Excess Sludge Production Research Articles

High-Rate Anaerobic Side-Stream Reactor (ASSR) Processes to Minimize the Production of Excess Sludge.

High-rate anaerobic side-stream reactor (ASSR) processes were developed to minimize excess sludge production during wastewater treatment. New ASSRs were operated in 2.5-day solids retention time (SRT), much shorter than 10-day SRT used by the commercial sludge reduction process. The 2.5-day was selected based on literature review and preliminary studies, showing that maximum solublization of key floc components, such as divalent cations, extracellular polymeric substances (EPS), and protease, occur within 2 to 3 days of anaerobic digestion. The laboratory reactor study showed that 2.5-day ASSR systems produced approximately 60 and 20% less sludge than the control (no ASSR) and the 10-day ASSR, respectively. The experimental systems showed acceptable effluent quality, despite minimal sludge wastage. This was possible because sludge EPS were continuously released/degraded and regenerated as sludge underwent recirculation between ASSR and the aerobic basin. The results supported that the activated sludge process incorporating small ASSRs significantly decrease the production of excess sludge during wastewater treatment.

Influence of an oxic settling anoxic system on biomass yield, protozoa and filamentous bacteria

An oxic settling anoxic system coupled with an activated sludge process has been studied to reduce sewage sludge production. The reduction of sludge yield, excess sludge production and active biomass yield were 51.7%, 52.9% and 67.1%, respectively, compared with the control system. The oxic reactor of the oxic settling anoxic system, even with a lower active biomass concentration than the oxic reactor of control system, showed a higher metabolic activity in their active biomass. Diversity and crawling ciliates group have been shown as promising bioindicators of active biomass yield reduction. The identification of floc-forming bacteria in the control system suggested that oxic settling anoxic system will improve settling properties compared to a Conventional Activated Sludge process.

Effects of hydraulic retention time on aerobic granulation and granule growth kinetics at steady state with a fast start-up strategy.

A hydraulic retention time (HRT) of 4, 6, and 8h was employed, respectively, in three reactors to study the effects of HRT on granulation with a newly developed fast granulation strategy, i.e., a strategy by combining strong hydraulic selection pressure with high organic loading rate (OLR). Granules with clear boundary appeared within 24h after reactor start-up and all reactors reached a pseudo steady state after 6-day operation. A 4-h HRT resulted in a relatively higher increasing rate in terms of granule size during granule development period, i.e., 208μm day(-1), and the bigger granule size and the higher sludge volume index at the pseudo steady state. For HRT of 6 or 8h, no obvious difference was observed. However, it was found that HRT influenced sludge retention time (SRT) and kinetics significantly. A HRT changing from 4 to 8h led to an increased SRT from 3 to 21days, a decreased observed specific biomass growth rate (μ obs) and an decreased observed biomass yield (Y obs) of stable granules from 0.37 to 0.062days(-1), and 0.177 to 0.055g MLVSS g(-1) COD, respectively. Both μ obs and Y obs had a linear relationship with the reciprocal of HRT. In addition, the great difference of microbial community between seed sludge, sludge retained in the reactors, and sludge washed out indicated a strong microbial selection for fast granulation within 24h. However, during the granule development period from day 1 to 6, no more microbial selection was observed except an adjustment of microbial community. Little influence of HRT on microbial population in granular sludge indicated a minor role of HRT played for granulation with the fast start-up strategy adopted in this study. The results demonstrated that hydraulic selection pressure for granulation was mainly from short settling time, which led to strong microbial selection during the granulation period. Meanwhile, although HRT did not affect granulation with the fast start-up strategy, it played an important role on sludge retention time and excess sludge production. Therefore, HRT should be carefully optimized to balance benefits and shortfalls it brings to aerobic granular sludge system.

A modified oxic-settling-anaerobic activated sludge process using gravity thickening for excess sludge reduction.

Oxic-settling-anaerobic process (OSA) was known as a cost-effective way to reduce the excess sludge production with simple upgrade of conventional activated sludge process (CAS). A low oxidation-reduction potential (ORP) level was the key factor to sludge decay and lysis in the sludge holding tank of the OSA process. However, the ORP control with nitrogen purge or chemical dosing in the OSA process would induce extra expense and complicate the operation. Hence, in this study, a sludge holding tank using gravity thickening was applied to OSA process to reduce the excess sludge production without any ORP control. Results showed that the modified OSA process not only reduced the excess sludge production effectively but also improved the sludge settleability without affected the treatment capacity. The reduction of the excess sludge production in the modified OSA process resulted from interactions among lots of factors. The key element of the process was the gravity thickening sludge holding tank.

The effect of different aeration conditions in activated sludge – Side-stream system on sludge production, sludge degradation rates, active biomass and extracellular polymeric substances

On-site minimization of excess sludge production is a relevant strategy for the operation of small-scale and decentralized wastewater treatment plants. In the study, we evaluated the potential of activated sludge systems equipped with side-stream reactors (SSRs). This study especially focused on how the sequential exposure of sludge to different aeration conditions in the side-stream reactors influences the overall degradation of sludge and of its specific fractions (active biomass, extracellular polymeric substances (EPS), EPS proteins, EPS carbohydrates). We found that increasing the solid retention time from 25 to 40 and 80 days enhanced sludge degradation for all aeration conditions tested in the side-stream reactor. Also, the highest specific degradation rate and in turn the lowest sludge production were achieved when maintaining aerobic conditions in the side-stream reactors. The different sludge fractions in terms of active biomass (quantified based on adenosine tri-phosphate (ATP) measurements), EPS proteins and EPS carbohydrates were quantified before and after passage through the SSR. The relative amounts of active biomass and EPS to volatile suspended solids (VSS) did not changed when exposed to different aeration conditions in the SSRs, which indicates that long SRT and starvation in the SSRs did not promote the degradation of a specific sludge fraction. Overall, our study helps to better understand mechanisms of enhanced sludge degradation in systems operated at long SRTs.

Mechanisms for Reduced Excess Sludge Production in the Cannibal Process.

Reducing excess sludge production is increasingly attractive as a result of rising costs and constraints with respect to sludge treatment and disposal. A technology in which the mechanisms remain not well understood is the Cannibal process, for which very low sludge yields have been reported. The objective of this work was to use modeling as a means to characterize excess sludge production at a full-scale Cannibal facility by providing a long sludge retention time and removing trash and grit by physical processes. The facility was characterized by using its historical data, from discussion with the staff and by conducting a sampling campaign to prepare a solids inventory and an overall mass balance. At the evaluated sludge retention time of 400 days, the sum of the daily loss of suspended solids to the effluent and of the waste activated sludge solids contributed approximately equally to the sum of solids that are wasted daily as trash and grit from the solids separation module. The overall sludge production was estimated to be 0.14 g total suspended solids produced/g chemical oxygen demand removed. The essential functions of the Cannibal process for the reduction of sludge production appear to be to remove trash and grit from the sludge by physical processes of microscreening and hydrocycloning, respectively, and to provide a long sludge retention time, which allows the slow degradation of the "unbiodegradable" influent particulate organics (XU,Inf) and the endogenous residue (XE). The high energy demand of 1.6 kWh/m³ of treated wastewater at the studied facility limits the niche of the Cannibal process to small- to medium-sized facilities in which sludge disposal costs are high but electricity costs are low.

Changes in enzymatic and microbiological activity during adaptation of a conventional activated sludge (CAS) to a CAS-oxic-settling-anaerobic (OSA) adapted process

The production of excess sludge in the conventional activated sludge (CAS) process is an important aspect of the operation of wastewater treatment plants. The oxic-settling-anaerobic (OSA) process is one of the most promising strategies among those to achieve a reduction in the excess sludge produced. Cell decay occurs at a low oxidation–reduction potential and subsequent degradation reactions seem to be the major causes of sludge reduction in the OSA process. As a consequence of previous exposure of the sludge to anaerobic conditions in the sludge holding tank, an increase of enzymatic and microbiological activity in the aeration reactor could enhance the degradation of the released material. In this study, the results of a pilot plant running sequentially as a CAS and a CAS-OSA system are shown not only in the terms of the crucial aspect of excess sludge reduction, but also in terms of enzymatic and microbiological activities in the sludge from the aeration tank. Specifically, the application of the OSA strategy with a sludge anaerobic exposure time (SAET) of 5.5 h achieved a 33.6% reduction of the produced sludge in comparison with the CAS approach. Similarly, this system showed an average increase of 23.2, 22.6 and 7.59% in the specific oxygen uptake rate (SOUR), dehydrogenase, and protease activity, respectively, and a negligible change in glucosidase activity compared to the conventional process. At the same way, the OSA process with a SAET of 8 h led to a reduction of 38.9% of produced sludge and an average increase of 27.9, 11.31, 28.7 and 26.57% in SOUR, dehydrogenase, protease, and glucosidase activity, respectively, compared to control unit.

The Treatment Performance and the Bacteria Preservation of Anammox: A Review

Because of the low energy costs in the absence of the need for aeration, the non-requirement of a carbon source and alkali, and the reduced production of excess sludge, anaerobic ammonia oxidation (Anammox) has been extensively studied as an alternative to the conventional nitrification–denitrification pathway for biological nitrogen removal from wastewater. However, many challenges remain which need to be overcome to prepare the process for engineering application. These include the long doubling time of Anammox bacteria/autotrophic ammonia-oxidizing bacteria (AAOB), the low tolerance capacity to substrate concentration, and high sensitivity to various environmental factors. This review article focuses on the main drawbacks of the Anammox process and evaluates the progress made to date with regard to the enrichment of AAOB and the treatment performance of the Anammox process itself. The factors affecting the nitrogen removal performance of the Anammox process, such as substrate concentration, organic matters, and variation of temperature, are also reviewed and discussed. Finally, the need for the development of long-term storage methods for AAOB is addressed.

Influence of mechanical disintegration on the microbial growth of aerobic sludge biomass: A comparative study of ultrasonic and shear gap homogenizers by oxygen uptake measurements

Wastewater treatment plant incorporates physical, chemical and biological processes to treat and remove the contaminants. The main drawback of conventional activated sludge process is the huge production of excess sludge, which is an unavoidable byproduct. The treatment and disposal of excess sludge costs about 60% of the total operating cost. The ideal way to reduce excess sludge production during wastewater treatment is by preventing biomass formation within the aerobic treatment train rather than post treatment of the generated sludge. In the present investigation two different mechanical devices namely, Ultrasonic and Shear Gap homogenizers have been employed to disintegrate the aerobic biomass. This study is intended to restrict the multiplication of microbial biomass and at the same time degrade the organics present in wastewater by increasing the oxidative capacity of microorganisms. The disintegrability on biomass was determined by biochemical methods. Degree of inactivation provides the information on inability of microorganisms to consume oxygen upon disruption. The soluble COD quantifies the extent of release of intra cellular compounds. The participation of disintegrated microorganism in wastewater treatment process was carried out in two identical respirometeric reactors. The results show that Ultrasonic homogenizer is very effective in the disruption of microorganisms leading to a maximum microbial growth reduction of 27%. On the other hand, Shear gap homogenizer does not favor the sludge growth reduction rather it facilitates the growth. This study also shows that for better microbial growth reduction, floc size reduction alone is not sufficient but also microbial disruption is essential.

An evaluation of the performance and optimization of a new wastewater treatment technology: the air suction flow-biofilm reactor

In this laboratory study, a novel wastewater treatment technology, the air suction flow-biofilm reactor (ASF-BR) – a sequencing batch biofilm reactor technology with a passive aeration mechanism – was investigated for its efficiency in removing organic carbon, nitrogen and phosphorus, from high-strength synthetic wastewaters. A laboratory-scale ASF-BR comprising 2 reactors, 350 mm in diameter and 450 mm in height, was investigated over 2 studies (Studies 1 and 2) for a total of 430 days. Study 1 lasted a total of 166 days and involved a 9-step sequence alternating between aeration, anoxic treatment and settlement. The cycle time was 12.1 h and the reactors were operated at a substrate loading rate of 3.60 g filtered chemical oxygen demand (CODf)/m2 media/d, 0.28 g filtered total nitrogen (TNf)/m2 media/d, 0.24 g ammonium-nitrogen (NH4-N)/m2 media/d and 0.07 g ortho-phosphate (PO4-P)/m2 media/d. The average removal rates achieved during Study 1 were 98% CODf, 88% TNf, 97% NH4-N and 35% PO4-P. During Study 2 (264 days), the unit was operated at a loading rate of 2.49 g CODf/m2 media/d, 0.24 g TNf/m2 media/d, 0.20 g NH4-N/m2 media/d and 0.06 PO4-P/m2 media/d. The energy requirement during this study was reduced by modifying the treatment cycle in include fewer pumping cycles. Removal rates in Study 2 averaged 97% CODf, 86% TNf, 99% NH4-N and 76% PO4-P. The excess sludge production of the system was evaluated and detailed analyses of the treatment cycles were carried out. Biomass yields were estimated at 0.09 g SS/g CODf, removed and 0.21 g SS/g CODf, removed for Studies 1 and 2, respectively. Gene analysis showed that the use of a partial vacuum did not affect the growth of ammonia-oxidizing bacteria. The results indicate that the ASF-BR and passive aeration technologies can offer efficient alternatives to existing technologies.

A full-scale plug-flow reactor for biological sludge ozonation.

The reduction of biological excess sludge production using ozone is a well-known technology and is applied in several full-scale plants around the world. Nevertheless, optimisation of the process is not yet adequately documented in the literature. Operational parameters are usually chosen by assuming a direct proportionality between ozone dose and excess sludge reduction. This paper investigates the role of ozone concentration on process efficiency and demonstrates the (non-linear) inverse relationship between ozone dose and specific particulate chemical oxygen demand solubilisation in plug-flow contact reactors. The influence of total suspended solids concentration is also studied and described. No short-term lethal effects on heterotrophic biomass have been observed.

Sludge formation in the activated sludge process with a sludge disintegration unit

The activated sludge process is one of the major aerobic processes used in the biological treatment of wastewater. A significant drawback of this process is the production of excess `sludge', the disposal of which can account for 50--60% of the running costs of a plant. We investigate how the volume and mass of excess sludge produced is reduced by coupling the bioreactor to a sludge disintegration unit. References A. O. M. Alharbi, M. I. Nelson, A. L. Worthy, and H. S. Sidhu. Sludge formation in the activated sludge process: Mathematical analysis. In Proc. Australasian Chem. Eng. Conf., Chemeca 2013 . Chemical College, Engineers Australia, 2013. www.conference.net.au/chemeca2013/papers/29490.pdf Y.-C. Chung and J. B. Neethling. Viability of anaerobic digester sludge. J. Environ. Eng. , 116(2):330–342, 1990. doi:10.1061/(ASCE)0733-9372(1990)116:2(330) M. I. Nelson, T. Kerr, and X. D. Chen. A fundamental analysis of continuous flow bioreactor and membrane reactor models with death and maintenance included. Asia-Pac. J. Chem. Eng. , 3:70–80, 2008. doi:10.1002/apj.106 S.-H. Yoon. Important operational parameters of membrane bioreactor-sludge disintegration (MBR-SD) system for zero excess sludge production. Water Res. , 37(8):1921–1931, 2003. doi:10.1016/S0043-1354(02)00578-X

Low Energy High Rate Anaerobic – Aerobic Digestion (HRAAD) and Applications

Conventional aerobic technologies based on activated sludge processes have been widely applied for the treatment of domestic wastewater due the high efficiency realized, the possibility for nutrient removal, and high operational flexibility [1]. Unfortunately, the high capital and operation costs that coincide with the introduction of aeration-based technologies likewise impose significant financial constraints upon city and county that process large amounts of dilute wastewater [2]. The dissolution of oxygen is energy intensive, and results in the growth of significant quantities of biomass known as sludge, which requires further treatment/stabilization prior to disposal [3]. As such; reliable, unsophisticated, and cost effective treatment technologies have been proposed, including anaerobic treatment under anoxic conditions, as well as combined forms of anoxic with anaerobic treatment [4]. Of these, anaerobic pretreatment is perhaps the simplest and most conceptually attractive approach for domestic wastewater due to its relatively low construction and operational cost, operational simplicity, low production of excess sludge, production of energy in the form of biogas and applicability in small or large scale [5]. Anaerobic digestion can also be used as a pretreatment step prior to facilitate the use of low energy aerobic treatments used as a polishing step - reducing the organic load flowing into the aeration treatment basins can result in the production of less sludge that will, in turn, will reduce the amount of energy used in sludge handling (e.g., pumping, dewatering through belt presses, drying, trucking, and placing in landfills) [6]. In this presentation on-going research to design high rate anaerobic –aerobic digestion (HRAAD) reactors will be presented, as will results from their deployment in industry at demonstration scale. Commentary on how the HRAAD can be integrated into businesses to achieve sustainable use of fresh water supplies in isolated city communities will also be presented along with the application of sand filtration to this system to produce an R1 effluent for agriculture. Finally, the design of a novel unit operation that combines the elements of both a sand filter and a microbial fuel cell will also be discussed. ________________________________________________________________________________________1. Gavrilescue, M., et al., Acta Biotechnology, 19(2), 111-145 (1999).2. Kassab, G., et al., Bioresource Technology, 101(10), 3299-310 (2010).3. Dufresne, L., et al., Evaluation of Energy Conservation Measures for Wastewater Treatment Facilities. 2010, Environmental Protection Agency: Washington D.C.4. Chan, Y.J., et al., Chemical Engineering Journal, 1(55), 1-18 (2009).5. Lettinga, G., Antonie van Leeuwenhoek, 67, 3-28 (1995).6. Gohil, A., et al., Bioresource Technology, 97, 2141-2152 (2006).

Reduction of the excess sludge production by biological alternating process: real application results and metabolic uncoupling mechanism

The biological solution proposed to reduce the wasted sludge production is based on a process of alternating phases realized in a specific reactor (alternate cycles in sludge line (ACSL)) where a quote of the recycle sludge is treated and sent back to the main activated sludge process. The ACSL process was applied in two urban wastewater treatment plants (WWTPs). The reduction was tested by changing the hydraulic retention time and the conditions of oxidation reduction potential. The main mechanism of the process is recognized in the metabolic uncoupling. In fact, an increase in the specific oxygen uptake rate in the biological reactors was recorded (up to 20 mg/g VSS/h), which was stimulated by the fasting condition in the ACSL. The process is able to reduce the observed sludge yield on average of 25–30% with final average values reaching 0.179 kg VSS/kg chemical oxygen demand (COD) for WWTP1 and 0.117 kg VSS/kg COD for WWTP2.

Refining wastewater treatment using EGSB-BAF system

The performance of an expanded granular sludge bed (EGSB) in combination with a biological aerated filter (BAF) system for petroleum refining wastewater treatment was investigated. The system was monitored for two months at a hydraulic retention time of 17.5 h and a digestion temperature of 34 ± 1°C. The results showed that the total chemical oxygen demand (CODCr) and oil removal efficiencies were up to 90 and 87%, respectively, with the average CODCr and oil concentrations of 85 and 11 mg/L in the system effluent. Moreover, almost 97% of suspended solids (SS) were removed by the system and the effluent SS concentration was only 15 mg/L. The sludge yield coefficient of 0.0036 mg/mgCODCr showed a low excessive sludge production for the EGSB reactor. The excellent treatment performance indicated that this EGSB-BAF system could be appropriate for refining wastewater treatment. Besides, methane yield was only about 0.21 mLCH4/mgCODCr in the EGSB reactor, lower than the theoretical yields. The poor methane production together with the low oil concentration and increased biodegradability of effluent indicated that the biochemical reaction of refining wastewater mainly remains in hydrolytic acidification phase in the EGSB reactor. For this reason, it was proposed that a highly efficient anaerobic process such as EGSB could be used as a pre-treatment process to improve the biodegradation performance of the following aerobic biochemical treatment.

Development of a down-flow hanging sponge reactor for the treatment of low strength sewage

The process performance of a down-flow hanging sponge (DHS) reactor for treating low strength sewage (biochemical oxygen demand (BOD) 20-50 mg/L) was investigated in Bangkok, Thailand. The hydraulic retention time (HRT) was set at 4 h during the start-up period and was reduced to 1.5 h in a stepwise manner. Throughout the 300-day operational period, the DHS reactor shows high performance with respect to the removal of total suspended solid (>90% total suspended solid removal efficiency). No clogging of sponge media was observed in response to the self-digestion phenomena of the biofilm. At a HRT of 1.5 h, the BOD removal efficiency was sufficiently high (about 85%). The pathogen Escherichia coli and other coliform bacteria were removed almost completely as well (removal was 99.4% and 98.1%, respectively). Regarding the retained sludge activity measurement, the nitrite oxidation rate was higher than the ammonium oxidation rate (0.031 and 0.022 gram of nitrogen per gram of volatile suspended solids per day, respectively). In the 300 days of operation, the amount of excess sludge production was negligible. Thus, no sludge treatment system is required. Introduction of the DHS system in developing countries is recommended because this system requires a relatively small area, and has low electricity consumption and operation costs.

A single-stage biological process for municipal sewage treatment in tourist areas

This pilot scale study aims to test the effectiveness of an innovative compact biological system (SBBGR – Sequencing Batch Biofilter Granular Reactor) for treating municipal wastewater in tourist areas characterised by intense seasonal water demand and wastewater discharge. The results obtained after a long term operation of 463 days have shown that the proposed system is able to assure average removal efficiencies higher than 90% for COD (chemical oxygen demand), total suspended solids and TKN (total Kjeldahl nitrogen) independently of the influent concentration values and organic loading, which ranged from 0.2 to 5.1 kgCOD/m3biofilter.d Furthermore, the plant showed a high degree of operation flexibility and stability in response to the organic load variations occurring in tourist areas. In fact, no significant deterioration in the plant's effluent quality was observed even during a sudden several-fold increase in organic loading. High nitrogen removal efficiencies (80%, on average) were also achieved thanks to the establishment of simultaneous nitrification-denitrification process favoured by the plant's high biomass concentration and operating conditions. Finally, the system was characterized by an excess sludge production much lower (60–80% lower) than that of conventional biological systems operating without a primary clarifier. An acceptable level of stabilization of excess sludge was also obtained so that a further stabilization process was no longer required.

Effect of alkaline and ozone pretreatment on sludge reduction potential of a membrane bioreactor treating high-strength domestic wastewater

The wide application and utilization of the activated sludge process has resulted in the production of excess sludge, posing a serious disposal problem. Many efforts have been dedicated to reduce the excess sludge by treatments such as digestion and dewatering. In this study, an aerobic submerged membrane bioreactor (MBR) was used to study the effect of alkaline and ozone pretreatment on the efficiency of sludge reduction. For this purpose, two MBRs were fabricated. Among the two MBRs, one acted as a control reactor (CMBR) and the other acted as an experimental reactor (EMBR). The MBRs were operated with mixed liquor suspended solids (MLSS) concentrations in the range of 7,000–7,200 mg/L for a period of 120 d. In the EMBR, part of the MLSS was withdrawn at a ratio of 1.5% of Q and was pretreated by alkali-ozone. The sludge pretreatment was carried out at pH 11 and an ozone dosage of 0.09 gO3/g MLSS. During the pretreatment, 40% COD solubilization and 30% suspended solid reduction were observed. The pretreated sludge was returned to the reactor for further degradation, where it was found to be 37% degraded. During the 120 d of reactor operation, both of the MBRs maintained a relatively constant transmembrane pressure. The sludge digestion does not have any impact on the COD removal efficiency of the reactor.

암모니아 센서를 이용한 간헐폭기 Membrane bioreactor공정에서의 전력비 저감과 관형막을 이용한 슬러지 농축에 관한 연구

It is essential to decrease energy consumption and excess sludge to economically operate sewage treatment plant. This becomes more important along with a ban on sea dumping and exhaustion of resource. Therefore, many researchers have been study on energy consumption reduction and strategies for minimization of excess sludge production from the activated sludge process. The aeration cost account for a high proportion of maintenance cost because sufficient air is necessary to keep nitrifying bacteria activity of which the oxygen affinity is inferior to that of heterotrophic bacteria. Also, additional costs are incurred to stabilize excess sludge and decrease the volume of sludge. There were anoxic, aerobic, membrane, deairation and concentration zone in this MBR process. Continuous aeration was provided to prevent membrane fouling in membrane zone and intermittent aeration was provided in aerobic zone through ammonia sensor. So, there was the minimum oxygen to remove below limited quantity that could be eliminated in membrane zone. As the result of this control, energy consumption of aeration system declined by between 10.4 % and 19.1 %. Besides, we could maintain high MLSS concentration in concentration zone and this induced the microorganisms to be in starved condition. Consequentially, the amount of excess sludge decrease by about 15 %.

Effects of sludge retention times on nutrient removal and nitrous oxide emission in biological nutrient removal processes.

Sludge retention time (SRT) is an important factor affecting not only the performance of the nutrient removal and sludge characteristics, but also the production of secondary pollutants such as nitrous oxide (N2O) in biological nutrient removal (BNR) processes. Four laboratory-scale sequencing batch reactors (SBRs), namely, SBR5, SBR10, SBR20 and SBR40 with the SRT of 5 d, 10 d, 20 d and 40 d, respectively, were operated to examine effects of SRT on nutrient removal, activated sludge characteristics and N2O emissions. The removal of chemical oxygen demand or total phosphorus was similar under SRTs of 5–40 d, SRT mainly affected the nitrogen removal and the optimal SRT for BNR was 20 d. The molecular weight distribution of the effluent organic matters was in the range of 500–3,000 Da under SRTs of 5–40 d. The lowest concentration of the effluent soluble microbial products concentration was obtained at the SRT of 5 d. Nitrifier growth was limited at a short SRT and nitrite existed in the effluent of SBR5. With increasing SRTs, mixed liquor suspended solids concentration increased while the excess sludge production was reduced due to the high endogenous decay rate at high SRTs. Endogenous decay coefficients were 0.020 d−1, 0.036 d−1, 0.037 d−1 and 0.039 d−1 under SRTs of 5–40 d, respectively. In BNR, the N2O emission occurred mainly during the aerobic phase and its emission ratio decreased with increasing SRTs. The ratio between the N2O-N emission and the removed ammonium nitrogen in the aerobic phase was 5%, 3%, 1.8% and 0.8% at the SRT of 5 d, 10 d, 20 d and 40 d, respectively. With low concentrations of dissolved oxygen and high concentrations of oxidized nitrogen, the N2O emission was significantly accelerated due to heterotrophic denitrification activities.