INNOVATION: Purple bacteria may help harvest green fuel from wastewater

London: A purple bacteria — which store energy from light — can help harvest hydrogen fuel from sewage, and recover carbon from any type of organic waste, scientists have found.

Organic compounds in household sewage and industrial wastewater are a rich potential source of energy, bioplastics and even proteins for animal feed — but with no efficient extraction method, treatment plants discard them as contaminants.

A study, published in the journal Frontiers in Energy Research, is the first to show that supplying electric current to purple phototrophic bacteria can recover nearly 100 per cent of carbon from any type of organic waste, while generating hydrogen gas for electricity production.

“One of the most important problems of current wastewater treatment plants is high carbon emissions,” said Daniel Puyol of King Juan Carlos University in Spain.

“Our light-based biorefinery process could provide a means to harvest green energy from wastewater, with zero carbon footprint,” said Puyol.

Purple phototrophic bacteria capture energy from sunlight using a variety of pigments, which turn them shades of orange, red or brown — as well as purple.

“Purple phototrophic bacteria make an ideal tool for resource recovery from organic waste, thanks to their highly diverse metabolism,” said Puyol.

The bacteria can use organic molecules and nitrogen gas — instead of carbon dioxide and water — to provide carbon, electrons and nitrogen for photosynthesis.

This means that they grow faster than alternative phototrophic bacteria and algae, and can generate hydrogen gas, proteins or a type of biodegradable polyester as byproducts of metabolism.

Which metabolic product predominates depends on the bacteria’s environmental conditions — like light intensity, temperature, and the types of organics and nutrients available.

“Our group manipulates these conditions to tune the metabolism of purple bacteria to different applications, depending on the organic waste source and market requirements,” said Abraham Esteve-Nunez of University of Alcala in Spain.

“But what is unique about our approach is the use of an external electric current to optimise the productive output of purple bacteria,” he said.

The researchers analysed the optimum conditions for maximising hydrogen production by a mixture of purple phototrophic bacteria species.

They also tested the effect of a negative current — that is, electrons supplied by metal electrodes in the growth medium — on the metabolic behaviour of the bacteria.

The first key finding was that the nutrient blend that fed the highest rate of hydrogen production also minimised the production of CO2.

“This demonstrates that purple bacteria can be used to recover valuable biofuel from organics typically found in wastewater — malic acid and sodium glutamate — with a low carbon footprint,” said Esteve-Nunez.