Sewage plants are contributing to plastic pollution in the oceans with millions of tiny beads spilling into the seas around the UK, according to a new report.

Dozens of UK wastewater treatment plants use tiny plastic pellets, known as Bio-Beads, to filter chemical and organic contaminants from sewage, according to a study from the Cornish Plastic Pollution Coalition (CPPC).

The report found that many millions of these pellets, which are only about 3.5mm wide, have been spilled and ended up in the environment.

The author of the report, Claire Wallerstein, said once the Bio-Beads had been released they are hard to spot and almost impossible to remove – yet can cause significant harm to marine wildlife.

“We are learning more all the time about the environmental impact of consumer microplastics in wastewater such as laundry fibres, cosmetic microbeads and tyre dust,” said Wallerstein.

“However, it now seems that microplastics used in the wastewater plants’ own processes could also be contributing to the problem.”

However, South West Water said there was “no evidence that Bio-Beads are currently being released into the marine environment” from any of its sites. It said only nine of its 655 plants use Bio-Beads but did accept there had been spills in the past that “were subsequently cleaned up”.

A spokesperson added: “We worked with the authors to encourage evidence-based rigour to this well-intentioned report. However, in parts, it remains anecdotal rather than factual, some of its conclusions are not supported by evidence and it insufficiently differentiates between nurdles [tiny pellets that form the basis of most plastic products] and Bio-Beads.”

However, Wallerstein said samples had been analysed by a plastics expert who had been studying nurdles for 20 years and he had confirmed they were Bio-Beads.

The Bio-Bead system is used in at least 55 wastewater treatment plants around the UK, according to CPPC.

Wallerstein said the scale of the subsequent pollution could be far-reaching adding that in Cornwall Bio-Beads account for the majority of industrial plastic pellets found littering the beaches.

“We know that these Bio-Beads have now reached the coast of northern Europe as well as the beaches here in the UK. What we need is more research into the scale of this problem and for a concerted effort by water companies to do something about it.”

Industrial pellets and small bits of plastic such as Bio-Beads are mistaken for food by birds, fish, and other marine animals. These particles can kill animals, not only by causing digestive blockages, but also as a result of the high concentrations of pollutants, such as DDT and PCBs, which adhere to them in seawater.

Plastic pollution can also enter the food chain. Last August, the results of a study by Plymouth University reported plastic was found in a third of UK-caught fish, including cod, haddock, mackerel and shellfish.

Wallerstein said: “We understand that Bio-Bead plants have been good at improving the quality of the effluent discharged by our wastewater plants – but this should not involve the risk of polluting our seas and waterways with microplastics, which could have long-term and far-reaching consequences.”

Bio-Beads are used in the last step of the sewage cleaning process before treated effluent water is released back into rivers or straight into the sea. There is currently no mechanism in place to trap lost Bio-Beads in the event of a spill and the CPPC report details several spills and near misses in recent years.

Wallerstein said: “We believe that the Bio-Bead system is far too vulnerable to losses. We are calling for a range of safeguards to be put in place at all plants using it, and ultimately for water companies to phase out its use altogether.”

South West Water said it welcomed the report but called for more research.

“We commend the report’s authors in raising this subject but they insufficiently acknowledge other potential sources of small plastic pellets on south-west beaches such as plastic manufacturing plants in the UK and abroad, or spills from container ships, all of which are worthy of further investigation.”