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New biodiesel production process uses microbes, sewage sludge

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Researchers at the Canadian Institut National de la Recherche Scientifique (INRS) have developed a new biodiesel production process that uses microbes, sewage sludge and a biofuel by-product.

Professors Rajeshwar Dayal Tyagi and Patrick Drogui have come up with a new method that significantly reduces the cost of making microbial biodiesel from glucose using microorganisms, down from $6.78 (€6.09) per litre to just $0.72 (€0.65) per litre.

The new process uses sewage sludge and glycerol, a by-product of biodiesel, with the processing of sludge resulting in reduced greenhouse gas emissions. "It keeps sludge out of landfills, where it releases methane,” explained Tyagi. “When it's used to produce biodiesel, most of the carbon is transformed into lipids by the microorganisms.” The process also makes it possible to reuse the glycerol without having to purify it.

In the first stage of fermentation, the microorganisms eat the glycerol and the sewage sludge, accumulating oil in their bodies in the form of lipids. The researchers then used bioflocculant (an organic polymer secreted by organisms) to separate the cells from the mixture and extract the lipids, which eliminates the need for centrifuges or chemical products.

In order to recover the lipids extracted from the cells, Tyagi and Drogui replaced those toxic chemical products with gas. "I was filling up my car and I asked myself why not use gas to separate the lipids,” explained Tyagi. “We tried it in the laboratory and it worked really well. The mixture of gas and lipids floated on top of the rest of the mixture.”

The gas added for separation is also part of the final product. Tyagi continued: "Biodiesel isn't entirely composed of oil. For type B10, 10% of the fuel is organic and the rest is gas.” The process produces biodiesel and glycerol, which could in turn be used to produce more fuel.

Their findings were published in the journal Bioresource Technology in February 2020.