Researchers at the University of Auckland say naturally occurring microbes could help reduce the carbon footprint of wastewater treatment while still meeting environmental standards.
The work is being led by Associate Professor Wei-Qin Zhuang from the university’s Department of Civil and Environmental Engineering, with trials now underway alongside water utilities and industry partners.
Nitrates must be removed from wastewater before it is released, as they fuel algae growth and damage waterways. In domestic wastewater, the main source of nitrate is human urine. While effective, current treatment methods come with an emissions cost.
Most wastewater plants rely on microbes that consume organic carbon to convert nitrate into nitrogen gas. In doing so, they also produce carbon dioxide and nitrous oxide.
Carbon emissions from wastewater treatment make up a relatively small share of national totals, but Zhuang describes them as “a small, but preventable percentage” of overall emissions.
Zhuang’s team identified two less common microbes already present in wastewater that can remove nitrate with fewer emissions. One uses hydrogen gas to convert nitrate into nitrogen gas and water. The other feeds on sulphur, converting nitrate into sulphate and nitrogen gas.
Because these microbes occur in low numbers, they must be grown and reintroduced into bioreactors. “When wastewater flows through, we provide hydrogen or sulphur for these microbes to use, and then they will reduce nitrate to nitrogen gas,” Zhuang said. Both systems generate less waste and avoid the use of greenhouse-gas-intensive, trucked-in chemicals.
The findings may also apply to drinking water. A national survey earlier this year found 5 per cent of private bore samples exceeded maximum nitrate guidelines, potentially affecting tens of thousands of rural New Zealanders. High nitrate levels can be dangerous for babies and harmful over time.
“Using microbes to clean water on-site gives communities a safety net, while long-term solutions, like better land management, are put in place,” Zhuang said.