116 3rd St SE
Cedar Rapids, Iowa 52401
STOCKTON — Powering an anaerobic digester is like feeding a cow with a million-gallon stomach.
Into the tank goes a massive amount of manure, culled potatoes, distillers grain, wet corn gluten and other food waste. If the ratio is just right and the temperature in the tank stays between 99 and 105 degrees, bacteria and microorganisms will break down the feedstock and produce biogas, a less-concentrated form of natural gas.
“If you get too much nitrogen, you can cause an upset stomach for the bacteria,” said Bryan Sievers, principal owner of the AgriReNew digesters on Sievers’ family cattle farm near Stockton. When they installed the digesters in 2013, the owners hired a consultant from a Belgian firm to figure out the right chemistry for the $7.5 million system.
“They have to have the kind of ’food’ they like,” he said of the digesters — two green tanks with foot-thick concrete walls and domed top membranes. Pipes and pumps move manure and other feedstocks into the top part of the tank, while waste — called digestate — is removed from the bottom.
AgriReNew is one of four on-farm digester systems in Iowa. Two of the other operations — Amana Farms, with 4,000 cattle near Amana, and Top-Deck Holsteins, a 700-head dairy farm in Westgate — also generate biogas, which then is turned into electricity or heat. Boland Farms, with 2,400 cattle in Williamsburg, has a covered manure lagoon to prevent odor, but does not reuse biogas.
Other Iowa farms in the past 20 years have tried digesters, but have abandoned the efforts because of the tricky chemistry or because they didn’t have enough waste.
“We put a digester in back in 2000,” said Dave Lawstuen, chair of Iowa's Dairy Center and dairy science faculty member at Northeast Iowa Community College in Calmar. “We’re a fairly innovative center, but we didn’t have the total volume. We didn’t have enough manure because our herd size isn’t big enough.” The center used the digester for a year or two, then had it removed, Lawstuen said.
There are about 270 farm-based anaerobic digesters in the United States, according to the AgStar database maintained by the U.S. Environmental Protection Agency. This number is small compared with the thousands of on-farm digesters in Europe, especially Germany.
Patrick Serfass, executive director of the American Biogas Council, thinks Iowa is prime territory for more on-farm digesters.
“If you look at the potential for on-farm digesters, there are over 1,200 farms in Iowa are probably big enough to be able to afford to build a digester,” Serfass said. “That’s an enormous opportunity.”
Digesters are touted as a way to turn animal, food and industrial waste into biogas, which can generate electricity and heat. Sievers’s digesters produce, on average, 300,000 to 350,000 standard cubic feet of biogas per day. He said this translates to about 23 megawatts of electricity, or enough to power 800 to 1,000 homes, each day. Sievers sells the electricity to Alliant Energy and uses the heat on the farm.
There is a 30 percent investment tax credit to offset the cost of installing anaerobic digesters that generate electricity.
Digesters, which have been used at wastewater treatment plants for decades, can control odor at large-scale animal feeding operations because smells don’t get out of the airtight tanks, if they are operated correctly, and the digested waste smells more like compost than manure, Serfass said.
Digestion removes most of the pathogens from manure, academic studies show. This means the digested waste can be applied on farm fields later in the season when there is less risk of heavy rains washing it into nearby creeks and rivers, causing water pollution, Serfass said.
But several Iowa environmental groups say claims about the benefits of digesters and biogas are inflated. The potential biogas from digesters at swine and dairy operations in the United States, according to a 2018 EPA report, is less than 1 percent of the total natural gas used in the United States in 2019, according to a report from the U.S. Energy Administration.
Opponents fear promotion of the biogas industry is really a backdoor way to encourage more large-scale animal confinements. Iowa — the No. 1 producer of hogs and eggs — already has more than 8,500 medium and large animal feeding operations. The state doesn’t track small operations, which are under 1,250 hogs.
“This really isn’t about providing a new source of renewable energy,” said Emma Schmit, an organizer for Food & Water Watch who spoke April 6 to The Gazette’s editorial board. “It’s about the further entrenchment of the factory farm industry assured by creating a market for factory farm waste.”
The Iowa Legislature is laying the groundwork for more digesters with bills this year at the Statehouse.
House File 522 would allow an anaerobic digester as an alternative to a manure storage structure at animal feeding operations and sets a maximum penalty of $10,000 for an air quality violation. A House committee recommended passage of the bill and it’s been parked on the Senate “unfinished business” calendar so lawmakers could bring it up for floor debate at any time before the session ends.
The bill’s House floor manager, Rep. Mike Sexton, R-Rockwell City, runs agricultural consulting firm Twin Lakes Environmental Service with his wife, Becky. Becky Sexton prepared the nutrient management plan for an 11,600-head open feedlot in Monona that was approved last month by the Iowa Department of Natural Resources despite concerns from environmental groups.
House File 789, which stalled in subcommittee in early March, would let state revolving fund money be used for biogas facilities.
Sievers supports HF522, saying it would encourage some larger dairies already in operation to build digesters rather than having open manure lagoons.
“There’s no incentives in the bill to create large livestock farms,” he said. “It just allows large livestock farms to use anaerobic digesters as a better way to use their manure streams.”
The Iowa DNR oversees anaerobic digesters and has the authority to investigate if there is a complaint, department attorney Kelli Book said.
The agency has issued two notices of violation related to on-farm digesters in the past three years, both connected with a digester near Riceville.
That facility originally was owned by Linkenmeyer Family Feeders, which raises cattle and swine, but at some point was sold to Big Ox Energy, of Green Bay, Wis., Iowa DNR records show. After the sale, the feedstock going into the digester shifted from manure to industrial food waste, such as animal carcasses and spoiled vegetables, the department reported.
The first violation, in July 2019, said the facility’s basin of digested waste didn’t have enough surplus space to avoid overflowing. The Iowa DNR did not pursue enforcement action because the company made a deal with the city of Des Moines to ship some of the waste to the city’s wastewater treatment plant when needed, said Brian Jergenson, senior environmental specialist with the Iowa DNR’s Manchester office.
The Iowa DNR issued the second notice of violation Feb. 19 because of a smelly gray discharge from an underground tile line into a tributary of the Wapsipinicon River, records show. Lab tests confirmed the discharge came from a site where owners were doing a pilot project to drain water off bags of digested waste, the Iowa DNR reported.
“It happened at a fortunate time for organisms, because it was during cold temperatures,” Jergenson said. “The ammonia nitrogen isn’t toxic to fish when the water temperature lowers.”
But the gray slime growing on rocks and soil of the creek bed “definitely had a chronic effect on the water quality of the stream,” Jergenson said. He is following up to make sure the discharge has stopped and has referred the violation to the department’s legal department for enforcement and potential fines.
NLC Energy, which now owns the digester, said it plans to refurbish the plant and upgrade technology.
“We believe the plant can be run safely and in full compliance with all environmental requirements while creating high wage jobs and valuable low carbon transportation fuel for the state,” the company said in a statement.
An Iowa DNR inspector visited Sievers’ farm July 9, 2020, after Sievers self-reported a manure spill, records show. A transformer blew nearby, causing Sievers’ transfer pumps to stop working and manure to flow into a grass waterway on his farm, the Iowa DNR said. The manure was contained before it could go into a waterway and Sievers was not cited.
Next generation biogas
Sievers’ digesters earn $48 per megawatt of energy produced, down from $64 per megawatt in 2013, he said.
“That’s what helps pay the bills,” Sievers said.
The digesters were profitable every year until 2020, when Sievers couldn’t get as much industrial food waste because of COVID-19 and because his generator has been overheating. They recently sent the generator to Michigan, where a company is cleaned out carbon deposits so the machine can generate electricity again.
The next step is to invest in equipment that will remove carbon dioxide from the gas to create a liquid transportation fuel called renewable natural gas, Sievers said. Several states, including California and Oregon, have low-carbon fuel standard incentive programs that provide credits for renewable natural gas that comes via pipeline.
Sievers, and potential partner Roeslein Alternative Energy, based in St. Louis, are working to secure an easement and permissions for a 3.3-mile pipeline from his farm to a larger pipeline, he said. Roeslein and Smithfield Foods, the world’s largest pork processor, announced last year an additional $45 million investment in a venture to capture methane from hog feeding operations in Missouri to make renewable natural gas.
Sievers said these are examples of more corporate ownership of on-farm digesters, which can be challenging and expensive for individual farmers. “The model being followed now is most digesters are being built by third parties and the farmer provides that manure.”
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