Biogas plants or anaerobic digesters are facilities that convert organic matter into methane and carbon dioxide. After extracting the CO2 the methane can be used as a renewable fuel for heating, generating electricity or operating vehicles.
Anaerobic digestion gets its name from anaerobic bacteria which do the work in these systems. This is the class of bacteria that feed on a broad range of organic matter in the absence of oxygen. They exist naturally at the bottom of swamps, in the ocean and are floating around dormant throughout the biosphere. Thats why a wound that isn’t cleaned properly before bandaging can putrify–anaerobic bacteria is trapped under the skin and starts feeding on you.
In addition to producing green methane, your digester makes high-quality compost as a by-product.
Feedstock can be vegetable waste and/or manure, the latter being more efficient at producing methane. It is now common for large hog farms to operate biogas plants with the manure they produce, and there are large industrial-scale plants in operation. But it is also possible to make a small plant for residences, neighbourhoods or small mixed farms. Here are two great videos explaining a couple of designs. The one from Dickinson College provides tested data for flow rates, quantity and quality of gas produced and other operating parameters. Its one of the best presentations I’ve found for teaching the construction and operation of a functional small scale system.
Science of biogas – Dickinson College
This next video describes a larger in-ground design that is very low-tech.
Simple biogas plant – Vietnamese design
Hailing from Kenya, the vid below focuses on using a digester to process household waste water. It has a great explanation of the fundamentals, but does not give any specific specs. It should be noted that the system described channels the effluent to a “soak pit” which is really a simplified septic field. This is a waste of good nutrients unless you have a garden or orchard downstream of the effluent field.