New “permanently wet” coating method could transform wastewater treatment by helping bacteria survive better

The research, published in ACS Applied Materials & Interfaces, addresses a persistent problem with the technology of biocoatings – thin layers of polymer that contain living bacteria. Most sewage treatment already depends on bacteria to break down organic matter and process nitrates and ammonia, but those bacteria are grown in large open tanks that are expensive, space-hungry and slow to respond to changes in load. Applying a metabolically active bacterial coating onto objects (called carriers) inside a treatment plant, or onto modular panels that could be inserted into existing infrastructure, could concentrate far more bacterial activity into far less space. The problem has always been keeping the bacteria alive while the coating is being made. 

Conventional methods dry the coating in warm air after manufacture. That process strips water from the bacterial cells and concentrates salts to levels that prove fatal to many species. The new method never dries the coating at all. 

The team adapted a process used in latex glove manufacturing. A paper substrate is first coated with calcium salt, then dipped into a liquid mixture of bacteria and polymer particles. Where the salt is present, the polymer gels on contact, forming a thick, porous layer around the bacteria. That layer is immediately submerged in warm lysogeny broth – a nutrient-rich liquid routinely used to grow bacteria in the laboratory – rather than placed in an oven. The warmth causes the polymer particles to fuse together, producing a hard but permeable coating while the bacteria remain submerged and hydrated throughout. They are never exposed to air. 

The research team, which includes PhD students Alexia Beale and Kathleen Dunbar, argue that the porous structure of the new coatings matters as much as the manufacturing process itself. For bacteria inside a coating to do useful work, nutrients and reactants need to reach them, and waste products need to escape. Conventional dried coatings are dense and poorly permeable. The new coatings have a water permeability measured to be more than ten times higher and observed by electron microscopy. 

The bacteria in the new coatings do not merely survive – they remain metabolically active. When supplied with glucose, bacteria in wet-sintered coatings produced ethanol through fermentation, a result the researchers describe as proof of concept for future biofuel applications. The team are exploring hydrogen production via fermentation as a next step. 

The method works in principle for any bacterial species but is expected to be of particular value for desiccation-intolerant bacteria – those that cannot withstand drying and are currently excluded from conventional biocoating processes entirely. 

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Notes to editors 

• Professor Keddie is available for interview. Please contact mediarelations@surrey.ac.uk to arrange. 
• The full paper is available at: https://doi.org/10.1021/acsami.6c05652