Water companies are causing high levels of microplastic contamination in UK rivers by discharging untreated sewage and wastewater into the water system, new research reveals. As pressure builds on water companies, the Environment Agency and ministers to tackle the way water firms release untreated effluent into rivers, scientists have for the first time linked the practice to microplastic pollution. Researchers at the University of Manchester found that the poor management of untreated wastewater and raw sewage by water companies was the main source of microplastic pollution in the UK’s rivers. One sample from the River Tame in Greater Manchester, downstream from a storm overflow for untreated wastewater run by United Utilities, contained microplastic pollution more than double that of the previous recorded hotspot for the highest microplastic river pollution in the world. The paper published on Thursday in the journal Nature Sustainability, said water companies were responsible for these high levels of microplastic pollution of the riverbed because they were discharging untreated sewage and wastewater into rivers during dry spells. “Water companies must stop releasing untreated sewage and wastewater into rivers during periods of dry weather, as this causes riverbeds to be heavily contaminated with microplastics and maximises habitat damage,” said Prof Jamie Woodward, who led the research. “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.” Untreated wastewater, which contains raw sewage as well domestic and industrial run-offs, should only be discharged to rivers from wastewater treatment works and combined sewer overflows during storms or extreme rainfall, when the flow of water helps to disperse pollutants downstream. Treating wastewater removes about 90% of microplastics, Woodward said. So the presence of high levels of microplastics on the riverbed suggested that water companies were spilling raw effluent during dry spells, against UK and EU regulations. The team has previously revealed that microplastic pollution of the ocean is far greater than originally thought. Microplastics include broken-down plastic waste, synthetic fibres and beads. They are known to harm marine life, which mistake them for food, and can be consumed by humans too through seafood, tap water or other food. The team focused on 14 sites in the River Tame in Greater Manchester. One site downstream of the wastewater treatment plant at Dukinfield had the highest microplastic contamination of the riverbed recorded in the world. It found more than 138,400 microplastic particles per kilogram of sediment – more than double that of the previous hotspot recorded by the researchers in their earlier study. Latest data from the Environment Agency showed that water companies in England discharged untreated wastewater into rivers and coastal waters more than 400,000 times in 2020 over 3.1 million hours. United Utilities was the worst-performing company for discharges, releasing 113,940 times over more than 726,000 hours. United Utilities disputes the conclusions of the new study. The research found the urban section of the riverbed was heavily contaminated with microplastics because untreated wastewater was routinely discharged during dry weather, and low river flows that were not capable of dispersing them downstream. “The hotspots of microplastic contamination we have described provide unambiguous evidence that untreated wastewaters laced with raw sewage and microplastics are routinely discharged into river flows that are incapable of dispersing them downstream,” said the research. “The discharge of raw sewage is already controversial – we have shown that the discharge of untreated effluent is the dominant pathway for the diffusion of microplastics to the heart of the riverine ecosystem.” “Every ‘spill’ releases microplastics into the river environment,” the research said. Spills during dry periods were especially concerning as they were a significant threat to the biodiversity and quality of riverine habitats. “Preventing deliberate ‘dry weather’ spills must be a key priority of wastewater management and environmental regulation,” the report said. “Treating these wastewaters would effectively shut down the major supply of microplastic fragments and microbeads … to riverine ecosystems and thus prevent their downstream transport to the ocean.” The paper called for water companies to invest more in wastewater treatment and storage capacity and in technology to capture microplastics. MPs on the environmental audit committee are investigating water quality in rivers and will report soon. Jo Harrison, director of environmental planning and innovation at United Utilities, said it was disappointing to see a “number of flaws” in the University of Manchester’s research. Harrison said the paper took a narrow view of the problem and made 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,” said Harrison. “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 said: “We stand by our findings. This research was subjected to detailed, line by line peer review in one of the most rigorous and respected academic journals. Nature journals do not publish papers that are not well supported by evidence.”