Driving Resource Recovery With Scum Reuse

By Peter Chawaga

Increasingly, wastewater treatment operations are becoming more than that name would imply.

Many have now evolved into resource recovery centers, ever more focused on extracting the valuable material from influent and recycling it into potable water, sellable energy, and more. As reuse technologies become more advanced, the ways in which wastewater treatment facilities can recover these materials and apply them are as well.

Researchers at the University of Minnesota have demonstrated this evolution with the development of a process that can turn wastewater scum, a treatment byproduct, into biodiesel. While the practice of converting wastewater byproducts into energy is not totally novel, transforming scum (which is typically disposed of in landfills) into vehicle fuel is a newly attractive prospect.

“Our preliminary research convinced us there was enough energy-dense waste oil contained within waste scum to economically justify its processing into biodiesel using our unique processes,” said Erik Anderson, one of the researchers. “As approximately 70 to 80 percent of the cost of making biodiesel is purchasing the initial oil, we’re able to utilize a waste material that not only has zero commercial value, but presents a negative cost to the taxpayer and the environment.”

The researchers were understandably vague about how this process, which they have yet to name, takes place, but they did provide a basic description of its three critical stages. First, usable oil is separated from non-oil organics, metals, and water through physical and chemical extraction. Then, the scum is converted to biodiesel in a one-step process. Finally, post-conversion distillation must occur. The result is a transparent product that meets ASTM International guidelines for commercial biodiesel.

Development of the process took place at a research pilot processing facility at the University of Minnesota and the nearby St. Paul Waste Water Treatment Facility (SPWWTF) provided the scum necessary to fuel it. The SPWWTF, which produces about 3.5 tons of wet scum per day, is preparing to install the process once pilot-scale testing is complete. The researchers expect benefits to the operation to be immediate.

“The SPWWTF can benefit in a number of ways,” Anderson said. “Immediately, they can avoid the volume of scum material they send to landfills each year by as much as 87 percent. In addition to disposing of their organic waste more efficiently, they would be producing enough renewable fuel onsite to power approximately 20 percent of the petroleum diesel vehicles operated by the Metropolitan council.”

While avoiding landfill costs is a clear benefit, it’s the new potential for scum to fuel utility, or possibly even commercial, vehicles that seems to have the most promise. The researchers estimate that the fuel produced at the metro facility has an innate value of about $400,000, which could reach over $600,000 when accounting for federal tax incentives and renewable credit programs. This can be a much more profitable enterprise than converting wastewater to biogas, a current favorite among wastewater recovery facilities.

“Based on our calculations and published economic review, producing biodiesel and selling it at market value for petroleum diesel, accounting for tax incentives and federal renewable credits, is roughly twice as profitable as compared to the electricity generated from [anaerobic digestion] biogas,” said Anderson.

Furthermore, according to the researchers, biodiesel is a really the only sensible reuse option for scum.

“There are several reasons [converting] scum into biogas is problematic,” said Anderson. “Scum has a large amount of oil content, which tends to float on the top of large digesters and cause foaming. For that reason, scum would have to be blended into the total feed mixer as a percentage. Also, scum contains a vast range of unknown organic and inorganic compounds that could have anti-bacteria properties. A complete analysis of the scum is difficult because there are so many unknowns, but when your business model relies on consistent biogas production, feeding something like scum to anaerobic digestion can raise the uncertainty of the operation.”

As the SPWWTF awaits successful pilot testing in order to apply the process, so too do operations all over the country. The researchers appear confident that wastewater operations all over the world will soon be able to convert their scum into biodiesel.

“The conversion yields and experimental mass balance [in pilot-scale testing] have indicated that our technology will work at any scale and we’re now looking to build the full-scale process as soon as possible,” Anderson said. “The successful completion of a full-scale scum-to-biodiesel process would mark the beginning of our commercial application. We would work toward introducing the waste to energy technology to every possible [wastewater] treatment facility of qualifying size.”