Anyone who keeps livestock--whether you have a small hobby farm with five cows or manage a factory farm with thousands--knows that safe, environmentally responsible manure disposal is a constant problem. You could simply compost the manure and add it back to the soil. This is one way to solve the manure problem, but simple composting only generates one benefit for you: soil amendment. Put the manure through an anaerobic digester first, and you gain two benefits: free electricity and soil amendment.
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Things you need
- Insulated methane digester, either a commercial model or homemade
- Manure--at least 6.8kg. per day
- pH test kit
- Bicarbonate of soda
- Natural gas powered electric generator
Choose the right methane digester. Many commercial models are available, but they are designed for large farms with at least 150 cows. They tend to be expensive, although some of this expense can be offset by tax credits, grants, and low-interest government loans. If you run a smaller operation, or do not have the funding to purchase a large-scale methane digester, consider building your own. Check with your local university extension office, or visit the Food and Agriculture Organization of the United Nations website for detailed plans. Find a link in References.
If you process manure from grain-fed cows, you will need to feed the digester 5 gallons of manure and 10 gallons of water daily. To digest manure from grass-fed cows, feed the digester 5 gallons of manure and 20 gallons of water. Five gallons of pig manure requires only 5 gallons of water to mix.
Feed the manure and water into the input tube of the digester, then activate the mixer (if your digester has one) to create a uniform slurry. This uniformity promotes even digestion throughout the tank.
Use a pH test kit to test the pH of the slurry weekly. Most methane digesters are self-regulating, but if you have changed the feed of the animals or the temperature has fluctuated recently, that could throw off the pH, making it too acidic. Add bicarbonate of soda to raise the pH if necessary.
Maintaining a consistent temperature is critical to efficient digestion, as fluctuations will disrupt the digestion process. Anaerobic bacteria will survive at a wide range of temperatures, but they are most efficient at around 98 and around 54.4 degrees C. A well-insulated digester will mitigate temperature fluctuations in the atmosphere, but you will still need to use a heating unit to maintain high heat within the digester.
The ideal temperature for manure digestion is 130 degrees, although maintaining a constant 98 degree temperature requires less energy. Digester efficiency drops off noticeably above 36.7 degrees C, then becomes highly efficient again around 54.4 degrees C.
A methane digester will produce between 500 and 800 Btu of energy per cubic foot. Attach the output tube of the methane digester to the natural gas generator to capture the methane and burn it to generate usable electricity.
A methane digester produces two products: methane, which you will divert to a natural gas generator, and sludge or effluent. This is the solid waste left over after digestion. It is a very concentrated form of compost and can be used as a soil amendment.
Tips and warnings
- The more you mix the materials in the digester, the more efficiently the waste will be converted to methane gas and sludge.
- Have the sludge tested for safety before using it on food crops. Any pesticides or other toxic substances will become highly concentrated in the sludge.
- Antibiotics used on livestock have been shown to kill the anaerobic bacteria necessary for the digestion process. Use these sparingly.
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