In existing wastewater treatment plants that employ biological nitrogen and phosphorus removal processes, a low concentration of organic substrates in the influent wastewater has a destabilizing effect on the phosphorus removal process. Many efforts have been made to combat this problem, including reconstructing the process flow and improving operation and control systems. However, because the mechanism used for biological phosphorus removal is complex, it is difficult to establish effective empirical methods. For this paper, we constructed a simulator for the wastewater treatment process and tried to improve on current procedures, focusing on the planning, execution, and evaluation of methods of retrofitting existing WWTP with equipment for biological nutrient removal. The Shinnanyo WWTP uses the anaerobic/aerobic activated sludge process to remove nitrogen and phosphorus biologically. At this plant, however, the influent wastwwater has an insufficient concentration of organic substrates, thus decreasing the efficiency of the biological phosphorus removal. An analysis of organic consumption in the reaction tank on the simulation suggested that injecting primary sludge into the reaction tank would increase the efficiency of phosphorus removal process. Full scale experiments conducted at the plant verified the efficiency of this method. In addition, by shortening the A-SRT, ensuring that nitrification is not negatively affected, the efficiency of the nitrogen and phosphorus removal was significantly improved.

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