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Degradation of algal organic matter using microbial fuel cells and its association with trihalomethane precursor removal
Bioresource Technology 116 (2012) 80–85
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Degradation of algal organic matter using microbial fuel cells and its association with trihalomethane precursor removal Huan Wang, Dongmei Liu, Lu Lu, Zhiwei Zhao, Yongpeng Xu, Fuyi Cui ⇑ State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
a r t i c l e
i n f o
Article history: Received 19 March 2012 Received in revised form 5 April 2012 Accepted 7 April 2012 Available online 17 April 2012 Keywords: Algal organic matter Microbial fuel cells AOM composition THM precursor
a b s t r a c t In order to provide an alternative for removal of algal organic matter (AOM) produced during algal blooms in aquatic environment, microbial fuel cell (MFC) was used to study AOM degradation and its association with THM precursor removal. The chemical oxygen demand (COD) removals in MFCs were 81 ± 6% and 73 ± 3% for AOM from Microcystis aeruginosa (AOMM) and Chlorella vulgaris (AOMC), respectively. THM precursor was also effectively degraded (AOMM 85 ± 2%, AOMC 72 ± 4%). The major AOM components (proteins, lipids, and carbohydrates) were obviously removed in MFCs. The contribution of each component to the THM formation potential (THMFP) was obtained based on calculation. The THMFP produced from soluble microbial products was very low. If the energy input during operation process was not considered, MFCs treatment could recover electrical energy of 0.29 ± 0.02 kWh/kg COD (AOMM) and 0.35 ± 0.06 kWh/kg COD (AOMC). Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In China, approximately 43.3% lakes and reservoirs have reached the mesotrophic/eutrophic state. Most of eutrophic freshwater lakes (e.g., Dianchi Lake, Taihu Lake and Chaohu Lake) are still used as the water supply for the surrounding urban and rural areas, because of a serious shortage of water (Liu and Qiu, 2007). During algal bloom seasons, algal concentration increases hundreds of times and the highest algal cell density can reach 109 cells per liter. Large quantities of algal organic matter (AOM), including algal intracellular and extracellular organic matter, is released into the aquatic environment resulting in water quality deterioration and an increase of disinfection by-products (DBPs) precursor (Nguyen et al., 2005). This is directly related to the safety of drinking water. The odorous tap water accident occurred in Wuxi City in 2007 was attributed to a bloom of Microcystis aeruginosa in Taihu Lake (Zhang et al., 2011). It’s necessary to develop pretreatment technologies to reduce AOM quantity before conventional processes when AOM was in a high concentration. Microbial fuel cells (MFCs) have been recently developed as a bioelectrochemical technology for generating electricity while simultaneously treating waste organic matter. In MFCs, electrochemically active bacteria oxidize organic matter to transfer ⇑ Corresponding author. Address: P.O. Box 2602, School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin, Heilongjiang Province 150090, China. Tel./fax: +86 451 86282098. E-mail addresses: [email protected], [email protected] (F. Cui). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.04.021
electrons to an anode, while releasing protons into the solution. These electrons flow through a circuit to a cathode, where they combine with protons and oxygen or other chemicals, such as ferricyanide (Logan et al., 2006). The greatest benefit of using MFCs is the electrical energy recovery, which can partly offset the operation and energy costs of the treatment processes. An additional benefit is that the microorganism in MFCs can utilize various types of substrates, including carbohydrates, proteins, lipids, alcohols, organic acids, hydrocarbons and a variety of complex wastewaters (Pant et al., 2010). Bioelectrochemical degradation of organic materials using MFCs demonstrates different metabolic processes with conventional aerobic or anaerobic treatment (Pham et al., 2006). Electrochemically active bacteria can not only completely oxidize fermentable compounds, such as sugar or amino acids, to CO2 with the electricity generation, but can also further metabolize the fermentative products (e.g. organic acids) that cannot be used by fermentative bacteria to CO2 with the current production (Lovley, 2006). Because substrates are more completely degraded and most substrates are converted to current rather than biomass (Logan, 2009), MFCs also have the advantage of producing fewer soluble microbial products compared to conventional aerobic treatment (Rabaey et al., 2003). In previous study, Velasquez-Orta et al. (2009) have tested the algae as a possible plentiful and sustainable biomass (Hulatt and Thomas, 2010) for electricity production in MFCs in laboratory scale. It is conceivable that MFCs can be applied as a cost-effective and energy-efficient biological pretreatment, in combination with conventional water treatment procedures, for AOM degradation. Previous studies have focused primarily on bioelectricity production
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from AOM. The changes in AOM characteristic are not well understood. AOM contains a wide range of hydrophilic substances, such as polysaccharides, proteins, peptides, fatty acids, amino sugars, amino acids and traces of other organic acids (Brown et al., 1997). All of these organic compounds may contribute to DBP formation, especially trihalomethane (THM), which has been identified as the most typical DBP with the greatest yield (Fang et al., 2010). THM production is known to vary with the algal species and their growth phase (Huang et al., 2009; Nguyen et al., 2005). The main reason for this variation is that the different algae species, as well as a single species at different phases in its growth, display differences in the proportions of their major organic components (e.g., proteins, carbohydrates and lipids), which determine THM yield (Hong et al., 2008). Little is known about the impact of MFC on changes in AOM composition and associated THM precursor removal. AOM from M. aeruginosa (blue–green algae) and Chlorella vulgaris (green algae), the most abundant algae during algal blooms, were used for the tests conducted in this study. Degradation of the major components was investigated in MFCs. The effect of the bioelectrical degradation on THM precursor removal and the contribution of major AOM components (based on pure chemicals) to THM formation potential (THMFP) in the same chlorination were also examined. Control tests in anaerobic reactors (ARs) were conducted for comparison. 2. Methods 2.1. Reactor construction The MFCs comprised two cuboid (7 cm in length 4.5 cm in width 7 cm in height) polymethyl methacrylate chambers, each with a working volume of 200 mL. The cation exchange membrane (CEM) (CMI-7000, Membrane International Inc., USA) was pretreated as recommended (Logan et al., 2006), and it was placed between the anode and the cathode by bolts, with an effective area of 30.25 cm2. The anode and cathode were ammonia gas-treated graphite brushes (4 cm in diameter 4 cm in length; fiber: T70012K, Toray Industries Co., Ltd.) (Logan et al., 2007). The distance between the anodic and cathodic electrodes was 5 cm. An Ag/AgCl reference electrode (+0.2 V vs. a normal hydrogen electrode, NHE) was used to measure the electrode potentials. The electrodes were connected to an external resistor (1 kX) with copper wire. An anaerobic reactor was constructed by sealing a 200 mL cylindrical polymethyl methacrylate bottle, and placed a same graphite brush in the bottle similar to that in the MFC (Velasquez-Orta et al., 2009). 2.2. AOM solution preparation M. aeruginosa and C. vulgaris were obtained from the Culture Collection of Algae at the Institute of Hydrobiology, of the Chinese Academy of Sciences. The algal cells were cultivated in 500 mL flasks containing 300 mL of BG11 medium, under a fluorescent lamp with an automated cycle of 12 h of light and 12 h of darkness, in an air-conditioned light incubator (GZX-250, Taisite Instruments Inc., China) at 30 ± 1 °C. AOM was extracted from both algae at the stationary growth phase. The algal cell densities of 3.85 106 cell/ mL and 3.19 106 cell/mL were achieved on Days 33 and 45 for M. aeruginosa and C. vulgaris, respectively. The algal cells in the BG11 medium were broken using ultrasound (3.5 W/mL) for 20 min at 4 °C with a sonicator (Sonics Vibracell VCX-130PB, 130 W, 20 kHz). The solution was collected and filtered through a filter with a pore diameter of 0.45 lm to remove the residual solids. The filtrate was AOM, including extracellular organic matter
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(EOM) and intracellular organic matter (IOM). The characteristics of the AOM from M. aeruginosa (AOMM) were as follows: TOC 141 ± 33 mg/L, COD 525 ± 11 mg/L, protein 334 ± 37 mg-COD/L, carbohydrate 52 ± 19 mg-COD/L, and lipid 35 ± 14 mg-COD/L. The characteristics of the AOM from C. vulgaris (AOMC) were as follows: TOC 128 ± 19 mg/L, COD 519 ± 8 mg/L, protein 247 ± 23 mg-COD/L, carbohydrate 45 ± 10 mg-COD/L, and lipid 142 ± 35 mg-COD/L. 2.3. Inoculation and operation The MFCs and ARs were inoculated with water derived from the eutrophic lake (algal cell density of 9.8 105 cell/mL). The MFC anode chambers and AR were fed with 200 mL of a substrate solution. The substrate solution was prepared by mixing 50 mL of lake water and 150 mL of the AOM solution, and contained 50 mM nutrient phosphate (Na2HPO4 4.58 g/L, NaH2PO4H2O 2.45 g/L, KCl 0.13 g/L, and NH4Cl 0.31 g/L) (Liu et al., 2005). The cathode chamber was filled with a 50 mM nutrient phosphate buffered solution (NPBS) and continuously aerated (at 100 mL/min) to provide oxygen for the cathode. Magnetic stirring was used to mix the medium in the anodic chamber. The solution was replaced when the voltage over the external resistor decreased to less than 50 mV. All of the reactors were operated in fed-batch mode. After 25 days of start-up, the lake water inoculum was omitted from the substrate solution. The reactors were operated using only 200 mL of AOM solution for another 65 days. Ten MFCs under steady-state conditions were used for tests and analyses after 90 days and were operated in a closed circuit (MFC-CC) or open circuit (MFC-OC). An open circuit was used to study the effect of other bacteria, except for electrochemically active bacteria, on the biodegradation of AOM. An anaerobic treatment of AOM was conducted using four ARs for comparison with the MFC process. To assess the contribution of soluble microbial products (SMPs) to THM formation, all of the reactors were fed only 50 mM NPBS. Before conducting these blank tests, the reactors were operated for four cycles using NPBS to remove the residual substrate attached to the electrode. 2.4. Analysis methods 2.4.1. Chemical and spectroscopy analyses Total organic carbon (TOC) was measured using a TOC/TN analyzer (TOC-VCPH/TNM-1, Shimadzu, Japan). Amino acids including alanine (Ala), arginine (Arg), aspartic acid (Asp), cystine (Cys), glutamic acid (Glu), glycine (Gly), histidine (His), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), tryptophan (Trp), tyrosine (Tyr) and valine (Val) were measured by high-performance liquid chromatography (Hitachi L-8800 Amino Acid Analyzer, Japan) using precolumn derivatization with o-phthalaldehyde (Jones and Gilligan, 1983). Chemical oxygen demand (COD) was measured using standard methods (APHA, 1998). Protein was measured using a bicinchoninic acid assay kit (BCA Protein Assay Kit; Pierce). Carbohydrate was measured using the phenol–sulfuric method (DuBois et al., 1956). Lipid was measured using the chloroform– methanol extraction method (Ishida et al., 1997). The COD conversion factors were 1.5 g-COD/g-protein, 1.07 g-COD/g-carbohydrate, 2.91 g-COD/g-lipid (Miron et al., 2000). All measurements were conducted in triplicate. THM precursors were measured using formation potential (FP) tests. All of the samples used for analysis were diluted with Milli-Q water before chlorination. TOC of influent, MFC-CC effluent, MFC-OC effluent, and AR effluent were 7.32 mg/L, 1.64 mg/L, 5.66 mg/L, and 4.82 mg/L, respectively. The samples were dosed with chlorine at a Cl2/TOC = 5 and stored headspace free for 4 days at 20 °C in the dark. The free chlorine concentration
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was determined using N,N-diethylp-phenylene-diamine (DPD) titration methods (APHA, 1998). THM was measured using liquid–liquid extraction (LLE) with a GC/ECD (7890 N, Agilent, USA) using modified EPA Method 551.1 (Wang et al., 2010). 2.4.2. Electrochemical analyses and calculations Voltage over the external resistor was automatically recorded every 10 min using a data acquisition system (2700, Keithley Instruments Inc., USA) connected to a computer. Voltage was converted to volumetric power density P (W m3) via the equation P ¼ UI=V a , where U is the voltage (V), I is the current (A), and Va is the net liquid volume in the anodic chamber (m3). The maximum volumetric power density (Pmax) for estimating the performance of MFCs was obtained by linear sweep voltammetry (LSV) at a scan rate of 0.1 mV/s using a potentiostat (CHI 660D, Chenhua, China). Coulombic efficiency (CE) is expressed as the ratio of total Coulombs in the circuit to the maximum possible Coulombs based Rt on removal of COD, CE ¼ M 0b Idt=ðFbV a DCODÞ, where M = 32 is the molecular weight of oxygen, F is Faraday’s constant, b = 4 is the number of electrons exchanged per mole of oxygen, and DCOD is the change in COD over a batch-cycle time (Logan et al., 2006). 3. Results and discussion 3.1. Electricity generation from AOM Sustained power generation was obtained in MFCs fed AOMM and AOMC (Fig. 1). The cell voltage was rapidly increased and reached a maximum of 581 ± 18 mV within 5 h, and then, it remained above 500 mV for a period of 1.5 days. Subsequently, the voltage rapidly decreased from 500 mV to 10 mV over 15 h and 1.5–3 days in the MFCs containing AOMM and AOMC, respectively. The maximum volumetric power densities (Pmax) were 4.14 ± 0.05 W/m3 (AOMM) and 3.70 ± 0.02 W/m3 (AOMC) (Fig. S1). Pmax was lower than that of 277 W/m3 using C. vulgaris (Velasquez-Orta et al., 2009) and 12.57 W/m3 using blue–green algae (Yuan et al., 2011) in single chamber MFC. The difference was probably due to reactor construction, larger reactor volume and less available biomass in our study. Moreover, the cathode may be a limiting factor because of the use of non-catalyst cathode in this study for considering the large-scale application with low cost. Further improvement of the cathode performance, such as development of a biocathode, can increase the power. CE reached 22 ± 2% (AOMM) and 25 ± 4% for (AOMC) with a 1 kX load. These efficiencies were consistent with the results (CE = 10–28%) obtained using
Fig. 1. Voltage output of MFCs fed AOM from Microcystis aeruginosa (AOMM) and Chlorella vulgaris (AOMC) over a 1 kX external resistance.
C. vulgaris and Ulva lactuca (Velasquez-Orta et al., 2009). Low CE in MFCs was primarily due to an oxygen leakage from the cathodic chamber to the anode. Oxygen directly consumes the electrons that should be transferred to the anode and also lead to a substrate loss by bacteria using oxygen. Methanogenesis and biomass synthesis are other possible reasons for low CE (Min et al., 2005). The electrical energy recovery, calculated as recommended by Cusick et al. (2010), was 0.29 ± 0.02 kWh/kg COD for AOMM and 0.35 ± 0.06 kWh/kg COD for AOMC. Traditional aerobic biological treatment still requires energy consumption (0.7–2.0 kWh/kg COD) for forced aeration (Metcalf and Eddy, 2003). A further improvement in CE is necessary to improve the efficiency of electrical energy recovery. Cases in which CE reaches 100% indicate that all of the electrons available from the removed COD have been fully converted into the current and the maximums of 1.32 and 1.4 kWh/kg COD would be achieved for AOMM and AOMC, respectively. 3.2. Changes in AOM composition 3.2.1. Total COD removal When MFCs were fed with AOMM, 81 ± 6% of the total COD was removed in MFC-CC, which was higher than that removed in MFCOC (23 ± 4%) and in AR (36 ± 7%) (Fig. 2). Total COD removal for AOMC was highest in MFC-CC (73 ± 3%) and lowest in MFC-OC (30 ± 5%). These results indicated that organic materials were degraded more completely in MFCs with closed circuit. This disparity for organic material removal was most likely due to the different metabolisms occurring in the three types of reactors. Under closed circuit condition, electrochemically active bacteria can directly oxidize a portion of non-fermentable substrates in AOM with small molecular weights, such as volatile acids, amino acids and chlorophyll a to produce electricity. Other fermentable components in AOM, such as protein and polysaccharose, can be degraded through syntrophic interaction between fermentation bacteria and electrochemically active bacteria (Lee et al., 2008). In open-circuit MFCs, these non-fermentable substrates and fermentation products cannot be used to generate electricity because electrochemically active bacteria do not work, which causes an accumulation of fermentation products, and this accumulation can result in feedback inhibition for the fermentation process. All of these factors reduce the total COD removal. In anaerobic treatment, biodegradation of AOM is mainly conducted by fermentative bacteria. The fermentation process produces large amounts of by-products, a portion of which can be used by methanogens or other bacteria. However, this anaerobic process is still an incomplete treatment for pollutants and thus might be the reason for the differences in COD removal between closed-circuit MFC and anaerobic treatment. 3.2.2. Removals of proteins, carbohydrates and lipids AOM is composed principally of proteins, carbohydrates and lipids. AOMM contained more proteins (64–86% of COD) but fewer lipids (7–13% of COD) compared with AOMC (protein: 41–48% of COD; lipid: 18–27% of COD) in influent and effluent (Fig. 2). Both types of AOM were low in carbohydrates (4–11% of COD). For AOMM, MFCs under closed circuit condition achieved the highest removals of proteins and lipids (78 ± 14% protein and 61 ± 6% lipid), followed by AR (29 ± 4% protein, 33 ± 2% lipid), and then by MFCs under open circuit condition (14 ± 4% protein, 25 ± 2% lipid). Carbohydrates were removed to the greatest degree in MFC-CC (86 ± 5%) and AR (83 ± 9%) and to a much lower degree in MFCOC (41 ± 8%). Although carbohydrates were degraded more thoroughly than protein, the carbohydrate content was much lower than that of proteins. The reduction in proteins (261 ± 47 mgCOD/L) was five times greater than the reduction in carbohydrates
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Fig. 2. Concentrations and removals of COD, proteins, carbohydrates and lipids using MFC in a closed circuit (MFC-CC) or open circuit (MFC-OC) and in an anaerobic reactor (AR) were compared.
under the closed-circuit MFC condition. For AOMC, the reductions of lipids (105 ± 17 mg-COD/L) and proteins (183 ± 28 mg-COD/L) were comparably high in closed-circuit MFCs. The difference between total COD and the sum of the three major components in AOM (8–35% of COD) was other trace organic materials, such as volatile acid, DNA, RNA, chlorophyll a and carotenoid (Hong et al., 2008).This part of COD was also better removed in MFCs with closed-circuit, indicating that these organic materials were more readily degraded by the electrochemically active bacteria. 3.2.3. Amino acids removal Because protein is the most abundant component of AOM, free amino acids as the degradation products of algal-derived protein are also important fractions of AOM. Fig. 3 presents the changes in concentrations of 18 free amino acids, including 10 hydrophilic amino acids and 8 hydrophobic amino acids, after MFCs and AR treatment of AOMM. The influent contained Ala, Asp, Glu and Leu as the dominant free amino acids, and Met, Cys and His as the minor free amino acids. With effective biodegradation, 65.2% of the total free amino acids were removed in MFCs with closed-circuit. The removal efficiency was higher than that in MFC with open-circuit (27%) and AR (42.6%). In MFC-CC, slightly higher removals were achieved for the hydrophilic amino acids (e.g., Gly, Ser, Thr, Glu, Lys, and Arg), with an average of 68.4%, compared with those for the hydrophobic amino acids (e.g., Ala, Val, Leu, Ile, and Phe), with an average of 60.5%. The removal efficiencies of hydrophilic amino acids were quite closed to those of hydrophobic amino acids in AR. It should be noted that there was little decline in Asp in MFCs with closed-circuit. Because Asp had high THM formation potential (Hong et al., 2009), the Asp left in MFC-CC contributed significantly
to the THM yield produced from amino acids. Tyr with a greater number of activated aromatic centers (defined as hydroxyl and nitrogen-substituted aromatics) was more reactive with chlorine to generate THM and the THM yield produced from Tyr was an order of magnitude larger than that from the other free amino acids (Hong et al., 2009). Tyr was obviously removed in MFC-CC, suggesting that the generation of THM from Tyr upon chlorination was less in closed-circuit MFCs. Trp, with the highest THMFP per unit TOC, was not detectable in this study.
3.3. Removal of THM precursor 3.3.1. Impact of bioelectrochemical degradation on THM precursor removal Initially, the influent THMFP yields were 158 ± 5 lg/L (AOMM) and 197 ± 23 lg/L (AOMC) (Fig. 4). Correspondingly, the specific THMFP yields were 22 ± 1 lg/mg TOC and 27 ± 3 lg/mg TOC for AOMM and AOMC, respectively. AOMC exhibited a higher productivity in THM formation as compared to AOMM. For AOMM, the highest removal of THMFP was in MFC-CC (85 ± 2%), followed by AR (34 ± 6%), and then MFC-OC (21 ± 5%). Similar results were obtained for AOMC among three types of reactors. In closed-circuit MFCs, THMFP for AOMM was consumed more completely than that for AOMC. Conversely, the removal of THMFP for AOMM was slightly lower in open-circuit MFCs. The different THMFP removals for both algae were attributed to the changes in the compositions after the biodegradation. Because THM production varied greatly with the different organic materials, it is necessary to investigate individual contribution for the total THMFP yields.
Fig. 3. Comparison of amino acid contents of algal organic matter from Microcystis aeruginosa (AOMM) among MFC in a closed circuit (MFC-CC), MFC in an open circuit (MFCOC), and in an anaerobic reactor (AR).
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Fig. 4. Comparison of the yields and removal efficiencies of THMFP generated from algal organic matter derived from Microcystis aeruginosa and Chlorella vulgaris (AOMM and AOMC) between measured values and calculated values in MFC in a closed circuit (MFC-CC), MFC in an open circuit (MFC-OC), and in an anaerobic reactor (AR).
Proteins, carbohydrates, and lipids are the major components in both AOM and are identified as the major THM precursors. Thus, we assumed that the total THMFP was generated from these three components (Hong et al., 2008):
Total THMFPðlg=LÞ ¼ ½THMFPProtein þ ½THMFPCarboh: þ ½THMFPLipid
ð1Þ
To evaluate the contribution of each component to total THMFP, three typical pure substances, bovine serum albumin (BSA), glucose, and fish oil were used to model the proteins, carbohydrates, and lipids, respectively, under the same chlorination conditions. THMFP from each component was calculated as follows:
½THMFPProtein ¼ ½THMFPBSA ½TOCX Protein
ð2Þ
½THMFPCarboh: ¼ ½THMFPglucose ½TOCX Carboh:
ð3Þ
½THMFPLipid ¼ ½THMFPfish oil ½TOCX Lipid
ð4Þ
where ½THMFPBSA , ½THMFPglucose , and ½THMFPfish oil are the THMFP yields per unit TOC from actual measurements of BSA (17.3 ± 5.3 lg/mg TOC), glucose (7.1 ± 2.5 lg/mg TOC), and fish oil (28.7 ± 11.4 lg/mg TOC), respectively. ½TOC is the measured TOC concentration (mg/L) in influent or effluents. X Protein , X Carboh: , and X Lipid are the fractions of TOC from each component in the total TOC and were calculated as follows:
X Protein ¼ aProtein ½Protein=ðaProtein ½Protein þ aCarboh: ½Carboh: þ aLipid ½LipidÞ
ð5Þ
X Carboh: ¼ aCarboh: ½Carboh:=ðaProtein ½Protein þ aCarboh: ½Carboh: þ aLipid ½LipidÞ
ð6Þ
X Lipid ¼ aLipid ½Lipid=ðaProtein ½Protein þ aCarboh: ½Carboh: þ aLipid ½LipidÞ
enhancing mass transfer, is expected to be effective in reducing THM precursors from AOMM. Both protein and lipid were dominant components in AOMC. Lipids had the highest THMFP yield per unit TOC, suggesting that lipids contributed more to THMFP than other organic materials in equal concentrations. Thus, lipid removal must be improved to reduce the THMFP production in AOMC.
ð7Þ
where aProtein = 0.55, aCarboh = 0.44, and aLipid = 0.80 are carbon percentages in proteins (C16H24O5N4), carbohydrates (C6H10O5), and lipids (C22H32O2), respectively. [Protein], [Carboh.], and [Lipid] are measured concentrations (mg/L) of the three components in influent or effluents. THM yield from the three major components were obtained based on calculation (Fig. 4). Protein content was believed to be a significant contributor to the THMFP yield in AOMM. THMFP removal was similar to the removal of protein content in the MFCs and AR treatment processes. Therefore, further optimization of operating conditions to improve protein removal, such as extending the reaction time, improving the reactor configuration, and
3.3.2. Contribution of soluble microbial products to THMFP THM precursors in effluents were presumably attributable to two sources: the residual substrates that were not completely oxidized and the soluble microbial products (SMPs) that were produced by the metabolism and biomass decay of microorganisms, such as exoenzymes, siderophores, extracellular materials, cell wall components, DNA fragments, and proteins released as a result of cell lysis (Liu and Li, 2010). SMP production varies among biotreatment processes depending on biomass concentration, substrate type and operation conditions (Barker and Stuckey, 1999). It has been reported that SMPs are produced at a rate that is proportional to the concentration of biomass due to the release of organic material from cell lysis (Barker and Stuckey, 1999). A previous study showed that the biomass yield was approximately 0.07–0.22 g-COD-biomass/g-COD-substrate in MFCs (Rabaey et al., 2003), which is comparable to that in anaerobic fermentation but less than that in an aerobic treatment process (0.53 g-CODbiomass/g-COD-substrate) (Verstraete and Van Vaerenbergh, 1986). The THMFP yields from SMPs were 10.6 ± 3.2 lg/mg TOC, 18.1 ± 5.1 lg/mg TOC, and 9.2 ± 2.4 lg/mg TOC in the effluent of MFC-CC, MFC-OC, and AR, respectively. There was no difference between MFC-CC and AR in THMFP, suggesting that both reactors might have similar biomass composition. A large amount of fermentative bacteria is present in both MFC-CC and AR to mediate the degradation of a complex substrate. In MFC-OC, electrochemically active bacteria cannot use the anode as the electron acceptor. Syntrophic interactions between electrochemically active bacteria and fermenters are inhibited, which generally leads to the biomass decay (Picioreanu et al., 2008). Increases in biomass decay lead to increases in the release of organic materials into the solution, thus resulting in more THMFP formation. 4. Conclusions A bioelectrochemical process using MFCs can effectively remove algal organic matter (AOM) derived from M. aeruginosa (AOMM) and C. vulgaris (AOMC). THM precursor from both AOM was degraded more completely in closed-circuit MFCs compared with the control reactors (open-circuit MFCs and anaerobic reactors). Other benefits related to bioelectrochemical degradation of AOM include electrical energy recovery, and the low THMFP produced from soluble
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microbial products (SMPs) in MFCs. The difference in THM precursor removal using AOMM and AOMC can be attributed to the disparity of two AOM compositions. Acknowledgements The authors would like to thank the National Natural Science Fund of China (Nos. 50808052 and 50778048), the Fund for Creative Research Groups of China (No. 51121062), and the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. HIT2010TS08), for their support for this study. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2012. 04.021. References American Public Health Association, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association and Water Environment Federation, Washington, DC. Barker, D.J., Stuckey, D.C., 1999. A review of soluble microbial products (SMP) in wastewater treatment systems. Water Res. 33, 3063–3082. Brown, M.R., Jeffrey, S.W., Volkman, J.K., Dunstan, G.A., 1997. Nutritional properties of microalgae for mariculture. Aquaculture 151, 315–331. Cusick, R.D., Kiely, P.D., Logan, B.E., 2010. A monetary comparison of energy recovered from microbial fuel cells and microbial electrolysis cells fed winery or domestic wastewaters. Int. J. Hydrogen Energy 35, 8855–8861. DuBois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Anal. Chem. 28, 350–356. Fang, J., Ma, J., Yang, X., Shang, C., 2010. Formation of carbonaceous and nitrogenous disinfection by-products from the chlorination of Microcystis aeruginosa. Water Res. 44, 1934–1940. Hong, H.C., Mazumder, A., Wong, M.H., Liang, Y., 2008. Yield of trihalomethanes and haloacetic acids upon chlorinating algal cells, and its prediction via algal cellular biochemical composition. Water Res. 42, 4941–4948. Hong, H.C., Wong, M.H., Liang, Y., 2009. Amino acids as precursors of trihalomethane and haloacetic acid formation during chlorination. Arch. Environ. Contam. Toxicol. 56, 638–645. Huang, J., Graham, N., Templeton, M.R., Zhang, Y., Collins, C., Nieuwenhuijsen, M., 2009. A comparison of the role of two blue–green algae in THM and HAA formation. Water Res. 43, 3009–3018. Hulatt, C.J., Thomas, D.N., 2010. Dissolved organic matter (DOM) in microalgal photobioreactors: a potential loss in solar energy conversion? Bioresour. Technol. 101, 8690–8697. Ishida, T., Hasegawa, N., Hayashi, N.R., Peerapornpisal, Y., Ishii, M., Igarashi, Y., Kodama, T., 1997. Growth characteristics and dense culture of a Thermophilic cyanobacterium, Chroococcidiopsis sp. strain TS-821. J. Biosci. Bioeng. 83, 571– 576.
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Jones, B.N., Gilligan, J.P., 1983. O-phthaldialdehyde precolumn derivatization and reversed-phase high-performance liquid chromatography of polypeptide hydrolysates and physiological fluids. J. Chromatogr., A 266, 471–482. Lee, H.S., Parameswaran, P., Kato-Marcus, A., Torres, C.I., Rittmann, B.E., 2008. Evaluation of energy-conversion efficiencies in microbial fuel cells (MFCs) utilizing fermentable and non-fermentable substrates. Water Res. 42, 1501– 1510. Liu, H., Cheng, S., Logan, B.E., 2005. Power generation in fed-batch microbial fuel cells as a function of ionic strength, temperature, and reactor configuration. Environ. Sci. Technol. 39, 5488–5493. Liu, J.L., Li, X.Y., 2010. Biodegradation and biotransformation of wastewater organics as precursors of disinfection byproducts in water. Chemosphere 81, 1075–1083. Liu, W., Qiu, R., 2007. Water eutrophication in China and the combating strategies. J. Chem. Technol. Biotechnol. 82, 781–786. Logan, B.E., 2009. Exoelectrogenic bacteria that power microbial fuel cells. Nat. Rev. Microbiol. 7, 375–381. Logan, B.E., Cheng, S., Watson, V., Estadt, G., 2007. Graphite fiber brush anodes for increased power production in air–cathode microbial fuel cells. Environ. Sci. Technol. 41, 3341–3346. Logan, B.E., Hamelers, B., Rozendal, R.A., Schrorder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., Rabaey, K., 2006. Microbial fuel cells: methodology and technology. Environ. Sci. Technol. 40, 5181–5192. Lovley, D.R., 2006. Bug juice: harvesting electricity with microorganisms. Nat. Rev. Microbiol. 4, 497–508. Metcalf, L., Eddy, H.P., 2003. Wastewater Engineering: Treatment and Reuse, third ed. McGraw-Hill, New York. Min, B., Kim, J.R., Oh, S.E., Regan, J.M., Logan, B.E., 2005. Electricity generation from swine wastewater using microbial fuel cells. Water Res. 39, 4961–4968. Miron, Y., Zeeman, G., Van Lier, J.B., Lettinga, G., 2000. The role of sludge retention time in the hydrolysis and acidification of lipids, carbohydrates and proteins during digestion of primary sludge in CSTR systems. Water Res. 34, 1705–1713. Nguyen, M.L., Westerhoff, P., Baker, L., Hu, Q., Esparza-Soto, M., Sommerfeld, M., 2005. Characteristics and reactivity of algae-produced dissolved organic carbon. J. Environ. Eng.-Asce 131, 1574–1582. Pant, D., Van Bogaert, G., Diels, L., Vanbroekhoven, K., 2010. A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production. Bioresour. Technol. 101, 1533–1543. Pham, T.H., Rabaey, K., Aelterman, P., Clauwaert, P., De Schamphelaire, L., Boon, N., Verstraete, W., 2006. Microbial fuel cells in relation to conventional anaerobic digestion technology. Environ. Life Sci. 6, 285–292. Picioreanu, C., Katuri, K.P., Head, I.M., van Loosdrecht, M.C.M., Scott, K., 2008. Mathematical model for microbial fuel cells with anodic biofilms and anaerobic digestion. Water Sci. Technol. 57, 965–971. Rabaey, K., Lissens, G., Siciliano, S.D., Verstraete, W., 2003. A microbial fuel cell capable of converting glucose to electricity at high rate and efficiency. Biotechnol. Lett. 25, 1531–1535. Velasquez-Orta, S.B., Curtis, T.P., Logan, B.E., 2009. Energy from algae using microbial fuel cells. Biotechnol. Bioeng. 103, 1068–1076. Verstraete, W., Van Vaerenbergh, E., 1986. Aerobic activated sludge. In: Rehm, H.J., Reed, G. (Eds.), Biotechnology, vol. 8. VCH Verlagsgesellschaft, Weinheim, Germany, pp. 43–112. Wang, H., Liu, D., Zhao, Z., Cui, F., Zhu, Q., Liu, T., 2010. Factors influencing the formation of chlorination brominated trihalomethanes in drinking water. J. Zhejiang Univ.-Sci. A 11, 143–150. Yuan, Y., Chen, Q., Zhou, S.G., Zhuang, L., Hu, P., 2011. Bioelectricity generation and microcystins removal in a blue-green algae powered microbial fuel cell. J. Hazard Mater. 187, 591–595. Zhang, X., Chen, C., Lin, P., Hou, A., Niu, Z., Wang, J., 2011. Emergency drinking water treatment during source water pollution accidents in china: origin analysis, framework and technologies. Environ. Sci. Technol. 45, 161–167.
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Degradation of algal organic matter using microbial fuel cells and its association with trihalomethane precursor removal Huan Wang, Dongmei Liu, Lu Lu, Zhiwei Zhao, Yongpeng Xu, Fuyi Cui ⇑ State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China
a r t i c l e
i n f o
Article history: Received 19 March 2012 Received in revised form 5 April 2012 Accepted 7 April 2012 Available online 17 April 2012 Keywords: Algal organic matter Microbial fuel cells AOM composition THM precursor
a b s t r a c t In order to provide an alternative for removal of algal organic matter (AOM) produced during algal blooms in aquatic environment, microbial fuel cell (MFC) was used to study AOM degradation and its association with THM precursor removal. The chemical oxygen demand (COD) removals in MFCs were 81 ± 6% and 73 ± 3% for AOM from Microcystis aeruginosa (AOMM) and Chlorella vulgaris (AOMC), respectively. THM precursor was also effectively degraded (AOMM 85 ± 2%, AOMC 72 ± 4%). The major AOM components (proteins, lipids, and carbohydrates) were obviously removed in MFCs. The contribution of each component to the THM formation potential (THMFP) was obtained based on calculation. The THMFP produced from soluble microbial products was very low. If the energy input during operation process was not considered, MFCs treatment could recover electrical energy of 0.29 ± 0.02 kWh/kg COD (AOMM) and 0.35 ± 0.06 kWh/kg COD (AOMC). Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In China, approximately 43.3% lakes and reservoirs have reached the mesotrophic/eutrophic state. Most of eutrophic freshwater lakes (e.g., Dianchi Lake, Taihu Lake and Chaohu Lake) are still used as the water supply for the surrounding urban and rural areas, because of a serious shortage of water (Liu and Qiu, 2007). During algal bloom seasons, algal concentration increases hundreds of times and the highest algal cell density can reach 109 cells per liter. Large quantities of algal organic matter (AOM), including algal intracellular and extracellular organic matter, is released into the aquatic environment resulting in water quality deterioration and an increase of disinfection by-products (DBPs) precursor (Nguyen et al., 2005). This is directly related to the safety of drinking water. The odorous tap water accident occurred in Wuxi City in 2007 was attributed to a bloom of Microcystis aeruginosa in Taihu Lake (Zhang et al., 2011). It’s necessary to develop pretreatment technologies to reduce AOM quantity before conventional processes when AOM was in a high concentration. Microbial fuel cells (MFCs) have been recently developed as a bioelectrochemical technology for generating electricity while simultaneously treating waste organic matter. In MFCs, electrochemically active bacteria oxidize organic matter to transfer ⇑ Corresponding author. Address: P.O. Box 2602, School of Municipal and Environmental Engineering, Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin, Heilongjiang Province 150090, China. Tel./fax: +86 451 86282098. E-mail addresses: [email protected], [email protected] (F. Cui). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.04.021
electrons to an anode, while releasing protons into the solution. These electrons flow through a circuit to a cathode, where they combine with protons and oxygen or other chemicals, such as ferricyanide (Logan et al., 2006). The greatest benefit of using MFCs is the electrical energy recovery, which can partly offset the operation and energy costs of the treatment processes. An additional benefit is that the microorganism in MFCs can utilize various types of substrates, including carbohydrates, proteins, lipids, alcohols, organic acids, hydrocarbons and a variety of complex wastewaters (Pant et al., 2010). Bioelectrochemical degradation of organic materials using MFCs demonstrates different metabolic processes with conventional aerobic or anaerobic treatment (Pham et al., 2006). Electrochemically active bacteria can not only completely oxidize fermentable compounds, such as sugar or amino acids, to CO2 with the electricity generation, but can also further metabolize the fermentative products (e.g. organic acids) that cannot be used by fermentative bacteria to CO2 with the current production (Lovley, 2006). Because substrates are more completely degraded and most substrates are converted to current rather than biomass (Logan, 2009), MFCs also have the advantage of producing fewer soluble microbial products compared to conventional aerobic treatment (Rabaey et al., 2003). In previous study, Velasquez-Orta et al. (2009) have tested the algae as a possible plentiful and sustainable biomass (Hulatt and Thomas, 2010) for electricity production in MFCs in laboratory scale. It is conceivable that MFCs can be applied as a cost-effective and energy-efficient biological pretreatment, in combination with conventional water treatment procedures, for AOM degradation. Previous studies have focused primarily on bioelectricity production
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from AOM. The changes in AOM characteristic are not well understood. AOM contains a wide range of hydrophilic substances, such as polysaccharides, proteins, peptides, fatty acids, amino sugars, amino acids and traces of other organic acids (Brown et al., 1997). All of these organic compounds may contribute to DBP formation, especially trihalomethane (THM), which has been identified as the most typical DBP with the greatest yield (Fang et al., 2010). THM production is known to vary with the algal species and their growth phase (Huang et al., 2009; Nguyen et al., 2005). The main reason for this variation is that the different algae species, as well as a single species at different phases in its growth, display differences in the proportions of their major organic components (e.g., proteins, carbohydrates and lipids), which determine THM yield (Hong et al., 2008). Little is known about the impact of MFC on changes in AOM composition and associated THM precursor removal. AOM from M. aeruginosa (blue–green algae) and Chlorella vulgaris (green algae), the most abundant algae during algal blooms, were used for the tests conducted in this study. Degradation of the major components was investigated in MFCs. The effect of the bioelectrical degradation on THM precursor removal and the contribution of major AOM components (based on pure chemicals) to THM formation potential (THMFP) in the same chlorination were also examined. Control tests in anaerobic reactors (ARs) were conducted for comparison. 2. Methods 2.1. Reactor construction The MFCs comprised two cuboid (7 cm in length 4.5 cm in width 7 cm in height) polymethyl methacrylate chambers, each with a working volume of 200 mL. The cation exchange membrane (CEM) (CMI-7000, Membrane International Inc., USA) was pretreated as recommended (Logan et al., 2006), and it was placed between the anode and the cathode by bolts, with an effective area of 30.25 cm2. The anode and cathode were ammonia gas-treated graphite brushes (4 cm in diameter 4 cm in length; fiber: T70012K, Toray Industries Co., Ltd.) (Logan et al., 2007). The distance between the anodic and cathodic electrodes was 5 cm. An Ag/AgCl reference electrode (+0.2 V vs. a normal hydrogen electrode, NHE) was used to measure the electrode potentials. The electrodes were connected to an external resistor (1 kX) with copper wire. An anaerobic reactor was constructed by sealing a 200 mL cylindrical polymethyl methacrylate bottle, and placed a same graphite brush in the bottle similar to that in the MFC (Velasquez-Orta et al., 2009). 2.2. AOM solution preparation M. aeruginosa and C. vulgaris were obtained from the Culture Collection of Algae at the Institute of Hydrobiology, of the Chinese Academy of Sciences. The algal cells were cultivated in 500 mL flasks containing 300 mL of BG11 medium, under a fluorescent lamp with an automated cycle of 12 h of light and 12 h of darkness, in an air-conditioned light incubator (GZX-250, Taisite Instruments Inc., China) at 30 ± 1 °C. AOM was extracted from both algae at the stationary growth phase. The algal cell densities of 3.85 106 cell/ mL and 3.19 106 cell/mL were achieved on Days 33 and 45 for M. aeruginosa and C. vulgaris, respectively. The algal cells in the BG11 medium were broken using ultrasound (3.5 W/mL) for 20 min at 4 °C with a sonicator (Sonics Vibracell VCX-130PB, 130 W, 20 kHz). The solution was collected and filtered through a filter with a pore diameter of 0.45 lm to remove the residual solids. The filtrate was AOM, including extracellular organic matter
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(EOM) and intracellular organic matter (IOM). The characteristics of the AOM from M. aeruginosa (AOMM) were as follows: TOC 141 ± 33 mg/L, COD 525 ± 11 mg/L, protein 334 ± 37 mg-COD/L, carbohydrate 52 ± 19 mg-COD/L, and lipid 35 ± 14 mg-COD/L. The characteristics of the AOM from C. vulgaris (AOMC) were as follows: TOC 128 ± 19 mg/L, COD 519 ± 8 mg/L, protein 247 ± 23 mg-COD/L, carbohydrate 45 ± 10 mg-COD/L, and lipid 142 ± 35 mg-COD/L. 2.3. Inoculation and operation The MFCs and ARs were inoculated with water derived from the eutrophic lake (algal cell density of 9.8 105 cell/mL). The MFC anode chambers and AR were fed with 200 mL of a substrate solution. The substrate solution was prepared by mixing 50 mL of lake water and 150 mL of the AOM solution, and contained 50 mM nutrient phosphate (Na2HPO4 4.58 g/L, NaH2PO4H2O 2.45 g/L, KCl 0.13 g/L, and NH4Cl 0.31 g/L) (Liu et al., 2005). The cathode chamber was filled with a 50 mM nutrient phosphate buffered solution (NPBS) and continuously aerated (at 100 mL/min) to provide oxygen for the cathode. Magnetic stirring was used to mix the medium in the anodic chamber. The solution was replaced when the voltage over the external resistor decreased to less than 50 mV. All of the reactors were operated in fed-batch mode. After 25 days of start-up, the lake water inoculum was omitted from the substrate solution. The reactors were operated using only 200 mL of AOM solution for another 65 days. Ten MFCs under steady-state conditions were used for tests and analyses after 90 days and were operated in a closed circuit (MFC-CC) or open circuit (MFC-OC). An open circuit was used to study the effect of other bacteria, except for electrochemically active bacteria, on the biodegradation of AOM. An anaerobic treatment of AOM was conducted using four ARs for comparison with the MFC process. To assess the contribution of soluble microbial products (SMPs) to THM formation, all of the reactors were fed only 50 mM NPBS. Before conducting these blank tests, the reactors were operated for four cycles using NPBS to remove the residual substrate attached to the electrode. 2.4. Analysis methods 2.4.1. Chemical and spectroscopy analyses Total organic carbon (TOC) was measured using a TOC/TN analyzer (TOC-VCPH/TNM-1, Shimadzu, Japan). Amino acids including alanine (Ala), arginine (Arg), aspartic acid (Asp), cystine (Cys), glutamic acid (Glu), glycine (Gly), histidine (His), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), tryptophan (Trp), tyrosine (Tyr) and valine (Val) were measured by high-performance liquid chromatography (Hitachi L-8800 Amino Acid Analyzer, Japan) using precolumn derivatization with o-phthalaldehyde (Jones and Gilligan, 1983). Chemical oxygen demand (COD) was measured using standard methods (APHA, 1998). Protein was measured using a bicinchoninic acid assay kit (BCA Protein Assay Kit; Pierce). Carbohydrate was measured using the phenol–sulfuric method (DuBois et al., 1956). Lipid was measured using the chloroform– methanol extraction method (Ishida et al., 1997). The COD conversion factors were 1.5 g-COD/g-protein, 1.07 g-COD/g-carbohydrate, 2.91 g-COD/g-lipid (Miron et al., 2000). All measurements were conducted in triplicate. THM precursors were measured using formation potential (FP) tests. All of the samples used for analysis were diluted with Milli-Q water before chlorination. TOC of influent, MFC-CC effluent, MFC-OC effluent, and AR effluent were 7.32 mg/L, 1.64 mg/L, 5.66 mg/L, and 4.82 mg/L, respectively. The samples were dosed with chlorine at a Cl2/TOC = 5 and stored headspace free for 4 days at 20 °C in the dark. The free chlorine concentration
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was determined using N,N-diethylp-phenylene-diamine (DPD) titration methods (APHA, 1998). THM was measured using liquid–liquid extraction (LLE) with a GC/ECD (7890 N, Agilent, USA) using modified EPA Method 551.1 (Wang et al., 2010). 2.4.2. Electrochemical analyses and calculations Voltage over the external resistor was automatically recorded every 10 min using a data acquisition system (2700, Keithley Instruments Inc., USA) connected to a computer. Voltage was converted to volumetric power density P (W m3) via the equation P ¼ UI=V a , where U is the voltage (V), I is the current (A), and Va is the net liquid volume in the anodic chamber (m3). The maximum volumetric power density (Pmax) for estimating the performance of MFCs was obtained by linear sweep voltammetry (LSV) at a scan rate of 0.1 mV/s using a potentiostat (CHI 660D, Chenhua, China). Coulombic efficiency (CE) is expressed as the ratio of total Coulombs in the circuit to the maximum possible Coulombs based Rt on removal of COD, CE ¼ M 0b Idt=ðFbV a DCODÞ, where M = 32 is the molecular weight of oxygen, F is Faraday’s constant, b = 4 is the number of electrons exchanged per mole of oxygen, and DCOD is the change in COD over a batch-cycle time (Logan et al., 2006). 3. Results and discussion 3.1. Electricity generation from AOM Sustained power generation was obtained in MFCs fed AOMM and AOMC (Fig. 1). The cell voltage was rapidly increased and reached a maximum of 581 ± 18 mV within 5 h, and then, it remained above 500 mV for a period of 1.5 days. Subsequently, the voltage rapidly decreased from 500 mV to 10 mV over 15 h and 1.5–3 days in the MFCs containing AOMM and AOMC, respectively. The maximum volumetric power densities (Pmax) were 4.14 ± 0.05 W/m3 (AOMM) and 3.70 ± 0.02 W/m3 (AOMC) (Fig. S1). Pmax was lower than that of 277 W/m3 using C. vulgaris (Velasquez-Orta et al., 2009) and 12.57 W/m3 using blue–green algae (Yuan et al., 2011) in single chamber MFC. The difference was probably due to reactor construction, larger reactor volume and less available biomass in our study. Moreover, the cathode may be a limiting factor because of the use of non-catalyst cathode in this study for considering the large-scale application with low cost. Further improvement of the cathode performance, such as development of a biocathode, can increase the power. CE reached 22 ± 2% (AOMM) and 25 ± 4% for (AOMC) with a 1 kX load. These efficiencies were consistent with the results (CE = 10–28%) obtained using
Fig. 1. Voltage output of MFCs fed AOM from Microcystis aeruginosa (AOMM) and Chlorella vulgaris (AOMC) over a 1 kX external resistance.
C. vulgaris and Ulva lactuca (Velasquez-Orta et al., 2009). Low CE in MFCs was primarily due to an oxygen leakage from the cathodic chamber to the anode. Oxygen directly consumes the electrons that should be transferred to the anode and also lead to a substrate loss by bacteria using oxygen. Methanogenesis and biomass synthesis are other possible reasons for low CE (Min et al., 2005). The electrical energy recovery, calculated as recommended by Cusick et al. (2010), was 0.29 ± 0.02 kWh/kg COD for AOMM and 0.35 ± 0.06 kWh/kg COD for AOMC. Traditional aerobic biological treatment still requires energy consumption (0.7–2.0 kWh/kg COD) for forced aeration (Metcalf and Eddy, 2003). A further improvement in CE is necessary to improve the efficiency of electrical energy recovery. Cases in which CE reaches 100% indicate that all of the electrons available from the removed COD have been fully converted into the current and the maximums of 1.32 and 1.4 kWh/kg COD would be achieved for AOMM and AOMC, respectively. 3.2. Changes in AOM composition 3.2.1. Total COD removal When MFCs were fed with AOMM, 81 ± 6% of the total COD was removed in MFC-CC, which was higher than that removed in MFCOC (23 ± 4%) and in AR (36 ± 7%) (Fig. 2). Total COD removal for AOMC was highest in MFC-CC (73 ± 3%) and lowest in MFC-OC (30 ± 5%). These results indicated that organic materials were degraded more completely in MFCs with closed circuit. This disparity for organic material removal was most likely due to the different metabolisms occurring in the three types of reactors. Under closed circuit condition, electrochemically active bacteria can directly oxidize a portion of non-fermentable substrates in AOM with small molecular weights, such as volatile acids, amino acids and chlorophyll a to produce electricity. Other fermentable components in AOM, such as protein and polysaccharose, can be degraded through syntrophic interaction between fermentation bacteria and electrochemically active bacteria (Lee et al., 2008). In open-circuit MFCs, these non-fermentable substrates and fermentation products cannot be used to generate electricity because electrochemically active bacteria do not work, which causes an accumulation of fermentation products, and this accumulation can result in feedback inhibition for the fermentation process. All of these factors reduce the total COD removal. In anaerobic treatment, biodegradation of AOM is mainly conducted by fermentative bacteria. The fermentation process produces large amounts of by-products, a portion of which can be used by methanogens or other bacteria. However, this anaerobic process is still an incomplete treatment for pollutants and thus might be the reason for the differences in COD removal between closed-circuit MFC and anaerobic treatment. 3.2.2. Removals of proteins, carbohydrates and lipids AOM is composed principally of proteins, carbohydrates and lipids. AOMM contained more proteins (64–86% of COD) but fewer lipids (7–13% of COD) compared with AOMC (protein: 41–48% of COD; lipid: 18–27% of COD) in influent and effluent (Fig. 2). Both types of AOM were low in carbohydrates (4–11% of COD). For AOMM, MFCs under closed circuit condition achieved the highest removals of proteins and lipids (78 ± 14% protein and 61 ± 6% lipid), followed by AR (29 ± 4% protein, 33 ± 2% lipid), and then by MFCs under open circuit condition (14 ± 4% protein, 25 ± 2% lipid). Carbohydrates were removed to the greatest degree in MFC-CC (86 ± 5%) and AR (83 ± 9%) and to a much lower degree in MFCOC (41 ± 8%). Although carbohydrates were degraded more thoroughly than protein, the carbohydrate content was much lower than that of proteins. The reduction in proteins (261 ± 47 mgCOD/L) was five times greater than the reduction in carbohydrates
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Fig. 2. Concentrations and removals of COD, proteins, carbohydrates and lipids using MFC in a closed circuit (MFC-CC) or open circuit (MFC-OC) and in an anaerobic reactor (AR) were compared.
under the closed-circuit MFC condition. For AOMC, the reductions of lipids (105 ± 17 mg-COD/L) and proteins (183 ± 28 mg-COD/L) were comparably high in closed-circuit MFCs. The difference between total COD and the sum of the three major components in AOM (8–35% of COD) was other trace organic materials, such as volatile acid, DNA, RNA, chlorophyll a and carotenoid (Hong et al., 2008).This part of COD was also better removed in MFCs with closed-circuit, indicating that these organic materials were more readily degraded by the electrochemically active bacteria. 3.2.3. Amino acids removal Because protein is the most abundant component of AOM, free amino acids as the degradation products of algal-derived protein are also important fractions of AOM. Fig. 3 presents the changes in concentrations of 18 free amino acids, including 10 hydrophilic amino acids and 8 hydrophobic amino acids, after MFCs and AR treatment of AOMM. The influent contained Ala, Asp, Glu and Leu as the dominant free amino acids, and Met, Cys and His as the minor free amino acids. With effective biodegradation, 65.2% of the total free amino acids were removed in MFCs with closed-circuit. The removal efficiency was higher than that in MFC with open-circuit (27%) and AR (42.6%). In MFC-CC, slightly higher removals were achieved for the hydrophilic amino acids (e.g., Gly, Ser, Thr, Glu, Lys, and Arg), with an average of 68.4%, compared with those for the hydrophobic amino acids (e.g., Ala, Val, Leu, Ile, and Phe), with an average of 60.5%. The removal efficiencies of hydrophilic amino acids were quite closed to those of hydrophobic amino acids in AR. It should be noted that there was little decline in Asp in MFCs with closed-circuit. Because Asp had high THM formation potential (Hong et al., 2009), the Asp left in MFC-CC contributed significantly
to the THM yield produced from amino acids. Tyr with a greater number of activated aromatic centers (defined as hydroxyl and nitrogen-substituted aromatics) was more reactive with chlorine to generate THM and the THM yield produced from Tyr was an order of magnitude larger than that from the other free amino acids (Hong et al., 2009). Tyr was obviously removed in MFC-CC, suggesting that the generation of THM from Tyr upon chlorination was less in closed-circuit MFCs. Trp, with the highest THMFP per unit TOC, was not detectable in this study.
3.3. Removal of THM precursor 3.3.1. Impact of bioelectrochemical degradation on THM precursor removal Initially, the influent THMFP yields were 158 ± 5 lg/L (AOMM) and 197 ± 23 lg/L (AOMC) (Fig. 4). Correspondingly, the specific THMFP yields were 22 ± 1 lg/mg TOC and 27 ± 3 lg/mg TOC for AOMM and AOMC, respectively. AOMC exhibited a higher productivity in THM formation as compared to AOMM. For AOMM, the highest removal of THMFP was in MFC-CC (85 ± 2%), followed by AR (34 ± 6%), and then MFC-OC (21 ± 5%). Similar results were obtained for AOMC among three types of reactors. In closed-circuit MFCs, THMFP for AOMM was consumed more completely than that for AOMC. Conversely, the removal of THMFP for AOMM was slightly lower in open-circuit MFCs. The different THMFP removals for both algae were attributed to the changes in the compositions after the biodegradation. Because THM production varied greatly with the different organic materials, it is necessary to investigate individual contribution for the total THMFP yields.
Fig. 3. Comparison of amino acid contents of algal organic matter from Microcystis aeruginosa (AOMM) among MFC in a closed circuit (MFC-CC), MFC in an open circuit (MFCOC), and in an anaerobic reactor (AR).
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Fig. 4. Comparison of the yields and removal efficiencies of THMFP generated from algal organic matter derived from Microcystis aeruginosa and Chlorella vulgaris (AOMM and AOMC) between measured values and calculated values in MFC in a closed circuit (MFC-CC), MFC in an open circuit (MFC-OC), and in an anaerobic reactor (AR).
Proteins, carbohydrates, and lipids are the major components in both AOM and are identified as the major THM precursors. Thus, we assumed that the total THMFP was generated from these three components (Hong et al., 2008):
Total THMFPðlg=LÞ ¼ ½THMFPProtein þ ½THMFPCarboh: þ ½THMFPLipid
ð1Þ
To evaluate the contribution of each component to total THMFP, three typical pure substances, bovine serum albumin (BSA), glucose, and fish oil were used to model the proteins, carbohydrates, and lipids, respectively, under the same chlorination conditions. THMFP from each component was calculated as follows:
½THMFPProtein ¼ ½THMFPBSA ½TOCX Protein
ð2Þ
½THMFPCarboh: ¼ ½THMFPglucose ½TOCX Carboh:
ð3Þ
½THMFPLipid ¼ ½THMFPfish oil ½TOCX Lipid
ð4Þ
where ½THMFPBSA , ½THMFPglucose , and ½THMFPfish oil are the THMFP yields per unit TOC from actual measurements of BSA (17.3 ± 5.3 lg/mg TOC), glucose (7.1 ± 2.5 lg/mg TOC), and fish oil (28.7 ± 11.4 lg/mg TOC), respectively. ½TOC is the measured TOC concentration (mg/L) in influent or effluents. X Protein , X Carboh: , and X Lipid are the fractions of TOC from each component in the total TOC and were calculated as follows:
X Protein ¼ aProtein ½Protein=ðaProtein ½Protein þ aCarboh: ½Carboh: þ aLipid ½LipidÞ
ð5Þ
X Carboh: ¼ aCarboh: ½Carboh:=ðaProtein ½Protein þ aCarboh: ½Carboh: þ aLipid ½LipidÞ
ð6Þ
X Lipid ¼ aLipid ½Lipid=ðaProtein ½Protein þ aCarboh: ½Carboh: þ aLipid ½LipidÞ
enhancing mass transfer, is expected to be effective in reducing THM precursors from AOMM. Both protein and lipid were dominant components in AOMC. Lipids had the highest THMFP yield per unit TOC, suggesting that lipids contributed more to THMFP than other organic materials in equal concentrations. Thus, lipid removal must be improved to reduce the THMFP production in AOMC.
ð7Þ
where aProtein = 0.55, aCarboh = 0.44, and aLipid = 0.80 are carbon percentages in proteins (C16H24O5N4), carbohydrates (C6H10O5), and lipids (C22H32O2), respectively. [Protein], [Carboh.], and [Lipid] are measured concentrations (mg/L) of the three components in influent or effluents. THM yield from the three major components were obtained based on calculation (Fig. 4). Protein content was believed to be a significant contributor to the THMFP yield in AOMM. THMFP removal was similar to the removal of protein content in the MFCs and AR treatment processes. Therefore, further optimization of operating conditions to improve protein removal, such as extending the reaction time, improving the reactor configuration, and
3.3.2. Contribution of soluble microbial products to THMFP THM precursors in effluents were presumably attributable to two sources: the residual substrates that were not completely oxidized and the soluble microbial products (SMPs) that were produced by the metabolism and biomass decay of microorganisms, such as exoenzymes, siderophores, extracellular materials, cell wall components, DNA fragments, and proteins released as a result of cell lysis (Liu and Li, 2010). SMP production varies among biotreatment processes depending on biomass concentration, substrate type and operation conditions (Barker and Stuckey, 1999). It has been reported that SMPs are produced at a rate that is proportional to the concentration of biomass due to the release of organic material from cell lysis (Barker and Stuckey, 1999). A previous study showed that the biomass yield was approximately 0.07–0.22 g-COD-biomass/g-COD-substrate in MFCs (Rabaey et al., 2003), which is comparable to that in anaerobic fermentation but less than that in an aerobic treatment process (0.53 g-CODbiomass/g-COD-substrate) (Verstraete and Van Vaerenbergh, 1986). The THMFP yields from SMPs were 10.6 ± 3.2 lg/mg TOC, 18.1 ± 5.1 lg/mg TOC, and 9.2 ± 2.4 lg/mg TOC in the effluent of MFC-CC, MFC-OC, and AR, respectively. There was no difference between MFC-CC and AR in THMFP, suggesting that both reactors might have similar biomass composition. A large amount of fermentative bacteria is present in both MFC-CC and AR to mediate the degradation of a complex substrate. In MFC-OC, electrochemically active bacteria cannot use the anode as the electron acceptor. Syntrophic interactions between electrochemically active bacteria and fermenters are inhibited, which generally leads to the biomass decay (Picioreanu et al., 2008). Increases in biomass decay lead to increases in the release of organic materials into the solution, thus resulting in more THMFP formation. 4. Conclusions A bioelectrochemical process using MFCs can effectively remove algal organic matter (AOM) derived from M. aeruginosa (AOMM) and C. vulgaris (AOMC). THM precursor from both AOM was degraded more completely in closed-circuit MFCs compared with the control reactors (open-circuit MFCs and anaerobic reactors). Other benefits related to bioelectrochemical degradation of AOM include electrical energy recovery, and the low THMFP produced from soluble
H. Wang et al. / Bioresource Technology 116 (2012) 80–85
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