Combined sewer overflows and their contribution to legacy faecal pollution in rivers
Extreme wet weather events in the UK challenge environmental management of land and water and in turn threaten the efficiency and reliability of the infrastructure designed to manage wastewater. Combined sewer overflows (CSOs) are part of the sewerage infrastructure associated with wastewater treatment plants; they are designed to spill untreated human sewage in ‘exceptional’ circumstances, i.e. very wet weather, when wastewater treatment plants become overwhelmed with stormwater. Reductions in wastewater treatment efficiency during extreme wet weather events can have localised and downstream consequences in terms of risk to the environment, ecosystem service provision and human health (Whelan et al., 2022).
Recently, sewage discharges into UK surface waters have received significant media attention with increased recognition of the frequency and magnitude of spills raising public awareness of the risks posed to water quality and downstream ecological and public health. This has been coupled with a series of record fines for some UK water companies for major sewage leaks and other pollution incidents. During 2020, in England alone, there were over 400,000 sewage discharges from 80% of CSOs monitored, totalling in excess of 3 million hours of discharge (Defra, 2022). This has attracted strong criticism from campaign groups, water quality experts and public health professionals who are increasingly concerned that CSOs are being used to regularly dispose of untreated sewage into receiving waters even during times of little to no rainfall.
To date, much of the debate concerning CSOs has focused on the immediate impacts of sewage spills to the hygienic status of the receiving water, but a secondary issue is the accumulation of solid fractions of untreated human sewage in riverbed sediments. Discharges from CSOs also deliver faecal indicator organisms (FIOs), such as E. coli, and potential pathogens into suspension in river drainage networks and, depending on factors such as, e.g., river flow, microbial-particle associations, and sedimentation rates, a proportion of faecally-derived microbial pollutants will become incorporated along with solids in the riverbed. Consequently, there is potential for legacy stores of FIOs to build up over time in riverbed sediments near to where CSOs discharge. These legacy stores, or hotspots, of FIOs in the riverbed sediments are likely to be dynamic, varying in size through a net effect of the input factors (described above) in combination with microbial die-off and resuspension mechanisms that erode the riverbed FIO supply. There is also potential for these sediment hotspots to serve as reservoirs for antibiotic resistant bacteria and resistance genes. The presence of legacy stores of FIOs in riverbed sediments can therefore provide a potential in-stream source of microbial pollution, resulting in delayed impairment of water quality following eventual FIO resuspension into the water column.
There is a growing body of research that has documented the delayed impacts that legacy phosphorus can have on water quality and the associated implications for catchment management; however, the importance of legacy risks associated with environmental stores of FIOs in sewage contaminated riverbed sediments has received much less attention. For example, we know very little about how such potential legacy FIO stores in riverbeds vary in terms of their spatial and temporal characteristics or how the survival and resuspension of FIOs from these hotspots can contribute to subsequent downstream risk, including the potential for impacting on bathing water quality.
The overarching aim of this studentship, therefore, is to provide critical data on the importance of legacy stores of FIOs in riverbed sediments that accumulate in response to CSO discharges. Specifically, the research objectives are to:
1. Evaluate how legacy stores of FIOs in riverbed sediments vary in space and time downstream of CSOs;
2. Characterise FIO die-off in sewage-contaminated riverbed sediments under different environmental conditions;
3. Determine and quantify the factors controlling FIO resuspension from hotspots of sewage-contaminated riverbed sediments;
4. Develop a risk-based approach to predict FIO contributions to the wider environment from hotspots of sewage-contaminated riverbed sediments.
Click on an image to expand
Wastewater treatment plant infrastructure