Characterization and treatment of urban land runoff

"Urban land runoff from a 1.67 square-mile urban watershed in Durham, North Carolina, was characterized with respect to annual pollutant yield. Regression equations were developed to relate pollutant strength yield. Regression equations were developed to relate pollutant strength to hydrograph characteristics. Urban land runoff was found to be a significant source of pollution when compared to the raw municipal waste generated within the study area. On an annual basis, the urban runoff yield of COD was equal to 91 percent of the raw sewage yield, the BOD yield was equal to 67 percent, and the urban runoff suspended solids yield was 20 times that contained in raw municipal wastes for the same area. Downstream water quality was judged to be controlled by urban land runoff 20 percent of the time (i.e., the pounds of COD from urban land runoff was approximately 4-1/2 times the pounds of COD from raw sewage.) It is conceivable that critical water quality conditions are not typified by the 10-year, 7-day flow, but by the period immediately following low-flow periods when rainfall removes accumulated urban filth into the receiving watercourse, greatly increasing the pollutant load while not substantially increasing water quantity. Specific urban land use did not appear to influence the quality of urban land runoff. The applicability and effectiveness of plain sedimentation and chemical coagulation of urban land runoff was evaluated. Plain sedimentation was found to remove an average of 60 percent of the COD, 77 percent of the suspended solids, and 53 percent of the turbidity. Cationic polyelectrolytes and inorganic coagulants were found to provide significant residual removal increases over plain sedimentation. Alum was judged the best coagulant and produced average removals of COD, suspended solids, and turbidity of 84, 97, and 94 percent, respectively. The EPA Storm Water Management Model (SWMM) was evaluated with respect to actual conditions as measured in the field. The model was judged to predict peak hydrograph flows and total hydrograph volumes with reasonable accuracy; however, it was not judged effective for predicting pollutant concentrations. In urban drainage basins, investments in upgrading secondary municipal waste treatment plants without concomitant steps to moderate the adverse effects of urban land runoff are questionable in view of the apparent relative impact of urban land runoff on receiving water quality. This report was submitted in fulfillment of project number 11030 HJP by the North Carolina Water Resources Research Institute under partial sponsorship of the Office of Research and Development, Environmental Protection Agfency. Work was completed as of September 1, 1973"--Page iv.