What’s clean and green, produces 11 megawatts of electricity, and saves the city of Oakland $3 million a year while decreasing the amount of solid waste? What decreases contamination of ground and surface water and minimizes odor while generating electricity and creating sellable fertilizer? What has none of the environmental hazards involved in fracking for gas?
The answer is: biogas digesters utilizing microorganisms. They can handle both human and animal waste, food waste and other agricultural waste. As of 2012, the Oakland plant became the first in the U.S. to sell extra electricity produced entirely from waste back to the grid. Of the 11 MW it produces, it uses 6 MW, so 5 MW are left to sell. It treats 60 million gallons of wastewater daily and uses sewage, food scraps from restaurants, winery and poultry farm wastes [avl.mx/5ph].
In Ohio, a sewage treatment plant using biogas digesters generates enough electricity to power 1,600 homes and also generates revenue from its safe, high-quality fertilizer.
AgStar estimates that biogas recovery systems are technically feasible at over 8,000 large dairy and hog operations, with the potential for generating 13 million megawatt-hours per year. Side benefits: substantial decrease in uncaptured methane emissions from the animal wastes, and the protection of the groundwater and surface water, with greatly decreased odors. Adding 25 percent food waste to sewage sludge in digesters increased biogas production by 60 percent, they found [avl.mx/5pi].
Has Asheville or Buncombe County considered using biogas digesters for municipal waste treatment? What could be better than using waste to generate the energy we need and saving money in the process?
— Cathy Holt
Editor’s note: Xpress checked in with the local public agencies that handle municipal waste with the letter writer’s question. We received the following response from Buncombe County Solid Waste Director Dane Pedersen: “Buncombe County operates a bioreactor landfill. The bioreactor landfill works to rapidly transform and degrade organic waste. This increased waste degradation and stabilization are accomplished through the reintroduction of liquid and, in our case, is recirculated leachate that enhances microbial processes. This provides opportunity to recapture air space due to the rapid stabilization of the waste mass, and we are able to harness the biogas that is generated through the organic waste decomposition process and power a 1.4-megawatt generator system that produces enough electricity to power 1,100 homes.
“We enjoy bragging about this waste-to-energy program and place a great deal of value in how we are able to create environmental utility from the services that we provide every day. Implementing practices to create beneficial uses of landfill biogas is and will continue to be a primary objective for our solid waste department. To learn more about the benefits of recirculating leachate visit [avl.mx/5pj].”
Xpress also received a response from Thomas E. Hartye, general manager of the Metropolitan Sewerage District of Buncombe County: “For many years, MSD has utilized digestion of its municipal sludge by anaerobic microorganisms. The byproducts of this digestion system are digested solids and methane gas. MSD would recover this gas and recover heat to generate heat/electricity. The sludge would then have to be lime-stabilized up to a pH of 14, and then could be used as an agricultural supplement or taken to the landfill.
“In the late 1990s, MSD did a cost-benefit analysis of the entire system. Due to lack of efficiency, cost of maintenance and the overall carbon footprint of treating and eliminating sludge byproduct, along with running that process, MSD moved to a different process. The current process utilizes intermediate process solids pumped to gravity thickeners; the thickened solids are pumped to high-performance belt-filter presses, which provide a drier, more BTU-laden sludge so that it can be thermally converted. This reduces the overall cost and carbon footprint.”