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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.
BETTY NABUMA: So, now, what is biogas? Biogas is just one of the renewable modern types of energy that we have. Most of you are used to traditional forms of energy which have their own challenges, especially in Africa. You have traditional energy sources like firewood. You have charcoal. People have been using cow dung in dry form. But there has been a desire to shift from these traditional forms of energy to the modern forms of energy because of the challenges of the traditional ones. Biogas is one of those modern forms of energy that comes from biomass. There are quite a number of biomass sources.
Wastes from agricultural activities, like leaves. You have maize cobs. You have waste from fruits like tomatoes. You have pineapple or pineapple peels. And then you also have waste that is animal manure that comes from animals, like cows, like pigs, like goats, and other animals that exist on farms, as well as the waste that comes from poultry. People who are having chicken– either for meat purposes, or for eggs– the waste that comes from that can also be a source– is biomass and can be a source of biogas. The process of production of biogas from biodegradable matter is an anaerobic process. And what do we mean by this? An anaerobic process is a process that takes place in the absence of oxygen.
Or, we could say air, because air is actually what has the biggest percentage of oxygen. So, in the absence of air– or in the absence of oxygen– we can have a biological process taking place which we refer to as anaerobic decomposition. That enables us to convert this raw material into the biogas that we desire to have.

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.


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.

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Renewable Energy: Achieving Sustainability through Bioenergy

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