An Innovative Approach to CSO Control can Reduce Costs

The Detroit Water and Sewerage Department (DWSD), in developing the Long Term CSO Control Plan, estimated that the cost of capital improvements needed to control CSO discharges would exceed 1 billion. Of particular concern are those discharges to the Detroit River. Real estate along the heavily urbanized river is among the highest valued property within the City. Using that property for pollution control facilities would likely generate protests from many different quarters. Therefore, DWSD is recommending an innovative screening and disinfection approach to reduce the impacts of CSOs on the Detroit River.To accomplish the mission, underground structures were designed for two pilot facilities to convey the CSO flows through 4mm raked bar screens produced by two different manufacturers. The screens, developed in Europe, direct the solids to the wastewater treatment plant through a side stream. These screens do not require residuals to be collected and transported from these remote facilities. This is a major benefit because hauling the screenings through high value residential and commercial areas is judged to be unacceptable. In order to judge the efficiency of the screens, both types will be compared side by side in one of the structures.To maximize the mixing of the disinfectant, high-energy mixers were included in chambers located after the screening equipment. The rapid mixing caused by these high-energy mixers will help maximize the bacterial kill and minimize the disinfectant discharged to the Detroit River.The discharges from the facilities are directed into large diameter pipes that will provide sufficient volume to provide the minimum 10 minutes disinfection contact time at the design flow. The plan maximizes the use of existing sewers to provide the volume for disinfectant contact, thereby minimizing the need for new construction on riverfront properties. Also included are control structures, instrumentation and control devices that will control the flows to maximize disinfectant contact time.If the use of this innovative screening and disinfection approach is successful, the citizens of Detroit will save a substantial portion of the estimated cost to construct retention basins, as well as reduce the impact of such facilities on scarce riverfront real estate.