected and immediately preserved with Lugol’s solution in amber bottles. The cells were counted using a Sedgwick-Rafter chamber under a binocular microscope (Olympus Model IX70). After sedimentation, the supernatant was discarded and the remaining concentrate of approximately 100 mL was homogenized and transferred using a pipette to the Sedgwick-Rafter chamber. After 15 min, the chamber was taken to the microscope for counting with 400× magnification. Climatologic variables Climatologic data (total incident solar irradiance and air temperature) were obtained from the University’s Meteorological Station, located at approximately 3.5 km from the experimental unit.
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Statistical analysis The version 2.15.2 of the statistical software R© , developed by the R Foundation for Statistical Computing (R Development Core Team, 2012) was used to assess whether the means of two groups (mean values of the variables measured at each pond) were statistically different from each other (t-test) at the 5% significance level. The Microsoft® Excel (Microsoft, 2010) was used to produce the graphs. Results and discussion Figure 1(a) shows the values for total incident solar irradiance, PAR and air temperature, observed over the monitoring period. As expected, the air temperature decreased from summer (beginning of the monitoring) through winter, and then increased from winter to the end of the monitoring period. The incident PAR measured at the site is shown in Figure 1(b), as well as the effluent temperatures in the ponds, which did not statistically differ (p > 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 UV HRAP, respectively, were considered statistically different (p < 0.050). Figure 2 shows the variation of this percentage over the monitoring period. The standard deviation was ±0.65% for both ponds; however, the coefficient of variation was 41% for the UV HRAP and 68% for the HRAP, thus the former can be considered more homogeneous. The greater percentage of chlorophyll a in the biomass obtained from the UV HRAP allows us to infer that pre-disinfection of pond influent may provide for a higher efficiency in algal biomass production. The ecological competition between algae and bacteria (for space and nutrients) is relevant for algal productivity. Thus, maximizing the photosynthetical activity and, consequently, algal production, is one of the main purposes of HRAPs projects. The results are increased efficiency in pollutant removal and reduced land area requirements, which differentiate HRAPs from conventional stabilization ponds.[8] Unlike the results presented for chlorophyll a, VSS mean concentrations of 152 and 124 mg L−1 were found in the HRAP and in the UV HRAP, respectively, as shown in Figure 3. Thus VSS production was greater in the HRAP than in the UV HRAP (p < 0.0010), with productivities of 11.4 and 9.3 gm−2 day−1 , respectively. These values are in the same magnitude order as those obtained by Craggs et al.,[6] which corroborates the results of this study. The lower total biomass production in the UV HRAP may be explained by the fact that the VSS variable does not account for only algal biomass, but for the total biomass
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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).
produced in the pond, and the UV HRAP received UASB effluent pre-disinfected by UV radiation, therefore, with a reduced number of bacteria. The wide range of diurnal pH values in the UV HRAP showed that algal production in this pond was limited in terms of CO2 , as observed by Craggs et al.[6] Thus, HRAPs associated with methods for predisinfection and CO2 addition are expected to present greater VSS production. This is a subject for future research, given that the productivity in such systems can also be limited by the self-shading phenomenon pointed out by Park and Craggs [17] and Cromar et al.[18] Temporal variability of DO and pH in the ponds Mean values for percent saturation of DO were 112% for the UV HRAP and 87% for the HRAP (Figure 4(a)).
Craggs et al. [6] observed median and minimum values of 98.2% and 86.2% for the units assessed in their study. The maximum values of DO percent saturation during the day (145% and 118% in the UV HRAP and the HRAP, respectively) were achieved around 14:00h. The same behaviour was observed for pH, which presented mean values of 8.2 and 8.7 at 14:00h, for the UV HRAP and the HRAP, respectively (Figure 4b). Mean DO and pH values measured throughout the day were significantly higher for the UV HRAP (p < 0.0010), which indicates the influence of algal biomass activity in this pond. A maximum DO percent saturation was maintained between 12:00 and 14:00h in the UV HRAP, which may indicate photoinhibition during high irradiance periods. This behaviour was not observed for the HRAP, which demonstrates the greater proportion of algal biomass in the UV HRAP.
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Table 1. Concentrations (mean ± standard deviation) and removal values of water quality variables analysed for pond influent and effluent samples. Pond influent
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Mean ± standard deviation
HRAP effluent Mean ± standard deviation
UV HRAP
Removal (%)
Mean ± standard deviation
effluent Removal (%)
Temp. (◦ C) (36) 24 ± 1.7 24 ± 2.2 24 ± 2.3 pH (36) 7.1 ± 0.4 7.7 ± 0.7 8.1 ± 1.0 DO (% sat.) (33) 23 ± 4.4 87 ± 26 112 ± 31 Cond (mScm−1 ) (35) 799 ± 31 655 ± 367 18 631 ± 376 21 Alk. (mgCaCO3 L−1 ) (34) 221 ± 71 60 ± 54 73 64 ± 52 71 TOCf (mgL−1 ) (34) 41 ± 10 20 ± 7 52 19 ± 6 55 CODf (mgL−1 ) (35) 99 ± 25 73 ± 29 26 69 ± 25 30 NTK (mgL−1 ) (36) 48 ± 18 28 ± 25 42 23 ± 13 52 N–NH4 (mgL−1 ) (36) 40 ± 13 11 ± 8 71 10 ± 9 74 Norg (mg L−1 ) (36) 8±9 17 ± 10 −113 13 ± 7 −62 N–NO3 (mgL−1 ) (34) 2±1 17 ± 9 −564 16 ± 14 −556 Ps (mgL−1 ) (35) 4.1 ± 1.1 3.5 ± 1.3 14 3.3 ± 1.3 19 Turbidity (UT) (33) 57 ± 26 95 ± 62 −68 73 ± 44 −27 TSS (mgL−1 ) (36) 96 ± 149 200 ± 79 −108 145 ± 54 −51 VSS (mgL−1 ) (36) 75 ± 98 152 ± 57 −102 124 ± 46 −65 Chlorophyll a (mg L−1 ) (31) – 1.5 ± 1.2 2.1 ± 1.0 E. coli MPN (100 mL)−1 (30) 3.4 × 106 ± 5.9 × 106a 2.7 × 104 ± 7.9 × 106a 2.1 log unitb 2.5 × ±1.03 × 105a 1.1 log unitb
p-Value >0.050 0.050 >0.050 >0.050 0.069 >0.050 0.060 >0.050 0.11 0.052 0.050). Craggs et al. [6] observed N-NH4 removals
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Environmental Technology 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 and 10 ± 9 mg N-NH4 L−1 were found in the HRAP and the UV HRAP, respectively. Considering that the diurnal pH values were below 8.2 in the HRAP and 8.7 in the UV HRAP, 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 = 6 h) 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 HRAP and the HRAP, respectively, with no statistical difUV ferences (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 UV HRAP, 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 UV HRAP, 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
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presumably removed by pre-disinfection), we can infer that UV disinfection interfered in the UV HRAP 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 UV HRAP 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. and 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 UV HRAP indicates that pre-disinfection may favour 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.
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Figure 5.
Relative abundance of the main algae present in the HRAP (a) and in the UVHRAP (b) during the monitoring period.
Conclusions Greater Chlorella sp. abundance after June in the UV HRAP 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
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UV HRAP; 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 UV HRAP, 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-UV HRAP). Considering the widespread use of UASB reactors, mostly in countries with hot climate, such systems are clearly applicable.
[9]
[10]
[11] [12]
Acknowledgements
[13]
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 Gerais State/SECTES (Secretaria de Estado de Ciência, Tecnologia e Ensino Superior).
[14] [15] [16]
References [1] Wijffels RH, Barbosa MJ. An outlook on microalgal biofuels. Science. 2010;329:796–799. [2] Chinnasamy S, Bhatnagar A, Hunt RW, Das KC. Biomass and bioenergy production potential of microalgae consortium in open and closed bioreactors using untreated carpet industry effluent as growth medium. Biores Technol. 2010;101:3097–3105. [3] Park JBK, Craggs RJ, Shilton AN. Wastewater treatment high rate algal ponds for biofuel production. Biores Technol. 2011;102:35–42. [4] Oswald WJ. Micro-algae and waste-water treatment. In: Borowitzka MA, Borowitzka LJ, editors. Micro-algal biotechnology. Cambridge: Cambridge University Press; 1988. p. 305–328. [5] Park JBK, Craggs RJ. Wastewater treatment and algal production in high rate algal ponds with carbon dioxide addition. Water Sci Technol. 2010;61:633–639. [6] Craggs RJ, Sutherland D, Campbell H. Hectare-scale demonstration of high rate algal ponds for enhanced wastewater treatment and biofuel production. J Appl Phycol. 2012;24:329–337. [7] Ketheesan B, Nirmalakhanda N. Development a new airliftdriven raceway reactor for algal cultivation. Appl Energy. 2011;88:3370–3376. [8] Fallowfield HJ, Cromar NJ, Evison M. Coliform die-off rate constants in a high rate algal pond and the effect of
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operation and environmental variables. Water Sci Technol. 1996;34:141–147. Cho S, Luong TT, Lee S, Oh Y, Lee T. Reuse of effluent water from a municipal wastewater plant in microalgae cultivation for biofuel production. Biores Technol. 2011;102: 8639–8645. Rocha EO, Calijuri ML, Santiago AF, Assis LC, Alves LGS. The contribution of conservation practices in reducing runoff, soil loss, and transport of nutrients at the watershed level. Water Res Manage. 2012;26:3831–3852. Park JBK, Craggs RJ, Shilton AN. Recycling algae to improve species control and harvest efficiency from a high rate algal pond. Water Res. 2011;45:6637–6649. Picot B, Halouani HE, Casellas C, Moersidik S, Bontoux J. Nutrient removal by high rate pond system in a Mediterranean climate (France). Water Sci Technol. 1991;23: 1535–1541. Gonçalves RF, Filho BC, Chernicharo CAL, Lapoli FR, Aisse MM, Piveli RP. Desinfecção por radiação ultravioleta. In: Gonçalves RF, editor. Desinfecção de Efluentes Sanitários. Vitória: ABES; 2003. p. 209–276. APHA. Standard methods for the examination of water and wastewater. 21st ed. Washington, DC: APHA; 2005. Nederlandse Norm. NEN 6520. Netherlands; 1981. Nush EA. Comparison of different methods for chlorophyll and phaepigmen. Arch Hydrobiolol Bech Stuttgart. 1980;14:14–36. Park JBK, Craggs RJ. Nutrient removal in wastewater treatment high rate algal ponds with carbon dioxide addition. Water Sci Technol. 2011;63:1758–1764. Cromar NJ, Fallowfield HJ, Martin NJ. Influence of environmental parameters on biomass production and nutrient removal in high rate algal pond operated by continuous culture. Water Sci Technol. 1996;34:133–140. García J, Green BF, Lundquist T, Mujeriego R, HernandezMariné M, Oswald WJ. Long term diurnal variations in contaminant removal in high rate ponds treating urban wastewater. Biores Technol. 2006;97:1709–1715. Craggs RJ, Daves-Colley RJ, Tanner CC, Sukias JP. Advanced pond system: Performance with high rate ponds of different depths and areas. Water Sci Technol. 2003;48: 259–267. el Hamouri B, Khallayoune K, Bouzoubaa N, Chalabi M. High-rate algal pond performances in faecal coliforms and helminth egg removals. Water Sci Technol. 1994;28: 171–174. García J, Mujeriego R, Hernandez-Mariné M. High rate algal pond operating strategies for urban wastewater nitrogen removal. J Appl Phycol. 2000;12:331–339. de Godos I, Blanco S, García-Encina PA, Becares E, Muñoz R. Long-term operation of high rate algal ponds for the bioremediation of piggery wastewaters at high loading rates. Biores Technol. 2009;100:4332–4339.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
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 , 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 Accepted author version posted online: 12 Jun 2013.Published online: 04 Jul 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, 34:13-14, 1877-1885, DOI: 10.1080/09593330.2013.812670 To link to this article: http://dx.doi.org/10.1080/09593330.2013.812670
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Environmental Technology, 2013 Vol. 34, Nos. 13–14, 1877–1885, http://dx.doi.org/10.1080/09593330.2013.812670
Algal biomass production and wastewater treatment in high rate algal ponds receiving disinfected effluent Aníbal Fonseca Santiagoa∗ , Maria Lucia Calijuria , Paula Peixoto Assemanya , Maria do Carmo Calijurib and Alberto José Delgado dos Reisc 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
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(Received 28 April 2013; final version received 31 May 2013) 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) pre-disinfection 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 radiation – UV HRAP. Physical, chemical and microbiological parameters were monitored, as well as algal biomass productivity and daily pH and dissolved oxygen (DO) variation. The UV HRAP 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 UV HRAP were 0.95 ± 0.65% and 1.58 ± 0.65%, respectively. However, total biomass productivity was greater in the HRAP (11.4 gVSS m−2 day−1 ) compared with the UV HRAP (9.3 gVSS m−2 day−1 ). Mean pH values were 7.7 ± 0.7 in the HRAP and 8.1 ± 1.0 in the UV HRAP, and mean values of DO percent saturation were 87 ± 26% and 112 ± 31% for the HRAP and the UV HRAP, 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 kilogram 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 to 10 days, and paddlewheels to provide mixing.[4–6] Algal photosynthesis produces the oxygen required for degradation of organic matter by heterotrophic bacteria. Nutrients and the CO2 resulting from oxidation are assimilated by
∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
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 aims 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 with 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
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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, an ultraviolet (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 pilotscale wastewater treatment systems which include pretreatment methods, the objective of this study was to assess the influence of UV pre-disinfection 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 upflow anaerobic sludge blanket (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 as a low-cost and easy-operation option. The HRAP influent wastewater was primary effluent from the UASB reactor, and the UV HRAP received UASB effluent pre-disinfected by UV radiation. The experimental HRAPs were made from fibreglass 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 = 3.3m2 , useful volume = 1 m3 and HRT = 4 days. Such units were embedded in the soil at 0.20 m. The paddle wheels 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 ms−1 , similar to values used in different studies [11,12] to assure the necessary agitation. For Oswald,[4] velocities of 0.12–0.15 ms−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 was 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 UV lamps with wavelengths below 290 nm (known as UVC radiation) encased in a quartz tube, 15 W each, with 436 mm of length and 26 mm of diameter. Monitoring Monitoring was carried out from 31 January to 23 November 2012 and comprised periods of hot and rainy climate (Feb–Mar), intermediate (Apr), cold and dry climate (May– Sep) and the beginning of the rainy season (Oct–Nov). Effluent samples were collected at weekly intervals. Composite samples were collected every 2h (from 8:00 to 18:00h) for physical–chemical analysis. Simple random samples for the analyses of chlorophyll a and E. coli were collected at 10:00 and 12:00h, respectively. The variables pH, dissolved oxygen (DO), electrical conductivity (Cond) and temperature (Temp) were measured at the site, every 2h, using the Hach HQ40d portable meter (Luminescent Dissolved Oxygen – for DO). Photosynthetically active radiation (PAR) (400–700 nm) was measured on the water
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Environmental Technology surface using the LI-193 Underwater Spherical Quantum Sensor available from LI-COR. Flow regulation was performed every week by direct flow measurement (volumetric method). The physical–chemical analyses of influent and effluent wastewaters were carried out according to the Standard Methods for the Examination of Water and Wastewater.[14] The methods used for analysis of each variable are between parentheses: filtered chemical oxygen demand (CODf) (5220D – sample filtered with 0.45 μm membrane), total alkalinity (Alk) (2320B), turbidity (TUR) (2130A), total suspended solids (TSS) (2540D), volatile suspended solids (VSS) (2540E), ammoniacal nitrogen (N-NH4 ) (4500 – NH3 C), total Kjeldahl nitrogen (TKN) (4500-NorgB), organic nitrogen (Norg ) (calculated as the difference between TKN and N-NH4 ), nitrate (N-NO3 ) (4500-NO3 A), soluble phosphorus (Ps ) (4500 P C – samples filtered with 0.45μm membrane) and E. coli abundance (Colilert® ). Filtered total organic carbon (TOCf) was determined using the Shimatzu TOC 5000 analyser (samples filtered with 0.45 μm membrane). Chlorophyll a analysis was performed using hot ethanol (80%) as the extractant according to the Dutch standard norm NEN 6520/1981,[15] and based on Nush.[16] The variables DO and pH were measured every 2h along the nyctemeral cycles on 25 May, 22 July and 23 November 2012. Quantitative and qualitative phytoplankton analyses The samples for phytoplankton analyses were collected at 2 weeks interval. For qualitative analysis, samples were collected using a plankton net with 20 μm mesh size and preserved with formalin solution (4%). The centre of the round opening of the plankton net was lowered at middepth of the pond and dragged horizontally through the entire length of the pond. Identification keys, appropriate bibliography and specialists were consulted for the proper identification of the species present in the experimental units. For the quantitative analysis, 1L samples of the effluent were collected and immediately preserved with Lugol’s solution in amber bottles. The cells were counted using a Sedgwick-Rafter chamber under a binocular microscope (Olympus Model IX70). After sedimentation, the supernatant was discarded and the remaining concentrate of approximately 100 mL was homogenized and transferred using a pipette to the Sedgwick-Rafter chamber. After 15 min, the chamber was taken to the microscope for counting with 400× magnification. Climatologic variables Climatologic data (total incident solar irradiance and air temperature) were obtained from the University’s Meteorological Station, located at approximately 3.5 km from the experimental unit.
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Statistical analysis The version 2.15.2 of the statistical software R© , developed by the R Foundation for Statistical Computing (R Development Core Team, 2012) was used to assess whether the means of two groups (mean values of the variables measured at each pond) were statistically different from each other (t-test) at the 5% significance level. The Microsoft® Excel (Microsoft, 2010) was used to produce the graphs. Results and discussion Figure 1(a) shows the values for total incident solar irradiance, PAR and air temperature, observed over the monitoring period. As expected, the air temperature decreased from summer (beginning of the monitoring) through winter, and then increased from winter to the end of the monitoring period. The incident PAR measured at the site is shown in Figure 1(b), as well as the effluent temperatures in the ponds, which did not statistically differ (p > 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 UV HRAP, respectively, were considered statistically different (p < 0.050). Figure 2 shows the variation of this percentage over the monitoring period. The standard deviation was ±0.65% for both ponds; however, the coefficient of variation was 41% for the UV HRAP and 68% for the HRAP, thus the former can be considered more homogeneous. The greater percentage of chlorophyll a in the biomass obtained from the UV HRAP allows us to infer that pre-disinfection of pond influent may provide for a higher efficiency in algal biomass production. The ecological competition between algae and bacteria (for space and nutrients) is relevant for algal productivity. Thus, maximizing the photosynthetical activity and, consequently, algal production, is one of the main purposes of HRAPs projects. The results are increased efficiency in pollutant removal and reduced land area requirements, which differentiate HRAPs from conventional stabilization ponds.[8] Unlike the results presented for chlorophyll a, VSS mean concentrations of 152 and 124 mg L−1 were found in the HRAP and in the UV HRAP, respectively, as shown in Figure 3. Thus VSS production was greater in the HRAP than in the UV HRAP (p < 0.0010), with productivities of 11.4 and 9.3 gm−2 day−1 , respectively. These values are in the same magnitude order as those obtained by Craggs et al.,[6] which corroborates the results of this study. The lower total biomass production in the UV HRAP may be explained by the fact that the VSS variable does not account for only algal biomass, but for the total biomass
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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).
produced in the pond, and the UV HRAP received UASB effluent pre-disinfected by UV radiation, therefore, with a reduced number of bacteria. The wide range of diurnal pH values in the UV HRAP showed that algal production in this pond was limited in terms of CO2 , as observed by Craggs et al.[6] Thus, HRAPs associated with methods for predisinfection and CO2 addition are expected to present greater VSS production. This is a subject for future research, given that the productivity in such systems can also be limited by the self-shading phenomenon pointed out by Park and Craggs [17] and Cromar et al.[18] Temporal variability of DO and pH in the ponds Mean values for percent saturation of DO were 112% for the UV HRAP and 87% for the HRAP (Figure 4(a)).
Craggs et al. [6] observed median and minimum values of 98.2% and 86.2% for the units assessed in their study. The maximum values of DO percent saturation during the day (145% and 118% in the UV HRAP and the HRAP, respectively) were achieved around 14:00h. The same behaviour was observed for pH, which presented mean values of 8.2 and 8.7 at 14:00h, for the UV HRAP and the HRAP, respectively (Figure 4b). Mean DO and pH values measured throughout the day were significantly higher for the UV HRAP (p < 0.0010), which indicates the influence of algal biomass activity in this pond. A maximum DO percent saturation was maintained between 12:00 and 14:00h in the UV HRAP, which may indicate photoinhibition during high irradiance periods. This behaviour was not observed for the HRAP, which demonstrates the greater proportion of algal biomass in the UV HRAP.
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Table 1. Concentrations (mean ± standard deviation) and removal values of water quality variables analysed for pond influent and effluent samples. Pond influent
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Mean ± standard deviation
HRAP effluent Mean ± standard deviation
UV HRAP
Removal (%)
Mean ± standard deviation
effluent Removal (%)
Temp. (◦ C) (36) 24 ± 1.7 24 ± 2.2 24 ± 2.3 pH (36) 7.1 ± 0.4 7.7 ± 0.7 8.1 ± 1.0 DO (% sat.) (33) 23 ± 4.4 87 ± 26 112 ± 31 Cond (mScm−1 ) (35) 799 ± 31 655 ± 367 18 631 ± 376 21 Alk. (mgCaCO3 L−1 ) (34) 221 ± 71 60 ± 54 73 64 ± 52 71 TOCf (mgL−1 ) (34) 41 ± 10 20 ± 7 52 19 ± 6 55 CODf (mgL−1 ) (35) 99 ± 25 73 ± 29 26 69 ± 25 30 NTK (mgL−1 ) (36) 48 ± 18 28 ± 25 42 23 ± 13 52 N–NH4 (mgL−1 ) (36) 40 ± 13 11 ± 8 71 10 ± 9 74 Norg (mg L−1 ) (36) 8±9 17 ± 10 −113 13 ± 7 −62 N–NO3 (mgL−1 ) (34) 2±1 17 ± 9 −564 16 ± 14 −556 Ps (mgL−1 ) (35) 4.1 ± 1.1 3.5 ± 1.3 14 3.3 ± 1.3 19 Turbidity (UT) (33) 57 ± 26 95 ± 62 −68 73 ± 44 −27 TSS (mgL−1 ) (36) 96 ± 149 200 ± 79 −108 145 ± 54 −51 VSS (mgL−1 ) (36) 75 ± 98 152 ± 57 −102 124 ± 46 −65 Chlorophyll a (mg L−1 ) (31) – 1.5 ± 1.2 2.1 ± 1.0 E. coli MPN (100 mL)−1 (30) 3.4 × 106 ± 5.9 × 106a 2.7 × 104 ± 7.9 × 106a 2.1 log unitb 2.5 × ±1.03 × 105a 1.1 log unitb
p-Value >0.050 0.050 >0.050 >0.050 0.069 >0.050 0.060 >0.050 0.11 0.052 0.050). Craggs et al. [6] observed N-NH4 removals
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Environmental Technology 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 and 10 ± 9 mg N-NH4 L−1 were found in the HRAP and the UV HRAP, respectively. Considering that the diurnal pH values were below 8.2 in the HRAP and 8.7 in the UV HRAP, 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 = 6 h) 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 HRAP and the HRAP, respectively, with no statistical difUV ferences (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 UV HRAP, 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 UV HRAP, 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
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presumably removed by pre-disinfection), we can infer that UV disinfection interfered in the UV HRAP 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 UV HRAP 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. and 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 UV HRAP indicates that pre-disinfection may favour 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.
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Figure 5.
Relative abundance of the main algae present in the HRAP (a) and in the UVHRAP (b) during the monitoring period.
Conclusions Greater Chlorella sp. abundance after June in the UV HRAP 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
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UV HRAP; 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 UV HRAP, 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-UV HRAP). Considering the widespread use of UASB reactors, mostly in countries with hot climate, such systems are clearly applicable.
[9]
[10]
[11] [12]
Acknowledgements
[13]
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 Gerais State/SECTES (Secretaria de Estado de Ciência, Tecnologia e Ensino Superior).
[14] [15] [16]
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