The composition of gross pollutants contained in wet weather overflows for different locations, spill frequencies and discharge volumes

Abstract

Public complaints arise from visible gross pollutants deposited on receiving waterway margins and in associated riparian vegetation after sewer overflows. The current study was conducted to assess the capability of a modified commercial stormwater trash-net to capture gross pollutants that spill from overflows of a sanitary (separate from stormwater) sewer system. Trash nets were installed on four overflows with differing gauged spill frequencies and volumes. Five categories of gross pollutants (wet wipes and paper products, sanitary items, condoms, miscellaneous identifiable items, and matted immeasurable items) were collected in similar proportions across the two highest volume overflows, although the weight of material was highest from the largest volume overflow. Only wet wipes and paper products were collected from the lowest volume overflow. Wet wipes and paper products were the main gross pollutant collected comprising 54% (159 kg) of the total weight over the study period. This weight was potentially an under representation as wet wipes were observed matted together and intertwined in the net that inhibited separation, with this material allocated into the ‘immeasurable’ category comprising 30% (89 kg) of the collected material. The companion aesthetic survey observed gross pollutants originating from the sewer at 36% of the 95 aesthetic study locations while gross pollutants originating from the stormwater system were found at 82% of the aesthetic study locations. A build-up of wet wipes and paper products at overflow outlet screen bars was the dominant gross pollutant observed, which supported measurements obtained with the deployed nets. The results of the study provide important new information that will improve management of gross pollutants spilling from designed overflow points of the sewerage system.