Anaerobic Digestion

A Primer on Anaerobic Digestion

Creating fuel from animal manure waste is gaining public attention. A scientist explains.
By Dr. Wei Liao, assistant professor, MSU Department of Biosystems & Agricultural Engineering

The days of animal manure being seen as merely waste are long gone. In addition to being a great fertilizer for growing crops, animal manure rich in carbohydrate and protein is a potential source of feedstock for production of renewable biobased energy.

It has been estimated that 160 million dry tons of animal manure are produced annually in the United States. That’s a lot of biomass for biobased energy production. With an energy content of 1,454 kilocalories per pound (87 percent of the  kilocalories contained in a pound of sugar) or potentially 1.7 kWh per pound of manure, that’s a lot of energy, too.  However, the energy stored in animal manure as carbohydrate and protein cannot be directly used as fuel energy. The carbohydrate and protein must be converted into usable fuels by various thermal and/or biological processes.

Anaerobic digestion (AD) is one of these processes. It is a natural, biological conversion process proven effective in converting animal manure into renewable energy such as methane biogas. It also has potential to coproduce fertilizers. It typically takes 20 to 30 days of storage in a warm, oxygen-free environment to convert 50 percent of manure
nutrients into methane, carbon dioxide, ammonia and other chemical compounds.

Methane is a main component in natural gas, usually accounting for about 80 percent of natural gas. Biogas from anaerobic digestion of animal manure is made up of 60 to 70 percent methane and 30 to 35 percent carbon dioxide. Its heating value is 600 to 700 British thermal units (BTU) per cubic foot, which is just slightly lower than the 900 to 1,000 BTUs per cubic foot of natural gas. This means that biogas from animal manure anaerobic digestion can be used as an alternative to natural gas.

Anaerobic digestion can also alleviate many of the environmental concerns associated with animal operations, such as odor, greenhouse gas emissions and subsurface contamination. Anaerobic digestion is capable of enriching excess nitrogen and phosphorus from animal operations into its effluents. Recovery technologies such as ammonia stripping and phosphorus precipitation will recover those excess nutrients for fertilizer production.

An integrated anaerobic digestion system—one with anaerobic digestion and nutrient recovery units—will create a win-win solution for animal manure: co-producing bioenergy and fertilizers for animal operations.

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