Aerobic treatment of dairy wastewater with sequencing batch reactor systems

Performances of single-stage and two-stage sequencing batch reactor (SBR) systems were investigated for treating dairy wastewater. A single-stage SBR system was tested with 10,000 mg/l chemical oxygen demand (COD) influent at three hydraulic retention times (HRTs) of 1, 2, and 3 days and 20,000 mg/l COD influent at four HRTs of 1, 2, 3, and 4 days. A 1-day HRT was found sufficient for treating 10,000-mg/l COD wastewater, with the removal efficiency of 80.2% COD, 63.4% total solids, 66.2% volatile solids, 75% total Kjeldahl nitrogen, and 38.3% total nitrogen from the liquid effluent. Two-day HRT was believed sufficient for treating 20,000-mg/l COD dairy wastewater if complete ammonia oxidation is not desired. However, 4-day HRT needs to be used for achieving complete ammonia oxidation. A two-stage system consisting of an SBR and a complete-mix biofilm reactor was capable of achieving complete ammonia oxidation and comparable carbon, solids, and nitrogen removal while using at least 1/3 less HRT as compared to the single SBR system.     

Effect of COD/N ratio on cultivation of aerobic granular sludge in a pilot-scale sequencing batch reactor

Aerobic granular sludge was successfully cultivated with the effluent of internal circulation reactor in a pilot-scale sequencing batch reactor (SBR). Soy protein wastewater was used as an external carbon source for altering the influent chemical oxygen demand/nitrogen (COD/N) ratios of SBR. Initially, the phenomenon of partial nitrification was observed and depressed by increasing the influent COD/N ratios from 3.32 to 7.24 mg/mg. After 90 days of aerobic granulation, the mixed liquor suspended solids concentration of the reactor increased from 2.80 to 7.02 g/L, while the sludge volumetric index decreased from 105.51 to 42.99 mL/g. The diameters of mature aerobic granules vary in the range of 1.2 to 2.0 mm. The reactor showed excellent removal performances for COD and $ {\text{NH}}_4^{ + }{\text{ - N}} $ after aerobic granulation, and average removal efficiencies were over 93% and 98%, respectively. The result of this study could provide further information on the development of aerobic granule-based system for full-scale applications.     

Identifying an interfering factor on chemical oxygen demand (COD) determination in piggery wastewater and eliminating the factor by an indigenous Pseudomonas stutzeri strain

AIMS: This study attempted to demonstrate nitrite interference on chemical oxygen demand (COD) determination in piggery wastewater, and the capability of aerobic denitrification of the SU2 strain which is capable of promoting the efficiency of nitrogen and COD removal from piggery wastewater. METHODS AND RESULTS: This study was performed in a 17-litre reactor with a 30% packing ratio, with a ratio of immobilized SU2 cells to sludge of 100:1. The ratio of aeration to nonaeration was 4 : 1.5. Removal efficiency of COD was 86.8%. Removal efficiency of BOD and SS was higher than 90%, and removal efficiency of NH4+-N and TKN was almost 100%. CONCLUSIONS: NO2- -N interference is significant when its concentration in piggery wastewater exceeds 100 mg l-1. COD in piggery wastewater can be indirectly reduced following nitrite reduction by SU2 strain. SIGNIFICANCE AND IMPACT OF THE STUDY: Utilizing immobilized SU2 cells in coordination with an SBR system simultaneously reduces nitrite and COD concentrations.     

Mass balance of nitrogen, and estimates of COD, nitrogen and phosphorus used in microbial synthesis as a function of sludge retention time in a sequencing batch reactor system

This study investigated characteristics of a sequencing batch reactor (SBR) system which was varied with respect to sludge retention time (SRT) (5.9, 8.2, 10.5, 12.2, and 16.2 days). The removal efficiencies of chemical oxygen demand (COD) were more than 90% under all SRT conditions, and the greatest efficiency (92.2%) occurred with a SRT of 16.2 days. As the SRT increased, the denitrification rate per mixed liquor suspended solids (MLSS) during the anoxic(I) period decreased significantly from 166.3 mg NO(X)(-)-N/g MLSS d to 68.8 mg NO(X)(-)-N/g MLSS d. As the SRT increased, the phosphorus removal efficiency decreased from 47.1% (SRT of 5.9 days) to 31.0% for a SRT of 16.2 days, because active phosphate release and uptake occurred under shorter SRT conditions. The mass balance of nitrogen (with respect to nitrogen in the influent) at a SRT of 16.2 days (the highest nitrogen removal efficiency) showed 14.9% of nitrogen was removed in clarified water effluent, 49.7% was removed by the sludge waste process and 33.3% was removed by denitrification. Nitrogen processing was well accounted for in the SBR system as the nitrogen mass balance was close to 100% (97.9%).     

菌藻共生系统对序批式反应器处理养猪废水脱氮除磷效果及微生物群落结构的影响

【背景】养猪废水作为高浓度有机废水,是导致我国农业面源污染的主要因素之一。目前采用菌藻共生系统处理养猪废水越来越受到关注,与传统序批式反应器(Sequencing Batch Reactor,SBR)相比,藻辅助SBR具有提高脱氮除磷效果、增加污泥活性和降低能源消耗的特点。【目的】针对SBR中菌藻共生系统对养猪废水脱氮除磷效能的影响,比较分析菌藻共生系统与常规SBR系统中污泥特性及微生物群落结构特征差异。【方法】在室温条件下分别平行运行SBR+微藻(R1)和作为对照系统不添加微藻的SBR(R2)。监测R1和R2系统废水处理效果,污泥的粒径、沉降性和代谢产物等污泥特性。利用变性梯度凝胶电泳(Denaturing Gradient Gel Electrophoresis,DGGE)技术分析R1和R2系统中的微生物种类和分布。【结果】与对照R2反应器相比,R1的化学需氧量(Chemical Oxygen Demand,COD)去除率提高了5.1%,NH4+-N提高了20.3%,总氮(Total Nitrogen,TN)提高了19.4%,总磷(Total Phosphorus,TP)提高了23.9%。进一步对反应器中的污泥特性进行分析发现,与R2相比,R1的胞外聚合物(ExtracellularPolymericSubstances,EPS)平均含量提高3.7%,可溶性微生物产物(Soluble MicrobialProduct,SMP)平均增加了38.5%。同时R1的污泥粒径较R2提高了14.8%,污泥体积指数(Sludge Volume Index,SVI)值较R2降低了11.7%,污泥的好氧呼吸速率(Specific Oxygen Uptake Rate,SOUR)较R2提高了64.8%,而且稳定的菌藻共生系统的形成进一步减少反应器出水中的悬浮固体浓度,表明藻类的添加对R1污泥特性具有改良作用【结论】R1反应器形成的菌藻共生体系可进一步优化微生物群落结构,其中放线菌纲(Actinobacteria)、α-变形菌纲(Alphaproteobacteria)和γ-变形菌纲(Gammaproteobacteria)为R1反应器的主要菌群,对养猪废水的处理起到重要作用。R1反应器中的藻类主要为链带藻属(Desmodesmus)和尖带藻属(Acutodesmus),对养猪废水的脱氮除磷起到重要作用。     

Analysis of factors affecting the performance of partial nitrification in a sequencing batch reactor

The objective of this study was to analyze the factors affecting the performance of partial nitrification in a sequencing batch reactor. During a 140-day long-term operation, influent pH value, dissolved oxygen (DO), and chemical oxygen demand/nitrogen (COD/N) ratio were selected as operating factors to evaluate the maintenance and recovery of nitrite accumulation. Results showed that high DO concentration (2–4 mg/L) could damage nitrite accumulation immediately. However, nitrite accumulation ratio (NAR) could be increased from 1.68?±?1.51 to 35.46?±?7.86 % when increasing the pH values from 7.5 to 8.3 due to the increased free ammonia concentration. Afterwards, stable partial nitrification and high NAR could be recovered when the reactor operated under low DO concentration (0.5–1.0 mg/L). However, it required a long time to recover the partial nitrification of the reactor when the influent COD/N ratios were altered. Fluorescence in situ hybridization analysis implied that ammonium oxidizing bacteria were completely recovered to the dominant nitrifying bacteria in the system. Meanwhile, sludge volumetric index of the reactor gradually decreased from 115.6 to 56.6 mL/g, while the mean diameter of sludge improved from74.57 to 428.8 μm by using the strategy of reducing settling time. The obtained results could provide useful information between the operational conditions and the performance of partial nitrification when treating nitrogen-rich industrial wastewater.     

Integration of a Coagulation/Flocculation step in a biological sequencing batch reactor for COD and nitrogen removal of supernatant of anaerobically digested piggery wastewater

The supernatant from mesophilic anaerobic digestion of piggery wastewater is characterised by a high amount of COD (4.1 g COD L(-1)), ammonium (2.3g NH(4)(+)-NL(-1)) and suspended solids (2.5 g SS L(-1)). This effluent can be efficiently treated by means of a Sequencing Batch Reactor (SBR) strategy for biological COD, SS and nitrogen removal including a Coagulation/Flocculation step. Total COD and SS reduction yields higher than 66% and 74%, respectively, and a total nitrogen removal (via nitrite) of more than 98% were reached when working with HRT 2.7 days, SRT 12 days, temperature 32 degrees C, three aerobic/anoxic periods, without external control of pH and under limited aeration flow. The inhibition of nitrite oxidizing biomass was achieved by the working free ammonia concentration and the restricted air supply (dissolved oxygen concentration below 1 mg O(2)L(-1)). Since a part of the total COD was colloidal and/or refractory, a Coagulation/Flocculation step was implemented inside the SBR operating strategy to meet a suitable effluent quality to be discharged. Several Jar-Tests demonstrated that the optimal concentration of FeCl(3) was 800 mg L(-1). A respirometric assay showed that this coagulant dosage did not affect the biological activity of nitrifying/denitrifying biomass.     

Nitrification,denitrification and biological phosphorus removal in piggery wastewater using a sequencing batch reactor

Nutrients in piggery wastewater with high organic matter, nitrogen (N) and phosphorus (P) content were biologically removed in a sequencing batch reactor (SBR) with anaerobic, aerobic and anoxic stages. The SBR was operated with 3 cycles/day, temperature 30 degrees C, sludge retention time (SRT) 1 day and hydraulic retention time (HRT) 11 days. With a wastewater containing 1500 mg/l ammonium and 144 mg/l phosphate, a removal efficiency of 99.7% for nitrogen and 97.3% for phosphate was obtained. Experiments set up to evaluate the effect of temperature on the process showed that it should be run at temperatures higher than 16 degrees C to obtain good removals (> 95%). Batch tests (ammonia utilization rate, nitrogen utilization rate and oxygen utilization rate) proved to be good tools to evaluate heterotrophic and autotrophic biomass activity. The SBR proved to be a very flexible tool, and was particularly suitable for the treatment of piggery wastewater, characterized by high nutrient content and by frequent changes in composition and therefore affecting process conditions.     

Aerobic granulation and nitrogen removal with the effluent of internal circulation reactor in start-up of a pilot-scale sequencing batch reactor

Aerobic granular sludge was successfully cultivated with the effluent of internal circulation (IC) reactor in a pilot-scale sequencing batch reactor (SBR) using activated sludge as seeding sludge. N removal was investigated in the start-up of aerobic granulation process. Initially, the phenomenon of partial nitrification was observed and nitrite accumulation rates (NO₂ ^?-N/NO _x ^? -N) were between 84.6 and 99.1?%. It was potentially caused by ammonium oxidizing bacteria (AOB) in the seeding activated sludge, high external environmental temperature (~32?°C) and free ammonia (FA) concentration. After 50?days’ running, the aerobic granules-based bioreactor demonstrated perfect performance in simultaneous removal of organic matter and ammonia nitrogen, and average removal efficiencies were maintained above 93 and 96?%, respectively. The maximum nitrogen removal efficiency of 83.1?% was achieved after the formation of aerobic granules. The average diameter of mature aerobic granular sludge mostly ranged from 0.5 to 1.0?mm. Furthermore, one typical cyclic test indicated that pH and DO profiles could be used as effective parameters for biological reactions occurring in the aerobic/anoxic process. The obtained results could provide further information on the cultivation of aerobic granular sludge with practical wastewater, especially with regard to nitrogen-rich industrial wastewater.     

Factors affecting the performance of membrane bioreactor for piggery wastewater treatment

This study was conducted to identify the factors affecting the performance of membrane bioreactor (MBR) for piggery wastewater treatment. The change of organic and nitrogen concentrations in piggery wastewater was studied to investigate the treatment efficiency. The increase of COD, BOD and NH₃–N from 1150 to 2050 mg/L, 683 to 1198 mg/L and 154 to 248 mg/L has led to the decrease of treatment efficiency. Removal efficiencies of COD, BOD and NH₃–N have decreased from 96.0% to 92.0%, 97.0% to 92.7% and 93.2% to 69.5%, respectively. The effects of biomass characteristics on membrane fouling were determined based on Pearson’s correlation coefficient (r_p). It was found that MLSS had a negative correlation with permeate flux (r_p = −0.745, at significant level of 0.05) while sludge floc size a positive correlation (r_p = 0.731, at significant level of 0.05). MLSS and sludge floc size were found to be the dominant factors that controlled the membrane filterability while sludge viscosity, EPS, SMP and SV₃₀ have taken as the sub-factors affecting membrane fouling.     

Analysis of the characteristics of short-cut nitrifying granular sludge and pollutant removal processes in a sequencing batch reactor

Aerobic granular sludge is a new type of microbe auto-immobilization technology; in this paper, short-cut nitrification and denitrification were effectively combined with the granular sludge technology. Simultaneous nitrification and denitrification granules were developed in a sequencing batch reactor (SBR) using synthetic wastewater with a high concentration of ammonia nitrogen at 25 °C with a dissolved oxygen concentration above 2.0 mg/L and a 15 days sludge retention time. The characteristics of the sludge and the removal efficiency were studied, and the removal mechanisms of the pollutants and the process of short-cut nitrification were analyzed. The average granule diameter of the granular sludge was 704.0 μm. The removal rates of pollutants and the accumulation rate of nitrite in the SBR were studied. During treatment of wastewater with a high concentration of ammonia nitrogen, simultaneous nitrification, and denitrification and the stripping process could contribute to the removal of total nitrogen. The high pH value, the high concentration of free ammonia, and the delamination of granular sludge were the main factors contributing to the short-cut nitrification property of granular sludge in the reaction process.     

Organics and nitrogen removal and sludge stability in aerobic granular sludge membrane bioreactor

Novel aerobic granular sludge membrane bioreactor (GMBR) was established by combining aerobic granular sludge technology with membrane bioreactor (MBR). GMBR showed good organics removal and simultaneous nitrification and denitrification (SND) performances for synthesized wastewater. When influent total organic carbon (TOC) was 56.8-132.6 mg/L, the TOC removal of GMBR was 84.7-91.9%. When influent ammonia nitrogen was 28.1-38.4 mg/L, the ammonia nitrogen removal was 85.4-99.7%, and the total nitrogen removal was 41.7-78.4%. Moreover, batch experiments of sludge with different particle size demonstrated that: (1) flocculent sludge under aerobic condition almost have no denitrification capacity, (2) SND capacity was caused by the granular sludge, and (3) the denitrification rate and total nitrogen removal efficiency were enhanced with the increased particle size. In addition, study on the sludge morphology stability in GMBR showed that, although some granular sludge larger than 0.9 mm disaggregated at the beginning of operation, the granular sludge was able to maintain the stability of its granular morphology, and at the end of operation, the amount of granular sludge (larger than 0.18 mm) stabilized in GMBR was more than 56-62% of the total sludge concentration. The partial disaggregation of large granules is closely associated with the change of operating mode from sequencing batch reactor (SBR) system to MBR system.     

Comparison of biological removal via nitrite with real-time control using aerobic granular sludge and flocculent activated sludge

The process of nitrification–denitrification via nitrite for nitrogen removal under real-time control mode was tested in two laboratory-scale sequencing batch reactors (SBRs) with flocculent activated sludge (R1) and aerobic granular sludge (R2) to compare operational performance and real-time control strategies. The results showed that the average ammonia nitrogen, total inorganic nitrogen (TIN), and chemical oxygen demand (COD) removal during aeration phase were 97.6%, 57.0%, and 90.1% in R2 compared with 98.6%, 48.7%, and 88.1% in R1. The TIN removed in both SBRs was partially due to the presence of simultaneous nitrification–denitrification via nitrite, especially in R2. The specific nitrification and denitrification rates in R2 were 0.0416 mgNH₄⁺–N/gSS-min and 0.1889 mgNO_X⁻–N/gSS-min, which were 1.48 times and 1.35 times that of R1. The higher rates for COD removal, nitrification, and denitrification were achieved in R2 than R1 with similar influent quality. Dissolved oxygen (DO), pH, and oxidization reduction potential, corresponding to nutrient variations, were used as diagnostic parameters to control the organic carbon degradation and nitrification–denitrification via nitrite processes in both SBRs. The online control strategy of granular SBR was similar to that of the SBR with flocculent activated sludge. However, a unique U-type pattern on the DO curve in granular SBR was different from SBR with flocculent activated sludge in aerobic phase.     

Nitrogen removal from wastewater with a low COD/N ratio at a low oxygen concentration

The goal of the study was to determine the effectiveness of nitrification and denitrification and the kinetics of ammonia removal from a mixture of wastewater and anaerobic sludge digester supernatant in an SBR at limited oxygen concentration. In addition, the COD removal efficiency and sludge production were assessed.In the SBR cycle alternating aerobic and anaerobic phases occurred; in the aeration phase the dissolved oxygen (DO) concentration was below 0.7 mg O₂/L. The low DO concentration did not inhibit ammonia oxidation-nitrification and the efficiency was ca. 96-98%. However, a relatively high COD concentration in the effluent was detected. The values of K_m and V_max, calculated from the Michaelis-Menten equation, were 43 mg N-NH₄/L and 15.64 mg N-NH₄/L h, respectively. Activated sludge production was almost stable (0.62-0.66 g MLVSS/g COD). A high net biomass production resulted from a low specific biomass decay rate of 0.0015 d⁻¹.     

Effect of pH on phase separated anaerobic digestion

A pilot scale experiment was performed for a year to develop a two-phase anaerobic process for piggery wastewater treatment (COD: 6,000 mg/L, BOD: 4,000 mg/L, SS: 500 gm/L, pH 8.4, alkalinity 6,000 mg/L). The acidogenic reactor had a total volume of 3 m³, and the methanogenic reactor, an, anaerobic up-flow sludge filter, combining a filter and a sludge bed, was also of total volume 3 m³ (1.5 m³ of upper packing material). Temperatures of the acidogenic and methanogenic reactors kept at 20°C and 35°C., respectively. When the pH of the acidogenic reactor was controlled at 6.0–7.0 with HCl, the COD removal efficiency increased from 50 to 80% over a period of six months, and as a result, the COD of the final effluent fell in the range of 1,000–1,500 mg/L. BOD removal efficiency over the same period was above 90%, and 300 to 400 mg/L was maintained in the final effluent. The average SS in the final effluent was 270 mg/L. The methane production was 0.32 m³ CH₄/kg COD_removed and methane content of the methanogenic reactor was high value at 80–90%., When the pH of the acidogenic reactor was not controlled over the final two months, the pH reached 8.2 and acid conversion decreased compared with that of pH controlled, while COD removal was similar to the pH controlled operation. Without pH control, the methane content in the gas from methanogenic reactor improved to 90%, compared to 80% with pH control.     

Some properties of a sequencing batch reactor system for removal of vat dyes

Bio-sludge from a wastewater treatment plant could be used as an adsorbent of vat dye from textile wastewater. Resting bio-sludge gave a higher adsorption capacity than dead bio-sludge. The resting bio-sludge from a textile wastewater treatment plant gave relatively high COD, BOD5 and dye adsorption capacity of 364.4 +/- 4.3, 178.0 +/- 9.0 and 50.5 +/- 1.3 mg/g of bio-sludge, respectively, in synthetic textile wastewater containing 40 mg/l Vat Yellow 1. Another advantage of the bio-sludge was that, after washing with 0.1 N NaOH solution, it was reusable without any activity loss. Through treatment with a sequencing batch reactor (SBR) system, both organic and dye in STIWW could be removed. The maximum dye (Vat Yellow 1), COD, BOD5 and TKN removal efficiencies of the SBR system under an MLSS of 2000 mg/l and an HRT of three days were 98.5 +/- 1.0%, 96.9 +/- 0.7%, 98.6 +/- 0.1% and 93.4 +/- 1.3%, respectively. Although, the dye and organic removal efficiencies of the SBR system with real textile wastewater were quite low, they could be increased by adding organic matters, especially glucose. The dye, COD, BOD5 and TKN removal efficiencies of the SBR system with glucose (0.89 g/l) supplemented textile industrial wastewater were 75.12 +/- 1.2%, 70.61 +/- 3.4%, 96.7 +/- 0.0%, and 63.2 +/- 1.1%, respectively.     

Characteristics of granular methanogenic sludge grown on phenol synthetic medium and methanogenic fermentation of phenolic wastewater in a UASB reactor

The growth of granules on a phenol synthetic medium and the methanogenic fermentation of industrial phenolic wastewater from a steel factory in an upflow anaerobic sludge blanket (UASB) reactor were investigated. Total granular sludge concentration retained in the UASB reactor was 6.7 g MLSS/l (6.0 g MLVSS/l) during the 10 months' operation on the phenol synthetic medium. This realized a maximum phenol removal rate of 2.2 g/l·d (phenol concentration of influent = 500 mg/l), which corresponded to 5.2 g COD/l·d at space velocity (SV) of 4.4 d⁻¹. The granules formed were of relatively small size ranging from 0.61 to 0.77 mm, and had a relatively low density of 0.013–0.023 g MLVSS/cm³ and low specific gravity (1.11) due to very low ash content (8.7–11.9%). Electron microscopic analysis showed that Methanothrix spp. appeared dominantly on the granule surface as well as within it. The specific metabolic activities of bacterial trophic groups were the highest for H₂ followed by acetate, benzoate, phenol, and propionate. In the case of industrial phenolic wastewater, although phenol efficiency was only 50% at SV of 0.4 d⁻¹, when the wastewater was diluted twofold and the treated wastewater was recycled at SV of 7.3 d⁻¹, the removal efficiencies of phenol and CODcr were restored to 90% (influent=400 mg/l) and 80% (influent=5,000 mg/l), respectively. It was suggested that recycling of the treated wastewater might be improved by partly degrading unknown toxic compounds contained in phenolic wastewater.     

Nitrogen removal performance of anaerobic ammonia oxidation (ANAMMOX) in presence of organic matter

A sequencing batch reactor (SBR) was used to test the nitrogen removal performance of anaerobic ammonium oxidation (ANAMMOX) in presence of organic matter. Mesophilic operation (30 ± 0.5 °C) was performed with influent pH 7.5. The results showed, independent of organic matter species, ANAMMOX reaction was promoted when COD was lower than 80 mg/L. However, specific ANAMMOX activity decreased with increasing organic matter content. Ammonium removal efficiency decreased to 80% when COD of sodium succinate, sodium potassium tartrate, peptone and lactose were 192.5, 210, 225 and 325 mg/L, respectively. The stoichiometry ratio resulting from different OM differed largely and R₁ could be as an indicator for OM inhibition. When COD concentration was 240 mg/L, the loss of SAA resulting from lactose, peptone, sodium potassium tartrate and sodium succinate were 28, 36, 50 and 55%, respectively. Sodium succinate had the highest inhibitory effect on SAA. When ANAMMOX process was used to treat wastewater containing OM, the modified Logistic model could be employed to predict the NRE_max.     

The effect and biological mechanism of granular sludge size on performance of autotrophic nitrogen removal system

The autotrophic process for nitrogen removal has attracted worldwide attention in the field of wastewater treatment, and the performance of this process is greatly influenced by the size of granular sludge particles present in the system. In this work, the granular sludge was divided into three groups, i.e. large size (>?1.2 mm), medium size (0.6–1.2 mm) and small size (<?0.6 mm). The medium granular sludge was observed to dominate at high volumetric nitrogen loading rates, while offering strong support for good performance. Its indispensable contribution was found to originate from improved settling velocity (0.84?±?0.10 cm/s), high SOUR-A (specific oxygen uptake rate for ammonia oxidizing bacteria, 25.93 mg O₂/g MLVSS/h), low SOUR-N (specific oxygen uptake rate for nitrite oxidizing bacteria, 3.39 mg O₂/g MLVSS/h), and a reasonable microbial spatial distribution.     

Performance and Microbial Structure of a Combined Biofilm Reactor

A novel combined biofilm reactor was established and applied as a single treatment unit for carbon and nitrogen removal of wastewater. The nitrogen removal performance of the reactor at different levels of organic carbon (COD) loading was investigated when the influent total nitrogen (TN) loading was 0.74 g TN/m² day. Continuous experimental results demonstrated that 80% nitrogen was eliminated when the influent COD loading ranged between 2.06 g and 3.92 g COD/m² day. Microbial composition in the reactor was analyzed using fluorescent in situ hybridization (FISH) and conventional batch tests. The relative abundance of ammonia-oxidizing bacteria in the aerobic zone of the reactor measured by FISH was consistent with the result from conventional batch tests.