Treatment Performance Stabilization after Wet Weather by Operation Control in Anaerobic-Anoxic-Oxic (A~2/O) Process

摘要

Yokohama City Government is currently introducing the anaerobic-anoxic-oxic (A~2/O) process towastewater treatment plants (WWTPs) as a Tokyo-Bay eutrophication measure. A~2/O processwhich is one of biological nutrient removal methods has some problems such as the phosphorusremoval instability and the maintenance cost. We have examined measures to solve thoseproblems.Generally, phosphorus removal of A~2/O process keeps a good condition in dry weather. But it turnsworse rapidly after a few days of rain and shows behavior to recover slowly. Because the ratio ofthe combined sewer system is high in Yokohama City, the control of wet weather phosphorusremoval is necessary for keeping the design effluent quality. And they have to be able to controlphosphorus removal deterioration peak without additional costs.From such a background, we have carried out the examination of the control of wet weatherdeterioration at the A~2/O process plants in Kanagawa WWTP. During 2004-2006, we examinedthe effect of the control of hydraulic retention time (HRT) by the inflow volume and circulationpump operation after wet weather on the water quality in the reaction tanks and that of treated waterabout six investigation periods. As the results, two conditions could control the deterioration, oneis influent water increase, the other is largely extending anaerobic tank HRT by stopping circulationpump. The achievement of the design effluent quality was enabled by their operations. Inaddition, it was suggested that reduction of the aerobic tank HRT also could improve the phosphorusremoval by a result of the collected data analysis.