Full-scale Activated Sludge Process Research Articles

Activated sludge microbial communities influencing the performance of a full-scale wastewater treatment plant treating optoelectronic wastewater

Microbial communities of activated sludge play vital roles in the biological treatment of wastewater. Next-generation sequencing (NGS) has assisted in increasing our knowledge of the activated sludge microbial communities treating various municipal and industrial wastewaters. However, our understanding of microbial communities in optoelectronic wastewater treatment systems remains limited. This study explored the microbial community dynamics of a full-scale activated sludge process that treated optoelectronic wastewater. Samples collected during periods of different system performance were sequenced via NGS of 16 S rRNA amplicons using the Illumina HiSeq platform. Indicator taxa analyses revealed that bacterial genera (e.g., Thauera) resisted the extreme ionizing radiation, and those capable of degrading aromatic compounds (e.g., Acinetobacter) were substantially lower in diversity and abundance during the low-performance period as compared to the high-performance period. In addition, the low-performance group lacked bacterial genera that are important in the nitrification process (e.g., Nitrospira). Moreover, co-occurrence network analyses indicated that the average path length and network diameter of microbial taxa were substantially shorter during the high-performance period, indicating efficient information dissemination among taxa. Collectively, these findings may aid in the design and/or optimization of systems with improved performance in treating optoelectronic industrial wastewater.

Simulation of a full-scale activated sludge reactor: an approach to airflow rate optimization

Wastewater systems are high-energy consuming processes that discharge considerable amounts of pollutants into aquatic ecosystems, thus requiring appropriate optimization. Therefore, a full-scale activated sludge process was studied in this work through mathematical modeling of heterotrophic and nitrifying microorganisms. The airflow rate was introduced as the controlling parameter to accomplish the degradation of pollutants. Moreover, a sensitivity analysis was performed to determine the most sensible parameters that should be evaluated through further experiments, aiming to improve the model. The obtained results indicated that readily biological oxygen demand degradation and the oxidation of ammonia to nitrite could be achieved with an airflow rate of 35 m 3 air d -1. The sensitivity analysis showed that heterotrophic kinetic parameters and oxygen mass transfer coefficient through diffusers are the most sensitive to the system. These results also proved that the air-diffusing systems can work at 0.3% of their maximum capacity to accomplish degradation. Future studies should consider other oxygen-consuming variables.

Microbial community similarity and dissimilarity inside and across full-scale activated sludge processes for simultaneous nitrification and denitrification.

Simultaneous nitrification and denitrification under low dissolved oxygen conditions is an energy-saving modification of the activated sludge process to achieve efficient nitrogen removal. Geographically distinct full-scale treatment plants are excellent platforms to address the links of microbial community with operating parameters. Mixed liquor samples were collected from a sequencing batch reactor plant, oxidation ditch plant, and step-feed activated sludge plant. Next-Generation Sequencing of the samples showed that the microbial communities were similar at the phylum level among the plants, being dominated by Proteobacteria. Microbial composition of functional groups was similar between the react fill and react phases of the sequencing batch reactors, among four sequencing batch reactors, and among four oxidation ditches. Nitrospira was the only identified genus of autotropic nitrifying bacteria with a relative abundance of 2.2-2.5% in the oxidation ditches and 0.4-0.7% at the other plants. Heterotrophic nitrifying-aerobic denitrifying bacteria were dominated by Dechloromonas with a relative abundance of 0.4-1.0%. Microbial community composition and nitrogen removal mechanisms were related to overall level and local zonation of dissolved oxygen, mixed liquor suspended solids concentration, nitrogen and organic loadings, and solids retention time. Low dissolved oxygen and low organic and nitrogen loadings favored growth of Nitrospira.

The impact of chemically enhanced primary treatment on the downstream liquid and solid train processes.

The use of chemical coagulants and flocculants to supplement chemically enhanced primary treatment (CEPT) processes is increasing in popularity as it has been demonstrated to improve carbon redirection and suspended solids and phosphorus removal. Dosing 15mg ferric chloride/L of wastewater and poly aluminum chloride (PACl; 0.5mg/L) to the influent of a primary clarifier successfully achieved improved carbon redirection and suspended solids removal at a full-scale WWTP. In this study, the impacts of PACl on the downstream liquid and solid train processes of the same WWTP were investigated. Compared to FeCl3 addition, a combined PACl and FeCl3 addition to the primary influent reduced the TSS and TP concentrations of the secondary clarifier effluent by 20% and 33%, respectively. Effluent BOD5 and ammonia-nitrogen concentrations of the downstream activated sludge process were not affected by the addition of a combined FeCl3 and PACl in the primary clarifier. PACl addition affects the bioavailability of carbon and hence reduced the methane production efficiency of the primary sludge by 20%-30%. However, the significant amount of carbon concentrated in the CEPT sludge would enhance the amount of energy recovered through incineration. PRACTITIONER POINTS: The chemically enhanced primary treatment process is an attractive method for carbon redirection and energy recovery. The combined FeCl3 and PACl addition in the primary clarifier improves the full scale activated sludge process effluent quality. PACl has a negative effect on methane production.

Enhancing nitrogen removal efficiency in a dyestuff wastewater treatment plant with the IFFAS process: the pilot-scale and full-scale studies.

The activated sludge (AS) process is widely applied in dyestuff wastewater treatment plants (WWTPs); however, the nitrogen removal efficiency is relatively low and the effluent does not meet the indirect discharge standards before being discharged into the industrial park's WWTP. Hence it is necessary to upgrade the WWTP with more advanced technologies. Moving bed biofilm processes with suspended carriers in an aerobic tank are promising methods due to enhanced nitrification and denitrification. Herein, a pilot-scale integrated free-floating biofilm and activated sludge (IFFAS) process was employed to investigate the feasibility of enhancing nitrogen removal efficiency at different hydraulic retention times (HRTs). The results showed that the effluent chemical oxygen demand (COD), ammonium nitrate (NH4+-N) and total nitrogen (TN) concentrations of the IFFAS process were significantly lower than those of the AS process, and could meet the indirect discharge standards. PCR-DGGE and FISH results indicated that more nitrifiers and denitrifiers co-existed in the IFFAS system, promoting simultaneous nitrification and denitrification. Based on the pilot results, the IFFAS process was used to upgrade the full-scale AS process, and the effluent COD, NH4+-N and TN of the IFFAS process were 91-291 mg/L, 10.6-28.7 mg/L and 18.9-48.6 mg/L, stably meeting the indirect discharge standards and demonstrating the advantages of IFFAS in dyestuff wastewater treatment.

The occurrence of enhanced biological phosphorus removal in a 200,000 m3/day partial nitration and Anammox activated sludge process at the Changi water reclamation plant, Singapore.

Mainstream partial nitritation and Anammox (PN/A) has been observed and studied in the step-feed activated sludge process at the Changi water reclamation plant (WRP), which is the largest WRP (800,000 m3/d) in Singapore. This paper presents the study results for enhanced biological phosphorus removal (EBPR) co-existing with PN/A in the activated sludge process. Both the in-situ EBPR efficiency and ex-situ activities of phosphorus release and uptake were high. The phosphorus accumulating organisms were dominant, with little presence of glycogen accumulating organisms in the activated sludge. Chemical oxygen demand (COD) mass balance illustrated that the carbon usage for EBPR was the same as that for heterotrophic denitrification, owing to autotrophic PN/A conversions. This much lower carbon demand for nitrogen removal, compared to conventional biological nitrogen removal, made effective EBPR possible. This paper demonstrated for the first time the effective EBPR co-existence with PN/A in the mainstream in a large full-scale activated sludge process, and the feasibility to accommodate EBPR into the mainstream PN/A process. It also shows EBPR can work under warm climates.

Biological Nitrogen Removal Activated Sludge Process in Warm Climates

Many Biological Nitrogen Removal (BNR) activated sludge processes in warm climates are conservatively designed, because little systematic investigation has been carried out on the BNR activated sludge process in warm or tropical climates, although many studies under temperate climate conditions are available. This book covers a wide spectrum of mechanistic approaches to deal with BNR activated sludge related issues such as sewage and sludge characterization, dynamic performance of full-scale processes, laboratory simulation and modelling that leads to a mechanistic and more economic design. Recommendations on the operation, upgrading and design of BNR activated sludge process are formulated. This will significantly aid the promotion of nitrogen removal in wastewater treatment plants in warm or tropical climates, particularly in developing countries. The book has three purposes: Reports are presented of comprehensive studies on: (i) on-site investigation of the activated sludge processes in three water reclamation plants (WRPs) in Singapore; (ii) laboratory-scale investigations to simulate the performance of full-scale activated sludge processes and to optimize the process design; and (iii) mathematical modelling and simulation with Activated Sludge Model No. 1 (ASM No. 1). Based on these detailed studies, guidelines on the operation, upgrading and design of the BNR activated sludge process in warm climates are presented. Results indicate that the laboratory-scale systems, designed based on the scale-down principles developed in this study and mathematic simulation with the model parameters calibrated by using the data obtained from the laboratory experiment, were able to describe the performance of the full-scale process. The applications of the approaches and principles are not limited to geographic locations and types of pollutants, and thus can reduce significantly the cost and time for the process development of the activated sludge process. This book will be an invaluable reference source for process and design engineers and consultants concerned with biological wastewater treatment as well as researchers in universities and research institutes. ISBN: 9781780401904 (eBook) ISBN: 9781843391876 (Print)

Influence of Temperature on the Performance of a Full-Scale Activated Sludge Process Operated at Varying Solids Retention Times Whilst Treating Municipal Sewage

In this study, the solid retention time (SRT) was varied with the ambient temperature for a full-scale municipal activated sludge plant with capacity of 200,000 PE (Population Equivalent) located in a humid sub-tropical environment. The effects of ambient temperature on treatment performance were investigated. Off-line samples were collected and analyzed from the treatment plant. The actual temperature variation during the study period was divided into three overlapping ranges and the SRT was adjusted accordingly with temperature in order to achieve the desired effluent quality. The plant’s observed effluent quality and thereby its overall removal efficiency was evaluated in terms of measuring standard biochemical parameters. The results indicate that significant improvement in effluent quality can be obtained by applying the variable SRT (5–7 days) dependent on temperature variation.

Effect of Powdered Activated Carbon to Reduce Fouling in Membrane Bioreactors: A Sustainable Solution. Case Study

Membrane Bio Reactors (MBRs) are mainly used for industrial wastewaters applications where their costs can be more easily afforded. High costs are basically due to energy consumption and membrane cleaning or replacement. Membrane fouling is responsible for reducing treated water production and increasing maintenance as well as operation costs. According to previous researches, the addition of Powdered Activated Carbon (PAC) in high dosages could reduce membrane fouling; but such concentrations are economically unsustainable for operative conditions. A MBR pilot plant, fed by mixed liquor of a full-scale activated sludge process from a municipal wastewater treatment plant, was operated dosing low PAC concentrations (0, 2, 5, 10 and 20 mg·L−1, respectively). Experiments were also carried out at two different temperatures corresponding to summer and winter conditions. Results indicated that PAC addition was effective at the low dosages (2 and 5 mg·L−1) by reducing the permeate flux loss (from 16 up to 27%, respectively) while higher PAC concentrations turns out in a useless cost increase.

Modeling and Simulation of the Activated Sludge Process at a Full-Scale WWTP

Model of a full-scale activated sludge process in a WWTP in Chongqing, China was established by coupling Activated Sludge Model No.2 (ASM2) and secondary settling tank (SST) model. Influent COD of the ASM2 was characterized through combined respirometric and physical-chemical assay and the model parameters were calibrated after evaluating their structural identifiability. The SST model is a flux model based on the consistent modeling methodology, in which a more detailed sludge settling velocity function was proposed to describe the most settling phenomena happened in SST, including discrete settling of floc, zone settling, and compression settling. The coupling model was used to simulate and optimize the operation of the activated sludge process It is concluded that: 1) the effluent COD, SS, TN and TP could be simulated well, 2) the sludge recycle ratio in the SST and the sludge concentration in the bioreactor were optimized to reduce waste sludge, increase the capability to resist load shocks and lower temperature, and improve effluent quality, and 3) an A2/O processes was proposed to upgrade the WWTP for improving the efficiency of N and P removal.

Influence of operational parameters on nitrogen removal efficiency and microbial communities in a full-scale activated sludge process

To improve the efficiency of total nitrogen (TN) removal, solid retention time (SRT) and internal recycling ratio controls were selected as operating parameters in a full-scale activated sludge process treating high strength industrial wastewater. Increased biomass concentration via SRT control enhanced TN removal. Also, decreasing the internal recycling ratio restored the nitrification process, which had been inhibited by phenol shock loading. Therefore, physiological alteration of the bacterial populations by application of specific operational strategies may stabilize the activated sludge process. Additionally, two dominant ammonia oxidizing bacteria (AOB) populations, Nitrosomonas europaea and Nitrosomonas nitrosa, were observed in all samples with no change in the community composition of AOB. In a nitrification tank, it was observed that the Nitrobacter populations consistently exceeded those of the Nitrospira within the nitrite oxidizing bacteria (NOB) community. Through using quantitative real-time PCR (qPCR), nirS, the nitrite reducing functional gene, was observed to predominate in the activated sludge of an anoxic tank, whereas there was the least amount of the narG gene, the nitrate reducing functional gene.

Oxygen transfer and uptake, nutrient removal, and energy footprint of parallel full-scale IFAS and activated sludge processes

Integrated fixed-film activated sludge (IFAS) processes are becoming more popular for both secondary and sidestream treatment in wastewater facilities. These processes are a combination of biofilm reactors and activated sludge processes, achieved by introducing and retaining biofilm carrier media in activated sludge reactors. A full-scale train of three IFAS reactors equipped with AnoxKaldnes media and coarse-bubble aeration was tested using off-gas analysis. This was operated independently in parallel to an existing full-scale activated sludge process. Both processes achieved the same percent removal of COD and ammonia, despite the double oxygen demand on the IFAS reactors. In order to prevent kinetic limitations associated with DO diffusional gradients through the IFAS biofilm, this systems was operated at an elevated dissolved oxygen concentration, in line with the manufacturer’s recommendation. Also, to avoid media coalescence on the reactor surface and promote biofilm contact with the substrate, high mixing requirements are specified. Therefore, the air flux in the IFAS reactors was much higher than that of the parallel activated sludge reactors. However, the standardized oxygen transfer efficiency in process water was almost same for both processes. In theory, when the oxygen transfer efficiency is the same, the air used per unit load removed should be the same. However, due to the high DO and mixing requirements, the IFAS reactors were characterized by elevated air flux and air use per unit load treated. This directly reflected in the relative energy footprint for aeration, which in this case was much higher for the IFAS system than activated sludge.

Biological Phosphorus Removal Activated Sludge Process in Warm Climates

Biological Phosphorus Removal Activated Sludge Process in Warm Climates presents the results of detailed researches on the Enhanced Biological Phosphorus Removal (EBPR) activated sludge process under warm climate conditions (20°C - 30°C), which is part of the R & D program of Public Utilities Board (PUB) Singapore. While the investigations and studies presented in this book are application-oriented, but at the same time the studies aim at an insightful understanding of the EBPR with the knowledge of the latest development in academic field. The focus points are: • EBPR performance of laboratory-scale and full-scale activated sludge processes under the site conditions in warm climates; • The carbon competition and distribution between PAO and GAO (and denitrifiers) in the process; • The stoichiometry and kinetics of P-release, COD uptake in the anaerobic environment and P-uptake in the aerobic environment under different temperatures and operating conditions; • PAO and GAO population fractions, shift and dominance studies using FISH and batch tests; • The inter-relationships between the system performance, process design and the microbial community; • EBPR for industrial wastewater (high ratio of feed COD/P) treatment under warm climates. Together with the preceding book — Biological Nitrogen Removal Activated Sludge Process in Warm Climates — published by IWA in 2008, this book fills the gap of biological nutrient (nitrogen and phosphorus) removal in warm climates and provides unique experiences and knowledge for Process and design researchers and engineers in wastewater research, students and academic staff in Civil/Sanitation/Environment Departments, as well as Managers, Engineers and Consultants in water companies and water utilities. ISBN: 9781843393818 (Print) ISBN: 9781780400891 (eBook)

Application of Iron Electrolysis to Full-Scale Activated Sludge Process for Phosphorus Removal

Addition of coagulant to the activated sludge process is an effective method to remove phosphorus from wastewater. Recently, iron electrolysis method has been developed as a new methodology of addition of coagulant into the activated sludge process. Although this process has been applied to industrial wastewater treatment plants or small-scale domestic wastewater treatment plants, the phosphorus removal performance by iron electrolysis combined with full-scale activated sludge process has not been clear yet. The objective of this study is to investigate the phosphorus removal efficiency by iron electrolysis combined with the activated sludge process. The enhanced biological phosphorus removal took place and phosphorus removal efficiency was increased without iron addition because of the occurrence of anaerobic condition in the surveyed wastewater treatment plant. Phosphorus removal performance in the investigated wastewater treatment plant was improved by the iron electrolysis. Effluent phosphorus concentration was decreased below 1.0 mg/L with additional iron of Fe/P molar ratio 0.4. The theoretical weight of iron release agreed with the actual weight decrease of iron electrodes. The introduction of iron electrolysis to the activated sludge process could enhance the phosphorus removal efficiency without any effects on substrate removal performance and make the phosphorus removal to be stable and sustainable.

Enhanced biological phosphorus removal in the retrofitting from an anoxic selector to an anaerobic selector in a full-scale activated sludge process in Singapore

This paper presents the investigation results of retrofitting an anoxic selector to an anaerobic selector through stepwise reduction of air supply in a full-scale activated sludge process with a focus on enhanced biological phosphorus removal (EBPR). The process experienced gradual shift from a Ludzack-Ettinger (LE) to an anaerobic-anoxic-oxic (A(2)O) process and subsequently, an anaerobic-oxic (A/O) process. The major findings are: (i) the average influent-based PO(4) (3-)-P release in the anaerobic selector compartment was 16.3 mg P l(-1) and that in the secondary clarifier was 1.7 mg P l(-1). 75% of the SCOD and 93% of the acetic acid in the primary effluent were taken up in the anaerobic selector compartment, respectively; (ii) PO(4) (3-)-P uptake contributed by both aerobic and denitrifying phosphorus accumulating organisms (DPAOs) occurred mainly in the first and second aerobic lanes together with simultaneous nitrification and denitrification (SND) while there was not much contribution from the last aerobic lane; (iii) The average PO(4) (3-)-P concentration of the final effluent was 2.4 mg P l(-1) corresponding to a removal efficiency of 85%; (iv) the SVI was satisfactory after retrofitting; and (v) the increase of NH(4) (+)-N in the final effluent from the commencement to the completion of the retrofitting resulted in an approximate 40-50% reduction in oxygen demand and a significant aeration energy saving was achieved.

Evaluation of Nitrification Kinetics for a 2.0 MGD IFAS Process Demonstration

The James River Treatment Plant (JRTP) operated a 2 MGD Integrated Fixed Film Activated Sludge (IFAS) demonstration process from November 2007 to April 2009 to explore IFAS performance and investigate IFAS technology as an option for a full scale plant upgrade in response to stricter nutrient discharge limits in the James River Basin. During the study, nitrification kinetics for both ammonia and nitrite oxidizing bacteria and plastic biofilm carrier biomass content were monitored on a near-weekly basis comparing the IFAS media, the IFAS process mixed liquor, and mixed liquor from the full-scale activated sludge process. Carrier biomass content is variable with respect to temperature and process SRT and relates to the localization of nitrification activity in the IFAS basin. Similar to trends observed for carrier biomass content (Regmi, 2008), ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) activity also shifted from the fixed film to the suspended phase as water temperatures increased and vice versa as the temperature decreased. The data suggest that AOB activity occurs on the surface of the biofilm carriers, while NOB activity remains deeper in the biofilm. During the highest temperatures observed in the IFAS tank, AOB activity on the media contributed as little as 30% of the total nitrification activity in the basin, and after temperatures dropped below 20 °C, AOB activity in the fixed film phase made up 75% of the total activity in the IFAS basin. During the warmest period of the summer, the media still retained more than 60% of the total NOB activity, and more than 90% of the total NOB activity during the period of coldest water temperature. This trend also points out that some AOB and NOB activity remained in the mixed liquor, even during the coldest periods. The retention of nitrification activity in the

Removal of endocrine disrupter compounds from municipal wastewater by an innovative biological technology

The removal of selected endocrine disrupter compounds (EDCs), namely estrone(E1), 17beta-estradiol (E2), 17alpha-ethynylestradiol (EE2), bisphenol A (BPA) and 4-tert-octylphenol (4t-OP) from municipal wastewater was investigated using a sequencing batch biofilter granular reactor (SBBGR), a new system for biological treatment based on aerobic granular biomass. This new biological treatment is characterized by high biomass concentration (up to 40 g/L), high sludge retention times (up to 6 months) and low sludge production (i.e., an order of magnitude lower than commonly reported for conventional biological technologies). The investigation was carried out comparing a demonstration SBBGR system with a conventional full-scale activated sludge process. Results showed that the SBBGR performed better than a conventional activated sludge process in removing E1, E2, BPA and 4t-OP. In fact, the average removal percentages of the above mentioned EDCs, obtained during a four month operating period, were 62.2, 68, 91.8, 77.9% and 56.4, 36.3, 71.3, 64.6% for the demonstrative SBBGR system and the conventional activated sludge process of the municipal sewage treatment plant, respectively.

PHA-accumulating microorganisms in full-scale wastewater treatment plants

A study was conducted to clarify phylogenetic affiliations of polyhydroxyalkanoate (PHA)-accumulating microorganisms in full-scale activated sludge processes. Activated sludge samples obtained from three full-scale activated sludge processes were aerobically incubated with excess acetate to increase their PHA content. The buoyant density separation method was applied to selectively collect PHA-accumulating cells, which were then analysed by the group-level FISH and the PCR-DGGE-sequencing methods, and possible PHA-accumulating microbial groups were screened. A set of oligonucleotide probes targeting the microbial groups suspected to accumulate PHA was introduced, and seven oligonucleotide probes were newly designed for this purpose. PHA accumulation of probe-positive cells was confirmed by the post-FISH PHA staining method, wherein PHA staining with Nile Blue A (NBA) was applied after FISH. As a result, the following seven bacterial groups were found to have PHA: Dechloromonas, Accumulibacter, Thauera, Zoogloea, Comamonas, Competibacter and a novel cluster in Beta-proteobacteria. Based on the results of the post-FISH PHA staining method, these seven bacterial groups were estimated to account for around four-tenths to two-thirds of total PHA-accumulating microorganisms.

Removal of endocrine disrupter compounds from municipal wastewater using an aerobic granular biomass reactor

The removal of selected endocrine disrupter compounds (EDCs), namely estrone (E1), 17β-estradiol (E2), 17α-ethynylestradiol (EE2), bisphenol A (BPA) and 4- tert-octylphenol (4t-OP) from municipal wastewater was investigated using a sequencing batch biofilter granular reactor (SBBGR), a new technology for biological treatment based on aerobic granular biomass. This new biological treatment is characterized by high biomass concentration (up to 40 g L −1), high sludge retention times (up to 6 months) and low sludge production (i.e., an order of magnitude lower than commonly reported for conventional biological technologies). The investigation was carried out comparing both a lab-scale SBBGR system and a demonstrative unit with a conventional full-scale activated sludge process. Results showed that the SBBGR, beside its already known benefits (removal efficiencies around 90% COD even at a high organic load (i.e., 5.7 kg COD m −3 d −1) and a sludge production value of around 0.06 kg TSS kg −1 COD removed), performed better than a conventional activated sludge process in removing E1, E2, BPA and 4t-OP. The average removal percentages of the above-mentioned EDCs, obtained during a 4-month operating period, were 60, 69, 93, 81% and 53, 41, 72, 67% for the demonstrative SBBGR system and the conventional activated sludge process of the municipal sewage treatment plant, respectively. In addition, lab-scale and demonstrative SBBGR systems showed similar efficiency for the removal of the investigated EDCs.

Analysis of phosphorus removal and anaerobic selector performance in a full-scale activated sludge process in Singapore

This paper analyses the performance of the anaerobic selector (A/O process) in a full-scale activated sludge process receiving mostly industrial sewage discharge (> 60%) in Singapore. In addition to the sludge settleability, enhanced biological phosphorus removal (EBPR) was studied. The sludge volume index (SVI) reduced from 200 to 80 ml g(-1) and foaming was suppressed significantly, indicating the effectiveness of the anaerobic selector in improving sludge settleability. The phosphorus removal efficiency was 66%, and 7.5 mg HAc-COD was consumed per mg PO4(3-) -P removed. In the anaerobic compartment, 31% of the SCOD and 73% of the acetic acid in the settled sewage were removed with PO4(3-) -P release of 14.1 mg PO4(3-)-P l(-1). The linear correlation between PO4(3-) -P release in the anaerobic compartment and PO4(3-) -P uptake in the aerobic compartment indicates that there is about 0.8 mg PO4(3-) -P release in the anaerobic compartment per mg PO34(3-) -P uptake in the aerobic compartment. The fates of volatile fatty acids (VFAs) and its short chain acids (SCAs) in the process were studied and discussed.