Characterization and sources apportionment of overflow pollution in urban separate stormwater systems inappropriately connected with sewage.

In recent years, combined sewer overflow (CSO) has been identified as a significant contributor to the deterioration of the urban water environment. It is thought that remolding it to a separate sewer system is a thorough and effective method of controlling the CSO in the appropriate area. However, according to current research, the separate stormwater sewer systems will also have overflow pollution due to functional defects, damaged or inappropriately connected with sewage, which has serious consequences for the separate system's operational efficiencies and the urban water environment. The event mean concentration, first flush effect, source apportionment, and correlation analysis of variables in overflow pollution generated in three residential catchments in Nanning, China, were investigated in this study. The results showed that the event mean concentration values in drainage outlets inappropriately connected with sewage were 2-4 times higher than those in stormwater outlets, especially for NH3-N, TN, and TP. Meanwhile, more than 80% of overflow events at outlets inappropriately connected with sewage had a weak first flush or even a weak dilution effect, with peak pollutant concentrations occurring 40-60 min after the overflow began. Besides, the discharge pollution load was primarily derived from the inside of the sewer. When the rainfall was heavy, the contribution rate of sewer sediment erosion exceeded 60%, which was much higher than the contribution rate of rainfall runoff and sewage. The variability in event mean COD and TSS concentrations was primarily attributed to the antecedent dry period and rainfall intensity. The COD concentration increased from 140.7 to 277.1 mg/L with the increase of antecedent dry period from 3 to 10 days. This study could help guide the implementation of targeted measures to treat overflow pollution in urban residential catchments, as well as the development of strategies to mitigate the effects on receiving water bodies.

[Analysis on the Source Tracing and Pollution Characteristics of Rainfall Runoff in the Old Urban Area of Nanning City].

Rainfall runoff is one of the important sources of urban river pollution. In order to understand the pollution characteristics of rainfall runoff, the synchronous sampling and monitoring of rainfall runoff in the old urban area of Nanning were carried out; the pollution condition, initial scouring effect, and pollution contribution ratio of different underlying surfaces under different rainfall conditions were analyzed; and the calculation method and influence factor analysis of initial scouring of runoff pollution were discussed. According to the underlying surface of the vegetable market in the old urban area, the selection standard and necessity of the underlying surface of rainfall runoff were discussed. The results showed that the average concentration (EMC) of COD and TSS in roads and vegetable markets were greater than those in green spaces and roofs in the runoff pollution of the old urban area of Nanning, and the EMC values of nutrient pollutants in field rainfall runoff were ranked in decreasing order as vegetable markets, green spaces, roofs, and roads. Under the condition of heavy rain, each underlying surface had an obvious initial scouring effect, the average value of initial scouring coefficient (b) was 0.67, and there were many pollutants transported by roads and green spaces at the initial stage. Under light rain and moderate rain conditions, there was no obvious initial scouring effect, and the average b values were 0.83 and 0.94, respectively. The b value calculated by the whole process was preferred for evaluating the scouring effect. Based on multivariate statistical analysis, the EMC value of TSS in road runoff was significantly positively correlated with the total rainfall duration (RD), and the EMC value of TN in green land runoff was significantly negatively correlated with the average rainfall intensity (ARI). Whether the vegetable market was considered as the underlying surface had a great impact on the calculation results of nutrient pollution load but had little impact on the calculation results of COD and TSS load. The difference percentage of nutrient pollution load under heavy rain reached 80%. Under this condition, the difference percentage of pollutant EMC between the road and vegetable market reached 1012%.