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Anaerobic Digestion Basics

The demand for clean energy, coupled with concern for management of livestock wastes, has revived an interest in generating methane from livestock manures. The most widely accepted technology currently available for converting organic wastes present in livestock manure is anaerobic digestion (AD). AD is a biological process by which microorganisms convert organic material into biogas, containing methane and carbon dioxide. Biogas produced by this process can be utilized to generate electricity or can be cleaned up and supplied to natural gas lines. Collection and utilization of methane generated from livestock manure offers the potential to reduce global emissions of methane (a greenhouse gas), reduce CO2 released from fossil fuels, diminish odor from agricultural facilities, and improve water quality. In many cases, anaerobic digestion either decreases on-farm energy costs or increases revenues from energy resale. However, installation of an anaerobic digester will result in increased maintenance and is not a good fit with all livestock operations. Care should be taken to ensure that installation of anaerobic digestion technology is appropriate.

Biogas Production

Biogas generated by anaerobic digestion typically contains between 60-70% methane. Other constituents include carbon dioxide, hydrogen sulfide, ammonia, and other trace organics. Predicted energy production for different types of animal wastes can vary (based on 1,000 lb. animals):

  • Dairy cattle: 16,000 BTU/animal/day
  • Beef cattle: 12,000 BTU/animal/day
  • Swine: 17,000 BTU/animal/day

To put the energy value of animal waste into perspective, a well-insulated, three-bedroom home takes about 900,000 BTU per day for heating during cold weather. Because 50 percent of the biogas goes back into maintaining the necessary temperature of the digester, it would take the manure from approximately 50 cows to produce enough biogas to heat an average home.

Biogas Use

Anaerobic digesters are typically large reactors constructed of either concrete or steel. The volume of the reactor depends on the volume of wastes to be processed in the system. With most conventional digesters, a holding time of 20 – 30 days is required to convert manure solids into methane. Methane gas can be utilized onsite, serve as fuel for an electricity generator, or be purified and supplied to natural gas lines. Through cogeneration, heat produced by the electricity generator is captured and utilized to meet digester heating requirements to 35°C. Cogeneration has been the most common use for methane produced by anaerobic digestion. Recently, there is a growing interest in purification of biogas for resupply to natural gas lines due to high maintenance requirements for generators. This requires that all gas components aside from methane are removed. Of note is that hydrogen sulfide should be removed from biogas for cogeneration due to its corrosive nature. This can be done by passing the biogas through iron filings.

AD End Product

The end product of AD will contain 5-15% solids, depending on the solids content of the waste which is input to the system. Either the processed material containing solids can be applied by a honey wagon, or solids can be separated for land application of solids separately from liquids. When solids are separated, the liquid end product can be used as a fertilizer and solids can be composted and land applied by a manure spreader. Solids separation in combination with composting can result in a lower weight product which can be transported for land application, while the weight of the processed material containing liquid and solids (5-15% solids) may be too difficult to transport over large distances. Utilizing the liquid fertilizer for irrigation is referred to as fertigation or chemigation and is regulated by the Colorado Department of Agriculture. When fertigation systems are connected to a freshwater source, appropriate measures must be taken to avoid contamination of the freshwater source such as inclusion of a backflow preventer and shutoff valve. Fertigation systems must adhere to Colorado Department of Agriculture regulations.

Last updated: October 18, 2017 at 11:36 am