New research by experts from The University of Manchester has found that the poor management of untreated wastewater and raw sewage by water companies is the main source of microplastic pollution in the UK’s rivers.

Image: Discharge of raw sewage and untreated wastewater into rivers and coastal waters across England in 2020.

Three years ago, the researchers from the University’s Department of Geography were the first to demonstrate high levels of microplastic contamination on the UK’s river beds.

Now, new research published in the journal Nature Sustainability has found that water companies themselves are the cause of this contamination, as they are releasing wastewater during periods of dry weather into river flows that are too sluggish to disperse microplastics downstream.

The quality of river bed habitats underpins the entire river ecosystem because many creatures live, feed and reproduce in this environment - when they are contaminated with microplastics, ecosystem exposure is maximised because the particles are stored on the bed for weeks or months before they are flushed away by flooding.

The river bed is the worst place for extended periods of microplastic contamination because it increases opportunities for ingestion by aquatic creatures, and for them to move through the food chain. Microplastics are also vectors for other contaminants present in wastewater.

The researchers say that conventional treatment removes the great bulk of the microplastic load in wastewater – therefore, river beds that are heavily contaminated with microplastics provide a clear indication of poor wastewater management.

The discharge of raw sewage to rivers is already controversial and has generated widespread condemnation. An inquiry into Water Quality in Rivers is currently underway by the UK Parliament’s Environmental Audit Committee. However, the researchers are calling for more to be done to tackle the problem.

Image: CSO outfall and channel bed microplastic contamination

Professor Jamie Woodward, who led the research, commented:

“Water companies must stop releasing untreated sewage and wastewater into rivers during periods of dry weather, as this causes river beds to be heavily contaminated with microplastics and maximises habitat damage.”

“Rivers are also the main supplier of microplastics to the oceans - to tackle the global marine microplastic problem, we need to limit their input to rivers.”

"We are asking the water companies to be guided by the science, and to shoulder their responsibilities. We are also calling for stronger regulation from the Environment Agency to police water company discharges."

United Utilities “disappointing to see number of flaws” in research which makes assumptions “without evidence”

Commenting on the findings, United Utilities said it was “disappointing” to see a number of flaws in the research.

Jo Harrison, Director of Environmental Planning and Innovation, said:

“As a society, it is vitally important that we increase our understanding of microplastics pollution, allowing us to develop meaningful solutions to tackle this significant issue – and United Utilities is already working with others to address the problem. It’s therefore disappointing to see a number of flaws in the University of Manchester’s research, which takes a narrow view of the issue and makes several assumptions without evidence.

“The research ignores important sources of microplastics pollution in the immediate study area such as the presence of plastics recycling plants. The study asserts that microplastics are found primarily in close proximity to wastewater inputs. In fact the data shows numerous examples of increasing levels of microplastics when there is no wastewater discharge, plus a sample point showing one of the lowest levels of microplastics downstream of two wastewater treatment plants.

“We understand that wastewater will be a contributing factor to microplastics pollution, however this piece of research seems to be a missed opportunity to shed more light on the subject. We recognise we have a role to play which is why we are involved in a much wider two-year study beginning this summer to give a much more holistic understanding of the sources, pathway and consequences of microplastics in the environment. Only through collaboration and cooperation can this issue be tackled successfully.”

Woodward, J., Li, J., Rothwell, J. et al. Acute riverine microplastic contamination due to avoidable releases of untreated wastewater. Nat Sustain (2021). Click here to access the full article online