Volatile Suspended Solids Concentration Research Articles

Optimization of Biological Elimination of Ammonia and Chemical Oxygen Demand from Wastewater Using Response Surface Methodology

Using a statistical technique, i.e. response surface methodology (RSM), optimization studies were carried out for different parameters like nitrogen ammonia (N‐NH3), chemical oxygen demand (COD) and time, to optimize overall method for maximum elimination of N‐NH3 and COD from synthetic waste water using heterotrophic biomass conversion (HBC) process, and simultaneously minimizing the N‐N2O emission. Ammonium hydroxide and glucose were used as nitrogen and carbon source. Central composite design technique was used to optimize the parameters with RSM. In this experiment, N‐NH3 and organic nutrient concentrations were varied keeping the biomass (total volatile suspended solids) concentration invariable. The pH during HBC process showed a decreasing trend may be due to consumption of alkalinity. Analyses of variance were carried out for quadratic model and were observed to be highly significant. The model was verified through experiments and tested statistically. Under optimized conditions (N‐NH3: 210 mg/L, COD: 2.90 g/L, time: 38 h) N‐NH3 and COD removal efficiency were observed to be 82.72 and 84.78%, respectively. N‐N2O emissions were minimized to 2.96 µg. The total desirability factor obtained was 0.9.

Exploring the links between population dynamics of total and active bacteria and the variables influencing a full-scale membrane bioreactor (MBR).

Long-term dynamics of total and active bacterial populations in a full-scale membrane bioreactor (MBR) treating urban wastewater were monitored during nine months by temperature-gradient gel electrophoresis (TGGE) of partial 16S-rRNA genes, amplified from community DNA and RNA templates. The bacterial community, dominated by Alphaproteobacteria, displayed the required characteristics for a successful and steady contaminant removal under real operating conditions. The evolution of population dynamics showed that a fully-stable microbial community was not developed even after technical stabilization and steady performance of the MBR were achieved. Non-metric multidimensional scaling and BIO-ENV demonstrated that the trends of the populations were often mostly explained by temperature, followed by the concentration of volatile suspended solids and C/N ratio of the influent. These variables were mainly responsible for triggering the shifts between functionally redundant populations. These conclusions contribute to the prediction of the complex profiles of adaptation and response of bacterial populations under changing conditions.

Optimization of thermophilic anaerobic-aerobic treatment system for Palm Oil Mill Effluent (POME)

Optimization of an integrated anaerobic-aerobic bioreactor (IAAB) treatment system for the reduction of organic matter (Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) concentrations) in Palm Oil Mill Effluent (POME) to legal standards with high methane yield was performed for the first time under thermophilic condition (50°C–55°C) by using response surface methodology (RSM). The experiments were conducted based on a central composite rotatable design (CCRD) with three independent operating variables, organic loading rates in anaerobic compartment (OLRan) and mixed liquor volatile suspended solids (MLVSS) concentration in anaerobic (MLVSSan) and aerobic compartments (MLVSSa). The optimum conditions for the POME treatment were determined as OLRan of 15.6 g COD·L−1·d−1, MLVSSan of 43100 mg·L−1, and MLVSSa of 18600 mg·L−1, where high aerobic COD, BOD and TSS removal efficiencies of 96.3%, 97.9%, and 98.5% were achieved with treated BOD of 56 mg·L−1 and TSS of 28 mg·L−1 meeting the discharge standard. This optimization study successfully achieved a reduction of 42% in the BOD concentrations of the final treated effluent at a 48% higher OLRan as compared to the previous works. Besides, thermophilic IAAB system scores better feasibility and higher effectiveness as compared to the optimized mesophilic system. This is due to its higher ability to handle high OLR with higher overall treatment efficiencies (more than 99.6%), methane yield (0.31 L CH4·g−1 CODremoved) and purity of methane (67.5%). Hence, these advantages ascertain the applicability of thermophilic IAAB in the POME treatment or even in other high-strength wastewaters treatment.

Performance of activated sludge diffusion for biological treatment of hydrogen sulphide gas emissions

Odours from wastewater treatment plants are comprised of a mixture of various gases with hydrogen sulphide (H(2)S) often being the dominant constituent. Activated sludge diffusion (ASD) as a biotreatment system for odour abatement has been conducted for over 30 years but has limited broad application due to disagreement in the literature regarding the effect that ASD may have on wastewater treatment performance. The effects of continuous H(2)S diffusion at 25 ppmv, with weekly peaks of approximately 100 ppmv, on H(2)S removal efficiency and wastewater treatment performance was evaluated over a 2-month period using an activated sludge pilot plant. H(2)S removal averaged 100% during diffusion at 25 ppmv, and 98.9% during the 100 ppmv peak periods. A significant increase in mixed liquor volatile suspended solids concentration (P < 0.01) was observed during H(2)S diffusion, which may be due to an increase in H(2)S-degrading microorganisms. There was no adverse effect of H(2)S on nitrification throughout the ASD trials. Ammonia (NH(3)) removal was slightly better in the test receiving H(2)S diffusion (87.6%) than in the control (85.4%). H(2)S diffusion appeared to improve robustness of the AS biomass to operational upsets.

Enzymatic activities in a moving bed membrane bioreactor for real urban wastewater treatment: Effect of operational conditions

The microbial hydrolytic enzymatic activities (acid phosphatase, alkaline phosphatase and glucosidase) in a moving bed membrane bioreactor (MBMBR system) for real urban wastewater treatment were investigated. This study was conducted in both suspended biomass and attached biofilm present in a MBMBR system. For this, four experimental phases with 20% and 35% (v/v) carrier filling ratios (CFRs) were conducted, which combined two mixed liquor total suspended solid (MLTSS) concentrations (c.a. 2500 and 4500mg/L) and two hydraulic retention times (HRTs) (10 and 24h). Hydrolytic activities of acid phosphatase, alkaline phosphatase and α-glucosidase in the mixed liquor (suspended biomass) were higher than in the biofilm (independent of the carrier concentration), during the four experiments, possibly due to better substrate diffusion in the mixed liquor. A redundancy analysis (RDA) was performed to evaluate the relationship between enzymatic activities and chemical oxygen demand (COD), biological oxygen demand at 5 days (BOD5), mixed liquor volatile suspended solid concentration (MLVSS), biofilm total solids (BTS), HRT, pH, CFR and temperature. According to the results obtained with the Monte Carlo permutation test, CFR, MLVSS, BTS, COD, temperature and HRT significantly contributed to the variation of enzymatic activities in the MBMBR.

Effect of the Addition of Zero Valent Iron (Fe0) on the Batch Biological Sulphate Reduction Using Grass Cellulose as Carbon Source

Mineral mining generates acidic, saline, metal-rich mine waters, often referred to as acid mine drainage (AMD). Treatment of AMD and recovering saleable products during the treatment process are a necessity since water is, especially in South Africa, a scarce commodity. The aim of the study presented here was to investigate the effect of zero valent iron (Fe(0)) on the biological removal of sulphate from AMD in batch reactors. The performance of the reactors was assessed by means of sulphate reduction, chemical oxygen demand (COD), volatile fatty acid (VFA) utilisation and volatile suspended solids (VSS) concentration. To this end, three batch reactors, A, B and C (volume 2.5 L), were operated similarly with the exception of the addition of grass cuttings and iron filings. Reactors A and B received twice as much grass (100 g) as C (50 g). Reactor A received no iron filings to act as a control, while reactors B and C received 50-g iron filings for the experimental duration. The results showed that Fe(0) appears to provide sustained sulphate removal when sufficient grass substrate is available. In reactors A and C, sulphate removal efficiency was higher when the COD concentration was lower due to utilisation. In reactor B, sulphate removal efficiency was accompanied by an accumulation of COD as hydrogen (H2) provided by the Fe(0) was utilised for sulphate reduction. Furthermore, these results showed the potential of Fe(0) to enhance the participation of microorganisms in sulphate reduction.

Performance of 14 full-scale sewage treatment plants: Comparison between four aerobic technologies regarding effluent quality, sludge production and energy consumption

The performance of 14 Full-Scale Sewage Treatment Plants (STPs) was evaluated. STPs were divided into four aerobic technologies: a) Aerated Lagoon (AL), and three configurations of activated sludge technologies, b) conventional (CAS), c) Extended Aeration (EA), d) Sequencing Batch Reactor (SBR). Comparison between these configurations were made regarding: a) control parameters, organic loading rate (OLR), Mixed Liquor Volatile Suspended Solids (MLVSS) concentrations, Food to Microorganism ratio (F/M), sludge age (θc), Hydraulic Retention Time (HRT) and return sludge ratio (R); b) effluent quality, through 5-day Biological Oxygen Demand (BOD5), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Total Kjeldahl Nitrogen (TKN), Total Phosphorus (TP); and c) indicators related to sludge production (on a dry basis) and electrical energy consumption. Also, complementary costs analyses were made. The results show that in terms of effluent quality, for all configurations organic matter (BOD5 and COD) and TKN removal efficiency were up to 90%, while TSS and TP were up to 90% and 50%, respectively. However, CAS, EA, SBR, and AL had stability problems with effluent concentrations. The results of the electrical energy consumption and sludge production analyses show that SBRs reduce these indicators by 40%. Cost analysis showed that CAS, EA, SBR and AL had similar cost structures, with more than 50% of total operating and maintenance cost being related to electrical energy and sludge management. Therefore, SBR could be defined as the configuration with a more stable performance.

Comparative analysis of the enzyme activities and the bacterial community structure based on the aeration source supplied to an MBR to treat urban wastewater

A comparative analysis was performed in a pilot-scale membrane bioreactor (MBR) treating urban wastewater supplied with either pure oxygen (O2) or air, to assess the influence of each aeration source on the diversity and activity of the bacterial communities in the sludge. The MBR was operated in three experimental stages with different concentrations of volatile suspended solids (VSS) and temperature, and under both aeration conditions. α-Glucosidases, proteases, esterases and phosphatases were tested as markers of organic matter removal in the sludge, and the diversity of the bacterial community was analysed by fingerprinting (temperature-gradient gel electrophoresis of partially-amplified 16S-rRNA genes). Redundancy analysis (RDA) revealed that temperature and VSS concentration were the only factors that significantly influenced the levels of enzyme activities and the values of both the Shannon–Wiener diversity index (H′) and the functional organisation index (Fo), while the bacterial community structure experienced significant changes depending on the aeration source supplied in each experimental stage.

Ozonation in seawater recirculating systems for black seabream Acanthopagrus schlegelii (Bleeker): Effects on solids, bacteria, water clarity, and color

A 44-day production trial was done in three identically configured pilot-scale seawater recirculating aquaculture systems (RAS) to evaluate the effects of two different ozonation levels on solids, bacteria, water clarity, and color variables. The control system (CS) was not ozonated, while the treatment systems received ozone at either 20gO3/kgfeedday−1 (T 20) or 40gO3/kgfeedday−1(T 40).Solids concentrations in the two treatments receiving ozone were lower and less variable than the concentrations in the CS. Ozonation reduced suspended solids (SS) and volatile suspended solid (VSS) concentrations by 42.0–52.5% and dissolved organic carbon (DOC) by 29.3–30.6%. The ratios of DOC to SS and to VSS decreased significantly (P<0.05). In addition, the amount of solid removed from the radial flow settler increased 27.1% in both ozone-treatment systems. Bacteria counts on different culture media (Marine, TCBS and MacConkey agars) varied considerably during the study period, but were generally lower in the treatments receiving ozone than the CS. Differences in bacteria abundance between two treatments, T 20 and T 40, were not significant (P>0.05). Based on 24-h sampling, bacteria counts in CS gradually increased after feeding during the daytime and declined during the night. Counts of Vibrio spp. in T 20 fluctuated daily in a similar pattern to the Vibrio spp. in the CS, while overall bacteria counts in T 40 remained low throughout the 24-h sampling period.Turbidity fluctuated greatly in all treatments; there were no significant differences between any of the treatments (P>0.05). Apparent water color (AC) and true water color (TC) in the treatment were reduced 70% compared to the CS, but there was no significant difference (P>0.05) between the two ozone treatments.Based on the variables measured, the higher dose of ozone (40gO3/kgfeedday−1) did not improve solid concentrations, bacterial counts or water colors beyond the values found in the T 20 treatment. Thus, the use of ozone at 20gO3/kgfeedday−1 in a seawater RAS appears to be sufficient to improve water quality of these systems without adversely affecting black sea bream (Acanthopagrus schlegelii) growth.

Techno-economic evaluation of the application of ozone-oxidation in a full-scale aerobic digestion plant

This paper deals with the application of the ozone-oxidation in a full scale aerobic sludge digester. Ozonation was applied continuously to a fraction of the biological sludge extracted from the digestion unit; the ozonated sludge was then recirculated to the same digester. Three different ozone flow rates were tested (60,500 and 670g O3h−1) and their effects evaluated in terms of variation of the total and soluble fractions of COD, nitrogen and phosphorous, of total and volatile suspended solids concentrations and Sludge Volume Index in the aerobic digestion unit. During the 7-month operation of the ozonation process, it was observed an appreciable improvement of the aerobic digestion efficiency (up to about 20% under the optimal conditions) and of the sludge settleability properties. These results determined an average reduction of about 60% in the biological sludge extracted from the plant and delivered to final disposal. A thorough economic analysis showed that this reduction allowed to achieve a significant cost saving for the plant with respect to the previous years operated without ozonation. Furthermore, it was determined the threshold disposal cost above which implementation of the ozone oxidation in the aerobic digestion units of similar WWTPs becomes economically convenient (about 60€t−1 of sludge).

Enhanced aerobic granulation and nitrogen removal by the addition of zeolite powder in a sequencing batch reactor

The aim of this study was to evaluate the impact of zeolite powders on feasibility of rapid aerobic granulation in the column-type sequencing batch reactors. After 90 days' operation, aerobic granular sludge was formed in both reactors by altering influent chemical oxygen demand/nitrogen (COD/N) ratios. R1 with zeolite powders had better removal capabilities of COD and total nitrogen than R2, which was without zeolite powders. Mixed liquor volatile suspended solid concentrations of the two reactors were 7.36 and 5.45 g/L, while sludge volume index (SVI30) values were 34.9 and 47.9 mg/L, respectively. The mean diameters of aerobic granular sludge in the above two reactors were 2.5 and 1.5 mm, respectively. Both reactors achieved the largest simultaneous nitrification and denitrification (SND) efficiency at an influent COD/N ratio of 8; however, R1 exhibited more excellent SND efficiency than R2. The obtained results could provide a novel technique for rapid aerobic granulation and N removal simultaneously, especially when treating nitrogen-rich industrial wastewater.

Fate of return activated sludge after ozonation: an optimization study for sludge disintegration

The effects of ozonation on sludge disintegration should be investigated before the application of ozone during biological treatment, in order to minimize excess sludge production. In this study, changes in sludge and supernatant after ozonation of return activated sludge were investigated for seven different ozone doses. The optimum ozone dose to avoid inhibition of ozonation and high ozone cost was determined in terms of disintegration degree as 0.05 g O3/gTS. Suspended solid and volatile suspended solid concentrations of sludge decreased by 77.8% and 71.6%, respectively, at the optimum ozone dose. Ozonation significantly decomposed sludge flocs. The release of cell contents was proved by the increase of supernatant total nitrogen (TN) and phosphorus (TP). While TN increased from 7 mg/L to 151 mg/L, TP increased from 8.8 to 33 mg/L at the optimum ozone dose. The dewaterability and filterability characteristics of the ozonated sludge were also examined. Capillary suction time increased with increasing ozone dosage, but specific resistance to filtration increased to a specific value and then decreased dramatically. The particle size distribution changed significantly as a result of floc disruption at an optimum dose of 0.05 g O3/gTS.

Influences of volatile solid concentration, temperature and solid retention time for the hydrolysis of waste activated sludge to recover volatile fatty acids

The study evaluated influences of sludge concentration, temperature and solids retention time (SRT) for the hydrolysis of waste activated sludge (WAS) in anaerobic digesters. The results indicated that volatile fatty acids (VFA) production increased when the concentration of mixed liquor volatile suspended solids (MLVSS) was higher. When SRT was 48h, VFA concentration increased 8.43times from 4.57 to 23.78gMLVSS/L. VFA generation was significantly affected with different temperature and SRT. When the temperature changed from 40 to 50°C, it induced 1.65-fold increase in VFA yield. The optimal SRT was 48h. As VFA concentration decreased only 1.31times compared with maximum VFA production at SRT of 120h. Iso-valeric acid, acetic acid and n-butyric acid were the dominant VFA produced and would improve ployhydroxyvalerate proportion in polymer. The feasibility of nitrogen and phosphorus recovery and the risk of metal ion released depended on the nature of WAS.

Effect of effluent recycle ratio in a continuous anaerobic biohydrogen production system

Abstract The effect of effluent recycle on hydrogen production in an anaerobic continuous stirred tank reactor (CSTR) was investigated. The CSTR was fed on sucrose (20 g chemical oxygen demand (COD)/L) and at hydraulic retention time (HRT) of 12 h. The pH and temperature were regulated around 6.7 and 35 °C, respectively. The effluent was then recycled at recycle ratios of 0, 0.2, 0.4, 0.6, 0.8 and 1.0. When the recycle ratios increased, the volatile suspended solid (VSS) concentration also increased with a peak of 14.2% at the recycle ratio 0.4 compared with the VSS 3.52 g/L without effluent recycle. However, a drastic drop in hydrogen production performance was observed with increased recycle ratio. The strategy of increasing organic loading rate by increasing the substrate concentration and shorting the HRT was then applied keeping a recycle ratio of 0.2. VSS concentration increased with increasing substrate concentration but as the HRT was shortened from 12 h to 2 h stepwise, VSS concentration increased first and then decreased at HRT 8 h. The maximal hydrogen production was obtained at sucrose concentration of 20 g COD/L and HRT 6 h (organic loading rate (OLR) 66.7 g COD/L-d) with recycle ratio 0.2. The hydrogen yield, hydrogen production rate and specific hydrogen production rate values also peaked at the same conditions (sucrose 20 g COD/L and HRT 6 h) with 3.88 mol H 2 /mol sucrose, 807 mmol H 2 /L-d and 244.3 mmol H 2 /g VSS-d, respectively. This study showed peak hydrogen production performance at OLR 66.7 g COD/L-d in the recycled continuous anaerobic biohydrogen production system by controlling HRT. Thus, effluent recycling at the optimum ratio along with optimum HRT and substrate concentration can maximize the hydrogen production performance in an anaerobic biohydrogen production system.

Biohydrogen production performance in a draft tube bioreactor with immobilized cell

The research was carried out using a draft tube fluidized bed bioreactor (DTFBR) system with immobilized cell. The total working volume of the DTFBR including the buffer tank and pipe was 2.4 L. The pH, volatile suspended solid (VSS) and total solid (TS) concentrations of the seed sludge were 6.8, 33.3 and 65.1 g/L, respectively. The thermally treated sludge was used for immobilization. Immobilization of cell was essentially achieved by silicone gel (SC) entrapment approaches. The entrapped-cell system operated stably at a low HRT without any cell washout. Hence, the H2 production rate was enhanced via increasing organic loading rates. The system was fed with sucrose-based synthetic medium, and was examined for its H2 production performance under different influent sucrose concentrations, different solid volume volumetric content and hydraulic retention times.It was found that the optimal solid volume volumetric content was 7.5% (v/v) when the solid volume volumetric contents ranged from 5 to 15% in the DTFBR. The maximum H2 production rate of 2.59 H2 L/h/L with H2 volumetric content of 36.8% and sucrose conversion of 80.5% was obtained at initial substrate concentration of 20 g COD/L, 7.5% (v/v) of solid volume, and HRT 0.5 h. The features of well mixing, high H2-producing performance and stable operation imply that using DTFBR system is a feasible approach to continuous H2 production in reality.

Process kinetics of an activated-sludge reactor system treating poultry slaughterhouse wastewater

The principal objective was to generate the essential kinetic parameters for model simulation and operation management of an activated-sludge reactor (ASR) system treating poultry slaughterhouse wastewater. By varying four different mean cell residence times (θc=4.6–24.3 d), the ASR system (26°C) removed effectively 93.5%–97.2% of chemical oxygen demand (COD) from wastewater. If a high COD removal efficiency and a low effluent volatile suspended solids (VSS) concentration are of great concern, a θc of 15–24 d or a food to microorganism (F/M) ratio of 0.3–0.7 kg COD/kg VSS-d is suggested; if resource sustainability and enhanced operation of the ASR system are of great concern, a θc of 9 d or an F/M ratio of 0.9 kg COD/kg VSS-d is suggested. The COD residual concentrations and COD removal efficiencies calculated by using the Monod model agreed well with the experimental results. When the parameters k and K s (Δ P/P) were respectively varied from−100% to+100%, the parametric sensitivity analysis showed that the COD residual concentration change (Δ S/S) was highly sensitive to k in the Δ P/P range between 0% and−40%, causing a marked increase in COD residual concentration.

Application of the International Water Association activated sludge models to describe aerobic sludge digestion

Batch and semi-continuous flow aerobic digesters were used to stabilize thickened waste-activated sludge at different initial conditions and mean solids retention times. Under dynamic conditions, total suspended solids, volatile suspended solids (VSS) and total and particulate chemical oxygen demand (COD and PCOD) were monitored in the batch reactors and effluent from the semi-continuous flow reactors. Activated Sludge Model (ASM) no. 1 and ASM no. 3 were applied to measured data (calibration data set) to evaluate the consistency and performances of models at different flow regimes for digester COD and VSS modelling. The results indicated that both ASM1 and ASM3 predicted digester COD, VSS and PCOD concentrations well (R 2, ). Parameter estimation concluded that compared to ASM1, ASM3 parameters were more consistent across different batch and semi-continuous flow runs with different operating conditions. Model validation on a data set independent from the calibration data successfully predicted digester COD (R 2=0.88) and VSS (R 2=0.94) concentrations by ASM3, while ASM1 overestimated both reactor COD (R 2=0.74) and VSS concentrations (R 2=0.79) after 15 days of aerobic batch digestion.

Partial nitrification of sludge reject water by means of aerobic granulation

Granular sludge formation was performed in a laboratory scale Sequencing Batch Reactor (SBR) fed with supernatant of anaerobic digestion of sewage sludge. This effluent was concentrated progressively in order to enhance biomass capacity without inhibiting it. During the first part of the study, ammonium nitrogen was converted to nitrate, so conventional nitrification took place. When a nitrogen load of 0.8 g N L(-1) d(-1) was treated, the effluent concentration of nitrite started to increase while the nitrate concentration decreased until it disappeared. So, partial nitrification was achieved. At the end of this study, a nitrogen load of 1.1 g N L(-1) d(-1) was treated obtaining an effluent with 50% ammonium and 50% nitrite. The volatile suspended solids concentration in the reactor reached 10 g VSS L(-1). At this point the granule morphology was quite round and no filamentous bacteria was observed. The Feret's diameter was in the range between 1 and 6 mm with an average value of 4.5 mm. Roundness value was all the time higher than 0.7. Granule density increased during the experimental period, obtaining a final value of 7.0 g L(-1).

Floc Volume Effects in Suspensions and Its Relevance for Wastewater Engineering

The aim of this paper is to better understand oxygen transfer reduction caused by floc suspensions. We demonstrate that the overall floc volume significantly influences oxygen transfer depletion. Submerged fine bubble and coarse bubble diffusers are affected in the same way by this phenomenon. The mixed liquor suspended solids concentration (MLSS concentration) is not an appropriate parameter for describing or relating phenomena that are caused by the overall floc volume in activated sludge (e.g., oxygen transfer depression and sludge sedimentation characteristics). A better correlation is achieved by using the mixed liquor volatile suspended solids concentration (MLVSS concentration). To characterize the effects of the overall floc volume in suspensions whose MLVSS concentration cannot be determined (e.g., inorganic iron hydroxide flocs), a new method-the hydrostatic floc volume (HFV)-that approximates the overall floc volume in floc suspensions is introduced. Application of this method demonstrates that oxygen transfer depression caused by iron hydroxide flocs and activated sludge flocs is similar.

Investigation of nitrogen converters in membrane bioreactor

In this study, the activity and diversity of nitrogen converters, ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), ammonia-oxidizing archaea (AOA) and Anammox bacteria in a pilot-scale membrane bioreactor (MBR) were investigated and monitored using amoA and 16S rDNA-based molecular tools. The pilot-scale MBR (100 m3/day) was located inside the full-scale Pasakoy Advanced Wastewater Treatment Plant (WWTP), and operated for approximately 5 months without sludge purge. During 148 days of operation, volatile suspended solids (VSS) concentration increased from 2,454 mg/L to 10,855 mg/L and the average organic carbon and ammonia nitrogen removal rates were 92% and 99%, respectively. Real-time PCR results indicated that the fraction of AOB increased from 2.94% to 4.05% when VSS concentration reached to 3,750 mg/L throughout 148 days of operation. At higher VSS concentrations, the fraction of AOB declined gradually to 1.15% while the fraction of Nitrospira population was varied between 8.23 and 13.01%. However, significant change or any positive and negative correlations between VSS concentration and Nitrospira population were not observed in this period. The phylogenetic analysis revealed that MBR harbored diverse AOB community which was related to the Nitrosomonas and Nitrosospira lineage. Candidatus Nitrospira defluvii was the only detected NOB in this study.