Water Quality Trading and Advanced Treatment Technology to Meet the Lowest Phosphorus Requirements in the Nation

Objectives Dissolved oxygen total maximum daily load (TMDL) requirements for the Spokane River call for effluent phosphorus levels of 10 μg/L and a revised draft TMDL published in May 2008 calls for still lower effluent phosphorus of 8 μg/L. Since this requirement is below the limits of current treatment technology, the Spokane County, Washington Division of Utilities has developed a creative approach to off-setting its point source discharge by reducing a nonpoint source phosphorus loading. Spokane County will provide sewer service to a large unsewered area to reduce nonpoint source loadings from on-site septic systems to trade with the point source discharge to the Spokane River to meet the TMDL loading requirements. Phosphorus soil breakthrough analysis was used to develop a water quality offset from elimination of on-site septic systems contributing phosphorus to groundwater and the Spokane River. The nonpoint source load reduction must meet the requirements of the Washington Administrative Code (WAC) 173-201A-450 to provide a water quality offset that makes up the difference between advanced treatment to the limits of current technology at 50 μg/L and the TMDL requirements of 10 μg/L.