April 21, 2011 | General

Increasing Pace Of Digester Installations (Tanzania)

BioCycle April 2011, Vol. 52, No. 4, p. 45
Biogas is proving itself to be a significant player as countries in sub-Saharan Africa seek alternatives to unsustainable use of wood for fuel.
Don Lotter

TANZANIA is facing an energy crisis. While similar to many developed countries in that the crisis involves consumption of an unsustainable form of energy, it is very different in that it is not fossil fuel being unsustainably consumed, but biomass energy – wood cut from savannah and forest. Households account for more than 90 percent of all energy use in Tanzania; industry, transport, commerce and agriculture combined account for less than 10 percent. And more than 90 percent of that household energy is from firewood and wood-derived charcoal, nearly all from natural areas. More than 2,500 tons of charcoal are produced daily in Tanzania, all from trees and shrubs, all by rural people trying to eke out a living and nearly all taken to market in 125 kilogram (275 pound) loads on the back of a bicycle. Other forms of renewable energy are badly needed to replace this unsustainable system. Biogas is one strategy that has been under development.
The first biogas digesters in Tanzania used the floating drum design built out of 55-gallon steel drums. They were built in 1975 by the Tanzania government’s Small Industry Development Organization for the large-scale cooperative farms of that socialist era. This design had been used in India (also known as the “Indian drum” design), and is now used as a rooftop biogas source – mostly utilizing blended kitchen waste innoculated with cow manure – in urban areas worldwide. The floating drum design utilizes two drums, one slightly smaller than the other, with the smaller one inverted inside the larger. As the carbon source decomposes in the larger drum, the inverted smaller one fills with gas and rises. Gas is captured via a hose connection at the top of the smaller drum (its actual bottom).
The steel drums were difficult to repair due to lack of welding tools, so the digester technology was abandoned. However, this design is ripe for reintroduction in the now densely populated cities of East Africa, where cooking gas is extremely expensive, demand for charcoal is causing the plunder of natural ecosystems, electrical blackouts are a regular part of life and large plastic drums – which can be used in place of the steel drums – are plentiful.

Fixed Dome Digesters
During the 1970s, the world was beginning to get a few reports about small-scale appropriate technology from China. The Chinese had implemented hundreds of thousands of what are known as “fixed dome” brick-and-mortar biogas units (named in reference to the floating drum design). This design, mostly small-scale (5-30 cubic meter digester volume, e.g., for 10-50 pigs), involved at least two connected below ground tanks. One tank is used to digest manure and produce gas, and the other, at a slightly higher elevation, holds liquid manure that is pushed up as gas pressure increases, which allows fluctuations in gas volume without losing gas.
In 1983, the Tanzanian government’s Centre for Agricultural Mechanization and Rural Technology (CAMARTEC), with the aid of the German government, began investigating small-scale fixed-dome biogas units, and by the mid-1980s had decided that the country was best served by a modified fixed-dome design that included seven coatings of waterproof cement to prevent gas leakage.
By 2000, only a few hundred biogas units had been built, many of them mid- to large-scale (50-500 cubic meters for processing industrial waste). Biogas development at the household and small farm level was lacking. According to Evarist Ngwandu, director general of CAMARTEC, this paucity of progress is a result of poor legislative and government support for renewable energy programs. The rather paradoxical reason given by politicians, according to Ngwandu, is that renewables account for only 0.5 to 1 percent of the country’s total energy.
In 2007, interest in biogas in Africa was renewed, and with the help of the Dutch and German governments, Tanzania embarked on the Tanzania Biogas Development Program. The program focuses on training masons and on subsidizing the building and use of small-scale biogas units that serve households and farms with 4 to 10 cows or 10 to 20 pigs – a size range of 4 to 13 cubic meters in digester volume. More than 500 masons are now trained and in 2010 alone, more than 1,000 digesters were built with 2,500 targeted for 2011. The subsidies include paying approximately one-third of the cost of the biogas unit, plus providing stove burners and gas lamps). NGOs (nongovernmental organizations) and community-based organizations, known as implementing partners, can apply for CAMARTEC’s help in building the units. Once selected, the partner organization must first dig the pits – a considerable amount of work – and buy the bricks and mortar. CAMARTEC supplies pipes, valves, connecters, etc., in addition to the mason.
Most of the biogas units being built by the program have human latrine connections, but these are installed only after what the CAMARTEC director calls “a lot of persuading” due to long-standing taboos. (This taboo revolves around using the “humanure,” which is considered unfit for any use). Numerous analyses have shown no problems with human pathogens in biogas effluent, due largely to the 50-day microbially active digestion time from input to effluent outflow. Biogas digester owners are taught to use the nutrient-rich effluent as fertilizer on crop fields.
The most popular size digester for Tanzanian rural households is 6 cubic meters. At least three cows or 10 pigs are needed for this size digester, generating about 45 kg/day of cow manure or about 20 kg of pig manure. The manure must be mixed with an equal volume of water before input.
With electricity shortages and daily blackouts common in Tanzania (three scheduled 12-hour outages per week for my household), biogas-generated electricity is on the horizon. Small generators that produce 1.5 kwh of electricity are now available. According to CAMARTEC specialists, to run for 6 hours a day such a generator needs 1 cubic meter/hour of biogas from approximately 150 kg/day of cow manure, the output of about 10 cows. This would require a 15 cubic meter biogas unit, which is slightly larger than the 4 to 13 cubic meter units that the CAMARTEC program builds.
Tanzania, along with many other sub-Saharan African countries, faces substantial challenges in developing a sustainable yet socially and economically adequate energy sector. In the next 10 years, these countries must make a transition from an energy economy that is 90 percent dependent on the unsustainable exploitation of wood from savannah and forest to one that uses mostly renewable sources. Biogas is proving itself be a significant player.

Don Lotter teaches at the Institute of Development Studies at St. John’s University in Dodoma, Tanzania (;

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