Influence of particle size on diffuse particulate pollutants in combined sewer systems

Since combined sewer systems arc still considered as a threat to urban water environment, understand their pollution transport process and load distribution characteristics during rainfall-runoff events would assist to mitigate urban stormwater pollution. In this study, built-up and wash-off processes of particulate pollutants on road surface and in sewer were synchronously observed during eight natural rainfall events in the combined sewer system in Zhuhai, Southern China. Field observation results showed the significant influence of particles size distribution characteristics on TSS concentration. High TSS concentrations (Road runoff, >200 mg/L; combined sewer runoff, >150 mg/L) usually contained relatively coarse particles (>100 pm) during the eight rainfall events, but the counter argument. Combined sewer runoff had a particle size coarser than road runoff. The relationship between the event means concentration (EMC) of TSS from road and combined sewer runoff varied with rainfall. EMC of TSS in road runoff were higher than that in sewer runoff during small rainfall, while that in combined sewer runoff were higher during large rainfall. The results also showed that 65.2 %of total nitrogen (TN) loadings and 62.3 %of total phosphorous (TP) loadings in the catchment came from combined sewer sediments, while 58.6 % of chemical oxygen demand (COD) loadings came from road runoff, which were mainly determined by the content of each pollutant in the sewer sediments and road-deposited sediments (RDS). Additionally, high pollutant loads phases (drainage of pollutant loads faster than runoff at a given incremental time during an event) of road runoff occurred earlier than that of combined sewer runoff. These findings can provide a basis for the type selection and scale determination of stormwater control measures in combined sewer systems.

Automated summary

This study observed the buildup and wash-off processes of particulate pollutants in road surface and sewer during rainfall events in a combined sewer system in Zhuhai, China. The results showed that the size distribution of particles had a significant influence on the TSS concentration, and the relationship between TSS concentrations in road runoff and combined sewer runoff varied with different rainfall intensities. The study also found that a significant portion of the pollutant loadings came from the sediments in the combined sewer and road runoff, with different contributions to different pollutants. These findings can provide insights for the design of stormwater control measures in combined sewer systems.