Study On Performance Of ABR-MBR Combined Reactor For Municipal Wastewater Treatment

Eutrophication is one of serious environmental problems in LakeTaihu. It is known thatcontrolling nutrient input is the most effective way of reducing the risk of blooms. However,domestic sewage contains abundant phosphorus and nitrogen pollutants that causeeutrophication of water bodies, and it is therefore necessary to identify the optimumbioreactor to treat municipal wastewater effectively. In order to explore a new and efficientwastewater treatment process, the combined ABR-MBR process （CAMBR） with thecharacteristics and advantages of both reactors was optimized and the shortcomings of bothreactors were overcome. It delayed the process of membrane fouling and improved theefficiency of nutrient removal. The studies have theoretical and practical significance onimproving the quality of effluent and its application. The main research results of thisdissertation were shown as follows:（1） During the72days start-up of the system, granular sludge was observed in the ABRand MBR with the diameters of3mm and2mm respectively at the end of start-up. Theaverage MLSS in ABR and MBR were28g·L^-1and6g·L^-1, respectively. The system couldacquire stable removal efficiencies of COD, NH₄⁺-N, TN and TP about91%,92%,72%and70%respectively, correspondingly with an average effluent of32,2.3,13.3and1.22mg·L^-1.（2） In order to ensure the good effluent quality and consider the performance of theprocess, optimal operation conditions with a hydraulic retention time of7.5h at a recycle ratioof200%and dissolved oxygen of3mg·L^-1was shown to achieve higher quality effluent withchemical oxygen demand, ammonium, total nitrogen and total phosphorus of26,0.5,10.5and0.4mg·L^-1, respectively. The effluent quality met the Level-one A criteria specified inDischarge Standard of Pollutants for Municipal Wastewater Treatment Plant （GB18918-2002）.（3） The whole process achieved an effective separation of microorganism’s phases, suchas anaerobic denitrification, anaerobic phosphorus release, anoxic denitrification andphosphorus removal, aerobic nitrification and phosphorus uptake. The average removal ratesof COD, NH₄⁺-N, TN and TP reached93%,99%,79%and92%, respectively,correspondingly with the COD, NH₄⁺-N, TN and TP effluent of24,0.4,10.6and0.31mg·L^-1with a HRT of7.5h at a recycle ratio of200%and DO of3mg·L^-1. The MBR enhancedNH₄⁺-N, TN and TP removal rates of14%,11%and18%, respectively. The process was ableto maintain a stable performance with high quality effluent during the period. The process haslarge biomass and is efficient on pollutants removal. In addition, it satisfies the requirementsof new, efficient and integrated process. Therefore, it is suitable to apply in small-scalemunicipal wastewater treatment plant.（4） The process had a good ability to adapt to the influent shift, and was able to achieve the simultaneous nutrient removal of both two kinds of municipal wastewaters effectivelywith the high quality effluent chemical oxygen demand, ammonium, total nitrogen and totalphosphorus of lower than28,0.6,12.6, and0.4mg L^-1, respectively. The CAMBR had goodremoval efficiencies of COD under different temperature conditions, the removal efficienciesof TN had a positive correlation to NH₄⁺-N, and the effluent quality is good in the stableoperation of the system. Furthermore, the effluent quality of the middle temperature conditionwas the best. In addition, TN removal efficiencies were stabilized around70%with theeffluent concentration about9mg·L^-1, and TP removal efficiency was about73%with theeffluent concentration about0.8mg·L^-1in high temperature environments. While the processoperated in low temperature environment, TN removal efficiency was only57%with theeffluent concentration of5mg·L^-1, and TP removal efficiency of67%with the effluentconcentration about1mg·L^-1. Thus, in order to achieve better nitrogen and phosphorusremoval efficiencies, insulation measures should be taken to keep the process warm.Intermittent pumping water, hydraulic recoil can effectively reduce the degree of membranefouling, but chemical clean is necessary to completely remove irreversible pollutants.Membrane fouling is closely related to three factors of sludge concentration, membrane fluxand aeration rate. Intermittent suction, scouring aeration, regular drug-wash, and on-linehydraulic backwash as a mean of inhibiting membrane fouling, ensure the sustainable runningof the CAMBR system.（5） The polymerase chain reaction with denaturing gradient gel electrophoresis andfluorescence in situ hybridization were used to analyze the structure of microbial communitythroughout the whole experient and the dominant bacterial composition of the activatedsludge in each compartment of the CAMBR. Throughout the process, the microbialpopulation within the system maintained diversity in distribution, while the dominant florawas prominent. During the same period, microbial population in each compartment had highsimilarity, and the structure of microbial community had no significant changes within arelatively short period. The physiological state of the each compartment was changed throughrespective different microenvironments. Thus, microorganisms were able to play a differentrole in the different compartments. The nitrifying bacteria and phosphorus accumulatingorganisms gradually increased with the running of the experiment, guaranteeing the efficiencyof nitrogen and phosphorus removal in the system. The specialized microorganisms, includingnitrifying bacteria （ammonia-oxidizing bacteria and nitrite-oxidizing bacteria）, andphosphorus accumulating organisms, were enriched in this process. The enrichedmicroorganisms in the system were the underlying reason why the CAMBR was efficient fornitrogen and phosphorus removal.