Sediment characteristics of sewer in different functional areas of Kunming

The problem of sediment deposition in urban sewer network in different levels, will not only reduce the sewer flow ability, but also release pollutants and generate secondary pollution. The impact of secondary pollution is more serious in the study area, Kunming, because of the combined sewer overflow in rainy season. In order to obtain the characteristics of the sewer sediments in Kunming, the sewer sediments from residential area, service area, cultural district, and business district were investigated and collected. The particle size, density, organic matter (VSS/TSS) and pollutant content of the sediments were analyzed in this study. The results showed that there were different characteristics for the sediments from different areas. The size of the sediments exhibited business district>cultural district>residential area>service area, and the D₅₀ was concentrated in the 20 to 100 μm except the business district. As for VSS/TSS, the order was residential area>cultural district>business district>service area. It was negatively correlated with the dry density with the correlation index of R²=0.982 7 and positively correlated with the water content. The contents of sediments showed significant differences in different functional areas. The size of COD presented residential area>cultural district>service area>business district, and the size of TN followed residential area>cultural district>business district>service area. As for TP, it exhibited residential area>service area>cultural district>business district. The COD, TN and TP were proportional to the population density of the area and TP was greatly influenced by sediment particle size. From the branch into the Sub-main sewer, COD and TN were irregular, and TP decreased slightly. TP mainly existed as particulate and was more likely to deposit onto the small particle. As for TN, there was no obvious rule about its distribution in different particle size sections. The pollution load was generally on the high side in Kunming. The content of heavy metals in business district was the highest among all functional areas, and the concentrations of Cu, Zn, Pb, Cd were 284.6, 786.4, 201.2, 2.54 mg·kg^-1 respectively. The average contents of Cu, Zn, Pb, Cd in urban area were 2.2, 4.4, 2.5, 8.6 times than the background values. It is suggested to control Cd and Zn with priority.