Anoxic Tank Research Articles

Biological treatment of actual petrochemical wastewater using anaerobic/anoxic/oxic process and the microbial diversity analysis.

A novel process integrating anaerobic hydrolysis-acidification (HA) and anoxic/oxic (A/O) reactors was developed to treat the actual petrochemical wastewater, which was operated for more than 8months, the removal efficiency of COD and NH4+-N was monitored, and the microbial community was analyzed. The results showed that the effluent concentrations were maintained at around 99 and 1.3mg/L, with the removal efficiency of 70.6 and 95.4%, respectively at a total hydraulic retention time (HRT) of 20h. The major pollutants in the influent were identified as hydrocarbons, aldehydes, heterocyclic matters, amines, alcohols, phenols, ketones, etc. by GC-MS analysis, while only heterocyclic compounds, ketones, and esters were detected in the effluent after HA-A/O treatment. Bacteria belonging to phyla Chloroflexi, Proteobacteria, and Bacteroidetes were highly enriched in the system. The predominant genera in HA, anoxic, and oxic tanks were Anaerolineaceae uncultured and Desulfobacter, Blastocatella and Anaerolineaceae uncultured, Saprospiraceae uncultured and Nitrosomonadaceae uncultured, respectively. The sulfate-reducing bacteria Desulfobacter, Desulfofustis and Desulfomicrobium were detected only in HA reactor. The ammonium-oxidizing bacteria Nitrosomonadaceae and Nitrosomonas and nitrite-oxidizing bacteria Nitrospira were highly enriched in A/O reactor, which is consistent with the good nitrification performance.

Performance of airlift MBR for on-site treatment of slaughterhouse wastewater in urban areas of Vietnam

In many cities in Vietnam, wastewater generated in slaughterhouses is normally discharged directly into surface waters without treatment. Management of slaughterhouse wastewater (SHWW) is difficult due to the lack of infrastructure for conveyance to centralized facilities. On-site treatment presents one cost-effective way of managing SHWW compared to mass improvement of infrastructure. This study evaluates the application of an airlift membrane bioreactor (AL-MBR) for on-site treatment of SHWW. The concentrations of chemical oxygen demand (COD), biochemical oxygen demand (BOD), total nitrogen (TN) and NH4+-N in the simulated SHWW were 1,697 ± 317 mg/L, 891 ± 137 mg/L, 246 ± 65 mg/L and 171 ± 4.2 mg/L, respectively. The mixed-liquor volatile suspended solids in the aerobic and anoxic tanks were maintained at 5,000-6,000 mg/L. Air flow rate and cross flow velocity were maintained at 0.2 L/min and 0.8 m/s, respectively, to keep the trans-membrane pressure (TMP) stable at 0.8 bar and the membrane flux at 15-18 L/m2 h bar (LMH/bar). The removal efficiencies of COD and TN were 95 ± 1.9% and 70 ± 3.3%, respectively, at a hydraulic retention time (HRT) of 2.5 days. This study shows that GL-MBR is a promising on-site solution for SHWW treatment.

Phosphorus removal performance and population structure of phosphorus-accumulating organisms in HA-A/A-MCO sludge reduction process

ABSTRACTWe developed a new sludge reduction HA-A/A-MCO (Hydrolysis-Acidogenosis-Anaerobic/Anoxic -Multistep Continuous Oxic tank) process, which has improved phosphate (P) and nitrogen (N) removal. Its biological treatment unit uses an A2/O P & N removal process with hydrolysis acidification, multistep continuous aeration, and continuous flow, coupled with sidestream P removal by draining out anaerobic P-bearing wastewater. The process has advanced synchronization of P and N removal and sludge reduction. The improved performance is closely associated with the population structure of P-accumulating organisms (PAOs). This study investigated the relationship between P removal performance and the population structure of PAOs. The results show that the average effluent P content of HA-A/A-MCO process was only 0.44 mg/L, when the influent P concentration was 8∼12 mg/L. The effluent met the A standard set by GB18918-2002. PAOs were able to effectively release 1 mg of P and absorb 2.8 mg of P. The system removed P by draining out anaerobic P-rich wastewater, as P had been reduced in the aerobic absorption process. This reduced the need for excess P uptake ability of the PAOs. The bacterial pure culture method was applied to isolate 5 PAOs with typical P absorption and removel features. 16SrDNA amplification and sequence analysis revealed that Acinetobacter sp. and Lampropedia sp played dominant roles in anaerobic P-releasing process. Moreover, Devosia sp. and Bdellovibrio sp were the primary strains in the aerobic tank, and, they were the major stains for P absorption. Uncultured Bacterium and other uncultured strains were detected in the anoxic tank.

Metagenomic analysis of an ecological wastewater treatment plant's microbial communities and their potential to metabolize pharmaceuticals.

Pharmaceuticals and other micropollutants have been detected in drinking water, groundwater, surface water, and soil around the world. Even in locations where wastewater treatment is required, they can be found in drinking water wells, municipal water supplies, and agricultural soils. It is clear conventional wastewater treatment technologies are not meeting the challenge of the mounting pressures on global freshwater supplies. Cost-effective ecological wastewater treatment technologies have been developed in response. To determine whether the removal of micropollutants in ecological wastewater treatment plants (WWTPs) is promoted by the plant-microbe interactions, as has been reported for other recalcitrant xenobiotics, biofilm microbial communities growing on the surfaces of plant roots were profiled by whole metagenome sequencing and compared to the microbial communities residing in the wastewater. In this study, the concentrations of pharmaceuticals and personal care products (PPCPs) were quantified in each treatment tank of the ecological WWTP treating human wastewater at a highway rest stop and visitor center in Vermont. The concentrations of detected PPCPs were substantially greater than values reported for conventional WWTPs likely due to onsite recirculation of wastewater. The greatest reductions in PPCPs concentrations were observed in the anoxic treatment tank where Bacilli dominated the biofilm community. Benzoate degradation was the most abundant xenobiotic metabolic category identified throughout the system. Collectively, the microbial communities residing in the wastewater were taxonomically and metabolically more diverse than the immersed plant root biofilm. However, greater heterogeneity and higher relative abundances of xenobiotic metabolism genes was observed for the root biofilm.

Where to dose powdered activated carbon in a wastewater treatment plant for organic micro-pollutant removal

Emissions of many organic micro-pollutants (OMP) into the aquatic environment can be efficiently reduced with advanced treatment at wastewater treatment plants (WWTP). Post-treatment with activated carbon is currently considered as one of the most promising options, but powdered activated carbon (PAC) could also be dosed into the existing biological treatment process instead. Due to much greater concentrations of suspended and dissolved constituents the adsorptive OMP removal was expected to be severely hindered. Systematic comparative adsorption tests with samples from different process steps of a large conventional WWTP were conducted to investigate differences in adsorption competition and removal efficiencies. The results show that much greater competition occurs in the WWTP influent and in the anaerobic tank but removal efficiencies in the anoxic and aerobic tank and in the WWTP effluent were more similar than expected. Suspended solids thus seem not to severely affect OMP adsorption. Similar results were obtained in a comparison of different commercial PAC in all for the respective matrices. OMP removals showed a relation with the PAC dosage normalized to the concentration of dissolved organic carbon. In the anoxic and aerobic tank and in the WWTP effluent, a uniform correlation of OMP removals and reductions of UV light absorption was observed.

Characteristic Features and Effect of Neo-Hydrofoil Impeller Applied in Sewage Treatment Plants

In this study, a newly developed agitator with hydrofoil impeller applied to actual biological process in advanced wastewater treatment plant was evaluated. Several series of experiments were conducted in two different wastewater treatment plants where actual problems have been occurred such as the production of scums and sludge settling. For more effective evaluation, computational fluid dynamics (CFD) and measurements of MLSS (Mixed Liquor Suspended Solids) and DO (Dissolved Oxygen) were used with other measuring equipments. After the installation of one unit of vertical hydrofoil agitator in plant A, scum and sludge settling problems were solved and more than seventy percent of operational energy was saved. In case of plant B, there were three cells of each anoxic and anaerobic tanks, and each cell had one unit of submersible horizontal agitator. After the integration of three cells to one cell in each tank, and installation of one vertical hydrofoil agitator per tank, all the problems caused by improper mixing were solved and more than eighty percent of operational energy was found to be saved. Simple change of agitator applied to biological process in wastewater treatment plant was proved to be essential to eliminate scum and sludge settling problems and to save input energy.

Greenhouse gases from sequential batch membrane bioreactors: A pilot plant case study

The paper reports the results of nitrous oxide (N2O) emissions from aerobic and anoxic tank of a Sequential Batch Membrane Bioreactor (SB-MBR) pilot plant. The influence of salinity variation on N2O emission was analyzed by gradually increasing the inlet salt concentration from 0 to 10g NaCl L−1.The observed results showed that the N2O concentration of the gaseous samples was strongly influenced by the salt concentration. This result was likely related to a worsening of the nitrification activity due to the effect of salinity on autotrophic bacteria. Dissolved oxygen concentration and salinity were found to be the key factors affecting N2O concentration in the gaseous samples withdrawn from the anoxic tank. Despite the fact that the N2O concentration in the anoxic tank was higher than in the aerobic one, it was found that the aerobic tank emitted around 25 times more N2O than the anoxic one.

Fate and removal of various antibiotic resistance genes in typical pharmaceutical wastewater treatment systems.

The high levels of antibiotic residues in pharmaceutical wastewater treatment plants (PWWTPs) make these plants the hotspots for the proliferation of antibiotic resistance genes (ARGs). This study investigated the fate and removal of 11 ARG subtypes for sulfonamide, tetracycline, β-lactam, and macrolide resistance in each processing stage of two full-scale PWWTPs in northern China. The levels of typical ARG subtypes in the final effluents ranged from (2.56 ± 0.13) × 10(1) to (2.36 ± 0.11) × 10(7) copies/ml. The absolute abundance of ARGs in effluents accounted for only 0.03-78.1% of influents of the two PWWTPs, while the majority of the ARGs were transported to the dewatered sludge with concentrations from (2.65 ± 0.43) × 10(5) to (4.27 ± 0.03) × 10(10) copies/g dry weight (dw). The total loads of ARGs discharged through dewatered sludge plus effluent was 1.01-14.09-fold higher than that in the raw influents, suggesting the proliferation of ARGs occurred in the wastewater treatment. The proliferation of ARGs mainly occurs in biological treatment process, such as aeration tank, anoxic tank, sequencing batch reactor (SBR), and bio-contact oxidation, facilitates the proliferation of various ARGs, implying significant replication of certain ARG subtypes may be attributable to microbial growth. Chemical oxidation seems promising to remove ARGs, with removal efficiency ranged from 29.3 to 85.7%, while the partial correlation analysis showed significant correlations between antibiotic concentration and ARG removal. Thus, the high antibiotic residues within the PWWTPs may have an influence on the proliferation, fate, and removal of the associated ARG subtypes.

Fate and removal of typical pharmaceutical and personal care products in a wastewater treatment plant from Beijing: a mass balance study

The fate and removal of pharmaceuticals and personal care products (PPCPs) in wastewater treatment plants (WWTPs) has received great attention during the last decade. Numerous data concerning concentrations in the water phase can be found in the literature, however corresponding data from sludge as well as associated mass balance calculations are very limited. In the present study, the adsorbed and dissolved concentrations of 9 PPCPs were investigated in each unit of a WWTP in Beijing, China. Based on the calculation of mass balance, the relative mass distribution and removal efficiency of each target compound was obtained at each process. The amount of PPCPs entering into the WWTP ranged from 12 g·d–1 to 3848 g·d–1. Five target compounds (caffeine, chloramphenicol, bezafibrate, clofibric acid, and N,N-diethyl-meta-toluamide) were effectively removed, with rates of 57%–100%. Negative removal efficiencies were obtained for sulpiride, metoprolol, nalidixic acid, and carbamazepine, ranging from -19% to -79%. PPCPs mainly existed in dissolved form (≥92%) in both the raw influent and the final effluent. The sludge cake carried a much lower amount of PPCPs (17 g·d–1) compared with the discharged effluent (402 g·d–1). In A2/O treatment tanks, the anaerobic and anoxic tanks showed good performance for PPCPs removal, and the amount of adsorbed PPCPs was increased. The results reveal that both the dissolved and the adsorbed phases should be considered when assessing the removal capacity of each A2/O tank.

도금폐수처리공정의 처리효율 개선방안

This study was performed to evaluate the alternative improvement plans for removal efficiency of plating wastewater treatment processes with high concentrations of heavy metals and total nitrogen in the influent. The average removal efficiency rates of the existing wastewater treatment plant were 58% of CODcr, 74% of CODmn, 78% of TN, 99% of TP, respectively. However, the concentration of SS (about 250 mg/L) was over the emission standard (120 mg/L). TN and Cu2+ concentrations were over the emission standard; about 62 mg/L and 5.2 mg/L, respectively. Carbon source quantity, fed into the anoxic tank of the biological wastewater treatment process, was controled by calculating the optimum required COD amount for denitrification. The removal efficiency rates of Zn2+, Cu2+, and Ni2+ were achieved using an electrolysis reactor 89%, 89%, and 99%, respectively. Therefore, it was recommended to modify the existing wastewater treatment process including the chemical precipitation to the electrolysis reactor as an efficient and environmentally effective alternative.

An Integrated Attached Growth Bioreactor for the Treatment of Wastewater

An integrated attached growth bioreactor was evaluated in this study for the removal of organic matter (COD). The bioreactor integrated the aerobic tank, anoxic tank and the secondary clarifier in a single unit. An attached growth media was submerged into the aerobic tank to improve the growth of biomass while bio-balls were submerged into the anoxic tank. Synthetic wastewater simulating medium and high strength domestic wastewater were pumped into the reactor for biodegradation. Hydraulic Retention Time (HRT) was varied in the range 12 and 7.2 days. Influent COD was introduced in two phases. Phase 1 consists of COD 400 mg/L whereas phase 2 consists of COD 650 mg/L. In phase 1, steady state was attained from day 18 until day 26. Effluent concentrations of COD were approximately 9 mg/L at HRT of 12 days. The effluent COD concentration increased (16 mg/L) when COD concentration was increased to 650 mg/L at HRT of 12 days. Effluent COD decreased from 19-9 mg/L when HRT was increased from 7.2-12 days. The effluent COD obtained in this study was below the Malaysia Department of Environment (DOE) guideline limit Standard A (120 mg/L). Residual ammonia, nitrate and total suspended solids were also monitored at steady state. Results show a 97, 87 and 97%, respectively removals for ammonia, nitrate and TSS, respectively. The attached growth media in the aerobic and anoxic tanks provided a large surface area for the attachment of biomass. The bioreactor performance shows it can serve as an alternative footprint for the treatment of wastewater.

Effects of aeration and internal recycle flow on nitrous oxide emissions from a modified Ludzak-Ettinger process fed with glycerol.

Nitrous oxide (N2O) is emitted from a modified Ludzak-Ettinger (MLE) process, as a primary activated sludge system, which requires mitigation. The effects of aeration rates and internal recycle flow (IRF) ratios on N2O emission were investigated in an MLE process fed with glycerol. Reducing the aeration rate from 1.5 to 0.5 L/min increased gaseous the N2O concentration from the aerobic tank and the dissolved N2O concentration in the anoxic tank by 54.4 and 53.4 %, respectively. During the period of higher aeration, the N2O-N conversion ratio was 0.9 % and the potential N2O reducers were predominantly Rhodobacter, which accounted for 21.8 % of the total population. Increasing the IRF ratio from 3.6 to 7.2 decreased the N2O emission rate from the aerobic tank and the dissolved N2O concentration in the anoxic tank by 56 and 48 %, respectively. This study suggests effective N2O mitigation strategies for MLE systems.

Comparative integrated omics: identification of key functionalities in microbial community-wide metabolic networks.

Background:Mixed microbial communities underpin important biotechnological processes such as biological wastewater treatment (BWWT). A detailed knowledge of community structure and function relationships is essential for ultimately driving these systems towards desired outcomes, e.g., the enrichment in organisms capable of accumulating valuable resources during BWWT.Methods:A comparative integrated omic analysis including metagenomics, metatranscriptomics and metaproteomics was carried out to elucidate functional differences between seasonally distinct oleaginous mixed microbial communities (OMMCs) sampled from an anoxic BWWT tank. A computational framework for the reconstruction of community-wide metabolic networks from multi-omic data was developed. These provide an overview of the functional capabilities by incorporating gene copy, transcript and protein abundances. To identify functional genes, which have a disproportionately important role in community function, we define a high relative gene expression and a high betweenness centrality relative to node degree as gene-centric and network topological features, respectively.Results:Genes exhibiting high expression relative to gene copy abundance include genes involved in glycerolipid metabolism, particularly triacylglycerol lipase, encoded by known lipid accumulating populations, e.g., Candidatus Microthrix parvicella. Genes with a high relative gene expression and topologically important positions in the network include genes involved in nitrogen metabolism and fatty acid biosynthesis, encoded by Nitrosomonas spp. and Rhodococcus spp. Such genes may be regarded as ‘keystone genes’ as they are likely to be encoded by keystone species.Conclusion:The linking of key functionalities to community members through integrated omics opens up exciting possibilities for devising prediction and control strategies for microbial communities in the future.

Effect of sludge retention time on inclined plate function, treatment performance and sludge characteristics of inclined plate membrane bioreactors treating municipal wastewater

AbstractTwo identical bench scale inclined plate membrane bioreactors (ip‐MBRs) were operated for the treatment of real municipal wastewater for 1 year. Sludge retention time (SRT) was varied over the course of operation to investigate the effects on inclined plate function, treatment performance and sludge characteristic. Removal rates of chemical oxygen demand and ammonia over 90% and of total nitrogen (TN) more than 70% were achieved when ip‐MBRs were operated under SRTs between infinite and 40 days while short SRT (20 days) negatively affected TN removal. When the sludge concentration in anoxic tank exceeded 15 g/L, the failure of the inclined plate function was observed, resulting in no difference in sludge concentrations between aerobic and anoxic tanks. To avoid severe effects on inclined plate function and treatment performance, an SRT range of 40–80 days was recommended for ip‐MBRs. Moreover, sludge floc size under prolonged SRT became smaller than that under short SRT due to increased attrition among the sludge floc particles caused by strong aeration needed for keeping a sufficient dissolved oxygen level.

Biological nitrogen removal in a modified anoxic/oxic process for piggery wastewater treatment

Piggery wastewater is characterized by its high ammonium concentration and low COD/nitrogen ratio. The indiscriminate discharge of untreated piggery wastewater from scattered household pig farms has posed a potential risk of surface water pollution in rural areas of southern China. In this study, an anoxic/oxic (A/O) process with elastic fillers tightly packed in each tank was investigated for piggery wastewater treatment. Two anoxic tanks and two aerobic tanks were used in series and the hydraulic retention time of each was 24 h. Dissolved oxygen concentration in the aerobic tanks was intentionally controlled at below 2 mg L–1 for the implementation of nitritation. After sludge acclimation, three recycle ratios from 100 to 300% were tested. More than 90% of organic matter was removed from raw piggery wastewater despite recycle ratio values. However, the NH4+-N concentrations in the effluent slightly increased from 30 to 65 mg L–1 as the recycle ratio increased. Higher recycle ratio benefited the removal of total nitrogen (TN) and dramatically increased the nitrite accumulation rate in the aerobic tanks. The tightly packed elastic fillers successfully prevented the loss of sludge and increased the biomass in reactors. Moreover, the formation of biofilms on the surface of the elastic fillers developed simultaneous nitritation and denitrification in the aerobic tanks, which counted for approximately 30% of the total removed nitrogen in the proposed system. The employment of elastic fillers in A/O process effectively improved TN removal at a relatively smaller recycle ratio, and consequently reduced the running cost of recirculation for nitrogen removal.

Effect of carbon sources on nitrous oxide emission in a modified Ludzak Ettinger process.

Effect of methanol and glycerol on nitrous oxide (N2O) emission in two laboratory-scale modified Ludzak Ettinger (MLE) processes was investigated during three distinct periods: dissolved oxygen (DO) control by intermittent aeration with a DO controller, and high and low aeration rates. N2O consumption rate in an anoxic tank and aeration mode influenced N2O emission rates from the MLE processes. In the DO control period, N2O emission rate from the glycerol-fed MLE process was higher than the methanol-fed counterpart, likely caused by a higher N2O consumption rate in an anoxic tank of the methanol-fed process. During the period of a higher aeration rate, N2O emission rates from both processes were comparable. In contrast, during the period of a lower aeration rate, N2O emission rate from the methanol-fed MLE process was higher than that from the glycerol-fed counterpart likely because of a higher degree of nitrite accumulation, corroborated by statistical analysis. N2O consumption activities of biomasses fed with the different carbon sources were distinct. However, the high activity did not necessarily result in a decrease in N2O emission rate from an aerobic tank and the effect of nitrite on the emission was stronger under the tested conditions.

Biological treatment of reverse osmosis concentrate from low salinity water

With increasing water demand and growing scarcity of potable water, the reuse of water recovered from low salinity water like sewage or surface water is becoming an important issue from both technical and economic points of view. The reverse osmosis (RO) membrane processes widely used for water recovery inevitably produce RO concentrate having very high concentrations of salts and other materials of concern. Disposal to nature or feeding back into the recovery facility can be an option, but may have an impact on environment and raise legal problems. This study deals with the biological treatment of RO concentrate produced during water reclamation processes. The RO concentrate studied had low BOD5 and T-P, but high T-N and COD concentrations. The sequencing batch reactor (SBR) process and the modified Ludzack-Ettinger (MLE) process were compared in controlled lab-scale experiments. The pilot-scale plant was operated to evaluate the performance of the SBR process under more realistic conditions. Kinetic parameters such as specific substrate utilization rate (SSUR), specific nitrification rate (SNR), and specific denitrification rate (SDNR) were obtained, and empirical equations were derived relating these parameters to the food-to-micro-organism (F/M) ratio. These parameters could prove useful tools for process design, operation, and improvement of anoxic and aeration tanks. Non-biodegradable COD components in the RO concentrate turned out to be hard to remove, which implies some physical or chemical process (e.g. flocculation, precipitation, or adsorption) may be needed in addition to the biological treatment process.

Shortcut biological nitrogen removal in continuous-flow anoxic/aerobic process for treating low-strength ammonium wastewater

The aim of this study was to achieve shortcut biological nitrogen removal (BNR) in a laboratory-scale continuous-flow anoxic/aerobic process. A BNR reactor and a hybrid biofilm nitrogen removal (HBNR) reactor fed with low-strength ammonium wastewater were employed. The pathway of nitrogen conversion in both reactors was also evaluated. Results showed that effective biological nitrogen removal could be reliably achieved in both reactors, with over 69% total nitrogen (TN) and 95% NH4+-N removal efficiencies. HBNR improved the TN removal efficiency by over 15% relative to conventional BNR. Based on nitrogen mass balance, the fates of nitrogen in the HBNR and BNR differed, with most nitrogen removal occurring by SBNR via nitrite in the anoxic tank of the BNR and by simultaneous nitrification–denitrification via nitrite in the aerobic tank of the HBNR. The specific oxygen utilization rate test and real-time quantitative polymerase chain reaction indicated that ammonia-oxidizing bacteria were dominant in activated sludge and biofilm, and low dissolved oxygen selected against nitrite-oxidizing bacteria (NOB), even if NOB was not completely washed out from the reactors.

Raising nutrients removal efficiency by improving the internal recycling strategy in an anoxic/oxic-membrane bioreactor package plant

The effects of two internal mixed liquor recycling strategies on nutrients removal from municipal wastewater were investigated in an anoxic/oxic-membrane bioreactor package plant with a capacity of 100 m3/d. In the original strategy, in which the mixed liquor was directly pumped back to the anoxic tank from the membrane tank, the removal efficiencies of total nitrogen (TN) and total phosphorus (TP) decreased significantly due to the high dissolved oxygen concentration in the anoxic tank with an increase of the internal recycling rate from 50 to 100%. In contrast, in the improved internal strategy, i.e., the mixed liquor in the membrane tank was returned to the oxic tank with a recycling rate of 150%, and meantime, the mixed liquor in the oxic tank was refluxed to the anoxic tank at a recycling rate of 250%, the average effluent concentration of chemical oxygen demand, NH4+-N, TN, and TP were 25.3, 0.55, 28.89, and 1.71 mg/L, respectively. When external acetic acid (2.66 L/d) was added, the effluent concentration of TN decreased to 8.5 mg/L. In addition, a reduction of membrane fouling with the high nutrients removal efficiency was achieved by the improved strategy as compared with that of the original strategy. PCR–DGGE fingerprints showed that bacterial community in the activated sludge from the tanks had a high similarity under the conditions of the improved strategy. The bacteria belong mainly to phyla-proteobacteria, Firmicutes, and Bacteroidetes, which were important for nitrogen and phosphate removal.

Influence of operating parameters on the fate and removal of three estrogens in a laboratory-scale AAO system.

A laboratory-scale anaerobic-anoxic-oxic (AAO) process was constructed to investigate the influence of hydraulic residence time (HRT) and sludge retention time (SRT) on the removal and fate of estrone (E1), 17β-estradiol (E2) and 17α-ethinylestradiol (EE2), and their removal mechanisms in a biological treatment system. In an HRT range of 5-15 h, the highest removal efficiencies for E1, E2 and EE2 were obtained at an HRT of 8 h, with values of 91.2, 94.6 and 81.5%, respectively. When the SRT was increased from 10 to 20 d, all three estrogen removal efficiencies stayed above 80%, while the optimal SRT for each estrogen was different. The contribution of each tank for removal of the three estrogens was in the order of aerobic tank>anoxic tank>anaerobic tank. The optimal HRT and SRT for the removal of both the three estrogens and nutrients were 8 h and 15d, respectively. At this condition, respectively, about 50.7, 70.1 and 11.3% of E1, E2 and EE2 were biodegraded, 28.8, 17.2 and 50% were accumulated in the system, 8.3, 5.4 and 17.3% were discharged in the effluent, and 12.2, 7.3 and 20.34% were transported into excess sludge. It indicated that biodegradation by sludge microorganisms was the main removal mechanism of E1 and E2, while adsorption onto sludge was the main mechanism for EE2 removal.