(Source: Warner Bros Pictures)
We won't ask where the inspiration for this idea came to the research team

Sewage sludge is a major issue for the massive metropolitan areas that are increasingly permeating our globe.  Sludge, the leftovers of wastewater processing, is largely the remains of human fecal matter.  

But some researchers are devising creative uses for the bowl movement brew.  There's a researcher in Japan who's supposedly working to turn sewer sludge into protein burgers.  And now a team of researchers in South Korea has suggested that producing biodiesel from sewage sludge may be cheaper than making it from spent food oil.

The study was led by Eilhann Kwon of the Research Institute of Industrial Science and Technology.  He reasoned that sewage sludge was rich in lipid content, so why not try to turn it into biodiesel -- whose starting component is more typically the lipids found in various waste oils (think used french fry oil).

Sewage sludge lipids are produced when aerobic bacteria in sewer drainage convert pockets ("moieties") of methane (CH4) and carbon dioxide (CO2) into lipids, which are then converted to energy.  Some of the lipids accumulate in the bacterial bodies which litter the drainage, hence the drainage can become relatively lipid rich.

Using n-hexane Professor Kwon extracted these lipids from dried sewage pellets.  The team found that sewage has 2,200 times more lipid per gram than soybeans -- which are considered a fairly "oily" crop.  And extracting those lipids cost a mere $0.03 USD per liter, versus $0.80 per liter of soy oil.

Soybean Oil
Sewage often has thousands of times the amount of lipids per gram as soybean oil and can be produced at a fraction of the cost. [Image Source: ThinkSoy]

So why hasn't this been done before?  Well it turns out fatty acid impurities also lurk in sewage, and foul the biodiesel catalytic conversion process, which involves combining the lipids with methanol.

To solve that problem the team came up with a non-catalytic process that uses heat, rather than a catalyst, to drive the reaction to completion.  The new process also increases the surface area involved in the reaction to further accelerate the reaction.  To do this it traps the reaction ingredients in a porous material -- activated alumina.

In a test, with a reactor heated to 380 °C, with excess carbon dioxide present, a 98 percent conversion rate was achieved.

Sewage to diesel
In lab tests, 98 percent of sewage lipids were converted to biodiesel using the new process.
[Image Source: ACS]

Professor Kwon hopes to move aggressively to offer the new waste salvage technology to sewage processing plants.  To do that a desiccation (drying) area would be needed, and extraction facilities would need to be added.  But Professor Kwon believes these additions could be paid off in a matter of years, yielding both a profit and environmental gains.  He comments, "Waste is not simply waste—it can be converted into useful resources like biodiesel."

The new method has been published [abstract] in the peer-reviewed journal Environmental Science Technology.  Rafael Hernandez of Mississippi State University praised the work in an interview with Chemical Engineering News, commenting that the results were "very encouraging" while cautioning that quality of waste varies from location to location and amongs given batches at a particular location.

Sources: Sewage, Chemical Engineering News

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