How to Use Manure for Small-Scale Biogas Production

How to Use Manure for Small-Scale Biogas Production

How to Use Manure for Small-Scale Biogas Production

Introduction to Using Manure for Biogas Production

Manure can be a great resource for producing biogas at home or on a small farm. Biogas is a renewable fuel that is generated through the breakdown of organic material by bacteria in an oxygen-free environment, a process called anaerobic digestion. The primary component of biogas is methane, which can be used similarly to natural gas for heating, cooking, and generating electricity. Using manure for biogas has many benefits:

  • It provides a useful purpose for waste materials
  • It reduces odor and pathogens in manures
  • It prevents methane emissions that would occur if manure was left to decompose on its own
  • It creates a renewable fuel source and reduces reliance on fossil fuels

The amount of biogas that can be produced from manure depends on the type and quantity available. Cattle, swine, and poultry manures generally produce the most biogas. The process works best with manures that have not been excessively dried out.

Below are some key steps and considerations for using raw manure to produce biogas through anaerobic digestion on a small scale, such as for a household or small farm.

Collecting and Preparing the Manure Feedstock

The first step is obtaining adequate quantities of fresh manure for digestion. This will usually require collecting manure from livestock on your own property or obtaining it from nearby farms. Some key points:

  • Cattle or swine manure works best. About 15-20 pounds per day should be adequate for a small household digester.

  • Avoid overly dried manure as it will not produce much biogas. Manure with a solids content of 10-13% is ideal.

  • Store manure feedstock in a sealed container to avoid losing methane. Mix in some water if the manure is too dry.

  • Chop or blend solid manures to increase breakdown in the digester. Slurries and liquids do not require blending.

  • Remove large contaminants like rocks, sticks, or undigested corn kernels which can clog pipes.

Choosing a Digester Design

There are several basic designs to choose from when constructing a small-scale manure biogas digester:

  • Floating drum digesters use a sealed drum that floats directly in the slurry tank. As gas is produced, the drum rises. Simple and inexpensive to build.

  • Plug flow digesters are long, heated tanks with manure and microbes added at one end and effluent drawn from the other. Requires some pumping.

  • Batch digesters are single covered pits that are loaded with manure and left to digest. The simplest method but gas production is intermittent.

  • Continuous stirred tank reactors (CSTR) have manure and microbes continuously added and removed. More efficient but requires constant feeding and pumping.

For household or small farm use, floating drum or plug flow digesters around 1-10 m^3 in size are likely most practical. Consider your manure quantities and construction constraints.

Digester Construction and Operation

Proper construction and feeding is key to maximizing biogas production:

  • Build the digester airtight and ensure the entry point is sealed after feeding. Anaerobic microbes cannot survive with oxygen.

  • Provide heating to maintain internal temperatures of 90-100°F (32-38°C). Biogas output drops considerably lower than 90°F. Insulate the digester.

  • Mix and dilute manure with an equal part water before adding to digester for best results. Other feedstocks like food waste can also be co-digested.

  • Feed the digester daily if using a continuous design. Allow 20-40 day retention time for plug flow and batch digesters.

  • Collect the biogas through piping attached to the digester outlet. Route it to a gas storage tank before consumption.

  • Safely burn excess gas if the digester is overproducing. Shutting down digesters can also help reduce gas.

Using the Biogas

The methane produced can be burned for various purposes once collected:

  • Biogas stoves and lamps provide clean cooking and lighting. Special stoves are designed to burn biogas.

  • Electricity generation via a biogas generator running an engine. Enough biogas can produce power for several lights or appliances.

  • Space and water heating using biogas boilers or heaters. It can supplement or replace conventional fuels.

  • Refrigeration and cooling through absorption chillers running on biogas. Useful for food storage and air conditioning.

Take safety precautions when using biogas – install gas detectors and ventilate enclosed spaces. Remove carbon dioxide and other contaminants for some applications. With the right materials, manure can sustainably produce sufficient biogas for basic household and farm energy needs.

Conclusion

Producing renewable biogas from manure has many benefits and can be achieved at home or on a small farm with some simple DIY equipment. Start by collecting adequate fresh manure feedstock from local livestock. Choose an appropriate digester design based on available manure and space. Carefully construct the airtight, heated digester, then safely collect the methane biogas for burning in lamps, stoves, generators or other equipment. With some initial investment and regular feeding and maintenance, a small biogas plant can provide useful energy.