Environmental benefits of methane gas recovery
- Biogas can create both electricity and heat. It can offset a farm's energy costs.
- Digestion lessens the potential for surface and groundwater contamination.
- Biogas can also reduce the need for other fuels. This lessens pollution that comes from drilling, mining, transporting and burning. It also reduces carbon dioxide — a factor in climate change.
- Collecting biogas prevents methane from entering the air. In the atmosphere, methane becomes a greenhouse gas.
- Anaerobic digestion typically decreases the volume of manure solids. The volume is usually reduced by more than 90%. The rest of the biosolids can be used as fertilizer.
Installing an anaerobic digestion system has environmental and economic benefits. The system makes electricity from methane. In turn, it can lower energy costs. It can also control odors and improve manure handling. The system can reduce potential for surface and groundwater contamination. It can also control harmful pathogens. Farms can use anaerobic digestion to comply with new regulations for manure handling. However, the process involves a number of financial and management resources. Costs and benefits should be weighed carefully.
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What is an anaerobic digester?
Anaerobic means "without oxygen". An anaerobic digester is made up of a closed tank that excludes oxygen. Naturally occurring bacteria break down manure. The manure is left as a range of gases. One such gas is methane.
Bacteria that produce methane are most active from 95 to 105°F. For this reason, some digesters run hot water through the pipes. It heats the manure, to stay in the ideal temperature range. A cover on the digester traps the biogas. The cover is both flexible and impermeable. When trapped, the biogas can be burned off in an open flame. It could also pass through an electrical generator. There are a numerous types of generators. However, the most common is the modified internal combustion engine. Electricity produced can then be used on the farm.
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Types of digesters
- Covered lagoon digester: Used on large-volume, liquid manure lagoons. The lagoons have less than 2% solids. A plastic cover traps gas made during decomposition. To collect methane, a floating lagoon cover is needed. The system also needs a gas pump. This system can be used on swine and dairy operations. It works best where the manure is handled as a liquid. The climate should be temperate to warm all year. This is the least expensive system. But, it's not fit for use in cold climates. Farmers in Wisconsin and the Upper Peninsula need a different system.
- Complete mix digester: Used for manure with 2 to 10% solids. This includes dairy or swine manure collected by a flush system. Tanks are heated in this system. A mixing system keeps the solids in suspension which speeds up the digestion process. These digesters are expensive to build. They cost more than lagoon digesters to operate and maintain. Extra water increases the required size of the digester. It also increases the cost of construction and operation, without increasing benefits.
- Plug-flow digester: Used for ruminant animal manure that contains 11 to 14% solids. This system doesn't work for manure with lower solids levels. In the best conditions, the plug goes through the digester in 15 to 20 days. This system has few moving parts. It requires little maintenance.
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Environmental benefits
Anaerobic digestion decreases the volume of manure solids by more than 90%. The biosolids that are left have higher levels of nitrogen, phosphorus and potassium (NPK) than manure. They also have higher levels of trace elements. This makes them an excellent soil amendment. They also contain ammonium-N. Like commercial fertilizers, this makes the N available when it's spread on fields.
Because manure digestion is anaerobic, most weed seeds and pathogens are killed during the process. Also, pathogens like E.coli, Salmonella and Cryptosporidium can't survive the high temperatures. Fecal coliform bacteria numbers in biosolids are only about 1% of those in fresh and stored manure. This lowers the potential for water pollution.
On some farms manure is stored in pits or lagoons. When methane is made it is released into the atmosphere. Methane is 21 times more potent than carbon dioxide in causing global warming. By capturing and burning the methane produced from animal manure, anaerobic digesters lower the rate of global warming.
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Odor control
Anaerobic microorganisms break down compounds that cause manure odors. This gets rid of odor problems. Research shows anaerobic digestion can reduce fresh manure odor by 97%. In farm expansion projects, odor control can be the main reason for installing a digester.
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Is it best for your farm?
The cost of a digester depends on the farm and the payback time varies. It can take a few years to more than 10 years. About 30% of the biogas is used to heat the digestion system. Many systems can source all of a farm's electricity or heating needs.
Money isn't the only thing to consider. Maintenance is required to keep a digester working smoothly. The system needs to be inspected. Manure needs to be mixed and pumped into the digester twice a day. Gauges should also be checked and recorded to measure biogas and electricity output. Generator engines need routine monthly maintenance. Also, the engines need oil changes, valve adjustments and spark plug cleaning.
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