National Pollution Discharge Elimination

The National Pollution Discharge Elimination permit requires the Village of Skokie to notify the public of the following information:

What is a combined sewer overflow (CSO)? A CSO is a discharge from a combined sewer system directly into a waterway. A combined sewer system is designed to collect a mixture of rainfall runoff, domestic and industrial wastewater in the same pipe for conveyance to a wastewater treatment plant. A CSO may occur during heavy rainfalls when the inflow of combined wastewater exceeds the capacity of the combined sewer system and the wastewater treatment plant. The CSO outfall to the waterway acts as a relief point for the excess flow in the sewer, thereby reducing the frequency and severity of sewer backups and flooding.

What is being done to reduce the occurrence of CSOs? The MWRDGC's ongoing Tunnel and Reservoir Plan (TARP) Project was implemented to alleviate the polluting effects of CSOs and to provide relief from local flooding by providing holding capacity for 18 billion gallons of combined sewage in its tunnels and reservoirs until it can to be pumped to the water reclamation plant for full treatment. Although TARP is scheduled for completion in 2015, significant benefits have already been realized. It is estimated that since the first of the tunnels went online in 1985 until 2001, more than 578 billion gallons of CSOs have been captured and conveyed to the water reclamation plants for full treatment. Since TARP went online, the waterways have seen an increase in both the fish population and number of species present; basement flooding has been reduced; and there are fewer floodwater discharges to Lake Michigan. To date, more than $2 billion have been spent on the project.

Why do floodwater discharges to Lake Michigan occur? During extremely heavy rainfall in the Chicagoland Area, storm runoff empties into the waterways system causing the water level to rise. The water level may rise to a level sufficient to submerge the CSO outfalls, thereby reducing the rate of discharge from the outfall. This can result in basement backups and local flooding. The discharge of floodwaters to Lake Michigan occurs when the waterways reach high levels and threaten flooding of structures along the waterway and submergence of CSO outfalls. Since the initial operation of TARP in 1985, the number of times that floodwaters are discharged to Lake Michigan has been reduced. When TARP is fully complete in 2015, the number will decrease further.

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