Features Anaerobic Digestion Energy
Easily digestible

This is an expanded version of a feature that originally appeared in the January/February version of Manure Manager magazine.

Anaerobic manure digesters have been hailed for some years now as a positive step for the livestock farming industry, and with good reason.

“A digester is essentially a big artificial stomach that allows naturally occurring microbes to continue digesting manure, producing biogas – mostly methane gas – that can be used for energy,” says Daniel Ciolkosz, an associate research professor with Penn State Extension’s Department of Agricultural and Biological Engineering. As such, digesters harvest a natural byproduct of manure waste, and harness it for good.

This being said, not everyone is a fan of digesters. The most common opponents are local residents who live close to proposed digesters, who worry about them leaking unpleasant odors. Others include the subset of environmental activists who want to reduce the world’s reliance on animal agriculture altogether, and thus see no benefit to manure processing at all.

The pros of digesters

The biggest ‘pro’ in favour of digesters is that they harvest the methane produced by manure and use it to either generate electricity through burning – heating water that drives steam turbines that generate power – or supplement the supply of natural gas being distributed to homes and businesses. This form of ‘biogas’ is known as renewable natural gas (RNG) because it comes from renewable sources, whereas natural gas derived from fossil fuels does not.

“The process behind harvesting methane from manure is pretty simple,” says Rebecca Larson, a professor and extension specialist with the University of Wisconsin’s Nelson Institute for Environmental Studies. “The microorganisms that are already in the manure degrade organic compounds naturally. “With a digester, that process is just contained in a tank or other vessel to allow you to capture biogas, which is a byproduct of that microbial breakdown.”

“Digestion definitely is a good way of capturing carbon from the manure, which is in the methane,” says Wei Liao. He is a professor and the director of MSU Anaerobic Digestion Research and Education Center (ADREC) in Michigan State University’s department of Biosystems and Agricultural Engineering. “This reduces the amount of manure remaining after the digestion process, which also helps to reduce the livestock industry’s carbon footprint.”

“Pros also include providing a new income stream for a farm, and reducing odors from manure,” Ciolkosz says. This is because the odous that are generated during the manure degradation that produces methane are captured in airtight tanks, rather than vented to the atmosphere in open fields and manure piles.

That’s not all digesters can do. Harmful pathogens like E. coli bacteria within manure are considerably reduced when this process is used to extract methane. Meanwhile, the solid portion of the digestate may be sold as a nutrient-rich soil conditioner. The liquid portion of the digestate may be treated and used for washing or flushing purposes, or further purified for other farm water needs.

If animal and food waste are added to the digester during the methane production process, the resulting solid digestate can still be used as fertilizer, thus reducing the amount of waste being taken to landfills. As well, tipping fees can be collected from producers or industries who want to sell manure or other biological materials as feedstocks for the digester.

Digesters can also help protect groundwater quality, which is why the concept was embraced by Wisconsin’s Dane County in 2011. Today, the county has two community digesters – one in Vienna, and the second in the town of Springfield. In 2022 alone, these digesters combined processed more than 105 million gallons of manure, and the separation systems following the digester removed 231,000 pounds of phosphorus. This resulted in reduced phosphorus runoff and improved water quality in the county, plus provided it with extracted phosphorus that can be applied to the ground where needed.

“The two Dane County digesters are very unique because the county requires phosphorus removal as part of its efforts to keep area lakes clean from toxic algae blooms during the summer,” says Dane County executive Joe Parisi, the elected official in charge of the county’s executive branch. “Both project operating agreements are enforced by the county, requiring phosphorus removal utilizing advanced technology systems. The EnTech digester uses a nutrient concentration filtration system purchased by Dane County that dewaters manure and discharges millions of gallons of clean water into a nearby stream. This means less phosphorus-filled manure and digester byproduct being spread on fields sensitive to runoff in the watershed. As well, both projects turn digester-produced methane into renewable natural gas.”

As well, “Digesters are an important tool in reducing agricultural climate-change emissions,” he adds. “Dane County’s Climate Action Plan has a goal of reducing community-wide emissions by 50 percent by 2030. Each digester project reduces an average of over 14,000 tons of CO2 equivalent annually. This reduction is comparable to removing emissions of nearly 34 million miles driven by passenger vehicles.”

The cons of digesters

You don’t need to be an insider to know that Wisconsin is “dairy country.” So, naturally, the state produces a lot of manure. And that scale is only increasing. As of 2022, Wisconsin cows produce twice as much manure and milk as they did in 1970, with 1.2 million cows in the state producing 12 billion gallons of manure and wastewater annually.

Dairy production is a big industry for Dane County. This is why establishing community digesters has been such a boon to this region. To date, “Dane County has not identified any disadvantages to using dairy digesters,” says Parisi.

This being said, manure digesters do have their ‘cons’. According to Ciolkosz, they “include the cost of installing a digester and the time needed to operate and manage the digester. If a farmer is accepting additional feedstock from off the farm, there could be added traffic on local roads.”

Running digesters properly require skilled workers and the money to pay them. This is because the digestion process is complex and requires careful monitoring and controlling to avoid production failures. Feedstocks, temperature and pH must also be managed carefully to produce consistent quality biogas.

In addition, the other gases such as carbon dioxide and hydrogen sulphide must be removed from the methane before using this gas for energy production or sending it into natural gas utility lines. As well, installing electrical and gas utility connections may be difficult and costly. Electricity and renewable natural gas production must also meet regulatory requirements, while market volatility in biogas prices along with carbon offset/credit markets can affect profit margins.

More cons to consider: digesters need heat to operate so colder climates can make the process more difficult and less efficient. Meanwhile, financial incentives, revenue from energy production and market demand vary between counties and areas. This is because digesters are a big capital expense, plus; differences in government funding or power returns can dramatically change the economic return on investment. Then there’s the availability of sufficient manure and other feedstocks: The closer the manure sources and other organic materials are, the greater the economic return. Finally, opportunities for the use of produced heat adds to the economic advantage, so close proximity to domestic or industrial partners to share heat production is important.

The takeaway: “For a producer installing a digester, it’s a lot of risk to take on,” says Larson. “It’s also a lot more work, so where your specialty used to be animal handling, you now have this entire energy business to run. That can be really complicated, which is why we’ve seen a number of digesters shut down for financial reasons. It can be hard to maintain and run the system in addition to managing your livestock facility.” This is why the community model adopted by Dane County can be a better option for livestock producers.

Then there’s what some might call the NIMBY (Not in My Back Yard) factor. A case in point: Rimrock Renewables has won approval from the Alberta Ministry of the Environment and Protected Areas (AEP) to build the Rimrock Biodigester Facility next to the Rimrock Cattle Company Ltd. feedlot about 5.5 km west of High River. “The Rimrock Cattle Company Ltd. feedlot will be the primary source of feedstock (livestock manure) for the biodigester facility, meaning the manure that is currently stored on the feedlot will be transferred approximately 200 m – 300 m to the biodigester facility where it will be anaerobically digested to capture odorous gases and produce RNG,” says www.rimrock-renewables.com. “Locating biodigester facilities immediately adjacent to the primary source of feedstock enhances environmental benefits (e.g., net decrease in greenhouse gas emissions) through reduced transportation requirements and hauling distances.”

This approval was granted despite opposition from some local residents. “My property value is going to drop significantly,” one neighbor was quoted as saying in an Oct. 31, 2022 Calgary Herald news story. “I don’t know who’s going to want to live next to a poop disaster.”

Addressing such resident concerns effectively – along with ensuring that digesters have adequate odour-management capabilities – is a must for such projects. To make this happen, “communication is the key,” says Ciolkosz. As well, visiting existing farm digesters is a great way for community members to see how they actually perform, and thus better assess whether a digester would be compatible with their community or not.

A word to the wise: when engaging or communicating on the project, focus on the specific community concerns. Work with the community to identify and address concerns. Ensure transparency and including the community in the development process. Finally, encourage community members to get informed and engaged in local and regional municipal planning, and in the approval process for digestion facilities.

This was how officials handled the introduction of digesters in Dane County, to everyone’s benefit. “Dane County’s successful digester development strategy implementing innovative programs and technologies resulted in cleaner lakes, reduced costs for dairy farmers, taxpayer savings and a reduction in harmful climate-changing emissions,” says Parisi. “This has resulted in alleviating community concerns and gaining widespread community acceptance.”

The bottom line

Given the range of pros and cons associated with digesters, it is understandable that some jurisdictions embrace them, while others don’t. The acceptance level varies due to operator costs, comprehension of the benefits by political decision-makers, and the willingness of local residents to accept digesters — or not. Still, there is no doubt that digesters are a realistic option for dealing with the methane emitted by manure, and to make reducing it both practical and profitable. •