We could make fuel and fertiliser by recycling wastewater

An environmentally friendly technique turns wastewater into ammonia and harmless by-products using a multi-chambered chemical reactor. The sustainable alternative requires much less energy than the conventional method for producing this crucial chemical.

Agriculture, refrigeration systems, paper, cleaning supplies and other industries use hundreds of millions of tonnes of ammonia every year. Making that much of the chemical uses about 2 per cent of energy total energy consumption and contributes 1.4 per cent of global carbon dioxide emissions.

Some of this environmental price is due to the conventional way of producing ammonia, which requires high temperatures and pressures. To make ammonia production more sustainable, Feng-Yang Chen at Rice University in Texas and his colleagues wanted to replace that technique with a room-temperature reactor.

Their reactor takes in water mixed with nitrates – nitrogen compounds often found in wastewater, such as industrial sewage or agricultural runoff contaminated with nitrogen-based fertilisers. After the nitrate water enters the first of three chambers, electrodes, similar to those found in batteries, create an electrochemical reaction that transforms the liquid into three components: only ammonia remains in the first chamber of the reactor, while purified water flows out through the second one and oxygen goes to the third.

Because ammonia contains only nitrogen and hydrogen, this electrochemical reaction does not require any ingredients other than the wastewater. And the purified water it produces is clean enough to meet the World Health Organization (WHO) regulations for drinking water.

Chen says that similar reactors have been tested before, but the electrodes could not shuffle charges at a sufficient voltage for the reaction to work – unless lots of salts were added to the wastewater. He and his colleagues made their device more practical by filling its middle chamber with a porous material that plays the role of those salts, so wastewater can be fed directly into the reactor without additives.

In experiments with water samples that had realistic concentrations of nitrates, the reactor processed 100 millilitres in about an hour, and it kept working well when it ran for 10 days straight. Its performance is similar to that of previous, more complicated reactor designs.

The team has only tested the reactor in the lab using nitrate-rich water, not real wastewater samples, which contain more than just nitrates, says Chen. But the researchers eventually envision local businesses and farms using these reactors to recycle wastewater, instead of sending it to far-away treatment plants that waste its ammonia-making potential.

In the best-case scenario, a farm might have its own reactor, powered by solar or wind power. Farmers could run local wastewater through the device and immediately re-use the ammonia it had extracted as fuel or fertiliser.

“We’re still at the academic research level, but this is my ultimate dream,” says Chen.

Topics:

• chemistry /
• sustainability