The wee problem with our freshwater

By Rebekah White

One chemical nobody wants to find in the water we drink or swim in? Nitrate.

Too much in drinking water makes people sick.

Too much in waterways causes algal blooms, which can smother and kill underwater life.

And it comes from urine, among other sources.

Nitrate ends up in waterways in the first place largely because of untreated urine and fertilisers spread on fields.

Cow urine leaches through soil into waterways; fish urine in aquaculture contaminates seawater; in towns and cities, even treated wastewater contains nitrate.

The problem is getting worse, at least where we’re measuring it.

In April, the Ministry for the Environment’s latest freshwater report showed nitrate levels had worsened at almost 40 percent of 998 groundwater monitoring sites, and breached safe limits at 12 percent of sites.

Now, University of Auckland associate professor Wei-Qin Zhuang from the Faculty of Engineering and Design and others are working on solutions, aiming to transform nitrates into harmless nitrogen gas that simply bubbles away.

Microbes to the rescue

Zhuang knew microbes could process nitrates into something else – he just had to find the right kinds of microorganisms.

And that involved finding the right fuel for them. If he offered nitrate-digesting microbes a free lunch, they would power up to do their work – converting nitrate to nitrogen gas.

Microbes use all kinds of chemicals as energy sources, and Zhuang discovered that hydrogen was a hit.

If he added hydrogen to the wastewater process, the microbes flourished: they would run mini chemical reactions using hydrogen and nitrate to create energy for themselves, producing nitrogen gas on the side.

However, hydrogen has a significant downside: it’s prone to exploding, Hindenburg-style, and Zhuang reckoned that city wastewater systems might not be so keen on it.

He went back to the drawing board, and found a better candidate: elemental sulphur.

New Zealand has a lot of it. Microbes like to munch on it, forming gooey layers called ‘biofilms’ – camping out on their energy source and using it little by little.

“So it’s another technology that you can use to remove nitrates.”

Saving fish from their own wee

Next, Zhuang and his collaborators built a sulphur-driven denitrification reactor and tested it at an aquaculture farm in Bream Bay in Northland.

There, kingfish grow in big tanks. Their waste means there’s lots of nitrate in the water, and if it’s released into the ocean, it can cause eutrophication – the process in which nitrates feed the sudden overgrowth of algae, degrading water quality or potentially making it uninhabitable for marine species.

When the kingfish tank water flowed through the reactor, the microbes removed the nitrates, showing that the system worked, and in a format that could be added to existing wastewater plants.

“We’ve designed this process so it can be a drop-in technology,” says Zhuang.

“That means you do not need to change the current wastewater treatment infrastructure too much. Wastewater flows through different units, and then at the end you add our technology to further lower the nitrate concentrations.”

The issue with city wastewater

In cities, wastewater is treated to remove most (but not all) nitrate before it’s discharged.

Normally, a little bit of residual nitrate is manageable, says Zhuang.

“However, in a fast-growing city like Auckland, population growth can mean that the overall nitrate load released to the environment gradually increases over time.”

That happens even when treatment systems are operating as they’re supposed to, and all that extra nitrate, little by little, can affect the water bodies where wastewater is discharged.

In Auckland’s case, that’s the Manukau and Waitematā Harbours.

And there’s an extra problem. Current wastewater systems remove nitrate by turning it into nitrogen, generating a small amount of nitrous oxide gas in the process.

Nitrous oxide is a potent greenhouse gas (as well as being the ‘laughing gas’ used in hospitals and elsewhere).

“Nitrous oxide’s greenhouse power is 295 times worse than carbon dioxide,” says Zhuang.

“It’s a huge impact to the environment. In the news, people say, ‘Methane is bad.’ Methane’s greenhouse power is only 25 times worse than carbon dioxide.”

Zhuang’s microbes produce very little nitrous oxide, whether they’re fed by hydrogen or sulphur, meaning that they offer a more environmentally friendly mode of treating nitrate in wastewater.

Sorting out groundwater with hydrogen

When it comes to treating drinking water, rather than wastewater, Zhuang says hydrogen is particularly well suited to the task.

Sulphur can produce sulphate as a byproduct, which would have to be kept to low levels in drinking water.

How do you get a volatile gas, like hydrogen, into groundwater?

Imagine a long, porous straw made of high-quality polymers. Pressurised hydrogen is pumped through it, and the hydrogen is squeezed out through the walls of the straw as individual molecules, which then readily dissolve in water.

A reactor filled with these straw-shaped membranes might be useful in parts of Canterbury which have higher nitrate levels in groundwater. Then groundwater could be pumped out and treated.

Are these microbes just hanging around, waiting for something to do?

So far, Zhuang has discovered they’re more widespread than he expected – simply add hydrogen or sulphur, and they make their presence known.

So far, they’ve been found everywhere he’s tested for them.

“So yeah, we don’t need to import them from overseas. We can just sort them out in New Zealand.”

The world is facing unprecedented environmental challenges. Planetary Solutions, an initiative of the Sustainability Hub at Waipapa Taumata Rau, University of Auckland, and Newsroom, explores these issues – and the practical ways we can all be part of the solution.