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Combined Sewer Overflows (CSO's)Combined sewers are remnants of the country's early infrastructure. In the past, communities built sewer systems to collect both storm water runoff and sanitary sewage in the same pipe. During dry weather, these "combined sewer systems" transported wastewater directly to the sewage treatment plant. In periods of rainfall or snowmelt, however, the wastewater volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For this reason, combined sewer systems were designed to overflow occasionally and discharge excess diluted wastewater directly to nearby streams, rivers, lakes, or estuaries. Combined sewer overflows contain not only storm water but also untreated human and industrial waste, toxic materials, and debris. This is a major water pollution concern for cities with combined sewer systems. The City of Defiance’s sewer system began operation in the late 1800’s. These initial sewers collected rainwater runoff, domestic sewage, and industrial wastewater during wet weather periods. Such sewers, called combined sewers, discharged directly into receiving streams. As of the 1950’s, 32 combined sewers within the City of Defiance discharged into the Maumee River, the Auglaize River and Preston Run. In 1956, the City constructed a system of 32 sewage regulators, seven interceptor sewers, 4 pumping stations and the Water Pollution Control Facility (WPCF). The City’s combined sewer overflow system currently consists of 32 regulators and 12 overflow manholes.
The twelve CSO manholes provide overflow relief by utilizing features that are not mechanical by design such as leaping weirs, surcharge relief pipes, troughs, or drop down structures. The interceptor sewers direct dry weather flow and some wet weather flow from the four pump stations to the WPCF. The interceptor sewers and the pumping stations were named for their geographic location in the City some examples are as follows:
Since 1956, additional interceptor sewers and pump stations have been included in the City’s sewer system; the Maumee Drive Interceptor Sewer and Pump Station in 1969 and the Preston Run Intercepting Sewer and Pump Station in the late 1980’s. As of 1969, development within the City and areas annexing to the city were required to provide separate sanitary and storm sewers. These areas are usually located around the perimeter of the City. These new sanitary sewers have been and continue to be connected to combined sewers in the older parts of the City. In 1988, the system was upgraded with new pumps at four pump stations and all 32 combined sewer regulators listed with the twelve other Combined Sewer Overflow (CSO) manholes, were restored to their original condition. The 32 regulators and the twelve overflow manholes are assigned National Pollution Discharge Elimination System (NPDES) identification numbers by the Ohio Environmental Protection Agency (OEPA). These ID numbers allow for tracking of improvements, such as percent reduction in flow, to the system. Combined Sewer Overflow Requirements One of the requirements of the City’s National Pollution Discharge Elimination System (NPDES) permit No. 2PD00013*OD is that the City implement the federally mandated 9 minimum control measures Nine Minimum Controls to control and manage CSO discharges during wet weather periods. In response to this requirement, the City has developed and implemented a Combined Sewer System Operational Plan and a Combined Sewer System Long-Term Control Plan. These plans were filed with the Ohio EPA in October of 1997 and December of 1998 respectively. The 9 minimum controls include the requirement to modify, as necessary, the industrial pretreatment program to minimize CSO discharges during wet weather. In accordance with this requirement, City of Defiance industrial sewer users are required to consider the impact of plant discharges into the City’s sewer system during wet weather periods. It is important to consider limiting any pretreatment system discharges and minimizing other plant wastewater discharges to the extent possible during periods of wet weather to minimize the impact on CSO discharges. Also, the City prohibits the release of any controllable batch wastewater discharges during wet weather conditions. This is a requirement contained in all of the discharge permits issued by the City. Additional information regarding combined sewer overflows is contained in the CSO portion of this web site and at this USEPA CSO web site. |
The regulators, installed in each of the 32 combined sewers, diverted sanitary flow during dry weather periods and some wet weather surface runoff flow to the intercepting sewers. The intercepting sewers carry sewage to pump stations that transport wastewater to the WPCF. The regulators consist of a mechanical gate that is activated by a float. The gate and float are enclosed in a concrete chamber located on the combined sewer. Sanitary wastewater normally enters the chamber and is diverted by a weir to the intercepting sewer. When larger flows occur during wet weather, excess flow rises over the weir and enters the float chamber. This water entering the chamber causes the float to rise, activating a gate that closes off flow from entering the intercepting sewers. The water entering the chamber is diverted to the overflow pipe and out to the nearest stream. (see FIG. 8-19) 
