Biodiesel from waste grease

14 April 2015 | Research
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From left: PhD students Akbar Vahidi and Tian Kaiyuan, working with the bioreactor that grows micro-organisms such as E. coli and yeast, which may also be used for carrying out large scale biodiesel reaction

A team of NUS researchers from the Department of Chemical and Biomolecular Engineering has developed a set of environment-friendly techniques to produce biodiesel entirely from waste grease more cheaply and efficiently.

Waste grease'a combination of used grease, vegetable oils and animal fats'is discharged by homes and food and beverage outlets. It usually accumulates in grease traps of buildings' sewer systems, and is collected for disposal by contractors licensed by the National Environment Agency in Singapore.

Given that some 30,000 tonnes of waste grease get discharged annually in the country, it made economic and environmental sense to recycle the waste material.

"Rather than letting the waste grease get disposed of, it is much better to turn it into a green fuel which reduces the reliance on external sources, explained team member Tian Kaiyuan, a PhD student. As far as he knows, no other urbanised cities in the US or Europe has harvested the waste grease from grease traps for biodiesel production on an industrial scale.

Traditionally, the high free fatty acid content in waste grease makes it unsuitable for biodiesel production due to technical issues. Current catalysts could not process the waste grease in a single step until the NUS team's recent discovery, which had been published in leading journals such as Bioresource Technology and Green Chemistry.

The NUS team led by Associate Professor Li Zhi pioneered an innovative one-step approach with specially engineered high-performance catalysts. Besides achieving impressive biodiesel conversion yields under optimal conditions, they can be easily separated from the biodiesel mixture at the end of the transformation process for multiple reuse with little deterioration in performance.

One method employs whole-cell Escherichia coli (E. coli) bacteria, producing a 97 per cent biodiesel conversion yield in 72 hours.

Another involves specially formulated magnetic nano-sized solid acid particles. When combined with methanol, they efficiently catalyse waste grease, yielding up to 98 per cent biodiesel in 24 hours.

The third is a recyclable magnetic nano-biocatalyst created using an enzyme derived from a fungus. This catalyses waste grease laden with methanol into biodiesel, achieving a remarkable 99 per cent biodiesel yield in 12 hours.

Among the three techniques, the natural system of whole-cell E.coli shows the most potential as the biocatalyst can be produced in large amounts at low costs.

With biodiesel gaining greater favour as a renewable and cleaner fuel source for transport and machinery, the team is in talks with industry partners to adopt the whole-cell biocatalysts technology for more in-depth pilot testing. A China-based company has expressed interest to adopt the technologies to recycle its gutter oil.