The increasing demands for clean energy, along with concerns over waste management, decreasing land space, the environmental impact of pathogens and the growing use of compost have sparked interest in the application of anaerobic digestion (AD).
And while the process is still finding a foothold in the United States, Colorado is helping to pave the way with a 20-megawatt AD facility — the largest in the country — close to completion in Weld County.
AD takes place naturally and is the force behind phenomena such as swamp gas, in which natural wetlands produce methane through the breakdown of organic material; and fool’s fire, in which the methane is ignited, creating glowing orbs of light. But, in the non-natural state, AD occurs in specialized tanks called anaerobic digesters.
Within these tanks, a series of biological processes take place where microorganisms break down organic material, such as food scraps or manure, in the absence of oxygen, producing biogas. Both methane and carbon dioxide (CO2) are yielded in the process, along with a solid residue similar to compost but without the nutrients, called digestate, and a liquid liquor known as “compost tea” that can be used in fertilizers.
Once the CO2 is removed from the methane, the remaining gas is a biogas called biomethane which is almost identical to natural gas, with the notable difference being that it was created in a matter of days, rather than relying on biological and geological processes set in motion millions of years ago. The renewable energy applications are instantly obvious for such a fuel.
On a dairy farm, for example, Robin Bacon, founder of Organic Marketplace Solutions, Inc., and consultant to Ag Waste Solutions and other renewable energy organizations, notes that “anaerobic digesters eliminate the need for propane, make electricity, dry grain, accept food waste, [and] mitigate odors and groundwater pollution.” Additionally, “the digester engine is a good source of heat for milking parlors, farm shops and milking centers,” he says.
Europe has been utilizing this process for a number of years. With their limited land space and stricter environmental guidelines, European countries have faced the added pressure to design and use creative waste management techniques. They don’t have the luxury of sacrificing hundreds of thousands of acres of land to landfills, as the United States has been wont to do. And although the number of landfills in the States has decreased, the acreage devoted to covering up our trash has increased, according to the U.S. Environmental Protection Agency’s Advancing Sustainable Materials Management report.
Questions are now being asked as to how to not only decrease our reliance on landfills, but also on how to perhaps save money and even create power while reducing the environmental impacts of landfills.
And the magic biogas called methane? Landfills make the perfect environment for naturally occurring anaerobic digestion, but un-harvested methane has a frightening global warming capacity 21 times higher than CO2. And while some landfills are capitalizing on what’s occurring naturally on their grounds by harvesting the methane into tanks, others allow it to leak into the atmosphere. Diverting food scraps alone from landfills to digesters could decrease methane emissions drastically and create a valuable renewable energy source. The Environmental Protection Agency (EPA) estimates that if only 50 percent of the food waste generated yearly in the U.S. were to be anaerobically digested, enough energy could be created to power more than 2.5 million homes for a year.
The benefits of using anaerobic digesters would trickle into a number of other areas, too. Fats, oils and grease (FOG) clog municipal sewer pipes, but in diverting FOG from our wastewater facilities to digesters, sewer overflows could be prevented and municipalities could experience money savings, as well as improved water quality.
The digestion of livestock manure, currently the leading use of AD in the States, not only allows for the production of biogas, but can save farms money, decrease odors, reduce methane emissions (and thereby increase safety) on the farm and improve water quality. In a report by S. Sharvelle and L. Loetscher of Colorado State University, they estimate a large dairy could potentially produce enough energy to heat 100 homes.
And that solid byproduct known as digestate produced by the anaerobic digesters can be mixed with added nutrients to create compost, or can be used alone as a soil amendment to aid in improving soil quality, help prevent runoff, retain surface water, improve plant growth and a number of additional applications.
Currently, according to the American Biogas Council, there are approximately 2,100 sites in the U.S. producing biogas with 247 anaerobic digesters on farms, 1,231 at wastewater treatment plants and 645 landfill gas projects. With the invention of food depackaging units that separate plastics and nonorganic material from food waste before sending the waste into the digester, larger chain grocery stores such as Wal-Mart are considering these machines for their distribution centers.
However, therein lies the problem: There needs to be a suitable amount of waste. As with most things, it comes down to dollar signs. The return on investment needs to be high enough to justify the introduction of an AD unit.
Anaerobic digesters are not designed for home, or even small-scale farm use. And while mini digesters are on the drawing boards of engineers across Europe, a time may never come when every city is equipped with its very own energy-producing vessel.
And it’s still a new idea in the United States.
“We are currently 20 years behind Europe,” says Will Hancock, a pre- and post-consumer food waste specialist for AD applications in North America. He believes the trend will continue to rise in the States as real estate becomes more valuable than burying waste, and with increasing support from the government subsidies.
Already a number of subsidy programs are on the slate for government review. Additionally, the U.S. Department of Agriculture (USDA), the EPA and the Department of Energy (DOE) are working together to foster support for the utilization of biogas in clean energy markets. According to the Biogas Opportunities Roadmap, a joint publication from the USDA, EPA and DOE outlining steps to increase awareness of biogas and AD benefits, the government agencies are diligently working to improve communication across state, federal and local governments to foster the adoption of biogas systems.
But Hancock is wary of the staying power of government subsidies, noting the roller coaster ups and downs of the wind energy programs as subsidies were given out and then taken away.
“We need to be more diligent in the beginning with dump fees and capital costs to make sure [AD programs] don’t close when subsidies go away or we will be back where we are right now,” he says.
As awareness continues to grow in the United States, there will be kinks in the system that will need to be worked out. But, for now, several states continue to implement anaerobic digesters in their systems. Hancock points to the Heartland Biogas anaerobic digester and renewable natural gas facility in Weld County, Colorado, as an example in our own backyard.
The facility will produce up to 4,700 MMBtu (1 million British Thermal Units, which are the units natural gas are measured in) of biogas daily, making it the largest anaerobic digester facility in the United States. The digestate and fertilizer produced will be used by local farms, while the biogas will be supplied to a municipal power authority through a 20-year Gas Purchase Agreement.
A1 Organics, an organic recycling company based in Colorado, has the exclusive contract to supply all nonmanure feed stocks to the Heartland digester.
“It is our plan for these solids [digestate] to be used as a peat moss replacement,” says Ted Mathews, biogas project manager with A1 Organics. “The nutrient rich liquid [byproduct] … will be irrigated on crop land to replace commercial fertilizer.”
Mathews goes on to note that it is A1’s hope to gain the contracts necessary to provide the “incidental loads,” or packaged food waste and waste acquired from transport losses or cooler breakdowns, in addition to their nonmanure feed stock supply. Incidental loads are often landfill bound, but with the addition of food de-packaging units, this waste can be harvested and sent to the digester.
“This presents a beneficial use much better than the landfill option,” Mathews says.
Bacon, of Organic Marketplace Solutions, is also optimistic about the Weld County digester.
“[It] generates renewable energy and reduces greenhouse gas emissions while supporting Colorado family farming and its roots,” she says. And as more facilities add food de-packaging units alongside digesters, “it creates a fully integrated renewable energy model and helps landfills meet their commitment of 80 percent diversion by 2020.”
Correction: In the original text, this story listed an accurate but outdated
number for anaerobic digesters on farms and wastewater treatment facilities in the U.S. That number has been corrected to the most recent number available.
