Study On Optimizing A~2/O Process For Upgrading And Reconstrcution Of Wastewater Treatment Plant

In order to corperate with the east route of south-to-north water diversion project and water quality improvement of Xiaoqing river, the key upgrading and reconstrcution technologies for municipal wastewater treatment plants to meet the discharge standard of level of A were studied, including the operation parameters of A2/O process for nitrogen and phosphorus removals, the selection of chemicals and the optimum dosing point for phosphorus removal, the advanced treatment technologies for meeting the level of A standard standard and the measures to control the activated sludge bulking in winter. These findings provided the related technical supports for the A2/O process of upgrading and reconstrcution of municipal wastewater treatment plant to meet the level of A standard. The major achievments are listed as follows.Based on the charaticristics of wastewater, the optimized operation conditions of the pilot scale A2/O reactor were presented as follows: hydraulic retention time(HRT) was 10 h; the internal return ratio of mixed liquer(r) was 200%. And other conditions were the sludge return ratio(R) of 100%, the sludge retention time(SRT) of 15 d and the dissolved oxygen(DO) concentration maintained in the aerobic zone of 2 ~ 4 mg/L.The optimized combined effect of assisted chemical phosphorus removal and biological phosphorus removal was found. The dosing concentration of PAC was 29.3mg/L with the dosing point at the end of the aerobic zone. The TP concentration in effluent was stably controlled under 0.45 mg/L, which met the national "urban sewage treatment plant pollutant discharge standard"(GB18918-2002), level of A standard. The PAC dosing caused the negligible impact on the activated sludge activities in the biological treatment system.The proper water level of fiber filter was 0.48 m with the running cycle of 24 h at the low turbidity(5 NTU) and 6 h at the high turbidity(15 NTU). The optimized operation conditions for the sand filter were obtained as follows: the filtration rate of 10 m/h, the backwash cycle of 20 h with the air backwash time of 3 min, the combined gas and water backwash time of 4 min and the water backwash time of 3 min for each cycle.Winter was the most difficult time for the operation of wastewater treatment plants as the lower temperatures, high concentration of pollutants, and more toxic and harmful substances. When the activated sludge bulking occurred, the dosing of PAFC had a significant effect on the improvement of sludge settling performance. After 24-day’s dosing, the SV30 value of activated sludge was reduced from 90% to 73%.