Enhanced Biological Phosphorus Removal Driven by Short-Chain

Sep 7, 2007 - First, the released phosphorus was recovered from the SCFA-containing alkaline fermentation liquid by the formation of struvite precipit...
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Environ. Sci. Technol. 2007, 41, 7126-7130

Enhanced Biological Phosphorus Removal Driven by Short-Chain Fatty Acids Produced from Waste Activated Sludge Alkaline Fermentation JUAN TONG AND YINGUANG CHEN* State Key Laboratory of Pollution Control and Resources Reuse, School of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China

This paper examines the feasibility of using alkaline fermentative short-chain fatty acids (SCFAs) as the carbon sources of enhanced biological phosphorus removal (EBPR) microorganisms. First, the released phosphorus was recovered from the SCFA-containing alkaline fermentation liquid by the formation of struvite precipitation, and 92.8% of the soluble ortho-phosphorus (SOP) could be recovered under conditions of Mg/P ) 1.8 (mol/mol), pH 10.0, and a reaction time of 2 min. One reason for a Mg addition required in this study that was higher than the theoretical value was that the organic compounds consumed Mg. Then, two sequencing batch reactors (SBRs) were operated, respectively, with acetic acid and alkaline fermentative SCFAs as the carbon source of EBPR. The transformations of SOP, polyhydroxyalkanoates (PHAs), and glycogen and the removal of phosphorus were compared between two SBRs. It was observed that the phosphorus removal efficiency was around 98% with the fermentative SCFAs, and about 71% with acetic acid, although the former showed much lower transformations of both PHAs and glycogen. The reasons that fermentative SCFAs caused much higher SOP removal than acetic acid were due to less PHAs used for glycogen synthesis and a higher PHA utilization efficiency for SOP uptake. Finally, the toxicity of fermentation liquid to EBPR microorganisms was examined, and no inhibitory effect was observed. It can be concluded from this study that the SCFAs from alkaline fermentation of waste activated sludge were a superior carbon source for EBPR microorganisms than pure acetic acid.

Introduction To protect water bodies from eutrophication and to meet increasingly stringent treatment requirements for nutrients, phosphorus and nitrogen in effluents need to be removed. Both biological denitrification and phosphorus removal depend on the presence of available biodegradable carbon, such as short-chain fatty acids (SCFAs). Many researchers use sole carbon sources (e.g., acetate and propionate) to achieve enhanced biological phosphorus removal (EBPR) (1, 2). It is uneconomical and unsustainable to add chemically synthesized SCFAs as the external carbon source in full scale wastewater treatment plants (3). * Corresponding author phone: 86-21-65981263; fax: 86-2165986313; e-mail: [email protected]. 7126

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Using primary sludge fermentation, SCFAs can be produced in the wastewater treatment plant itself, which reduces sludge volume and minimizes the operating cost of supplementary carbon dosing. However, sometimes the SCFA production from primary sludge fermentation is not enough to ensure efficient nutrient removal. For example, Thomas et al. observed that the effluent phosphorus was still high when the fermentation liquid of primary sludge was added to the Noosa BNR plant (3). To produce more SCFAs to lower the effluent phosphorus level, additional molasses had to be supplied to mix with primary sludge for fermentation, and a better phosphorus removal was achieved than dosing with an equivalent amount of acetate (3). It seems that supplemental SCFAs from waste activated sludge (WAS) would be advantageous when the SCFAs from the influent wastewater or prefermented primary solids are insufficient, and so the SCFAs production technology needs to be improved. In a previous investigation, we reported that the production of SCFAs from WAS can be significantly enhanced under alkaline conditions (4). Nevertheless, it is unclear as to whether the produced SCFAs can be used as the carbon sources of EBPR microorganisms. In the literature, researchers usually used primary sludge (PS) to produce SCFAs, and they directly applied the fermentation liquid to drive EBPR (5, 6). However, during WAS fermentation, some toxic compounds, such as heavy metals, might be released, which might be harmful to activated sludge microbes when the SCFA-containing fermentation product was added to the EBPR facility. Also, it has been observed during WAS fermentation that significant amounts of phosphorus were released because the phosphorus content in WAS is usually much higher than in PS (7). Thus, without phosphorus recovery, the direct use of WAS fermentation liquid would increase the phosphorus loading of the wastewater treatment plant. Recently, phosphorus recovery from swine wastewater or the supernate of dewatered sludge in the form of the precipitation of magnesium ammonium phosphate (MgNH4PO4‚6H2O, MAP) (i.e. struvite) has drawn much attention (8-10) because MAP can be utilized as a fertilizer and both phosphorus and ammonium can be simultaneously removed (see eq 1). In the alkaline fermentation liquid of WAS, however, it was observed that there are high concentrations of organic compounds, such as protein, carbohydrates, and SCFAs (4). Until now, the influence of wastewater organics on phosphorus recovery by the struvite method, and the effect of MAP precipitation on the concentration of wastewater organic compounds, especially SCFAs, have not been documented. Mg2+ + NH4+ + PO43- + 6H2O f MgNH4PO4‚6H2OV

(1)

In this paper, the phosphorus was first recovered from the alkaline fermentation liquid of WAS before it was applied to drive EBPR, and the influence of the fermentation liquid concentration on phosphorus removal was investigated. Also, the variations of SCFAs, soluble chemical oxygen demand (SCOD), and soluble protein and carbohydrate during struvite precipitation were studied. Then, the fermentative SCFAs were used as the carbon sources of EBPR, and their performance was compared with that of chemically synthesized acetic acid. Finally, the heavy metals in the fermentation liquid and EBPR effluent were examined, and the possible inhibitory effect of alkaline fermentation liquid on EBPR activated sludge microorganisms was measured. 10.1021/es071002n CCC: $37.00

 2007 American Chemical Society Published on Web 09/07/2007

Materials and Methods Sludge Source. The WAS used for SCFA alkaline fermentation was obtained from the secondary sedimentation tank of a municipal wastewater treatment plant (WWTP) in Shanghai, China, which was operated with a traditional activated sludge process. The sludge was concentrated by settling at 4 °C for 24 h, and its main characteristics after settlement are as follows: pH 6.86, TSS (total suspended solids) 11 036 mg/L, VSS (volatile suspended solids) 9531 mg/L, SCOD 118 mg/L, TCOD (total chemical oxygen demand) 14 890 mg/L, sludge carbohydrate 1085 mg of COD/L, sludge protein 9874 mg of COD/ L, and sludge lipid and oil 152 mg of COD/L. In the EBPR experiments, the activated sludge from a WWTP in Shanghai, China was used as the inoculum of two sequencing batch reactors (SBRs). This WWTP was operated with an anaerobic (1.5 h)-aerobic (2.5 h) biological nutrient removal process. SCFA Production. The production of SCFAs was conducted at alkaline pH as described previously (4). After fermentation, the mixture was centrifuged at 100g for 10 min, and the centrate (pH ) 10.0) was collected for phosphorus recovery. Phosphorus Recovery from Alkaline Fermentation Liquid. To form MAP, Mg2+, NH4+, and PO43- are required (eq 1). Since both ammonium and phosphorus were released and the former was much greater than the latter, only magnesium dichloride (MgCl2‚6H2O) was added to recover phosphorus in this study. The batch experiments with 500 mL of the previous fermentation liquid in each test were conducted to investigate the influence of reaction time, initial pH, and Mg/P ratio on phosphorus recovery during struvite precipitation. The initial pH of the fermentation liquid was adjusted with 3 M NaOH or 3 M HCl, and the reaction system was thoroughly mixed. The crystals produced underwent X-ray diffraction (XRD) and were confirmed to be MAP. After phosphorus recovery, the alkaline fermentative liquid was used as the carbon source of EBPR microorganisms and tested for its possible toxicity to biomass. The following experiments were conducted to investigate the influence of concentration of organic compounds in the alkaline fermentation liquid on phosphorus recovery and to explain why the Mg/P molar ratio was greater than the theoretical value of 1/:1 during phosphorus recovery in the form of MAP. A total of 500 mL of different concentrations of alkaline fermentation liquid (0, 20, 40, 60, 80, and 100%) diluted with distilled water was put into six glass beakers. All six samples were adjusted to have the same SOP and ammonium initial concentrations (110 and 601 mg/L, respectively) as well as the same initial pH (10.0). Then, MgCl2‚ 6H2O was added to each reactor at a Mg/P molar ratio of 1.8:1. After 2 min of reaction, the SOP removal efficiency was assayed. EBPR Driven by Fermentative SCFAs. Most of the studies on EBPR used acetic acid as the main carbon source. Thus, in this paper, two anaerobic-aerobic SBRs were operated. The F-SBR and A-SBR were fed, respectively, with fermentative SCFAs and acetic acid as the main carbon source, but they had almost the same influent biological oxygen demand (BOD), total SCFAs, and phosphorus concentrations. The performance of the F-SBR was compared to that of the A-SBR. Both the F-SBR and the A-SBR had a working volume of 3.5 L and were maintained at 20 ( 1 °C in a temperaturecontrolled room. The operation of two SBRs was the same as described in our previous publication (11) with minor revisions. They were operated on three 8 h cycles per day, with each cycle consisting of 2 h anaerobic and 3 h aerobic periods, followed by 1 h settling, 5 min decanting, and 115 min for the remaining idle phase. Each of the reactors was constantly mixed with a magnetic stirrer except during the

settling, decanting, and idle periods. During the aerobic time, the dissolved oxygen (DO) concentration in both SBRs was around 6.2 mg/L. After the setting period, 2.5 L of the supernatant was discharged, resulting in a hydraulic retention time of 11.2 h, and replaced with a fresh 2.5 L volume of wastewater in the initial 10 min of the anaerobic period. The influent pH of each reactor was adjusted to 7.5 by adding either 2 M HCl or 2 M NaOH. The sludge retention time in the two SBRs was maintained at approximately 12 days. The feed was prepared daily from stock solutions called concentrated feed and P-water. The concentrated feed was adapted from Smolders et al. (12). The initial BOD concentration in the two SBRs was increased progressively over a 30 day period from around 80 to approximately 270 mg/L. The P-water consisted of (g/L) 23.47 KH2PO4 and 17.60 K2HPO4, and the pH was adjusted to 10.0 with 2 M NaOH. The beginning BOD/P ratio was 20, and it increased to a final value of 24 when the two SBRs were working at 270 mg of BOD/L. After around 60 days’ acclimatization, the phosphorus anaerobic release and aerobic uptake as well as net removal in two SBRs reached steady-state conditions. Analytical Methods. The measurements of soluble orthophosphate (SOP), total phosphate (TP), SCFAs, carbohydrate, glycogen, protein, lipid, COD, TSS, VSS, and ammonia nitrogen (NH4-N) were the same as described in our previous publications (4, 7, 11). The total SCFA content was calculated as the sum of measured acetic, propionic, n-butyric, isobutyric, n-valeric, and iso-valeric acid. The analyses of poly-3-hydroxybutyrate (PHB), poly-3hydroxyvalerate (PHV), and poly-3-hydroxy-2- methylvalerate (PH2MV) were conducted according to the method of Oehmen et al. (13). Lyophilized sludge samples were digested, methylated, and extracted with chloroform. The extracted methyl esters were analyzed using GC (HP4890) equipped with a FID (length 30 m, internal diameter 0.53 mm, film thickness 0.88 µm). Helium gas was used as the carrier gas (30 mL/min) and makeup gas. A calibration curve using a PHB/PHV (88%/12%) standard purchased from SigmaAldrich Chemical Co. was obtained. Six different concentrations of the standard were subjected to the same procedures used for the samples and injected on the GC. The direct correlations between PHB concentration and corresponding peak area and also between PHV concentration and corresponding peak area were established. Another calibration curve using 2-hydroxy-caproic acid provided by SigmaAldrich Chemical Co. was used. It should be noted here that no sample was available for the direct measurement of PH2MV. 2-Hydroxy-caproic acid and PH2MV are isomers; hence, the former compound was utilized for the standard curve of PH2MV (13). The total PHA was calculated as the sum of measured PHB, PHV, and PH2MV. The heavy metal concentrations in the WAS fermentation liquid and in the EBPR effluent were determined by inductively coupled plasma-optical emission spectrometry (ICPOES) (PerkinElmer Optima 2100DV) after the samples were acidified with 5% HNO3 and filtered with 0.45 µm filters. The possible toxicity of WAS fermentation liquid to biological wastewater treatment processes was examined by monitoring the changes of specific oxygen uptake rate (SOUR) because this measurement permits instantaneous following of the biomass response to injection of toxins (14). It should be noted that in the influent of F-SBR, the concentration of the fermentation liquid was around 4%. Therefore, in the toxicity study of the alkaline fermentation liquid to activated sludge, the fermentation liquid concentration was in the range of 1-10%. In five 300 mL BOD bottles, 4.8, 9.6, 14.4, 19.2, and 24 mL of alkaline fermentation liquid was added, respectively, to 235.2, 230.4, 225.6, 220.8, and 216 mL of phosphorus buffer solution (pH 7.5). Thus, five different concentrations (2, 4, 6, 8, and 10%) of the fermentation liquid VOL. 41, NO. 20, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Effect of pH on phosphorus and ammonium removal at Mg/P ) 2 and a reaction time of 5 min.

FIGURE 2. Effect of Mg/P on phosphorus and ammonium removal at pH 10.0 and reaction time of 5 min. were made. After being fully aerated, 60 mL of fully aerated mixed liquor withdrawn from the F-SBR was added to each BOD bottle. The MLSS in each BOD bottle was around 900 mg/L. The DO value in the bottle was measured at 1 min intervals until the DO was completely exhausted. Then, the MLSS used was measured to express the mass of microbes. The SOUR can be calculated by using the following equation: SOUR )

-60G C

(2)

where G is the slope of the linear portion of the DO decline curve in mg/L min, C is the MLSS concentration in grams/ liter, and the unit for SOUR is mg of O2/g of MLSS h.

Results and Discussion Phosphorus Recovery from Alkaline Fermentation Liquid of WAS. In the literature, most of the studies on phosphorus recovery with the formation of MAP were conducted for the supernate of dewatered sludge or swine wastewater (8-10), and the pH, Mg/P molar ratio, and reaction time were reported to affect phosphorus recovery. For the alkaline fermentation liquid of WAS, the influence of these factors on phosphorus recovery are shown in Figures 1-3, in which the ammonium removal was also included. It can be seen from Figure 1 that the phosphorus removal efficiency increased with pH from 8.0 to 10.0 and that there was no significant increase after pH 10.0. As the formation of struvite consumed ammonium, the removal of ammonium was also observed. However, due to the volatilization of ammonium during mixing, there was still some ammonium removal at pH > 10.0 when the phosphorus removal efficiency stopped increasing. Because the original pH of the WAS alkaline 7128

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FIGURE 3. Effect of reaction time on phosphorus and ammonium removal at Mg/P ) 1.8 and pH 10.0. fermentation liquid was 10.0, and we focused on phosphorus removal in this study, the suitable pH for phosphorus recovery was controlled at pH 10.0. As shown in Figure 2, with the increase of Mg/P from 1.2 to 1.8 mol/mol, both phosphorus and ammonium removal increased, but further increasing Mg/P to 2 resulted in only marginal improvement of both phosphorus and ammonium removal, which indicated that Mg/P should be maintained at 1.8:1. Nevertheless, according to eq 1, the theoretical value of the Mg/P molar ratio is 1:1 when MAP is formed. The possible reason was that some of the Mg combined with SCOD (such as proteins and lipids) or even coprecipitated with SCOD (the data in Table 2 do indicate that some COD was removed during phosphorus recovery). In fact, at a fixed Mg/P molar ratio of 1.8:1, when the fermentation liquid concentration was 0, 20, 40, 60, 80, and 100%, the SOP removal was, respectively, 99.7, 98.2, 96.2, 95.7, 94.9, and 92.8%, which suggested that with the increase of organic compounds, the SOP removal decreased, and more Mg should be added if a higher SOP removal was required. From Figure 3, it can be seen that the removal efficiency of SOP, TP, and ammonium increased rapidly from 0 to 92.8, 84.3, and 16.8%, respectively, with the increase of time during the first 2 min, but no obvious increase observed after 2 min, which was almost the same as that observed in swine wastewater (15). At other Mg/P ratios, the same observation could be made (data not shown). It seems that 2 min was enough for efficient phosphorus recovery. Table 1 summarizes the changes in the main composition of the fermentation liquid after struvite precipitation under conditions of pH 10.0, Mg/P ) 1.8 mol/mol, and a reaction time of 2 min. Along with the removal of phosphorus and ammonium, there were some SCOD, SCFAs, and soluble carbohydrate and protein removed from the fermentation liquid. Also, it can be easily calculated from Table 1 that after phosphorus recovery, the SCFAs accounted for 53.4% of the SCOD and the SCFAs were composed of 29.1% acetic, 27.4% propionic, 17.3% iso-valeric, 13.5% iso-butyric, 9.0% nbutyric, and 3.7% n-valeric acid. EBPR Driven by SCFA-Containing Alkaline Fermentation Liquid. It was observed that all SCFAs were consumed during the initial 10 min of anaerobic time in the F-SBR, but it took 30 min to take up all acetic acid in the A-SBR. During one anaerobic and aerobic cycle, the variations of SCOD, BOD, and soluble protein and carbohydrates in the F-SBR are shown in Table 2. Obviously, all protein and carbohydrate and most COD and BOD were taken up after one anaerobicaerobic cycle. The profiles of changes of SOP and sludge PHAs (including PHB, PHV, and PH2MV) and glycogen in the F-SBR and A-SBR

TABLE 1. Variations of Main Composition of Alkaline Fermentation Liquid before and after Phosphorus Recovery before recovery (mg/L) after recovery (mg/L) removal rate (%)

SOP

TP

NH4-N

SCOD

SCFAsa

carbohydrate

protein

109.5 7.9 92.8

123.7 19.4 84.3

601.1 500.1 16.8

10606 9862 6.9

5396 5267b 2.4

382.0 324.2 15.1

2122.3 1833.1 13.6

a Unit of SCFAs is mg of COD/L. b SCFAs were composed of acetic 1534, propionic 1445, n-butyric 475, iso-butyric 709, n-valeric 193, and iso-valeric acid 911 mg of COD/L, respectively.

TABLE 2. Variations of BOD, SCOD, Protein, and Glycogen in Acetic Acid and Fermentative SCFAs SBRa,b F-SBR (mg/L)

A-SBR (mg/L)

time

protein

carbohydrate

BOD

SCOD

protein

carbohydrate

BOD

SCOD

anaerobic initial anaerobic end aerobic end

86.0 31.4 0

10.0 6.2 0

271 24 6

347 98 71

46.1 7.9 0

0 0 0

268 20 5

283 81 57

a In this investigation, two SBRs received almost the same anaerobic beginning total SCFAs and BOD concentrations. b At the beginning of anaerobic time, the concentration of SCFAs in F-SBR was acetic 62, propionic acid 59, n-butyric 19, iso-butyric 29, n-valeric 8, and iso-valeric 37 mg of COD/L, respectively, with total SCFAs of 214 mg of COD/L and acetic acid concentration in A-SBR of 220 mg of COD/L.

TABLE 3. Transformations of SOP, PHAs, and Glycogen in Two SBRsa item

F-SBR

A-SBR

initial SOP (mg/L) 11.14 11.35 specific SOP release (mmol/g of VSS) 0.93 0.89 specific SOP uptake (mmol/g of VSS) 1.06 0.98 SOP uptake/SOP release (mmol/g of VSS) 0.13 0.09 aerobic end SOP (mg/L) 0.15 3.28 SOP removal efficiency (%) 98.7 71.1 PHAs synthesis (mmol of C/g of VSS) 2.11b 3.65c PHAs degradation (mmol of C/g of VSS) 2.16d 3.59e glycogen degradation (mmol of C/g of VSS) 0.75 1.67 glycogen synthesis (mmol of C/g of VSS) 1.23 2.44 glycogen synthesis/PHAs degradation 0.57 0.67 (mmol of C/mmol of C) SOP uptake/PHAs degradation (mmol of 0.49 0.27 P/mmol of C) a MLSS and MLVSS in F-SBR were 4416 and 2738 mg/L, which were 4268 and 2824 mg/L in A-SBR. b Including PHB 0.77, PHV 1.00, and PH2MV 0.34. c Including PHB 2.81, PHV 0.72, and PH2MV 0.12. d Including PHB 0.75, PHV 0.98, and PH2MV 0.43. e Including PHB 2.72, PHV 0.67, and PH2MV 0.20.

FIGURE 4. Variations of SOP (a) and sludge glycogen (b) as well as sludge PHAs (c) during one cycle in two SBRs (open symbols refer to transformations in F-SBR, and filled symbols refer to transformations in A-SBR).

a higher specific SOP anaerobic release and aerobic uptake than A-SBR (0.93 vs 0.89 mmol/g of VSS and 1.06 vs 0.98 mmol/g of VSS, respectively), and the former showed a higher difference between SOP uptake and release than the latter (0.13 vs 0.09 mmol/g of VSS). Thus, the aerobic end SOP concentration in F-SBR was much lower than in A-SBR (0.15 vs 3.28 mg/L), and a much higher SOP removal efficiency in the F-SBR was obtained (98.7 vs 71.1%), which was in correspondence with a greater phosphorus content in the sludge of F-SBR than that of A-SBR (84.9 vs 74.3 mg/g of VSS). The phosphorus removal efficiency remained around 98% in F-SBR and 71% in A-SBR during a 5 month period after reaching steady-state operation.

are shown in Figure 4. There was an obvious phosphorus anaerobic release and aerobic uptake, and the latter was greater than the former in both SBRs. The PHAs accumulation and glycogen degradation occurred in the anaerobic phase. During the aerobic time, the PHAs were degraded, and glycogen was synthesized. The behavior of the current two SBRs was similar to that observed by other researchers (12, 16), which indicated that both F-SBR and A-SBR exhibited good EBPR performance. Table 3 summarizes the anaerobic and aerobic transformations of SOP, PHAs, and glycogen in two SBRs. F-SBR had

From the PHA transformation data in Table 3, it can be seen that the synthesis and degradation of PHAs in F-SBR was much lower than those in A-SBR (2.11 vs 3.65 mmol of C/g of VSS and 2.16 vs 3.59 mmol of C/g of VSS, respectively). Nevertheless, the data in Table 3 reveal that the SCFA concentration in F-SBR and A-SBR was, respectively, 214 and 220 mg of COD/L or 5.80 and 6.88 mmol of C/L. Obviously, less PHAs were synthesized and degraded with the fermentative SCFAs than with the same amount of acetic acid, which was due to other longer chain SCFAs other than acetic acid in F-SBR. Lemos et al. (17) also observed that the VOL. 41, NO. 20, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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PHA production decreased with the increase of the chain length of SCFAs. Since the energy for SOP uptake comes from PHA degradation, one might want to know why lower PHA aerobic degradation in F-SBR resulted in a higher SOP uptake. One possible reason is that the degradation of PHAs is used not only for SOP uptake but for glycogen synthesis. If a lower glycogen synthesis occurred, then more PHAs could be saved for taking up more SOP even though the PHA degradation was lower. The glycogen aerobic synthesis in F-SBR and A-SBR was, respectively, 1.23 and 2.44 mmol of C/g of VSS, and the corresponding ratio of glycogen synthesis/PHAs degradation was 0.57 and 0.67 mmol of C/mmol of C. Thus, less PHAs were used for glycogen synthesis, and more PHAs were used for SOP uptake in F-SBR. The difference can also be explained from the point of view of the PHA utilization efficiency for SOP uptake. The ratio of SOP uptake/PHAs degradation in two SBRs was 0.49 and 0.27 mmol of P/mmol of C, which suggested that for per unit PHAs degradation, the SOP uptake in F-SBR was much greater than that in A-SBR. It seems that the PHA utilization efficiency for SOP uptake in F-SBR was higher than that in A-SBR. Examination of the Toxic Effect of Fermentation Liquid on EBPR Activated Sludge Microorganisms. During WAS fermentation for SCFA production, some toxic compounds might be released or formed. It was observed that some heavy metals were released. The released Mn, Ni, Cu, Zn, Cd, As, and Pb in the fermentation liquid were, respectively, 0.073, 0.063, 0.065, 1.580, 0.001, 0.021, and 0.006 mg/L. However, when the SCFA-containing fermentation liquid was used to drive EBPR, all these heavy metals were removed, and all of them were nondetectable in the effluent of F-SBR. The SOUR analysis showed that at fermentation liquid concentrations of 2, 4, 6, 8, and 10%, the SOUR was 34.52, 34.67, 34.56, 34.48, and 34.52 mg of O2/g of MLSS h, respectively, which suggested that different concentrations of the fermentation liquid had little impact on SOUR in the investigated range. Therefore, the inhibitory effect of alkaline fermentation liquid on the activity of sludge aerobic microbes was negligible.

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Acknowledgments This work was financially supported by the National HiTech Research and Development Program of China (863) (2007AA06Z326), the program for NCET in University (060373), the National Science Foundation of China (50678125), and the ShuGuang Scholarship (05SG26).

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Received for review April 28, 2007. Revised manuscript received July 23, 2007. Accepted July 25, 2007. ES071002N