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OPTIMIZATION OF COMBINED SEWER OVERFLOW CONTROL SYSTEMS
Citation:
O'Connor*, T P. AND R Field*. OPTIMIZATION OF COMBINED SEWER OVERFLOW CONTROL SYSTEMS. Presented at Fifth International Conference: Diffuse/nonpoint Pollution & Watershed Managment, Milwaukee, WI, June 01 - 15, 2001.
Impact/Purpose:
To inform the public.
Description:
The highly variable and intermittent pollutant concentrations and flowrates associated with wet-weather events in combined sewersheds necessitates the use of storage-treatment systems to control pollution.An optimized combined-sewer-overflow (CSO) control system requires a management strategy that maximizes the use of the existing system before construction of new facilities and sizes storage volume in concert with the treatment rate to obtain the lowest-cost storage-treatment system. This paper describes the components, hardware and strategies to create such a system. The first step is to analyze where the most economical system components are (e.g., maximizing in-sewer storage by simple CSO-flow-regulator adjustments, installing dynamic regulators that respond to remote sensing devices and direct more flow to thewastewater treatment plant (WWTP)). Second, analyze low-cost modifications to WWTP. The most expensive options, constructing new parallel facilities at the WWTP, and additional storage or treatment facilities, should be considered last. For a given level of control, an important part of CSO-control system planning is the determination of the economic break-even point between the required storage volume and treatment capacity. It is not economical to design storage facilities beyond an optimum tank volume since the rate of cost increases more rapidly than the pollution removal benefit for retaining the flow or larger, infrequent storms. To effectively employ control technologies in a combined sewer system, the functions, applicability, and idiosyncrasies of their individual designs must be clearly understood. Unlike sanitary-sewer wastewater, with easior to establish diurnal pollutant distributions and hydraulic loadings, CSO characteristics are dependent on sewer system and sewershed and solids rainfall characteristics which vary during a storm event. Accordintly, it is important to characterize CSO on a site-specific basis since the build up of solids, which are subsequently resuspended during intensive storm flows, is a function of the local topography, geology and antecedent dry-weather periods. Many existing WWTPs have treatment limitations due to interceptor capacity, expansion costs, and space availability. Therefore, along with capacity enhancement, optimization alternatives must include upstream modifications (e.g., high-rate, satellite treatment/storage in the proximity of an upstream CSO point and catchbasin modifications) and management options (disconnecting roof drains and real-time control). Each system requires its own optimization plan since no one method of storage, treatment, or management has been demonstrated as a cure all. Only through the evaluation of the total sewershed system can the appropriate methodology and technology be chosen for CSO-control optimization.