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A wastewater treatment plant in Akaroa.
Photo: Supplied/Christchurch City Council
University of Auckland researchers have discovered different microbes can be used to reduce the carbon footprint of treating wastewater.
Associate Professor Wei-Qin Zhuang and his team from the Department of Civil and Environmental Engineering have been tackling the problem of reducing the carbon footprint of removing nitrates from water.
Nitrate removal needed to occur before the treated water can be discharged into the environment, Zhuang said, as it encouraged the growth of algae and caused other problems for the natural environment.
In domestic wastewater, the main source of nitrate was human urine.
The kind of microbes commonly used to turn nitrate into nitrogen gas fed on organic carbons for energy, in order to process nitrates – and they released by-products like carbon dioxide and nitrous oxide, which contributed to the carbon footprint.
Carbon emissions from wastewater treatment were thought to make up a small, but preventable percentage of many countries’ overall carbon footprints.
Zhuang said his team had discovered two different kinds of microbes that existed in wastewater already and could be used to remove nitrate. One used hydrogen gas as fuel to turn nitrate into harmless nitrogen gas and water, and the other fed on sulphur, turning nitrates into sulfate and nitrogen gas.
These microbes weren’t as common, so they needed to be duplicated and re-introduced into bioreactors to do the job.
“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 created less waste than their organic carbon-eating alternatives, avoided greenhouse-gas-intensive chemicals and reduced the need for trucked-in chemicals, making them well suited to local communities.
They could also be suitable methods for treating drinking water, he said.
Earlier this year, the country’s largest-ever drinking water survey found tens of thousands of rural New Zealanders [https://www.rnz.co.nz/news/country/574594/country-s-largest-study-into-drinking-water-nitrates-reveals-rural-freshwater-at-risk could be drinking water with harmful nitrate levels, with 5 percent of the private bore samples tested exceeding the national maximum nitrate guidelines.
Health data has shown high nitrate levels can be dangerous for babies and can affect people’s health over time.
According to Zhuang, these microbe set-ups could be useful for small communities or private land owners to treat their own drinking water supplies.
“Using microbes to clean water on-site gives communities a safety net, while long-term solutions, like better land management, are put in place,” he said.
Zhuang and his team were now testing these systems in real-world trials with water utilities and industry partners, aiming to make them easy for councils and communities across the country to use.
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