A new study by researchers in China has shown that rhamnolipids can help turn sewage into hydrogen and biodiesel, Chemical & Engineering News (C&EN) reported.

Rhamnolipids are biodegradable surfactants named for their rhamnose sugar groups and fatty acid tails.

The study showed that wastewater could be treated, injected with oxygen, and fermented as part of a process that produces both hydrogen and biofuels, the 10 February report said.

Researchers at the Harbin Institute of Technology and the Northeast Agricultural University, in Harbin, took waste-activated sludge – made from raw sewage that had been treated with oxygen to stabilise its microbe populations – from a treatment plant in Harbin, China.

After putting the mixture through a sieve to remove large particles, the sludge was heated, and, using fermentation, bacteria were left to produce hydrogen. Micro-algae were then added to generate lipids for use in biofuel production.

As a final step, the effects of nine surfactants were tested on the process.

The study showed that adding rhamnolipids created the best environment for the microalgae and the sludge’s bacteria to generate fuels, C&EN wrote.

The researchers speculated that the rhamnolipids’ hydrophilic and hydrophobic groups reduced the surface tension of the waste-activated sludge and the macro-molecules contained within it, the report said. This could have helped break down the waste’s organic matter, making vital nutrients more accessible for the hydrogen-producing bacteria to digest.

In addition, the surfactant helped biodiesel output as the microalgae fed on the fermentation solution produced by the bacteria.

The study showed that waste processed with rhamnolipids yielded 13.53 times the hydrogen and 11.2 times the biodiesel precursors compared to the control group.

The biosurfactant also worked well at relatively low concentrations, the researchers said.

“It looks promising economically because of this,” says Pattanathu Rahman, a biomolecular scientist at Liverpool John Moores University, UK, was quoted as saying.

Although Rahman was not involved in the study, he had previously discovered a novel biosurfactant-producing bacterium, the report said.

“This technology would work best in a tropical country… Microalgae need sunlight, and so you’d need a photo-bioreactor in winter time in colder countries, which would make it very expensive,” Rahman added.

However, others noted that this new technology had only been tested on waste-activated sludge.

This material was a relatively simple substance to work with compared with wastes rich in phosphates, nitrates, and complex compounds, Shirish H Sonawane, a chemical engineer at the National Institute of Technology Warangal, India – who did not contribute to the new study – was quoted as saying.

“That type of waste is really difficult to treat, and that’s what the major challenge to tackle is.”