Unique renewable digester system in Alberta, Canada. Along with processing manure from dairy, beef, pigs and chickens it converts
food processing waste and potentially hazardous animal carcasses into electricity, heat and fertilizer.
Our planet’s continued dependence on fossil fuels and global warming has become “top of mind”. Best known are renewable energy solutions produced by the sun and wind – effective only when the sun shines or the wind blows.
But there is a lesser-known renewable energy – anaerobic digestion or biogas. This is proven technology that often utilizes manure from cows, pigs and chickens along with food waste. Importantly, biodigesters on farms not only produce energy, but also remove the methane and carbon dioxide that cause global warming.
Biogas is not new. It is the natural process of collecting gases from decaying organic material. It was used to heat water as early as the 10th century and the first plant was built in Bombay, India in 1859.
Today in western and central Europe over 13,000 digesters on farms convert organic waste into electricity and renewable natural gas. In contrast, the United States had just 265 systems on farms as of 2020. Lately however, increasing environmental incentives have triggered a surge to almost 600, many being large renewable natural (RNG) projects on America’s mega farms.
The biogas process involves waste materials being fed into a sealed tank without oxygen. It remains there for 17-25 days at a temperature of 39 degrees Celsius (100+ degrees Fahrenheit). Bacteria is generated causing the release of methane which is then piped to a combined heat and power (CHP) generator to create electricity or into specialized cleaning equipment to meet renewable natural gas (RNG) standards.
Milk carton in Sweden showing the importance of biogas on Swedish dairies. Germany, on its own, has more than 8,000 farm digesters.
Connecticut and Massachusetts farmers comparing notes at nutrient management seminar presented by Muir. Three of these farms later built biodigesters.
Manure and other organic waste are chemically altered in the process resulting in significant pathogen and odor reduction. The output or “digestate” can be separated into solid digestate and liquid digestate. Here is a video description from an experienced nutrient management manager.
The liquid is spread on a farmer’s fields as high quality fertilizer while the solid portion can be used as compost, peat moss replacement and soil amendment for plant nurseries. More immediately, In the case of dairy farmers, it is a cost-free bedding material that if used fresh, can reduce somatic cell counts.
Slurry food waste pumped into a digester at Yippee farm in Lancaster County, Pennsylvania.
Which materials or feedstocks can an anaerobic digester process?
Biodigesters process anything organic. Animal manure (including human) is available in abundance, but food or food processing waste, can also be digested. This includes fats, oils and greases from restaurants as well as slaughterhouse and meat packing waste. In Europe there are 100% corn silage and 100% sugar beet pulp digesters. American biodigesters often co-digest manure with some food waste.
In complete mix digesters organic waste is transformed by a 25-day (on average) oxygen free biogas process into odorless, pathogen free, bedding for cows. Plug flow digesters process waste more precisely but for a shorter 17-day period.
In Europe there are 100% corn silage digesters. American farm digesters often co-digest manure with some food processing waste.
Manure transformed by oxygen-free biogas process into odorless, pathogen free, bedding for cows.
What benefit does an anaerobic digester provide?
The revenue streams from a biogas plant are diverse and identifying them depends on the ingenuity of the digester owner and developer. Green electricity or renewable natural gas can be produced and sold. The heat from the generator can be used in buildings near the digester offsetting heating costs in cold weather climates.
The output from the digester is usually separated into solid and liquid portions. The solids used as cow bedding, a major cost offset which in America can be more than $100,000 annually for one farm.
Excess solids can also be sold to nurseries as peat moss replacement and compost. It is pathogen free which reduces costly mastitis infections. When used fresh, it keeps somatic cell counts low.
The liquid fraction can now be separated a second time to make a granular organic fertilizer for homeowners.
In at least one case a farmer takes dried digested solids, presses the solids in a mold to make “cow pots” as plant holders!
Brothers Reg and Mike Chaput at their Mystic Meadow Farm on the Canadian border in North Troy. In 2012, they received the Vermont Dairy Farm of the Year award and strive to innovate their farms.
The experience on a real farm?
The Chaputs share the experience of other farms with biodigesters. Their American style, complete mix, digester was installed in 2010 and currently supplies over $100,000 worth of free bedding.
These “solids” contain few pathogens so somatic cell counts and Mastitis rates remain low, however, Mike points out that bedding solids only work well if they are used fresh. He says “the sooner the better and the material should be in the stalls at our two sites on the same day or that night. If they sit for 2 days or more in a pile, pathogens and SSC can definitely build up and so the material should be discarded”.
Reg adds that while their digester and separator only processes the manure of the 1,000 cows at their main location, it produces more than is needed, so they can supply their other dairy of 400 cows as well.
Another point is that the Chaputs are aware that some phosphorus migrates from the liquid to solid fraction during anaerobic digestion and mechanical separating. The phosphorus levels at the farm near where bedding is kept have not increased abnormally. This was confirmed by their nutrient management specialist and matches the experience of TeamAg (above) at their ten Pennsylvania farms.