Biogas Digester

Biogas is a by-product of organic materials such as plant and animal products being broken down by bacteria in an oxygen-free environment. This process is called  anaerobic digestion.

A biogas digestor uses natural anaerobic decomposition of organic matter under controlled conditions.

The digestor is usually a large, sealed container for the organic matter eg manure from livestock or poultry, green waste from agriculture, sewage or food waste. This is digested by bacteria in the absence of oxygen to produce a gas containing methane and carbon dioxide. The gas is piped away from the digestor and burnt to produce heat energy.

Biogas from landfill

Municipal landfill sites also act as very large biogas digestors and produce methane that can be extracted to provide energy. Biogas from landfill sites can be produced at a price that is less than the domestic price of natural gas from fossil fuel sources.

The biogas digestor is a living system so the conditions need to be right for growth of the anaerobic bacteria that produce the methane. Livestock manures are good for digestors because they have a neutral pH and high buffering capacity, which suits the bacteria. However, the energy content is relatively low because they have already been digested by the livestock. So, mixing livestock slurry with agricultural waste will increase the energy content of the biomass to be digested and so increase biogas production.

Biogas from livestock waste

Biogas production is well suited to farming in which there is a combination of livestock and agriculture. However, some care needs to be taken to ensure the digestor has the right mix of biomass feedstock. Growth of the anaerobic bacteria will be inhibited if there is too much ammonia or sulphur. Waste from intensive poultry farming is high in ammonia so this needs to be diluted or the ammonia removed.

Vermiculture

Biogas digestion produces a high-quality organic fertiliser ideal for market gardening. This can be improved by using worms for composting. This is called vermiculture and involves mixing residue from the biogas digestor with certain species of earthworm.

An advantage of the creation of fertiliser by using farm wastes for biogas is that the digestion process can help reduce the amount of nutrient-rich pollution from animal slurry and sewage, which contains a lot of nitrogen and phosphorus. This type of nutrient pollution is a major problem with intensive livestock rearing if it seeps into nearby rivers causing algal blooms, which can be toxic. Reduction in the amount of oxygen in the river from increased growth of micro-organisms in response to raised nutrient levels can also result in the death of fish.