Deterioration of Discharge Quality of Separate Storm Systems Caused by Intercepting Dry Weather Flows

To solve the pollution problem caused by separate storm sewer system dry weather flows, Shanghai Water Authority has set intercepting facilities in most storm water pump stations to intercept dry weather flows which induced by illicit connection & discharges. In this paper, two typical separate storm systems in Pudong New District are chosen to investigate water quality of dry weather flow and wet weather discharge before and after interceptor facilities been put into use to evaluate the intercepting effect. The study results show that: after intercepting, in both dry and wet weather flow, COD, SS, TP concentrations are significantly higher than those before, the change of NH₄⁺-N concentrations are influenced by the TN concentrations in illicit inflows. The impact extent of dry weather flows interception to wet weather discharge quality is proportional to the extent of illicit discharge to separate storm system, and also affected by rainfall intensity, rainfall amount and the flow rate of discharge. The quality of wet weather discharge was seriously deteriorated especially in the case of heavy rains, which brought severe pollution load to the receiving body. Therefore, just intercepting the dry weather flows is not able to solve the pollution problem caused by illicit connection & discharge of separate storm sewer system.