Typically, sanitary sewer systems transport household, commercial and industrial wastewater to a Sewage Treatment Plant (STP) for treatment, while storm sewers transport rainwater and snowmelt to a surface water to be discharged.  In most areas of NJ, sanitary sewer systems and stormwater systems function separately, but in 21 municipalities in NJ the sanitary sewers and storm sewers are combined.  Combined sewer systems are shared underground pipe networks that direct both sanitary wastewater and stormwater to a STP for treatment.  Under normal conditions, all the sanitary wastewater and stormwater are transported to a STP.  During rainfall events or snowmelt, the additional volume of water can overwhelm the capacity of the pipes and cause a CSO outfall to discharge a mixture of sewage and stormwater to the surface water.  Combined sewer systems are designed to overflow when they are over capacity and discharge the excess sanitary wastewater and stormwater directly to the surface water through a CSO outfall.  These discharges contain high levels of pollutants that may pose public health risks and impair water quality and the recreational uses of the surface waters.  However, CSOs provide a hydraulic release for combined sewer systems when they are over capacity.  Without CSOs this mixture of sewage and stormwater could backup into streets and homes.

In 1994, the United States Environmental Protection Agency (USEPA) established the Federal CSO Policy to provide guidance on how communities with CSOs can achieve the goals of the Clean Water Act. The Federal CSO Control Policy requires permittees to implement the Nine Minimum Controls, and to develop and implement a Long Term Control Plan. The Federal CSO Policy defines the expectations for the New NJPDES CSO permits. Since 1995, the NJDEP has issued NJPDES permits to the owners/operators of combined sewer systems and/or CSO outfalls with the goal of reducing or eliminating the discharges from CSO outfalls in NJ.

CSOs can be reduced or eliminated through gray and green infrastructure practices. Gray infrastructure is the traditional practice for wastewater and stormwater management, such as sewer separation, expansion of the sewage treatment plant, storage tanks, and treatment at the outfall.  Green infrastructure mimics natural processes utilizing soils and vegetation to manage stormwater.

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