| As an activated sludge process Carrousel Oxidation Ditch is widely used in China. To prevent the eutrophication of water environment it is significant to enhance the nitrogen removal capacity of Carrousel oxidation ditch. Oxidation ditch is a process with both characteristics of complete mixed and plug flow reactor. In the oxidation ditch, the spatial anoxic zones along the ditch are created by adjusting the aeration at different points; high internal recycle flow subjects mixed liquor to alternating aerobic and anoxic condition, which results in the advantage for the occurrence of Simultaneous Nitrification and Denitrificaiton (SND). The study on the optimum condition of removal nitrogen in the ditch was carried out by the investigation of full-scale Carrousel oxidation ditch and the experiment of pilot-scale and lab-scale Carrousel oxidation ditch. The study on the strategy and methods of control to enhance removal nitrogen regarding the fluctuation of influent was conducted basic on the simulation by using Computational Fluid Dynamics (CFD) and Activated Sludge Model (ASM).To enhance nitrogen removal, a systematic monitoring of the change of the biological and hydrological parameters of Carrousel Oxidation Ditch was carried out in full-scale and lab-scale. The variation and distribution of parameters such as flow velocity, the concentration of dissolved oxygen and Mixed Liquor Suspended Solids (MLSS) in oxidation ditch were measured and analyzed, which are major control factors for removal of nitrogen. The results indicated that the grads of dissolved oxygen along the ditch is a key factor for the occurrence of SND. To study the feasibility of SND in the ditch a series of research was conducted in lab-scale and pilot-scale of Carrousel oxidation ditch. The optimum operational condition for SND in the ditch under different operational conditions was obtained. The results indicated that the total nitrogen removal of 70% was achieved with the influent C/N of 5-6 and with two inlets of influent in the anoxic zone of the ditch. The feasible fluid conditions for SND are interval time of about 10 min alternating aerobic and anoxic zones in the ditch and bigger diameter of sludge floc. The analysis of characters of the microbial community structure in oxidation ditch was carried out by using analysis method of molecular biology (T-RFLP and FISH). The results indicated that the microbial community structure of pilot-scale oxidation ditch is similar to other wastewater treatment plant based on analysis using T-RFLP, microbial community structure feature is affected by quality of influent, the influences of different processes on the proportion of different microbial communities is obviously. The influence of the fluctuation of influent quality in a day on the proportion of different microbial communities is weak, the change of operational effect is caused by the change of influent composition on the microbial activity.The influence of influent fluctuation on the operational parameters of Carrousel oxidation ditch was evaluated through the investigation and simulation using basic process stoichiometry, ASM and CFD. The study presented an optimized control strategy of improving removal nitrogen based on the adjustments of activate sludge loading and aeration of oxidation ditch. A discrete control model based on the adjustment of MLSS and DO was built, which intended to implement of the strategy followed the fluctuation of influent quantity and quality. A pilot-scale experiment was conducted to examine this control strategy. The results showed that the effluent quality was effectively improved by using this control method, and the effluent quality meets the requirement of TN<20 mg/L of the new discharge standards of China (GB18918-2002).
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