The process could be installed at the St. Paul Wastewater Treatment Plant after pilot-scale testing

Scientists from the University of Minnesota are piloting a plant capable of converting wastewater treatment byproducts into biodiesel fuel.

While wastewater scum is typically treated by anaerobic digestion to make biogas or disposed of in a landfill, a new process developed by university researchers allows plants to convert scum to biodiesel that is usable by campus utility vehicles.

The process was developed by College of Food, Agricultural and Natural Resource Sciences (CFANS) professor R. Roger Ruan — who also directs the Center for Biorefining — along with doctoral candidate Erik Anderson.

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A statement from the university claims that 68 percent of dried and filtered scum can be converted to biodiesel, which is equal to approximately 140,000 gallons of biodiesel and $500,000 to $600,000 in profit per year. Additionally, the process uses available waste heat from a nearby water treatment facility.

The team has been using scum from the St. Paul Wastewater Treatment Plant and processing it at the pilot processing facility at the university’s Rosemount station. Ruan and Anderson say they’re hopeful that the St. Paul Wastewater Treatment Plant will soon be ready to install the process after pilot-scale testing is complete.

The university said in its statement that the plant produces about 3.5 tons of wet scum each day. Assuming a constant supply of scum material over a 365-day period, a yield of between 189,000-216,500 gallons of usable oil could be produced annually. At the conservatively estimated process efficiency of 68.8 percent, the scum oil could yield between 130,000 and 149,500 gallons of biodiesel each year.

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“Using a free oil source combined with the lack of transportation and handling costs makes a scum to biodiesel process very attractive financially,” said Anderson. “Scum oil is not only free, but is considered a waste material. By converting most of the scum to biodiesel, and part removed as either distillation bottoms or filtered solids, the process saves a majority of the liquid organic scum from a landfill. That’s an immediate savings of more than $150,000 per year in costs associated with landfilling scum at the St. Paul facility, in addition to the revenue generated.”

If the finished biodiesel is used directly, it can be valued at the same price as the petroleum diesel, roughly three dollars per gallon, offsetting the purchase of an estimated 145,000 gallons of biodiesel and saving approximately $435,000 in fuel costs alone.

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