Combined Sewer Overflows
Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, combined sewer systems transport all of their wastewater to a sewage treatment plant, where it is treated and then discharged to a water body. During periods of heavy 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 are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other water bodies.
These overflows, called combined sewer overflows (CSOs), contain not only storm water but also untreated human and industrial waste, toxic materials, and debris. They are a major water pollution concern for the approximately 772 cities in the U.S. that have combined sewer systems.
CSOs may be thought of as a type of "urban wet weather" discharge. This means that, like sanitary sewer overflows (SSOs) and storm water discharges,they are discharges from a municipality's wastewater conveyance infrastructure that are caused by precipitation events such as rainfall or heavy snowmelt.
EPA's CSO Control Policy, published April 19, 1994, is the national framework for control of CSOs. The Policy provides guidance on how communities with combined sewer systems can meet Clean Water Act goals in as flexible and cost-effective a manner as possible. EPA's Report to Congresson implementation of the CSO Control Policy assesses the progress made by EPA, states, and municipalities in implementing and enforcing the CSO Control Policy.
The City of Clarksburg has implemented a Long Term Control Plan to address minimization of the discharges from the CSOs. Of the 84 original CSOs in the system, 29 have been eliminated. The wastewater treatment plant has been upgraded to treat an additional 5.0 million gallons per day and $1,000,000+ has been spent to upgrade and seal the main interceptor system to eliminate the introduction of stream and ground water. As a result, overflows from the CSOs have been reduced by 25% and flows through the treatment plant have increased approximately 11%. Additional steps are being planned to further reduce the discharges from the CSOs.
Twice per year an advisory is posted in the local newspaper in regards to public health as follows: Public Advisory.