Complete Biomass Retention Research Articles

Anaerobic Membrane Bioreactors (AnMBRs) for Wastewater Treatment: Recovery of Nutrients and Energy, and Management of Fouling

Anaerobic membrane bioreactor (AnMBR) technology is emerging as an alternative to conventional anaerobic treatment due to its complete biomass retention, short start-up time, high effluent quality, and small footprint. This paper provides a general overview of the application of AnMBRs for industrial and municipal wastewater treatment. The potential benefits of AnMBRs are discussed, such as the degradation of organic matter for energy production, the concentration of nutrients for subsequent reclamation, or the effective removal of organic contaminants for water reuse. To explore the technology for energy-neutral wastewater treatment, the recovery of methane, hydrogen, and ethanol is summarized, highlighting the problems of dissolution of methane in permeate and competition between sulfate-reducing bacteria and methanogens for organic matter. Recovery of water and nutrients for reuse, e.g., for algae production, is reported. Since membrane fouling remains a challenge in membrane operation and leads to increased operation and maintenance costs, methods to reduce fouling are highlighted. Future research prospects related to the application of AnMBR in resource recovery plants and fouling management are emphasized.

MBR공정의 운전조건의 변화에 따른 여과성능 개선연구

Membrane bioreactor (MBR) combining an activated sludge process and membrane filtration could achieve high nutrient removal efficiency and complete biomass retention without a secondary clarifier. Membrane fouling remains the most challenging issue in MBR operation and attracts considerable attention in MBR studies. The objective of this study is to investigate the influence of permeate flux, aeration mode and strength, and mixed liquor suspended solid (MLSS) on membrane filtration of small-scale MBR (the capacity 100㎥/d), which consisted of anaerobic, anoxic, and aeration tank coupled with membrane. Experiments were conducted with real MBR using real wastewater (BOD <250 mg/L, COD <220 mg/L, T-N <65 mg/L, T-P <8 mg/L, E-coli <100,000 no/mL). As a result, the increase of both permeate flux and MLSS increased TMP of MBR while the increase of aeration strength decreased the TMP. Compared to the existing operation conditions of MBR, the operation conditions of shorter filtration time (8 min on/ 2 min off) for a cycle and lower MLSS (<3000 mg/L) in MBR resulted in longer filtration duration time (about two times) as well as the reduction of cleaning chemical.

Maintenance power requirements of anammox bacteria "Candidatus Brocadia sinica" and "Candidatus Scalindua sp."

Little is known about the cell physiology of anammox bacteria growing at extremely low growth rates. Here, "Candidatus Brocadia sinica" and "Candidatus Scalindua sp." were grown in continuous anaerobic membrane bioreactors (MBRs) with complete biomass retention to determine maintenance energy (i.e., power) requirements at near-zero growth rates. After prolonged retentostat cultivations, the specific growth rates (μ) of "Ca. B. sinica" and "Ca. Scalindua sp." decreased to 0.000023 h-1 (doubling time of 1255 days) and 0.000157 h-1 (184 days), respectively. Under these near-zero growth conditions, substrate was continuously utilized to meet maintenance energy demands (me) of 6.7 ± 0.7 and 4.3 ± 0.7 kJ mole of biomass-C-1 h-1 for "Ca. B. sinica" and "Ca. Scalindua sp.", which accorded with the theoretically predicted values of all anaerobic microorganisms (9.7 and 4.4 kJ mole of biomass-C-1 h-1at 37 °C and 28 °C, respectively). These me values correspond to 13.4 × 10-15 and 8.6 × 10-15 watts cell-1 for "Ca. B. sinica" and "Ca. Scalindua sp.", which were five orders of magnitude higher than the basal power limit for natural settings (1.9 × 10-19 watts cells-1). Furthermore, the minimum substrate concentrations required for growth (Smin) were calculated to be 3.69 ± 0.21 and 0.09 ± 0.05 μM NO2- for "Ca. B. sinica" and "Ca. Scalindua sp.", respectively. These results match the evidence that "Ca. Scalindua sp." with lower maintenance power requirement and Smin are better adapted to energy-limited natural environments than "Ca. B. sinica", suggesting the importance of these parameters on ecological niche differentiation in natural environments.

Investigation of Formation and Development of Anammox Biofilms by Light, Epifluorescence, and Electron Microscopy

The stages of formation and development of activated sludge biofilm carrying out the anammox process in a laboratory sequencing batch reactor (SBR) with complete biomass retention on the carrier were investigated using light, epifluorescence, and electron microscopy. Light microscopy revealed biofilm formation on the carrier to occur within one week. Rod-shaped and filamentous microorganisms were the first to attach on the carrier and acted as a skeleton for biofilm formation, thus playing an important role in colonization of the carrier. Epifluorescence microscopy revealed physiologically active anammox bacteria of the genera Candidatus “Brocadia” and Ca. “Jettenia” in the biofilms. Selective autofluorescence of the colonies of anammox bacteria Ca. “Jettenia” was observed. This autofluorescence was probably caused by specific proteins in the extracellular polymeric substances (EPS), their synthesis and/or amount depending on the colony age and the physiological state of the cells. Structural organization of the colonies of anammox bacteria was investigated by electron microscopy. High-throughput sequencing of the 16S rRNA gene fragments revealed the presence of sequences affiliated with members of the phyla Chloroflexi, Bacteroidetes, Planctomycetes, and Proteobacteria in the biofilms. Apart from anammox bacteria, anaerobic and facultatively anaerobic organotrophs, stage I nitrifiers, denitrifiers, and sulfate reducers were detected. Members of the genus Ca. “Brocadia” were predominant among the anammox bacteria, probably due to better adhesion of their cells to the carrier or to their competitive advantage over Ca. “Jettenia” in the presence of organic acids (acetate and formate) in the medium.

High performance nitrogen removal through integrating denitrifying anaerobic methane oxidation and Anammox: from enrichment to application

Integrating denitrifying anaerobic methane oxidation (DAMO) with Anammox provides alternative solutions to simultaneously remove nitrogen and mitigate methane emission from wastewater treatment. However, the practical application of DAMO has been greatly limited by slow-growing DAMO microorganisms living on low-solubility gaseous methane. In this work, DAMO and Anammox co-cultures were fast enriched using high concentration of mixed sludges from various environments, and achieved nitrogen removal rate of 76.7 mg NH4+-N L−1 d−1 and 87.9 mg NO3−-N L−1 d−1 on Day 178. Subsequently, nitrogen removal rate significantly decreased but recovered quickly through increasing methane flushing frequency, indicating methane availability could be the limiting factor of DAMO activity. Thus, this work developed a novel Membrane Aerated Membrane Bioreactor (MAMBR), which equipped with gas permeable membrane for efficient methane delivery and ultrafiltration membrane for complete biomass retention. After inoculated with enriched sludge, nitrogen removal rates of MAMBR were significantly enhanced to 126.9 mg NH4+-N L−1 d−1 and 158.8 mg NO3−-N L−1 d−1 by membrane aeration in batch test. Finally, the MAMBR was continuously fed with synthetic wastewater containing ammonium and nitrite to mimic the effluent from partial nitritation. When steady state with nitrogen loading rate of 2500 mg N L−1 d−1 was reached, the MAMBR achieved total nitrogen removal of 2496.7 mg N L−1 d−1, with negligible nitrate in effluent (~6.5 mg NO3−-N L−1). 16S rRNA amplicon sequencing and fluorescence in situ hybridization revealed the microbial community dynamics during enrichment and application. The high performance of nitrogen removal (2.5 kg N m−3 d−1) within 200 days operation and excellent biomass retention capacity (8.67 kg VSS m−3) makes the MAMBR promising for practical application of DAMO and Anammox in wastewater treatment.

A novel anaerobic filter membrane bioreactor: prototype start-up and filtration assays.

Anaerobic digestion allows efficient treatment of high loaded wastewater, and membrane technology allows obtaining high quality effluents with complete biomass retention. However, high biomass concentration interferes with membrane fouling. In the present work, a new bioreactor that integrates an attached biomass anaerobic culture on a fixed bed and a submerged membrane has been started up. The recirculation between the digestion and filtration chambers is coupled to the gas-lift effect of the bubbling employed for the scouring of the membranes, avoiding the use or electromechanical pumps that damage the suspended biomass. The support material retains the biomass in the digestion tank despite the downwards flow, avoiding the submerged membrane contacting with a high concentrated suspension. This novel system, called an anaerobic filter membrane bioreactor was immediately started up, achieving chemical oxygen demand (COD) removal efficiencies of 96% at an organic loading rate (OLR) of 7 kg COD/m3·d. In order to select filtration flux, specific gas demand and filtration cycle duration, the results of 15 short term assays, eight hours for each one, is presented for fluxes between 15.7 and 17.7 L/m2·h, cycle duration between 10 and 30 minutes, and three levels of scouring. It was checked that reversible and irreversible fouling were directly related when dTMP/dt > 2.5 mbar/min.

Rapid establishment of phenol- and quinoline-degrading consortia driven by the scoured cake layer in an anaerobic baffled ceramic membrane bioreactor.

Although toxic and refractory organics, such as phenol and quinoline, are decomposed by anaerobic bacteria, the establishment of specific degrading consortia is a relatively slow process. An anaerobic membrane bioreactor allows for complete biomass retention that can aid the establishment of phenol- and quinoline-degrading consortia. In this study, the anaerobic digestion of phenol (500mgL-1) and quinoline (50mgL-1) was investigated using an anaerobic baffled ceramic membrane bioreactor (ABCMBR). The results showed that, within 30days, 99% of phenol, 98% of quinoline and 88% of chemical oxygen demand (COD) were removed. The substrate utilisation rates of the cake layer for phenol and quinoline, and specific methanogenic activity of the cake layer, were 7.58mgphenolg-1mixed liquor volatile suspended solids (MLVSS)day-1, 8.23mgquinolineg-1MLVSSday-1 and 0.55gCODCH4g-1MLVSSday-1, respectively. The contribution of the cake layer to the removals of phenol and quinoline was extremely underestimated because the uncounted scoured cake layer was disregarded. Syntrophus was the key population for phenol and quinoline degradation, and it was more abundant in the cake layer than in the bulk sludge. The highly active scattered cake layer sped up the establishment of phenol- and quinoline-degrading consortia in the ABCMBR.

Autotrophic ammonia removal from landfill leachate using anaerobic membrane bioreactor

Anaerobic Membrane Bioreactor (AnMBR) is an innovative high cell density system having complete biomass retention, high reactor loading and low sludge production and suitable for developing slow growing autotrophic bacterial cultures such as ANAMMOX. The Anaerobic Ammonium Oxidation (ANAMMOX) process is an advanced biological nitrogen removal removes ammonia using nitrite as the electron acceptor without oxygen. The NH4+-N in the landfill leachate that is formed due to the release of nitrogen from municipal solid waste (MSW), when discharged untreated, into the surface water can result in eutrophication, aquatic toxicity and emissions of nitrous oxide (N2O) to atmosphere. Besides, NH4+-N accumulation in landfills poses long term pollution issue with significant interference during post closure thereby requiring its removal prior to ultimate disposal into inland surface waters. The main objective of this study was to investigate the feasibility and treatment efficiency of treating landfill leachate (to check) for removing NH4+-N by adopting ANAMMOX process in AnMBR. The AnMBR was optimized for Nitrogen Loading Rate (NLR) varying from 0.025 to 5 kg NH4+-N/ m3/ d with hydraulic retention time (HRT) ranging from 1 to 3 d. NH4+-N removal efficacy of 85.13 ± 9.67% with the mean nitrogen removal rate (NRR) of 5.54 ± 0.63 kg NH4+-N/ m3/ d was achieved with nitrogen loading rate (NLR) of 6.51 ± 0.20 kg NH4+- N/ m3/ d at 1.5 d HRT. The nitrogen transformation intermediates in the form of hydrazine (N2H4) and hydroxylamine (NH2OH) were 0.008 ± 0.005 mg/L and 0.006 ± 0.001 mg/L, respectively, indicating co-existence of aerobic ammonia oxidizers (AOB) and ANAMMOX. The free ammonia (NH3) and free nitrous acid (HNO2) concentrations were 26.61 ± 16.54 mg/L and (1.66 ± 0.95) x 10-5 mg/L, preventing NO2--N oxidation to NO3--N enabling sustained NH4+- N removal.

Cultivation of Aquincola tertiaricarbonis L108 on the fuel oxygenate intermediate tert-butyl alcohol induces aerobic anoxygenic photosynthesis at extremely low feeding rates

Aerobic anoxygenic photosynthesis (AAP) is found in an increasing number of proteobacterial strains thriving in ecosystems ranging from extremely oligotrophic to eutrophic. Here, we have investigated whether the fuel oxygenate-degrading betaproteobacterium Aquincola tertiaricarbonis L108 can use AAP to compensate kinetic limitations at low heterotrophic substrate fluxes. In a fermenter experiment with complete biomass retention and also during chemostat cultivation, strain L108 was challenged with extremely low substrate feeding rates of tert-butyl alcohol (TBA), an intermediate of methyl tert-butyl ether (MTBE). Interestingly, formation of photosynthetic pigments, identified as bacteriochlorophyll a and spirilloxanthin, was only induced in growing cells at TBA feeding rates less than or equal to maintenance requirements observed under energy excess conditions. Growth continued at rates between 0.001 and 0.002 h(-1) even when the TBA feed was decreased to values close to 30 % of this maintenance rate. Partial sequencing of genomic DNA of strain L108 revealed a bacteriochlorophyll synthesis gene cluster (bchFNBHL) and photosynthesis regulator genes (ppsR and ppaA) typically found in AAP and other photosynthetic proteobacteria. The usage of light as auxiliary energy source enabling evolution of efficient degradation pathways for kinetically limited heterotrophic substrates and for lowering the threshold substrate concentration Smin at which growth becomes zero is discussed.

Prevalence of Nitrosomonas cluster 7 populations in the ammonia-oxidizing community of a submerged membrane bioreactor treating urban wastewater under different operation conditions

A pilot-scale ultrafiltration membrane bioreactor (MBR) was used for the aerobic treatment of urban wastewater in four experimental stages influenced by seasonal temperature and different sets of operation conditions. The structure of the ammonia-oxidizing bacteria (AOB) community was profiled by temperature gradient gel electrophoresis (TGGE), based on the amplification and separation of partial ammonia-monoxygenase subunit A (amoA) genes. Canonical correspondence analysis revealed that temperature, hydraulic retention time and percentage of ammonia removal had a significant effect on the fingerprints of AOB communities. Phylogenetic analysis conducted on amoA/AmoA sequences of reamplified TGGE bands showed, however, that closely related ammonia-oxidizing populations inhabited the sludge of the MBR in all experimental stages. Nitrosomonas cluster 7 populations (N. europaea-N. eutropha cluster) prevailed under all conditions tested, even when the MBR was operated under complete biomass retention or at low temperatures, suggesting that the high ammonia concentrations in the system were determinant to select r-strategist AOB.

Application of anaerobic membrane bioreactors for the treatment of protein-containing wastewaters under saline conditions

BACKGROUND: Anaerobic treatment of saline wastewaters may be hindered by problems related with biomass retention, since at high salt concentrations formation of biofilms and granules may not proceed well. This research studied the use of anaerobic membrane bioreactors (AnMBR) as a way to promote complete biomass retention. A lab scale AnMBR fitted with a ceramic tubular membrane was operated for 2 years. RESULTS: Results showed that enhanced biomass retention produces conditions enabling anaerobic treatment of saline wastewaters. Despite the high resulting sludge retention time, no accumulation of a high proportion of dead cells was observed. Protein degradation and not methanogenesis was shown to be the rate limiting step for organic matter degradation, a fact that is relevant for protein-containing wastewaters such as those from seafood processing industries. Only low levels of flux could be applied, in the region of 5 L m−2 h−1 due to reversible cake formation promoted by single cell growth. CONCLUSION: Biomass retention provided by membrane filtration promotes conditions suitable for efficient treatment of saline wastewaters. However, operation may be restricted to low values of flux due to biomass development as single cells. © 2012 Society of Chemical Industry

Enrichment of denitrifying methanotrophic bacteria for application after direct low-temperature anaerobic sewage treatment

Despite many advantages of anaerobic sewage treatment over conventional activated sludge treatment, it has not yet been applied in temperate zones. This is especially because effluent from low-temperature anaerobic treatment contains nitrogen and dissolved methane. The presence of nitrogen and methane offers the opportunity to develop a reactor in which methane is used as electron donor for denitrification. Such a reactor could be used in a new concept for low-temperature anaerobic sewage treatment, consisting of a UASB-digester system, a reactor for denitrification coupled to anaerobic methane oxidation, and a nitritation reactor. In the present study denitrifying methanotrophic bacteria similar to ‘Candidatus Methylomirabilis oxyfera’ were enriched. Maximum volumetric nitrite consumption rates were 33.5mgNO2−-N/Ld (using synthetic medium) and 37.8mgNO2−-N/Ld (using medium containing effluent from a sewage treatment plant), which are similar to the maximum rate reported so far. Though the goal was to increase the rates, in both reactors, after reaching these maximum rates, volumetric nitrite consumption rates decreased in time. Results indicate biomass washout may have significantly decelerated enrichment. Therefore, to obtain higher volumetric consumption rates, further research should focus on systems with complete biomass retention.

Understanding the physiology of Lactobacillus plantarum at zero growth

Situations of extremely low substrate availability, resulting in slow growth, are common in natural environments. To mimic these conditions, Lactobacillus plantarum was grown in a carbon-limited retentostat with complete biomass retention. The physiology of extremely slow-growing L. plantarum--as studied by genome-scale modeling and transcriptomics--was fundamentally different from that of stationary-phase cells. Stress resistance mechanisms were not massively induced during transition to extremely slow growth. The energy-generating metabolism was remarkably stable and remained largely based on the conversion of glucose to lactate. The combination of metabolic and transcriptomic analyses revealed behaviors involved in interactions with the environment, more particularly with plants: production of plant hormones or precursors thereof, and preparedness for the utilization of plant-derived substrates. Accordingly, the production of compounds interfering with plant root development was demonstrated in slow-growing L. plantarum. Thus, conditions of slow growth and limited substrate availability seem to trigger a plant environment-like response, even in the absence of plant-derived material, suggesting that this might constitute an intrinsic behavior in L. plantarum.

Application of membrane enhanced biomass retention to the anaerobic treatment of acidified wastewaters under extreme saline conditions

Two sludge bed anaerobic reactors were operated for the treatment of a saline, completely acidified synthetic wastewater. One of the reactors was equipped with a microfiltration membrane, in order to achieve complete biomass retention. Reactors were operated at increasing organic loading rates and salinity. At a sodium concentration of 24 g/L propionate accumulation was severe. When sodium concentration was reduced to 16 g/L, propionate oxidation in the membrane based reactor recovered after 40 days. This was not the case for the sludge bed reactor, in which propionate oxidation rates remained very low. This shows that the enhanced biomass retention achieved by membrane filtration can have a positive effect in retaining active halotolerant bacteria. Fluxes in the range of 10–15 L/m 2 h were achieved during the membrane bioreactor operation, even though membrane was exposed to a solid concentration below 2 g/L. However, flux showed to be limited by cake formation and low levels of internal pore fouling were detected.

Performance and microbial surveying in submerged membrane bioreactor for seafood processing wastewater treatment

The main objective of this study is to evaluate the performance and the structure of the microbial community of a submerged membrane bioreactor (MBR) for seafood processing wastewater treatment and reuse. This system was operated continuously under favourable biological conditions, with complete biomass retention. In terms of organic removal and membrane performance, the MBR had a good efficiency to treat high strength with organic loading variation characteristic of real seafood processing wastewater. The overall results showed the importance of ecological microbial compositions in each stage of development that were different in MBR in comparison with conventional activated sludge. The MBR sludge could tolerate real stresses such as salinity and variation of COD and SS inlet in a wide range, with always good removal efficiencies in terms of BOD and COD. At the beginning period, bulking sludge was present. After 700 h of operation, the sludge characteristics not only showed healthy floc formations and good settling, but also confirmed the stable state of the system without overgrowth filamentous bacteria. In evaluating the fouling potential of membrane by cake filtration theory, it was found that soluble substances, soluble microbial products and/or residual organics play a key role in inducing membrane fouling. The αW values of the soluble fraction were three times higher than those of the particulates of bacterial floc.

AEROBIC DEGRADATION OF A NON-IONIC SURFACTANT IN A MEMBRANE BIOREACTOR(MBR)

A membrane bioreactor (MBR) was used to investigate the aerobic degradation of foam active substance - non-ionic surfactant, APG 2000 UP. The surface aeration using the propeller loop reactor (PLR) guaranteed sufficient O₂ for substrate removal and bacteria growth and avoided foam development. Moreover, the cross-flow membrane filtration enabled the separation of the bacteria still loaded with surfactant in the collecting container. The biological degradation of the surfactant with varying hydraulic retention time (HRT) and influent concentration cs0 showed high substrate removal of nearly 95 % at high volumetric loading rates up to 7.4 kgCOD m(-3) d(-1) and at sludge loading rates up to 1.8 kgCOD (kgVSS d)-1 for biomass concentration cB ≒ constant. The increasing cB from 3.4 to 14.5 gL-1 TSS respectively sludge retention time (SRT) from 5.1 to 442 d under complete biomass retention by the membrane filtration resulted in high removal of substrate α > 90 % with reducing excess sludge production.

Anaerobic Membrane Bioreactors: Applications and Research Directions

Membranes provide exceptional suspended solids removal and complete biomass retention that can improve the biological treatment process, but their commercial application to anaerobic treatment has been limited. This review summarizes the state of the art with respect to anaerobic membrane bioreactors (AnMBRs), determines the types of wastewaters for which AnMBRs would be best suited, and identifies the research required to increase implementation. AnMBRs have been tested with synthetic, food processing, industrial, high solids content, and municipal wastewaters at laboratory, pilot, and full scale. Chemical oxygen demand removal ranges from 56% to 99%, while the reported design membrane fluxes range from 10 to 40 L/m2/h. AnMBRs should be immediately applicable to highly concentrated, particulate waste streams like municipal sludges where the membrane can decouple the solids and hydraulic retention times. Opportunity for application to dilute wastewaters also appears strong, while application to highly concentrated soluble wastewaters is likely limited. Greater assessment of vacuum-driven immersed membranes, combining external or immersed membranes with retained biomass reactor designs, control of membrane fouling, and economic feasibility are the key research areas to be addressed.

Physical properties of activated sludge in a submerged membrane bioreactor and relation with membrane fouling

The objective of this study is to establish some relations between the physical properties of the sludge (i.e., specific resistance to filtration, capillary suction time …) and membrane fouling in a submerged membrane bioreactor, for high solid retention time (SRT) conditions. The analysis of the membrane performances, carried out with an analysis of the depollution performances and of the activated sludge physical properties, is proposed in an experimental membrane bioreactor (MBR) unit working with complete biomass retention. It is first confirmed that high-SRT conditions involve a high degree of organic degradation, with low sludge yield. A continuous increase of the viscosity can be observed during time but the sludge dewatering behaviour can be considered as constant, despite the total suspended solids (TSS) increase. No direct correlation is established between the transmembrane pressure (TMP) evolution and the conventional parameters used to describe sludge dewaterability but it can be assumed that the sludge filtration is not possible when TSS and viscosity reach “critical” values. Soluble microbial substances and mean floc size diameter evolutions cannot explain the decrease of membrane permeability, although these compounds may enhance fouling. The characterisation of the bulk suspension seems to not provide enough informations to anticipate the evolution of the membrane fouling and some complementary methodologies, allowing the characterisation of the environment close to the membrane, have to be developed.

Analysis of the Activated Sludge Process in an MBR under Starvation Conditions

An aerobic membrane bioreactor (MBR) at complete biomass retention was studied over a period of time under starvation conditions. Kinetic parameters were determined in a no-feed batch test. The decay rate of activated sludge, kd = 0.05 d-1, was determined by tracking the decrease of MLSS. The ratio of MLVSS/MLSS was in the range 0.76– 0.85. The pH values were between 7.02 and 8.23. As a function of different initial concentrations of MLSS, specific nitrification rates, qN, decreased from 4.23 to 0.02 mg-N/(g MLVSS d) and specific biodegradation rates, qb, increased from 0.23 to 1.90 mg COD/(g MLVSS d). From experimental data the kinetic constants for respiration, which followed Monod kinetics, were determined as qO2max = 9.8 mg O2/(g MLVSS h), Kx = 2.9 g/dm3. Additionally, a linear correlation between MLSS and mean floc size was found to exist during the biodegradation process.

Sustained nitrite accumulation in a membrane‐assisted bioreactor (MBR) for the treatment of ammonium‐rich wastewater

AbstractA membrane‐assisted bioreactor (MBR) for sustained nitrite accumulation is presented, treating a synthetic wastewater with total ammonium nitrogen (TAN) concentrations of 1 kg N m−3 at a hydraulic retention time down to 1 day. Complete biomass retention was obtained by microfiltration with submerged hollow fibre membranes. A membrane flux up to 189.5 dm3 day−1 m−2 could be maintained at a suction pressure below 100 kPa. Nitrification was effectively blocked at the nitrite stage (nitritation), and nitrate concentration was less than 29 g N m−3. The rate of aeration was reduced to obtain a mixture of ammonium and nitrite, and after adjusting this rate the TAN/NO2‐N ratio in the reactor effluent was kept around unity, making it suitable for further treatment by anaerobic oxidation of ammonium with nitrite. After increasing again the rate of aeration, complete nitrification to nitrate recovered after 11 days. It is suggested that nitrite accumulation resulted from a combination of factors. First, the dissolved oxygen (DO) concentration in the reactor was always limited with concentrations below 0.1 g DO m−3, thereby limiting nitrification and preventing significant nitrate formation. The latter is attributed to the fact that ammonium‐oxidising bacteria cope better with low DO concentrations than nitrite oxidisers. Second, the MBR was operated at a high ammonia concentration of 7–54 g N m−3, resulting in ammonia inhibition of the nitrite‐oxidising microorganisms. Third, a temperature of 35 °C was reported to yield a higher maximum growth rate for ammonium‐oxidising bacteria than for nitrite‐oxidising bacteria. Nitrite oxidisers were always present in the MBR but were out‐competed under the indicated process conditions, which is reflected in low concentrations of nitrate. Oxygen limitation was shown to be the most important factor to sustain nitrite accumulation. Nevertheless, nitritation was possible at ambient temperature (22–24 °C), lower ammonia concentration (&lt;7 g N m−3) and when using raw nitrogenous wastewater containing some biodegradable carbon. Overall, application of the MBR for nitritation was shown to be a reliable technology.© 2003 Society of Chemical Industry