Shining a light on safer, greener water treatment techniques

Engineers at Monash University have designed an improved method for removing harmful pathogens such as E. coli from water by using graphitic carbon nitride and sunlight, offering water utilities a chemical-free, energy-neutral solution to effective water treatment.

The international team, led by Monash University Department of Chemical Engineering’s Professor Xiwang Zhang, combined graphitic carbon nitride with polyethylenimine (PEI) to destroy bacteria, offering significant improvements in large-scale water treatment techniques.

Zhang said the the process, which was published in the journal Applied Catalysis B: Environmental, uses positively charged PEI on graphitic carbon nitrate to promote contact between photocatalyst and bacteria cells via electrostatic adhesion, allowing the PEI to trap bacteria cells in water.

The harmful bacteria cells are then deactivated by the generated reactive oxygen species through photocatalysis.

“We discovered the PEI functionalisation can, in effect, tune the photochemical reactions on graphitic carbon nitride,” Zhang said.

“We found that the positively charged PEI on graphitic carbon nitride could promote the contact between photocatalyst and bacteria cells (negatively charged surface) via electrostatic adhesion, which can enable reactive oxygen species to kill the trapped bacteria cells.”

Sunlight disinfection

Zhang said integrating their photocatalyst into solar water disinfection technology achieves disinfection without the need for chemical dosing or excessing energy use.

“This technology is a solar-driven disinfection process, which has a strong oxidising power. It is just as effective as chemicals we use in water disinfection, like chlorine and hydrogen peroxide,” he said.

“The best thing about this new method is that it’s super clean. There isn't a large amount of disinfection by-product, which is a great benefit. We utilise sunlight; we don’t need to apply any other chemicals.

“This is a green disinfection process, and it is energy passive — you don't need electricity to facilitate the process.”

In terms of how the method might be integrated into current water treatment techniques employed by the Australian water sector, Zhang said there are plenty of options, with the method offering an improvement on stormwater harvesting, as well as wastewater and potable water treatment.

“We can use this technology to improve stormwater harvesting. Across Australia, our current technology for stormwater harvesting treatment involves the use of bio-filters, which is a natural system,” he said. 

“And while bio-filters are great for removing heavy metals and nitrogen, unfortunately they are not effective in removing pathogens, like E.coli.

“Furthermore, the technology can also be utilised for the treatment of wastewater. We currently utilise activated sludge to remove organic pollutants by applying microorganisms. But there are usually still some pathogens present in the treated wastewater.

“We need to discharge this water back into the environment, and in many cases, we need to apply chemicals to remove those pathogens.”

Cleaner drinking water

Zhang said there is potential to apply the new method as a secondary disinfectant process for potable drinking water, too.

“Perhaps a more promising application of this technology would be to apply it as a secondary disinfection process. Chlorination is common; our treatment facilities are already set up to apply this form of treatment,” he said.

“But, in many regions we have big water tanks to store our drinking water. This offers the opportunity to apply this solar-driven disinfection technology.”

Aside from potential applications in the Australian water sector, Zhang said the reduced need for chemical dosing and electricity use also makes the method appropriate for water treatment needs in developing countries.

“Infectious diseases caused by waterborne pathogens threaten the health of people worldwide,” he said.

“This PEI functionalisation process is simple. It can be shared with desperate communities across the world after further research is conducted on the development of photocatalysis devices.