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Microplastic contamination of UK rivers caused by poor wastewater management

by Jamie Woodward, University of Manchester


This blog has been reposted with permission from the Policy@Manchester blog. You can read the original here.


The UK has some of Europe’s most polluted rivers. Rivers face significant ecological degradation from the routine discharge of raw sewage from wastewater treatment works, alongside other sources of pollution such as agricultural run-off and chemical discharges from industry. The most recent data from the Environment Agency (EA) show that every single river in England failed to meet safe chemical standards, and 86% were not in good ecological condition.

recent Panorama investigation showcased the impact of solid waste such as sanitary products and wet wipes on rivers and provided evidence of the widespread and routine discharge of raw sewage. Our research shows that such discharges can result in extremely high levels of riverine microplastic contamination.

Releasing large volumes of untreated wastewater laced with microplastics and sewage severely compromises water quality and poses a threat to freshwater habitats, marine life and human health. Microplastics from rivers then go on to supply most of the microplastics in the oceans. There are also significant concerns that concentrated sewage deposits provide a breeding ground for genetic mutation and antibiotic resistance.

Unambiguous proof of the routine discharge of raw sewage into UK rivers

Currently, the EA grants permits to water companies allowing the release of untreated sewage into rivers via storm overflows after heavy rainfall. This is to prevent sewers and treatment works becoming overwhelmed with excess runoff. In these circumstances, swollen rivers dilute and disperse pollutants thus minimising local environmental impacts. Microplastics are transported easily in higher river flows. If untreated sewage was only discharged in these exceptional circumstances, we would see only minimal build-up of microplastics on river channel beds.

Our research shows this is not the case. Studying sites along the River Tame in the Upper Mersey basin, we found very high levels of microplastic contamination in the sediments that lie on the bed of the river. Urbanised zones in particular showed alarming microplastic concentrations – the highest recorded anywhere in the world. The geography and composition of the microplastic hotspots shows unequivocally that untreated sewage is routinely discharged during periods of low river flow; these hotspots cannot form during heavy rainfall events. The timing of wastewater discharge is the key control on levels of microplastic contamination.

Professor Jamie Woodward sampling effluent from a Combined Sewer Overflow (Photo by James Rothwell)

Previous studies into microplastic pollution in UK rivers have commonly underestimated the extent of the problem. These studies have focused on sampling microplastics suspended in river water. However, concentrating solely on buoyant microplastics in the water column can yield a very limited picture of microplastic contamination. We have observed that microplastics of all types accumulate on river beds in very high concentrations and often reside there for weeks or months, until a significant flooding event disturbs the bed and washes them downstream. This has profound implications for the ecological health of the rivers. Many aquatic fauna live, feed and reproduce in these contaminated channel bed environments; they face prolonged exposure to microplastics as a result of sewage spills. Allowing microplastics to accumulate on the river bed maximises opportunities for primary and secondary ingestion by aquatic fauna. Microplastic hotspots can amplify the effects of sewage-borne toxins to further degrade river ecosystems.

Microplastics are now ubiquitous in the environment – in the ocean, in the air and even in snow from the Arctic to the Alps. However, we can manage the volume of microplastics entering rivers because they enter at distinct point sources – these are primarily combined sewer overflows and wastewater outfalls  that are water company assets. A UK-wide survey of the microplastic burden on river beds is needed. This national assessment should adopt the methodology employed in our paper to understand the extent of the problem and move towards defining tolerable levels of microplastic contamination in river catchments. These data should then be used to inform policy around targets for compliance.

A preventable problem

There is an increasing body of evidence showing that discharging untreated wastewater laced with sewage and microplastics is common industry practice. Data from the EA show that water companies discharged raw sewage into rivers and coastal waters in England more than 400,000 times last year for a total of 3.1 million hours. Despite this, the EA only reported 160 breaches of permit. Currently, the water industry is obliged to self-report on sewage ‘spills’. However, this has proven ineffective, with Professor Peter Hammond suggesting  there could be ten times more permit breaches than the EA currently identifies.

Much of this riverine microplastic pollution is avoidable because existing wastewater treatment is capable of removing up to 99% of the microplastics entering rivers. Where microplastics are found in high concentrations on river beds, it forms one of the clearest indicators of poor wastewater management. Without unlawful sewage discharges, microplastic hotspots would not be found. Analysing river bed sediment for its microplastic content offers a highly effective method for monitoring and policing wastewater treatment practices. In addition, specific legislation must be introduced to provide a legal limit for microplastic content in rivers. If wastewater is properly treated in the first place and only discharged during periods of heavy rainfall, the major source of microplastics would be removed and river bed ecosystems would not be subjected to prolonged exposure.

An assemblage of microplastic particles from a contaminated river bed. The large microbead at lower left is about 200 microns in diameter (Image by Jiawei Li)

Shifting goalposts in government targets

While microplastic testing represents an easy path to identifying unlawful sewage discharges, it needs to be backed up by action from regulators to prosecute offending companies. The recent 25 Year Environment Plan moves away from the previous goal of “100% of UK rivers to have good ecological health by 2027”. Instead, it only commits to “improving at least three quarters of our waters to be close to their natural state as soon as is practicable”. Now is not the time to be lowering the bar on our strategy to protect our environment.

The stripping back of a tangible ecological goal and timeline is reflective of a trend of a lack of accountability and lack of action on the issue of river pollution. The EA is failing in one of its core duties by failing to protect rivers across England. Cuts to EA funding have hamstringed its staffing and operational capabilities.

DEFRA has  affirmed its commitment to tackling storm overflows with new provisions in the Environment Bill, following its mention in the Queen’s speech. While these announcements are encouraging, it is critical that Government remains ambitious in setting targets to reduce sewage discharges from these overflows. The legislated duty of water companies to report on the operation of sewage overflows should be more frequent than the current annual returns.

The link between sewage disposal and acute microplastic contamination of river habitats is now clear. Microplastic pollution in rivers will become more severe as the climate warms and urban populations grow. This will have increasing impacts on river ecology, the environment and public health. River corridors have provided important therapeutic green spaces in our towns and cities during the Covid pandemic. It has never been more important to protect these ecosystems. We must act now to strengthen legislation to prevent the shameful – yet widespread – practice of routinely discharging untreated sewage into UK rivers.


About the author: Jamie Woodward is Professor of Physical Geography at The University of Manchester. He has published widely on river science and fluvial landscape change and has extensive field experience in the UK, the Mediterranean region, and in the Nile Valley. Jamie also delivered a Monday Night Lecture to the Society, in 2019: “From city to sea: microplastics in UK rivers.” Fellows and members can view the recording of that lecture here.

Suggested further reading

Woodward, J., Li, J., Rothwell, J. et al. (2021) Acute riverine microplastic contamination due to avoidable releases of untreated wastewater. Nature Sustainability. https://doi.org/10.1038/s41893-021-00718-2

Roberts, LR, Bishop, IJ, Adams, JK. (2020) Anthropogenically forced change in aquatic ecosystems: Reflections on the use of monitoring, archival and palaeolimnological data to inform conservation. Geo: Geography and Environmenthttps://doi.org/10.1002/geo2.89

1 comment

  1. Wastewater is any domestic, commercial, or industrial water that has been used and needs to be treated before it can be discharged into the natural environment. Wastewater contains wastewater solids such as fats, oils, and grease.

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