Characteristics And Regulation Of Overflow Pollution From Combined Sewer Sediment

Due to the high pollution load and complex characteristics,combined sewer overflow is one of the important sources of black and odorous water bodies.The sediment formed during the long-term and long-distance sewage transportation in the combined sewer system will be suspended and migrated again when it is washed by rainfall,becoming the main source of overflow pollutants.Therefore,the scouring and overflow of sewer sediments is an important cause of black and odorous urban water bodies in rainy days.However,the characteristics of overflow pollution caused by sediment scouring are still unclear,which greatly limits the scientific regulation of overflow pollution.In this study,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.And the overflow pollution characteristics and regulation mechanism of sewer sediment in combined sewer system were proved.The main conclusions were as follows:（1）The research on the distribution of pollutants in the longitudinal section of the sedimentary layer showed that the distribution of particle size to the sediments increased with the increase of the depth of the longitudinal section.The particles were transformed from easily suspended fine particles<76μm to large particles of 76～250μm with certain sedimentation.The organic matter was easily attached to the large-sized particles,and the concentration was mainly concentrated in the lower layer of the sediments,with an average concentration of about 2000μg/g.The surface layer of the sediments had small particle size and high porosity,and the concentrations of nitrogen and phosphorus pollutants adsorbed were the highest.The average concentrations were115μg/g and 17μg/g,respectively.With the increase of the depth of the longitudinal section,the SO₄^2-concentration gradually decreased,the highest was 0.27μg/g,and the S^2-concentration increased layer by layer,the highest was 1.6μg/g.（2）The scouring overflow pollution characteristics of sediment under different rainfall intensities were explored.The results showed that with the increase of rainfall intensity,the concentration of overflow pollutants increased as a whole,and the contribution rate of sediments to overflow pollutants also increased.The stratified distribution regulation of sediment pollutants made the SS and COD concentrations rise to 1139.76 mg/L and 1601.69 mg/L respectively within 1 min under high rainfall intensity,which was about 2-3 times that of daily sewage.The contribution rate of sediment to each pollutant was as high as 66%and 65%;The concentrations of nitrogen and phosphorus pollutants gradually decreased with the increase of rainfall intensity.Under high rainfall intensity,the concentrations of TN and TP reached the maximum values of 29.73 mg/L and 9.36 mg/L at 2 min and 1 min,respectively.It gradually decreased due to the enhancement of dilution,and the contribution of sediments to both increased slightly,40%and 35%,respectively.（3）The regulation and storage conditions of the overflow pollutants in the regulation and storage tank were studied,and the results showed that during the overflow period,under the low rainfall intensity（0.68 mm/h）,the concentration of SS was lower than the directly discharged standard of 150 mg/L at 15 min of overflow,then the concentration rose to 440 mg/L,and the recommended storage time was 15 min.Under the moderate rainfall intensity（0.9 mm/h）,the SS concentration decreased layer by layer with the increase of overflow time,and the concentration stabilized at 200mg/L after 45 min.The recommended storage time was 45-60 min.Under high rainfall intensity（1.6 mm/h）,the SS concentration was always above 1000 mg/L,and the recommended storage time was after 60 min.After the overflow,SS and COD settled preferentially,and the time required for their surface concentrations to drop to 150 mg/L under low,medium,and high rainfall intensities were 20 minutes,30 minutes,and after30 minutes,respectively.Nitrogen and phosphorus pollutants were easily soluble in water,and their sedimentation properties were relatively poor.The regulation time should be appropriately extended by 10 min on the basis of the former.（4）Based on the above research results,combined with the relationship between rainfall intensity,sediment scouring thickness and regulation and storage time,the prediction formulas for regulation and storage time of SS,COD,TN and TP in overflow pollutants under different rainfall intensities were obtained by fitting.The formulas were suitable for the rainfall intensity of 0.68～1.6 mm/h,and the fitting rate was above0.95.It played a guiding role in the overflow control process of the combined sewer system with sediment,and had important practical significance.