Uganda: The Fuel from Human Excreta

New Vision (Kampala) - Africa Uses Anaerobic Digestion

2 January 2008
John Kasozi
Kampala

HUMAN excreta and urine are now an asset to farmers because they are a cheap source of biogas energy. The two offer the best biogas followed by pig and cattle dung.

Banana peels, poultry droppings, water hyacinth and algae are the other organic raw materials that generate biogas. Banana peels and water hyacinth should be mixed with cow dung and poultry droppings to give off good gas after putting them out to dry under the sun for two days to reduce the amount of sap.

Andrew Ndawula, a technician subcontracted by Heifer International Uganda (HIU), revealed this during the recent tour of Heifer biogas beneficiaries by Dr. Sahr Lebbie, the Heifer International vice president of the Africa programme, based in USA.

Heifer International is a worldwide NGO whose mission is to work in partnership with others to end hunger and poverty and to care for the earth through sharing livestock and knowledge. Its main goal is to improve food security and the income of needy households, while focusing on the family, gender and environmental protection through sustainable agriculture.

In conjunction with its partners, HIU has initiated the construction of biogas plants among beneficiaries after realising that deforestation is a serious threat to the environment.

"Biogas will slow down the rate of deforestation and its by-products are utilised to improve soil fertility," said Lebbie. He added that biogas also indirectly reduces the burden of women and children fetching firewood from long distances.

Biogas is an inflammable gas produced by bacteria during bio-digestion fermentation of organic materials. This occurs under airless condition in an air tight container called a digester. It is composed of methane gas (60-65%) and carbon dioxide (35-40%). Biogas from animal excreta contains 60-90% methane and is combustible if the methane is more than 50%. In this range, biogas burns without further purification.

"There are three models and different sizes of biogas plants: floating, polythene tubular system and the Chinese fixed dome. "The fixed dome is the most common and economical plant constructed for HIU beneficiaries," said Ndawula.

The smallest size of fixed dome is 6-cubic metres (cu/m), and the largest is 100cu/m. Thirty cu/m and above are for institutions or large farms. The fixed dome is of two types; bricks/blocks type and cast systems. The price of cement, bricks and galvanised pipes put the total cost of the plant high.

The 6 and 8cu/m plants pressure prepares a simple meal in 24 hours if both the cow and plants are well fed and maintained. The digester should not be under- or over-fed by raw materials in order to have constant pressure supply to the house. The 12cu/m plant can be maintained by two or three well fed zero-grazed cows. Gas can light one lamp and a twin-burner for a family of 8-10 people.

However, the 16cu/m plant can be maintained by three to six cows. Its pressure can run three burners and two lamps. People owning animal farms should go for 30cu/m plant. Ten and more cows can sustain the plant that runs about five lamps, a canteen burner (commercial) and a twin-burner. This type of plant has two expansion chambers that maintain constant pressure.

Ndawula noted that 75 to 100cu/m have been built in Kenya and Tanzania by institutions such as universities. "I have approached a number of institutions in Uganda to put up similar plants, but they are skeptical," he added.

In China, Kenya and Tanzania institutions mix human and animal excreta, giving off good grade methane, while some institutions in China make biogas from only human excreta.

Africans are making good use of Anaerobic Digestion