How to Turn Manure Into Methane Gas: An Introduction to Small Scale Biogas Production

How to Turn Manure Into Methane Gas: An Introduction to Small Scale Biogas Production

How to Turn Manure Into Methane Gas: An Introduction to Small Scale Biogas Production

What is Biogas?

Biogas is a renewable energy source produced by the breakdown of organic matter such as manure, food scraps, and crop residues by microorganisms in an oxygen-free environment. The main component of biogas is methane, which is a clean burning fuel used for heating, electricity generation, and vehicular fuel. Converting manure and other organic wastes into biogas through anaerobic digestion provides multiple benefits such as renewable energy production, waste management, and reduced greenhouse gas emissions.

Benefits of Small Scale Biogas Production

  • Energy production – Biogas can be used similarly to natural gas for heating, cooking, lighting, and power generation. This offsets the use of fossil fuels and provides energy self-sufficiency.

  • Waste management – Anaerobic digestion results in reduced volume of wastes and pathogen destruction, providing an environmentally sound waste management option.

  • Reduced pollution – Capturing methane from manure prevents its release into the atmosphere as a potent greenhouse gas. The nutrient-rich digested manure can be used as organic fertilizer.

  • Revenue generation – Biogas can be sold or used to displace purchased electricity and fuels. The high-quality digestate has value as biofertilizer.

Feedstocks for Small Scale Biogas Plants

The best feedstocks for small scale biogas digesters are high in energy-rich compounds like carbohydrates, proteins, and fats. The most accessible for small farms are:

  • Manure – Cow, pig, chicken, and other manures contain bacteria that produce methane during anaerobic digestion.

  • Crop residues – Leftover plant materials like corn stalks and husks can be digested.

  • Food waste – Fruits, vegetables, grains and other leftover food scraps are excellent feedstock.

Choosing an Anaerobic Digester Design

There are several types of digesters suitable for small farms:

  • Covered lagoons – This low-cost digester is an pond sealed with a flexible cover collecting biogas. Best in warmer climates.

  • Plug flow digesters – These are long, rectangular concrete tanks with modules. Manure flows sequentially from inlet to outlet.

  • Complete mix digesters – A concrete tank with mixing for uniform conditions throughout. More complex mechanically.

  • Fixed dome digesters – A dome-shaped underground concrete digester. Simple and affordable but limited gas storage.

I opted for a small plug flow digester due to my warm climate and availability of construction materials. The modular design also allows expansion.

Construction and Operation

I built my small 150 m^3 plug flow biogas plant with the following process:

  • Excavated trench 2 m deep and filled with concrete for digester floor and walls.

  • Added steel rebar reinforcing and used forms to pour ~20 m^3 rectangular digester modules.

  • Connected modules with PVC pipe openings to enable manure flow.

  • Built brick half-dome gas collection tank at end.

  • Sealed with waterproof render and mortar.

  • Covered digester with greenhouse plastic to retain heat.

  • Installed inlet and outlet pipes, gas piping to stove, and slurry collection point.

For operation, I mix fresh manure slurry with water at ~10% solids and inlet it daily. The hydraulic retention time is 20 days to allow full digestion. The biogas is piped to my kitchen for cooking fuel. The nutrient-rich effluent provides excellent fertilizer.

Results

My simple DIY biogas digester has been very successful so far. Benefits include:

  • ~1-2 m^3 biogas daily, enough for 2 hours of cooking fuel.

  • ~30% reduction in manure volume after digestion.

  • No noticeable manure odor and reduced flies around the farm.

  • Improved crop yields in fields amended with the liquid digestate.

For a small investment in materials and labor, I now have a free source of renewable cooking fuel while also managing farm waste. This first-hand experience has taught me the possibilities and benefits of small scale biogas production.