Source apportionment of pollutants and flows of combined sewer wastewater.

To reduce the combined sewer overflows (CSOs) and improve surface water quality in the Greater Milwaukee area, construction of a 19.5-mile (31.4 km) inline storage system (ISS) was completed in 1994 to capture and convey sanitary sewage and stormwater for treatment. However, one to six overflows per year still occur. Chemical mass balance (CMB) and positive matrix factorization (PMF) modeling was done in order to find origins of flows and pollutants in CSOs. Based on overflow events from 2000 to 2006 (CMB) and 2004 to 2006 (PMF), we found that between 27% and 56% of the total overflow is from sanitary sewage and most of the remaining from stormwater with possible minor contribution (< or = 8%) from groundwater. Most total suspended solids and metals (Cd, Cr, Cu, Pb, Ni, Hg, and Zn) are from stormwater, while sanitary sewage carries large contributions (> or = 28%) of BOD5, NH3, and total phosphorus. The fraction of NH3 is especially high, i.e., > or = 58%. Implications for a possible future sewer separation in combined sewer areas into separate sanitary and storm sewers are discussed.