Inflows into Wastewater and Stormwater Systems: Sources, Causes, and Assessment

Illicit or inappropriate inflows into urban drainage systems cause significant operational issues, impacting utilities, communities, and the environment. The continued deterioration of system assets increases these inflows. Groundwater infiltration, rain-derived inflows, and misconnections contribute to reduced system performance, amongst other detrimental inflows. Climate change effects and the revised EU Wastewater Treatment Directive put pressure on utilities to reduce combined sewage and polluted stormwater discharges and overflows while promoting carbon neutrality. The effective management of undue inflows requires identifying cause–effect relationships and quantifying their consequences. This paper proposes a performance-based methodology with metrics and reference values to assess and categorise various undue inflows in wastewater, stormwater, or combined systems. This approach allows the tracking of performance over time, the comparing of systems, and requires data commonly available to utilities. The reliable quantification of inflows depends on the availability and accuracy of flow data from relevant system locations, rainfall data, and pertinent contextual information. This paper uses data from eight utilities and the Portuguese regulator to validate its approach, calculate metrics, refine reference values and enable better-targeted control measures. The results enhance the value of a unified approach to this problem in making better decisions to improve the urban water drainage system’s performance, enhance pollution control, and promote sustainable water management.