Methane Gas Recovery
Environmental Benefits of Methane Gas Recovery
- Biogas can produce electricity and heat offsetting a farm's energy costs and reducing the need for other fuels. This, in turn, reduces pollution that comes from drilling, mining, transporting and burning and reduces carbon dioxide — another major contributor to global climate change.
- Digestion reduces the potential for surface and ground water contamination.
- Collecting biogas prevents the methane from releasing into the atmosphere and becoming a powerful greenhouse gas.
- Anaerobic digestion typically decreases the volume of manure solids by more than 90 percent. The remaining biosolids are an extremely high quality fertilizer.
Handling the manure produced on a livestock farm requires a lot of manpower, but farmers are discovering there is a way to use that same manure to produce power.
Installing an anaerobic digestion system — and the equipment necessary to generate electricity from the methane produced and collected by it — has environmental and economic benefits.
Using methane to generate electricity can lower energy costs. Anaerobic digestion systems are also a way to handle the increased pressure on livestock operations to control odors. And such a system can improve manure handling, reducing ground and surface water contamination and controlling harmful pathogens.
Some farms can use anaerobic digestion as a cost-effective way to comply with new regulations for manure handling. Producing and managing methane requires a substantial commitment of financial and management resources, however, so costs and benefits should be weighed carefully.
What is an Anaerobic Digester?
Anaerobic means "without oxygen;" therefore, an anaerobic digester is an enclosed tank that excludes oxygen. Naturally occurring bacteria break down manure into a variety of gases, including methane. Since methane-producing bacteria are most active in a range of 95 to 105 degrees F, some digesters circulate hot water through pipes to heat the manure and maintain the desired temperature range. A flexible, impermeable cover on the digester traps the biogas, which is either burned off in an open flame or passed through an electrical generator.
There are several types of generators, but most use a modified internal combustion engine. The electricity can then be used on the farm.
Types of Digesters
- Covered lagoon digester: A covered lagoon digester is used on large-volume, liquid manure lagoons with less than two percent solids. A plastic cover trips gas produced during the decomposition of the manure. Collecting methane requires a floating lagoon cover and a gas pump to move the gas. This system can be used on both swine and dairy operations and works best where the manure is handled as a liquid and the climate is temperate to warm year round. This is the least expensive of the three types of digestion systems, but is not suitable for use in the cold climate of Wisconsin and the Upper Peninsula.
- Complete mix digester: A complete mix digester is used for manure that contains two to 10 percent solids, which includes dairy or swine manure collected by a flush system. Above- or below-ground tanks are heated in this system. A mixing system keeps the solids in suspension, which accelerates the digestion process. These digesters are expensive to construct and cost more than the covered lagoon digesters to operate and maintain. The excessive water addition increases the digester size requirements, along with the cost of construction and operation, without increasing benefits.
- Plug-flow digester: Plug-flow digesters are suitable for ruminant animal manure containing 11 to 14 percent solids. This system is not appropriate for manure with lower solids concentration, such as dairy manure collected through a flush system. In a plug-flow digester, the solids in manure with lower than desirable solids concentrations are poorly digested because they gradually settle to the bottom of the tank. Under optimal conditions the plug passes completely through the digester in 15 to 20 days. This system has few moving parts and requires minimal maintenance.
Environmental Benefits
Anaerobic digestion typically decreases the volume of manure solids by more than 90 percent. The biosolids that remain contain higher concentrations of NPK and trace elements than manure and are, therefore, an excellent soil amendment. They also contain relatively high concentrations of ammonium-N, making the N rapidly available, (more like commercial fertilizers) once it is spread on fields.
Because manure digestion is anaerobic, most weed seeds and pathogens are killed during the process. Pathogens like E.coli, Salmonella, and Cryptosporidium can't survive the high temperature of heated digester. Fecal coliform bacteria numbers in the biosolids are only about one percent of those in fresh and stored manure, lowering the potential for this source of water pollution.
On farms where manure is stored in manure pits or lagoons, methane is generated and released into the atmosphere. Methane is 21 times more potent than carbon dioxide in causing global warming, so by capturing and burning the methane produced from animal manure, anaerobic digesters lower the rate of global warming.
Odor Control
As housing developments sprout up on what was once exclusively farmland, and the trend toward larger confined livestock operations continues, manure odors are pitting the public against livestock producers. Anaerobic digesters offer a solution. Anaerobic microorganisms break down many odor-causing compounds in manure as it moves through the digester. This basically eliminates odor problems. Research has shown anaerobic digestion reduced odor by 97 percent over fresh manure. In farm expansion projects, odor control can be a primary reason for installing a digester.
Is It Best for Your Farm?
The cost of a digester depends on specific farm conditions and pay back can range from a few years to more than 10 years. Systems typically use about 30 percent of biogas to heat the digestion system. Many systems are able to supply a farm's complete electricity or heating needs. Typically a minimum herd of 300 dairy cows or 2,000 swine is needed to make such a system feasible.
Money isn't the only consideration. On average, about 45 minutes of daily maintenance is required to keep a digester working smoothly. This includes system inspection, mixing and pumping manure into the digester twice a day, and checking and recording gauges to measure biogas and electricity output. Generator engines also require monthly maintenance including oil change, valve adjustment and spark plug cleaning.