A Combined Sewer Overflow (CSO) is a discharge of untreated storm and wastewater from a combined sewer into the environment. CSOs typically occur when combined sewers fill up with too much water for the system to handle, most often during heavy rains, and the excess water is released into a stream or river.
Richmond's CSO system is the largest in the state of Virginia. The area serviced by Richmond's CSO system is approximately 12,000 acres.
During dry weather, combined sewer systems carry all the sanitary flow to wastewater treatment plants. During times of rainfall, however, the amount of rain adds to the amount of flow going to the treatment plant. This heavier flow is greater than the capacity of the combined sewer system. When the flow exceeds the capacity, the excess flow is discharged directly to the river at various overflow points in the sewer system. In Richmond, the major overflow points are found on the banks of the James River and Gillies Creek.
Richmond's CSO system is financed through state and federal grant funding, state low-interest loans, and rate payers.
The CSO program began in 1970. Here's the timeline:1972: Study recommends construction of a retention basin for the Shockoe CSO area.
1974: Initiation of a comprehensive CSO study, including extensive CSO sampling.
1978: Temporary suspension of the 1974 CSO study, awaiting the outcome of State of Virginia James River water quality studies.
1983: Completion of the construction of the Shockoe CSO area retention basin.
1985: Completion of State of Virginia Water Quality Model of the James River. Resumption of 1974 CSO study.
1987: Initiation of construction of wastewater treatment plant improvements to increase plant capacity during wet weather events to allow emptying of Shockoe CSO area retention basin in two days and to accept additional wet weather flow.
1988: Completion of the comprehensive CSO study defining the Long Term Control Plan (LTCP) for the CSOs.
1990: City completes the implementation of the initial elements (Phase I) of the approved plan.
1992: State Water Control Board issues a Special Order requiring implementation of additional elements included in Phase II of the plan. This Special Order includes a requirement 7(d) to Re-Evaluate the CSO Control Plan, at the end of Phase II to determine if changes should be made to the approved plans.
1996: The Department of Environmental Quality (DEQ) amended the Special Order to accelerate all the Northside CSO Control Projects and place on hold the swirl concentration project because this technology had not produced, nationwide, the expected results.
1998: City places in operation all CSO conveyance projects on the south and north sides of the James River.
1999: The DEQ Piedmont Regional office issues a Special Order by Consent requiring the city to advance the schedule of the re-evaluation of the CSO Control Plan consistent with the EPA National CSO Control Policy.
2002: City completes CSO Re-Evaluation Report.
2003: City places in operation CSO Retention Tunnel on the north side of the James River.
2006: Completion of the Program Project Plan that shows the master plan for the Phase III CSO Controls.
2007: The DEQ is evaluating the water quality standards and developing a water quality model as part of the Richmond Area Total Maximum Daily Load.