Optimization of a Cyclic Activated Sludge Technology process for organic matters oxidization & NH₃-N removal in town sewage treatment plant

Abstract

In recent years, water pollution incidents have frequently occurred in various parts of China. For example, surface water bodies such as Taihu Lake in Wuxi, Dianchi Lake in Yunnan, Chaohu Lake in Anhui, have experienced serious outbreaks of eutrophication, massive algal blooms and cyanobacteria crisis. Sewage treatment plants with the promulgation of a series of environmental policies from government are faced with more stringent discharge standards and requirements. The Cyclic Activated Sludge Technology (CAST) has been applied in the municipal wastewater treatment plants in the city of Wuxi. This study investigates the concurrent removal of organics, nitrogen and phosphorus in municipal sewage by the CAST process. The aim of this study is optimization of the CAST process for organic matters oxidization and NH₃-N removal in town sewage treatment plant. The general process of CAST has the ability to remove organic matters, nitrogen and phosphor. However, when the facilities achieved a high level of nitrogen removal, filamentous sludge bulking would be a common problem and the removal efficiency would be limited. This thesis aims to develop a novel CAST process to optimize CAST reaction operation and the DO control during aeration period and finally culture a stronger activated sludge. And also, the removal efficiency of organic matters and ammonia nitrogen would be greatly enhanced to mitigate the eutrophication of Taihu Lake in treating high nitrogen concentration wastewater. The full-scale study of the organic and nitrogen removal was carried out in a Sewage Treatment Plant in Wuxi near Taihu Lake. The optimum operation cycle was concluded and also the other operation strategy. Results showed that the CAST process was capable of removing organic matter, Suspended Solid (SS), and Total Phosphorous (TP) with high removal efficiencies (up to 85%). However, the removal of nitrogen and non-biodegradable organic substances were not as satisfactory. The residual Chemical Oxygen Demand (COD) concentration in effluent had an obvious relationship with the effectiveness of the prior anaerobic process, and could be reduced to as low as 50 mg/L by incorporating coagulation and sedimentation units to the CAST process. In addition, a number of operating conditions had been found to be as important factors of achieving favorable nitrification and the significant increase of ammonia nitrogen removal rates (up to 95%). These conditions include: the DO concentration which was controlled below 0.5 mg/L during first 30 minutes of the inflow/aeration stage, 2.04.0 mg/L during pure aeration, and had the DO level guaranteed above 2.5 mg/L for at least 20 minutes at the end of aeration stage. Under specific operating conditions, sludge return ratio set at more than 20% and with proper DO concentration gradient between anoxic zone and aerobic zone of CAST system, have also been found to be as factors of achieving the synergistic effect of Simultaneous Nitrification and De-nitrification and Sequential Nitrification and De-nitrification, and increasing Total Nitrogen removal rates up to 60%. These conditions were: the DO concentration was controlled below 0.5 mg/L during first 30 minutes of the inflow/aeration stage, 2.03.0 mg/L during pure aeration stage, and the DO concentration was maintained below 0.5 mg/L in anoxic zone during the whole process. In conclusion of this study, the essential operating parameters of the advanced CAST process were successfully optimized: the organic matters oxidization and NH₃-N removal rate were increased and the sludge bulking condition was well controlled and final the effluent from town sewage treatment plant was successfully met the stringent discharge standards, without having the process hardware upgraded. The results also show that the CAST could be also modified for treating the influent of high nitrogen loading and fluctuating conditions.