Microplastics flowing through urban sewer systems may be doing more than polluting waterways, with new research suggesting they could also alter underground microbial ecosystems in ways that increase methane emissions.
The study, published in the journal Environmental Science and Ecotechnology, found that common plastic particles can change the balance of microbes living in sewers, reducing the production of hydrogen sulphide while boosting methane-producing microorganisms.
The research, which took place in China, uncovered a previously overlooked consequence of microplastic pollution, prompting suggestions that sewer networks should be viewed as active environments where plastics continue to change before reaching wastewater treatment plants.
Previous research has largely focused on how microplastics are removed during wastewater treatment, but little attention has been paid to what happens while they are travelling through sewer systems.
To investigate, scientists from Beijing University of Technology and Beijing Waterworks Group created model sewer reactors supplied with real domestic sewage. Over 120 days they exposed the systems to different concentrations of two widely used plastics – polyethylene terephthalate (PET), commonly used in drinks bottles, and polybutylene adipate terephthalate (PBAT), a biodegradable plastic.
The researchers found that conditions inside sewers, where there is very little oxygen, cause the plastics to slowly break down and change chemically. As they do, they produce highly reactive molecules that put the microbes living in the sewer under stress.
This stress changes which microbes thrive. Bacteria that normally produce hydrogen sulphide – which produces the ‘rotten egg’ smell in sewers – become less common, while microbes that produce methane become more abundant.
At the highest levels of microplastics tested, the microbes responsible for making hydrogen sulphide became much less active, while methane-producing microbes increased significantly.
Although lower hydrogen sulfide levels could reduce unpleasant odours and corrosion in sewer pipes, the researchers warn that increased methane production may create different challenges. Methane is a powerful greenhouse gas and can also accumulate in poorly ventilated sewers, creating potential safety risks.
The researchers also found that biodegradable PBAT degraded more quickly than PET under sewer conditions, suggesting different types of plastic may have varying environmental impacts even before reaching rivers or the ocean.
To tackle microplastic pollution before it enters wastewater systems, the team suggest measures which include reducing the release of synthetic fibres during washing, treating industrial wastewater before discharge and installing capture systems within sewer networks to intercept plastics before they continue to fragment.
The authors summarise: ‘Microplastics are not inert. In the sewer environment, they undergo physicochemical changes while simultaneously reprogramming the metabolism of the resident microbial community. These particles trigger a cascade of effects – oxidative stress, membrane damage and community restructuring- that shifts the balance toward methanogenesis. What’s particularly concerning is that this process may create a legacy effect in sewer microbiomes, where communities become locked into a deterministic, stress-adapted state that may persist even if microplastic inputs are reduced.’
The full research can be read here.
Photo: Florian Olivo