Plasma, the fourth state of matter more commonly associated with neon lights and lightning, may hold the key to a safer, more sustainable way to clean contaminated water.
Recent experiments suggest that plasma-based water purification can eliminate E. coli and other contaminants without using chlorine, ozone, or conventional filtration.
The process relies on the generation of reactive oxidants, such as hydroxyl radicals and hydrogen peroxide, produced when an electrical discharge interacts with water. These compounds are highly effective at breaking down both microbial and chemical pollutants.
The findings come from two main studies, one led by physicist John E. Foster at the University of Michigan and another conducted by a team at the Polytechnic University of Puerto Rico.
Both suggest that plasma, though still experimental, could form the basis of a new generation of water purification systems, especially in places where infrastructure is aging or clean water is in short supply.
In Puerto Rico, researchers built a simple reactor using a mason jar and stainless steel electrodes powered by 850 volts. Wastewater collected from a local treatment facility was exposed to glow discharge plasma, a type of non-thermal ionised gas. Within minutes, a faint violet glow appeared around the anode, indicating that reactive species were forming in the liquid.
Samples taken before and after treatment showed a stark contrast. Petri dishes seeded with untreated water revealed widespread bacterial growth, while dishes containing treated samples were completely clear. Further lab analysis using the Most Probable Number method confirmed the effective removal of E. coli, even at lower concentrations.
The plasma also altered the chemistry of the water. The pH shifted from neutral to slightly alkaline, consistent with the formation of oxidants such as ozone and hydrogen peroxide. These substances are known to kill bacteria and degrade organic pollutants, but unlike chlorine, they break down quickly and leave no long-term residues.
Foster, who authored a comprehensive review of plasma-based purification in 2017, argues that the method could play a significant role in addressing contaminants of emerging concern. This includes compounds like pharmaceuticals and personal care products that conventional treatment systems struggle to remove. Plasma has the added advantage of producing its oxidants in situ, eliminating the need for storage tanks, chemical dosing equipment or specialist handling.
Still, the technology faces hurdles. The biggest is scale.
Plasma systems are currently effective only when treating low volumes of water at a time. That makes them ill-suited to large municipal treatment plants without major engineering advances.
Efforts to scale up the systems are ongoing, with researchers focusing on improving the diffusion of reactive species into flowing water and increasing energy efficiency.
Several pilot programs in the United States are testing plasma reactors on industrial wastewater, particularly targeting persistent contaminants like per- and polyfluoroalkyl substances (PFAS).
Designs vary, but most aim to create thin films or mist layers that maximise contact between plasma and water, allowing higher doses of oxidants to be delivered in a short time. Some systems under development can treat up to five gallons per minute, with performance comparable to UV or ozone systems, but without the associated chemical costs or safety concerns.
For now, plasma water purification remains a promising but niche solution. It is unlikely to replace existing infrastructure in the near term. But in a world increasingly defined by water scarcity and pollution, its potential may be too significant to ignore.
References:
Foster, J.E., 2017. Plasma-based water purification: Challenges and prospects for the future. Physics of Plasmas, 24(5). <https://doi.org/10.1063/1.4977921>
Gonzalez-Lizardo, A., Morales, G., Rondon, J., Santiago, J., Landron, A. and Pena, X., 2024. Water Purification Via Plasma. arXiv preprint arXiv:2405.09524. <https://doi.org/10.48550/arXiv.2405.09524>