Simultaneously Removal Of Nitrogen And Phosphorus Using Enhanced A~2O-BAF Process

The standard of wastewater discharge was controlled more strictly. Therefore, the wastewater techniques developed gradually from the period of pure COD removal to the period of simultaneous COD, nitrogen and phosphorus removal. Today, the nitrogen and phosphorus removal technology aimed for eutrophication control has been the study focus in the wastewater treatment area. The existence of some challenges troubled with biological nutrient removal (BNR) in wastewater treatment plants maybe result from the following reasons, such as high energy consumption, low removal efficiency and unstable operation. Therefore, it is urgent to find ways on how to improve the conventional nitrogen and phosphorus removal of the existing wastewater treatment process, which is in favor of saving money, improving effluent quality and reusing wastewater resource.Anaerobic-anoxic-oxic (A~2O) process can achieve good N/P removal without chemical addition, which makes it to be used widely in the worldwide full-scale wastewater treatment plants. However, activated sludge in such system comprises of complex microbiological community, which results in the low and variable N/P removal efficiency at high cost, new technologies of N/P removal based on the process control can overcome these problems to a certain extent. In order to accelerate the investigation development of A~2O process, in particular to accelerate the application of A~2O process for the low C/N wastewater treatment, Under this background, an innovative two-sludge A~2O-BAF process, combining the advantages of suspended growth and attached growth process, was developed to provide new brainstorm for the upgrade of built-up WWTPs(wastewater treatment plant)and the design of new WWTPs.The A~2O process was set up in two-sludge system, in which nitrifies were separated from denitrifying phosphorus removal bacteria in a nitrification biofilm reactor, whereas DPB sludge was recalculated between an anaerobic reactor and an anoxic reactor. The process solves the competition of organic substance among Poly-p organisms and denitrifiers as well as the problems of overgrowing of slow reactor volume can be reduced strongly. Since it is possible to control nitrification and denitrifying dephosphatation separately, there will be more possibilities for optimal process design for integrated phosphorus and nitrogen removal. A benefit of two-sludge system in comparison with single-sludge system is to be capable to operate each step of nitrification and denitrifying dephosphatation under optimal conditions. It is possible to control the SRT separately in the nitrification and denitrifying dephosphatation step. This will lead to optimal control of the nitrification step for the two-sludge system. The long SRT of nitrification reactor no longer affects the phosphorus removal efficiency, instead, this benefits to the complete nitrification which shall supply enough electron acceptors to guarantee the anoxic phosphorus remval efficiency.After stable operation of A~2O-BAF process, firstly, the effect of volume ratio in A~2O process on the nitrogen and phosphorus removal was discussed.The results showed that the A~2O-BAF process has potential for the domestic wastewater treatment with 1:6:2, 67.4% of TN and 98.6% of TP was removed. This can guarantee the advantage of PAOs growth and effectively restrain nitrifying bacteria proliferate, and the system of nitrogen phosphorus removal efficiency is the highest. On the other hand, it can be found optimizing anoxic biological phosphorus removal in the A~2O system was dependent on some parameters: sludge retention time(SRT),MLSS,sludge return ratio and internal return ratio. The SRT of 15 days,the sludge return ratio of 60~100%,the internal return ratio of 300%, MLSS around 3000mg/L,were determined as optimal for the phosphorus and nitrogen removal in the system. The effluent quality could satisfy the first level A criteria specified in the discharge standard of pollutants for municipal wastewater treatment plant (GB18918-2002).Operational performance of A~2O-BAF was investigated when treating domestic wastewater with low carbon-to-nitrogen ratio at low temperature. Under the condition of average temperature of 14.2℃and carbon-to-nitrogen radio of 4.81, enhanced nitrogen and phosphorus removal was achieved. Average effluent total nitrogen and total phosphorus concentrations were 13.21mg/L and 0.23mg/L, respectively. The removal efficiencies of COD, ammonium nitrogen, total nitrogen and total phosphorus were 86.2%, 99.8%, 96.6% and 81.5%, respectively. The effluent quality could satisfy the first level A criteria specified in the discharge standard of pollutants for municipal wastewater treatment plant (GB18918-2002). Although the system was operated under low temperature, good settle ability with SVI of 85.4 mL/g was obtained. Moreover, pH and oxidation reduction potential could be used as control parameters for process control of A~2O-BAF system.In order to solve the instability of removing nitrogen and phosphorus process in A~2O, the high effluent nitrogen and phosphorus concentration issues. Fluidized immobilization media were added to the aerobic reactor to maintain a high concentration of nitrifying bacteria and nitrification rate. The experiment put great emphasis on the effect of different carbon loading, carbon sources and filing carbon modes on the process of nitrogen and phosphorus. Denitrifying anoxic phosphorus uptake was the main phosphorus removal.