Prior to 1970, wastewater was discharged directly into the Charleston Harbor. Water quality concerns, coupled with the implementation of state and federal regulations requiring the treatment of wastewater, prompted the City of Charleston to turn the sewer system over to Charleston Water System. In the late 1960s, Charleston Water System began constructing a wastewater treatment plant and a deep tunnel system to intercept existing sewer lines and deliver wastewater to the new plant.

When the Plum Island Wastewater Treatment Plant began operating in 1971, it provided primary treatment (removal of solids and debris from wastewater) for up to 18 million gallons per day (mgd). The plant included an influent pumping station, settling tanks for the removal of solids, an outfall to the Harbor, and a solids handling building and incinerator.

In 1984, the plant was upgraded to provide secondary treatment, which involves the use of microorganisms to consume harmful bacteria in the wastewater. Another expansion was completed in 1990 to bring the treatment capacity to 27 mgd. In 1997, Charleston Water System staff completed an in-depth study of the plant’s performance and process capability and obtained a permit to increase to a capacity of 36 mgd, which is the current permitted capacity of the plant.

In 1992, following the City’s annexation of Daniel Island, Charleston Water System built the Daniel Island Wastewater Treatment Plant to handle wastewater generated on the Island.  But booming growth, coupled with stringent discharge permits, made the plant to costly to upgrade, so Charleston Water extended the Cooper Sewer Tunnel under the Cooper River to send the wastewater to the Plum Island plant.  Wastewater is undergoes pretreatment at the Daniel Island facility before entering the tunnel.

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