Temporal distribution of grit in the combined sewer system of the typical mountainous city

The sewage samples in the combined sewer system of the typical mountainous city were collected and analyzed during dry and wet weather. Results showed that grit from influent had obvious change curve during 24 hours in dry-weather. The average concentration of grit from influent was about 183.9 mg.L-1. The loads of grit were higher than value designed in the Fuling Wastewater Treatment Plant (WWTP). Grit variation curve was associated with the sewer flow discharge in collection system. In wet-weather, the peak concentration of grit from initial rainfall was at least eight times more than that in dry-weather. The average concentration of grit during the wet weather of sampling was 405.5mg.L-1 in combined sewer system. In the sanitary sewer in the commercial zones, the average concentration of grit was 426.2mg.L-1. Because of the dilution effect of rain in a rain pipe, the grit concentration was only 199.1mg.L-1. Up to 50% of the grit entering the Fuling WWTP may be 200 microns or smaller. It is difficult for the existing grit removal systems to efficiently remove the grit 200 microns or less. For removal of smaller grit, a grit removal device should be designed for removal of grit and sediment particles near 100 microns in particle size. Regardless of dry weather and wet weather, the WWTP of the mountainous city had to pay more attention to the grit removal system.