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.

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Image Captions

Wastewater treatment plant infrastructure


The interdisciplinarity of this studentship links environmental microbiology, environmental risk assessment and catchment management. The project will involve fieldwork in river and catchment environments, controlled laboratory experiments and GIS mapping of river drainage networks to help contextualise the relative risk of legacy FIO stores in riverbed sediments contaminated by CSO discharges across case study catchments. Following an initial critical literature review, the studentship will comprise of three core components: (1) detailed characterisation of persistence profiles of FIOs (and potential pathogens) in sewage contaminated riverbed sediment under a suite of environmental conditions; (2) quantification of mobilisation and resuspension potential of FIOs entrained in sewage-contaminated riverbed sediment under a range of environmental scenarios; and (3) an integrated programme of targeted catchment monitoring and GIS-supported risk modelling, drawing on outputs from (1) and (2), to deliver new understanding of the risk posed by sewage-contaminated riverbed sediment stores in catchment systems.

Field sampling: The Forth estuary catchment integrates the water quality signatures from many catchments of mixed land use and varying CSO numbers and accommodates a number of designated bathing waters on the Firth of Forth coastline. A case study catchment draining into the Forth Estuary will be selected following discussion with the supervisory team, SEPA and wider stakeholders. The case study catchment will allow for detailed mapping, monitoring and characterisation with respect to sewage-contaminated riverbed sediment stores, their spatial and temporal relationship with CSO discharges and the potential for downstream impacts.

In addition to catchment fieldwork, replicated microcosm experiments investigating the persistence of FIOs in hotspots of sewage-contaminated sediment of varying characteristics will be conducted under controlled environmental conditions (e.g., constant temperature and also variable diurnal regimes using climate-controlled chambers), and complement field-relevant experiments to detail FIO persistence profiles under variable interacting environmental factors. These experiments will also provide an opportunity to investigate the role of sewage contaminated sediments in promoting antibiotic resistance. Resuspension experiments using flow-through chambers will enable an evaluation of the potential for FIOs to be mobilised under different conditions (e.g., flow rates, sewage composition, etc.). We have limited understanding of how processes such as upwelling versus hydrological entrainment drive FIO resuspension from sewage-contaminated hotspots in river sediments and these experiments will provide novel data to quantify the ‘bleeding’ of FIOs from such sediments.

The integrated field and laboratory-based approach will contribute new information to develop a GIS-based modelling framework for assessing the potential for legacy pollution risk from combined sewer overflow discharges in rivers. The studentship will therefore enable a more detailed assessment of the longer-term risks to water quality posed by CSO discharges and provide evidence to underpin land and water management decisions at the catchment scale.

Project Timeline

Year 1

Develop a critical review of the literature
Learn key skills (field techniques, microbiological methods, GIS) and participate in training opportunities
Catchment familiarisation/scoping and initial sampling and data collection
Engage with the CASE partner and wider stakeholders

Year 2

Spatially targeted field sampling campaigns across the full hydrological year
Controlled laboratory studies of FIO die-off in sewage-contaminated sediments
Statistical training
Continued engagement with CASE partner

Year 3

Controlled laboratory investigation of FIO resuspension processes from sewage-contaminated sediments
Development of GIS-based modelling framework
Continued engagement with CASE partner
Disseminate findings via workshops and meetings

Year 3.5

Paper and thesis writing
Conference attendance

& Skills

This studentship will provide a platform to build an interdisciplinary research career in applied microbiology and hydrology in the context of environmental pollution with human health impact. The studentship will broaden the scope of the applicant’s skills base by providing specialist training in the safe handling of Hazard Group 2 microorganisms & microbiological methods, and by developing expertise in the use of a wide range of interdisciplinary field, laboratory, & GIS modelling methodologies. Extensive skill development in fieldwork will include comprehensive training in sampling & monitoring techniques. The student will join supportive research groups in Stirling and have access to partner expertise at the UK Centre for Ecology and Hydrology, Wallingford. A full programme of IAPETUS training courses will also be available. Finally, this studentship will provide opportunities to understand the role of the science-policy interface and experience KE-related activity through CASE support provided by SEPA, which also includes provision of a placement within SEPA.

References & further reading

Whelan MJ, Linstead C, Worrall F, Ormerod SJ, Durance I, Johnson AC, Johnson D, Owen M, Wiik E, Howden NJK, Burt TP, Boxall A, Brown CD, Oliver DM, Tickner D (2022). Is water quality in British rivers “better than at any time since the end of the Industrial Revolution”? Science of the Total Environment, 843, 157014 doi.org/10.1016/j.scitotenv.2022.157014

House of Commons Environmental Audit Committee (2022). Water quality in Rivers. House of Commons, available at: https://committees.parliament.uk/publications/8460/documents/88412/default/

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