Scientists at the University of Stirling have developed a new method of monitoring the presence of pathogens in rivers polluted by wastewater treatment plants (WWTPs).

The number of discharges from the sewage network into rivers and seas in the UK is increasing and studies have shown that effluent released by WWTPs contains pathogens that pose a risk to human health.

Currently, the presence of harmful bacteria following discharges from pipes is monitored by taking random samples of water which are then analysed in a laboratory.

However, Stirling scientists say this method may miss pathogens because discharge events from WWTPs are unpredictable and the receiving water is constantly changing.

University of Stirling scientists have instead used microplastic beads submerged downstream from sewage effluent pipes to capture pathogens, such as E. coli.

Professor Richard Quilliam, of Stirling’s Faculty of Natural Sciences, has previously published research which established for the first time that E. coli can stick to microplastics and make the journey to bathing waters and beaches.

He said: “Wastewater treatment plants are releasing sewage effluent into the environment at an unprecedented rate in the UK, leading to environmental contamination and risks to public health.

“Many studies have highlighted that bacterial pathogens are released in this effluent, but current monitoring tools typically rely on water sampling from the effluent. As water sampling is a transient process, bacteria may be missed using this process.

“We have instead utilised microplastic beads as a tool to monitor pathogen release because they form a biofilm coating their surface, and as bacteria passes over this surface they stick to the plastics.

“We placed microplastics downstream of an effluent pipe and measured which bacterial pathogens stuck to the surfaces of the plastic. This technique can capture more information over time than the current water sampling techniques and is a low cost and easy to apply method.”

The beads are secured in small spherical metal cages which are then placed in larger cubed cages. The cages are then submerged in rivers upstream and downstream of WWTP effluent pipes.

Researchers analysed the microplastic beads in a laboratory at the University of Stirling and found significantly higher levels of harmful pathogens downstream from the WWTPs, including E. coli and Klebsiella which can cause human infections.

The new study, published in the prestigious journal Water Research, was led by the University of Stirling’s Dr Luke Woodford, who said: “The microplastics worked as a surveillance system detecting bacterial pathogens within one day of placement and at every sampling point onwards for more than three weeks.

“Using an infection model, we also showed that bacteria being detected downstream were pathogenic to humans and that a range of different bacterial pathogens were being released from the WWTP.

“Additional genome sequencing revealed that these bacteria contained many genes conferring antimicrobial resistance and increased virulence, further highlighting their risk to the public and the wider environment.

“Our study demonstrates a new technique for monitoring WWTP effluent pollution which can be used by other researchers or by environmental monitoring groups as a tool to help improve our understanding of changes in water quality.

“Sewage releases are increasing in the UK, posing risks to human health, so having systems like our one in place to monitor what is being released is a key part of tackling this public health issue.”

Fieldwork was carried out by researchers between April and May 2024 in Scotland, with lab testing and data analysis performed on samples in the following months.

Cages containing the microplastics were positioned upstream and downstream of a WWTP effluent pipe and microplastic samples were removed at multiple time points between 24 hours and 23 days of placement.

The paper Exploiting microplastics and the plastisphere for the surveillance of human pathogenic bacteria discharged into surface waters in wastewater effluent was published in Water Research. The research was funded as part of the Plastic Vectors Project UKRI NERC grant.