What Is Anaerobic Digestion (Biogas)?

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. 

The planet’s continued dependence on fossil fuels and global warming have become “top of mind” among political leaders and everyday citizens. What tops most minds are renewable energy solutions produced by 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 electricity or renewable natural gas, they 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 15,000 digesters, primarily agricultural, convert organic waste into electricity and renewable natural gas. In contrast, the United States had just 265 systems on farms as of 2019. Lately however, increasing environmental incentives are triggering a surge, especially the development of large projects at America’s mega dairy farms.

The biogas process involves manure and other organic waste being fed into a sealed tank without the presence of oxygen. Depending on design, it remains there, typically for 17-25 days at a temperature of over 38 degrees Celsius (100 degrees Fahrenheit). Bacteria is generated causing the release of methane gas which is then piped into a combined heat and power (CHP) generator to create electricity, or into specialized cleaning equipment to meet renewable natural gas (RNG) standards.

Doug Woodger of Rockwood Farm (far right) speaking with other farmers 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 holding tank at digester located at Yippee Farm in Lancaster county, Pennsylvania.

Which materials or feedstocks can an anaerobic digester process?
Anaerobic digesters process anything organic. Animal manure (including human waste) 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 slaughter house and meat packing waste. In Europe there are 100% corn silage and 100% sugar beet pulp digesters. American biodigesters usually 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 and 100% sugar beet pulp digesters. American digesters usually co-digest manure with 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 utilized in buildings located near the anaerobic 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.

The excess solids can also be sold to nurseries as peat moss replacement and compost. The material is pathogen free which reduces costly mastitis infections. When used fresh, it keeps somatic cell counts low.

In at least one case a farmer takes dried digested solids, presses the solids in a mold to make “cow pots” as plant holders!

Vermont 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 establish innovative improvements on 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 much more than is needed there, so they can supply their other dairy of 400 cows as well.

Another interesting point is that the Chaputs are aware that some phosphorus migrates from the liquid to solid fraction during anaerobic digestion and mechanical separating. Nevertheless, 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.