Characteristics of overflow pollution from combined sewer sediment: Formation, contribution and regulation

Sewer sediments contain high concentrations of carbon, nitrogen, and phosphorus pollutants, which can be the main source of overflow pollution due to high-velocity scouring. To elucidate the scouring overflow pollution characteristics and regulation mechanism of sewer sediment under different precipitation intensities, a sewer-storage tank linkage control experimental device was established to simulate the practical sewer overflow under different precipitation intensities and the control process of storage tank overflow pollutants. Based on the division of flow from small to large, the pollution characteristics of overflow pollutants and the contribution rate of sewer sediments to overflow pollutants were analysed. The results showed that the total load of overflow pollutants increased with an increase in rainfall intensity and were 7.58 kg, 16.54 kg, 27.42 kg, respectively. The concentration of particulate pollutants increased sharply in a short time, and the concentration of dissolved pollutants decreased at a certain dilution. Sewer sediment was the main source of overflow pollutants, contributing up to 70%. After the overflow pollutants entered the regulation and storage tank, a certain stratification phenomenon was discovered at different sedimentation times. The concentration of large particle pollutants gradually increased from top to bottom in the regulation and storage tank, and the concentration of dissolved pollutants showed no obvious difference between the layers. With an increase in rainfall intensity, the recommended regulation and storage times of overflow pollutants were within 15 min, 45-60 min, and 60 min, respectively. Finally, based on the relationship among rainfall intensity, sediment scouring thickness, regulation and storage time, a prediction formula for the regulation and storage time of overflow pollutants was obtained, which provided a basis for the regulation and treatment of subsequent overflow pollutants.