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HOW IT WORKS: Biogas digester
Figures from the Uganda Bureau of Statistics show that most homes met their energy for cooking and heating, among other domestic activities, from wood, charcoal and other biomass. It was calculated that collectively our expenditure on this amounts to over Shs400b per year!
However, this is clearly not sustainable in the long run as a 2009 study carried out by the National Forestry Authority attributed 27 per cent loss of the forest cover in the last 15 years to meeting this energy demand through wood and charcoal. In fact, the picture of the future is grim as it is estimated that by 2020 (that is just in eight years), we will be importing wood fuel. And just five years from that year, we will number 55 million people and will need more than 42 million tonnes of wood for domestic energy.
Available and renewable
Yet all around us there is an almost inexhaustible renewable source of energy that can save the trees and forests and help avoid the effects of pollution from using wood and charcoal. That is biogas which can be got from the waste that comes from domestic and other activities. This includes manure, sewage, dead plant and animal material, and kitchen waste.
Biogas is released through the biological breakdown of organic matter in the absence of oxygen and it is this that can be turned into this kind of bio fuel.
Biogas is produced by the anaerobic digestion (without oxygen) or fermentation of decomposable material. It comprises mainly methane and some amounts of carbon dioxide and smaller quantities of other gases and substances.
Since methane, hydrogen, and carbon monoxide can be combusted, biogas can be used as a fuel for heating or cooking and it can be used to power machines and vehicles as it is possible to compress it just like natural gas.
A case study done by researchers from Kyambogo University in Apac District computed that dung from five to 10 cows produces 2.5 square metres of gas per day. This is enough to provide lighting and cooking for a rural household of six people. Other figures in the same study show that an average dairy cow can produce 55kg of dung and urine per day, or 20 metric tonnes per year, which can generate an equivalent of three kilowatt hours of electricity.
The way in which organic matter is broken down into different components that release the gases is known as anaerobic digestion. It is a series of processes that starts with action of bacteria on the material to break it down into substance as such as carbohydrates, and make them available for other bacteria.
These bacteria known as acidogenic bacteria then convert it into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these into acetic acid, along with additional ammonia, hydrogen and carbon dioxide. Then, methanogens convert these products to methane and carbon dioxide.
The place where all this takes place is known as a biogas digester, which is a specially constructed container for storing the slurry (waste). There are various types of digesters based on different principles depending on the use for which they are designed. The technologies are categorised into batch type digesters—which treat a large amount of material at once and are used for large-scale application—and continuous flow units, which add and remove waste regularly and are best as smaller applications.
The factors
The digester types are fixed dome, floating drum, and balloon. The choice of type and technology is dependent on various factors but the most important one is temperature. It is essential in creating the conducive conditions for bacteria to act on the organic matter or slurry that is put in the digester to produce biogas. For instance, fixed dome digesters tend to be more successful in areas with extreme temperature fluctuations. Besides temperature inside the digester, it is noteworthy that gas production is dependent upon fermentation or retention time and the material.
For farmlands, like Uganda where most households are agriculture-based or dependent in one way or another on agricultural activities, biogas digesters would have the plus of a high-nutrient fertiliser as a by-product and contribute to improved sanitation via better waste disposal.
As the world moves towards eco-friendly methods of producing energy, it is time biogas digesters are factored in the equation especially in developing countries like Uganda that have an abundant source of biogas and may need to facilitate the spread of the technology to the its people.
Compiled by Gumisiriza Mwesigye
