DEVELOPMENT OF A HIGH-RATE SEDIMENTATION PROCESS FOR WET WEATHER TREATMENT

A novel high-rate sedimentation process has been developed for treating combined sewer overflows (CSOs). A study was conducted for the development of this process with the primary goal of finding a way by which the BOD5 load from the sewer to the public water body in wet weather could be reduced by 65%. This study followed a specific target determined by a committee in Japan's Ministry of Land, Infrastructure and Transport. The objective was to develop a new process capable of treating CSOs directly, i.e. capable of a high solid-liquid separation rate up to 50 m3/(m2·h), compact enough to add on to an existing wastewater treatment plant, capable of simple operation, with simple structure for simple maintenance, and capable of being run under a low running cost.A sedimentation process was modified and improved to enable easy operation and maintenance. This was done using a test facility at an actual wastewater treatment plant in Tokyo. Pilot test results, carried out 13 times, from December 2002 to July 2003 in wet weather, suggested that the process was suitable for treatment of CSOs. The performance of the process was favorable at 50 m3/(m2 ·h) up to 880 mg/L influent TSS, removing between 78% and 91% of TSS at loading and between 64% and 85% of BOD5 also at loading. In cases of high influent pollutant concentration in the first flush, the performance showed almost identical performance, without any need for controlling chemical dosage rates. The pilot testing clarified that a decrease in the influent alkalinity after the first flush peak caused a drop in the pH after FeCl3 addition and thus improved coagulation, with a significant decrease in the effluent TSS.