Exmouth Beach looking across to Dawlish Warren

Understanding CSO Discharge Impacts

Background

The majority of SWW’s sewer network utilise a combined sewerage system, amalgamating sewage and storm/surface water runoff within the same pipes. Such systems are largely legacy networks. They are designed to agreed standards with sufficient capacity to convey effluent flows from domestic and trade sources under ‘design standard’ rainfall events to wastewater treatment works (WwTW) for treatment, before discharging to the environment. However, during high rainfall events, combined systems are designed to utilise Combined Sewer Overflows (CSO) to prevent sewer flooding of property and land by spilling excess water into the environment, at time when it is expected the receiving water will have greater capacity.

Recent studies have highlighted aging legacy infrastructure, increasing housing development and sewage loads, sewer misconnections to surface water outfalls and infiltration of groundwater, as well as increasing rainfall intensity. This combination of factors with improved reporting are leading to sustained frequencies of CSO spill events against a background of continued investments throughout the region, with similar findings across England.

Against this background the evidence on the quality and quantity of CSO spills and their potential impacts is limited. Understanding the risks and impacts of CSO spills on environmental and human health needs to be assessed as water companies move towards a more transparent risk-based approach for long-term Drainage and Wastewater Management Planning (DWMP).

Research

In collaboration with SWW, the project runs in two phases:

Phase I ran between 2025-2026 and included pilot studies undertaken in three major SW catchments (Dart, Exe and Tamar) to assess methods for the characterisation of CSO spills (chemically and microbiologically), quantify their frequency and volumes, and assess their potential environmental (ecotoxicological) impacts on receiving river waters. The results from Phase I are largely qualitative and have served as a basis for informing the Phase II.

Phase II will include more in-depth quantitative studies to assess the possible impact of chemicals in CSOs on riverine biota at four diverse CSO study sites across the Dart, Exe and Tamar catchments. In this phase of the work, that will run over years 2026-2028, a more targeted approach is being taken, focusing on chemicals including inorganics (including metals) and organics (including pesticides, pharmaceuticals, household and industrial chemicals) likely to be of most concern to riverine biota based on their bioavailability and innate toxicity. This work includes quantitative targeted chemical analyses of the CSO discharges, microbial content assessments via DNA sequencing, and undertaking a series of bioassays to assess the ecotoxicity of these discharges and comparing these elements with that of WwTW discharges. In this phase of work the project will furthermore seek to assess the longer-term ecological impact of CSO spills on resident riverine flora and fauna, including via field-based sampling.

The results from Phases 2 will be used to formulate recommendations for Drainage and Wastewater Management Planning (DWMP) and to help establish an approach for application of this work to other CSOs in the SW region.