What Is Anaerobic Digestion (Biogas)?

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 other organic waste. Importantly, anaerobic digesters on farms not only produce electricity or 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 may have been 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 manure and other organic waste into electricity and natural gas. By contrast, the United States had just 265 systems on farms as of 2019. This was due primarily to lower prices paid in the USA for renewable energy, but just as importantly was a chronic lack of marketing and outreach. Lately however, increasing environmental incentives are triggering a surge, especially the development of large renewable gas 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.

Pictured at top of page is unique 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. 

Doug Woodger of Rockwood Farm (far right) speaking with other farmers at nutrient management seminar presented by Muir. Four of these farms signed letters of intent to build digesters. 

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 solids and liquid. 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 a soil amendment for plant nurseries or in the case of dairy farmers, a cost-free bedding material.

Manure transformed by twenty five day anaerobic digestion process into odorless, pathogen free, bedding for cows.

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 digesters 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 digester 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 to some degree depend on the ingenuity of the digester owner or developer. As mentioned, electricity or natural gas can be produced and sold. The heat from the generator can be utilized in buildings located near the digester and can offset 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 offset a major cost which in America can be more than $100,000 annually for one farm.

The excess solids can also be sold to nurseries as soil amendment. The separated solids are pathogen free which reduces costly mastitis infections. It keeps somatic cell counts low.

In at least one case a farmer takes dried solids, presses them in a mold and sells the resulting “cow pots” for household plants!