WSU works with anaerobic digesters to offer more renewable resources
Traditional methods of dealing with organic waste sources include sewage treatment plants, aerobic lagoons, compost yards and landfills. Using these solutions to treat organic waste often consumes more energy than produces.
However, anaerobic digesters come in many different designs and are able to treat municipal sewage, urban food waste and animal manure while producing energy and renewable resources.
“Digesters have been around for a while, but we’ve got new technology that allows us to take this to a better place economically and environmentally,” says Dan Evans, president of Promus Energy.
The next generation of anaerobic digesters is attaining technology promising to be capable of producing more renewable energy, bio-fertilizer and a reduction in greenhouse gases, pathogens and odors.
“Anaerobic digesters are basically carbon conversion tools,” describes Craig Frear, biological systems engineering assistant professor at Washington State University (WSU). “They simply take organic carbon in manure and turn it into a gaseous inorganic carbon that is called biogas.”
Biogas is comprised of methane and carbon dioxide. It is the methane that is sought after from this process to be used as a fuel source or to make electricity.
The initial capital costs of an anaerobic digester can be extreme, costing anywhere from $114,000 to $326,000. However, the benefits in energy savings from digesters have been noted to be around $40,000 a year.
In the construction process of anaerobic digesters, approximately 15 to 20 full time jobs are created, and an additional two to three permanent positions are created for the operation of the digesters.
Anaerobic digesters work by mimicking the digestion process of an animal’s stomach.
The main component of the digester is a heated, sealed tank where organic material is broken down through agitation and mixed in an anaerobic, or oxygen-free, environment.
In the case of dealing with animal manure, traditional manure management methods were limited to storing the untreated manure in lagoons and later applying the untreated liquid onto fields.
Potential air and water quality issues can arise from this practice, as well as nutrient overload of phosphorous and nitrogen to soils.
The process of anaerobic digesters converts solid carbon in manure to biogas.
The biogas is then collected, cooled and either used to power electrical generators or another process where it is transitioned into renewable natural gas.
Electricity generated from the system can be sold later to local utility companies.
Evans explains that the industry is on the verge of transitioning from petroleum-based fuels to methane fuels, or renewable natural gas.
“We have the technology now to produce renewable natural gas from biogas that comes off of digesters that can be used for trucking fleets and for cars,” describes Evans. “It’s clean to pipeline standards and is available for vehicle fuel for half the price of diesel.”
The digesters are able to create their own energy and are, therefore, self-sustaining.
Excess biogas and heat reclaimed from the generator return back to the main sealed tank to keep the cyclic process going.
After the initial biogas is captured, the treated leftover manure from the tank is pumped to a separator where the liquid is removed. The remaining solids can be used as animal bedding, a soil amendment or peat moss replacement.
Crops can also be watered utilizing the treated liquid from the digester. This liquid has been significantly reduced in pathogens and is virtually odor free – unlike the liquid from traditional methods of manure management.
“In the future we will be able to take this to the next step to clean even drinkable water,” comments Evans.
Development is occurring to add additional components to anaerobic digesters to recover nutrients such as nitrogen and phosphorous from the organic material.
Washington State University and their industrial partners are developing a process to reclaim nitrogen in the form of ammonia.
“We can collect it and then make it into ammonia sulfate fertilizer that farmers traditionally use,” comments Frear. “An added benefit is that during this ammonia production process we also cause phosphorous to settle out, so we get a two-for-one.”
These concentrated bio-fertilizers can be used by farmers to wean themselves off using fossil fuel-based fertilizers.
“A combined digester and nutrient recovery system is a win-win because we make renewable energy and solve a concern with nutrients, as well as make revenue for the local community,” Frear adds.