The SFPUC’s Wastewater Service Charges cover the cost of collecting, transporting, treating and disposing of each unit of wastewater discharged into the sewer system. Wastewater service charges are calculated by multiplying water consumption by an assigned flow factor. The flow factor is the percentage of metered water use returned to the sewer system as wastewater. For purposes of determining applicable charges, the percentage of water use returned to the sewers (flow factor) is assumed to be 90% for single-family residential users and non-residential users and 95% for multifamily residential users.