Evaluation of the Water Quality Effects of Combined Sewer Overflow (CSO) Control Measures

Abstract

At present, design guidelines for combined sewer systems ere being formulated by the water pollution control authorities in the Netherlands. These guidelines are aimed at the reduction of pollutant loads by means of stormwater sedimentation tanks and the increase of storage capacity. To obtain better knowledge of the water quality aspects of the guidelines, the effects of combined sewer overflows from systems with stormwater sedimentation tanks of varying storage capacity were studied. Three receiving water bodies, with different dimensions and base flow, were reviewed. For the evaluation of effects on the oxygen balance a combined simulation model was used to predict the pollutant load and the dissolved oxygen response of individual overflow events. Eutrophication effects and accumulation of micropollutants were judged from the annual discharge data. It appeared that in small, stagnant ditches, the guidelines will induce a sharp reduction of the number of violations of the dissolved oxygen standard. In larger, flowing waters, the guidelines may adequately protect the water against violation of these standards. The discharge of total phosphorus and micropollutants is largely reduced by the prescribed measures. However, other sources of phosphorus must be eliminated as well to combat eutrophication effectively.