River water intrusion as a source of inflow into the sanitary sewer system.

Previous studies on the extraneous water problem (or infiltration/inflow) in sanitary sewer systems assumed that the wastewater flow is mainly composed of foul sewage (FS), groundwater infiltration (GWI) and rainfall-derived inflow and infiltration (RDII). Most existing assessment methods are based on this assumption. In 2018, China initiated the 'Protection of the Yangtze River Program', and the two-year research data showed that it was neither the GWI nor the RDII but the direct surface water intrusion (DSWI), which has rarely been reported in literatures, that serves as the main source of the extraneous water in many local sewer systems. The discovery has enriched the understanding of the extraneous water in sewer systems. Meanwhile, it brings new challenges for the assessment of extraneous water. In this study, starting from the analysis of the low influent concentration of chemical oxygen demand (COD) of the wastewater treatment plant in a southeastern city in China, a river water intrusion point was successfully localized and the volume of river water intrusion was quantified by a series of field experiments. The methodology used in this study can also be applied in other areas with DSWI.