December 15, 2009 | General

Activated Biogas Digester For Improved Organics Recycling (Nigeria)

BioCycle December 2009, Vol. 50, No. 12, p. 50
Researchers developed a simple three-compartment digester to produce biogas to meet the energy needs of households and smaller communities.
M.K.C. Sridhar, O. Olumuyiwa, A. Oluyemi, O. Okareh, E.O. Oloruntoba and G.R.E.E. Ana

AMONG the developing countries, India, China and Nepal showed how biogas technology can be promoted to reach every household, with economic gains through savings from alternate energy and backyard farming. However African countries, and in particular West Africa, have been lagging behind in alternate energy technology, thus contributing to increased deforestation and desertification on the continent. There is need to change this trend.
In Nigeria and other West African countries, the common digester designs are floating and fixed dome types. The tanks are made of concrete, metal or plastic. A cheaper gas collection has been tried using polythene tubes and rubber tubes. Large-scale biogas plants are not popular; they were tried only in a few places to serve institutions. Shortcomings in the biogas technology in these countries is lack of policy support from the governments, traditional dependence on fuel wood, charcoal and fossil fuels, lack of awareness on the derivable benefits and low level of capacity to adopt the technology which is still considered as alien.
At the University of Ibadan, we have developed a simple design applicable to households and smaller communities for managing their organic wastes. The new model consists of three PVC tanks (digesters) connected in series, with a gentle slope for gravity flow of the slurry with uniform mixing (Figure 1). In this prototype model, the capacity of the digesters is 226 liters, each round in shape and colored black to retain heat. The top digester unit shown in Figure 1 is the inlet tank and holds the slurry for seven days, after which it is allowed to pass into the second (middle) digester.
In the second digester, the feedstock is hydrolyzed by the microorganisms and is made available for the methanogenesis phase. The slurry has a retention time of about 10 days before it is allowed to pass into the third digester. It is retained in the third tank for a short period of five days, then discharged into a small spent slurry collector.
The active digestion takes place in the second tank, evident from the increased temperature during operation. At a predetermined interval, a certain amount of the feedstock is allowed to flow from the first tank into the second and to the third. Once the digestion process is stabilized, this process can be made to operate continuously. In the third unit, the digestion continues and produces additional biogas, thus degrading any feedstock that might have escaped adequate contact with the methanogenic organisms in the second digester. This enhances the efficiency and completes digestion.
Each 3-tank digester unit is designed to operate independently with a gas collection pipe, a manometer and a device to measure temperature. It is easy to maintain as well as to troubleshoot operational problems that may crop up during usage. We coined the name “Activated Biogas Digester,” as a portion of feedstock slurry from each of the tanks enter into the other in sequence and in the process, an active inoculum of organisms is maintained at all times. The spent slurry was found to be useful in growing cocoa yam and banana plants on a backyard farm.
The raw materials tested in development of the activated biogas digester were poultry droppings, water hyacinth, cow dung and cow rumen contents. Food waste also serves as a good feedstock. The feedstock was prepared to reach 30 percent solids content and fed to the first digester. The cow rumen contents were placed in the second digester essentially to hasten methane production.

Mobilizing Food Vendors
The biogas technology is being promoted among various community groups through workshops, group discussions and demonstration activities. In the past workshops were held in Jos, capital of Plateau State, and Ibadan (capital of Oyo State), which have sensitized the communities. Food sellers and small-scale cassava processors were identified from selected institutions and markets in Ibadan. They were in the habit of using firewood and charcoal in their daily cooking. Kerosene is rarely used because of its high cost and irregular supplies and LPG gas was not affordable.
Vendors and processors were shown the biogas plant, its operation and the derivable benefits. In the sessions, which lasted two to three hours, the target groups appreciated the nonpolluting nature of biogas, availability of regular supply with their own wastes generated in the vicinity, by-product utilization for backyard farming and smokeless cooking. They were also educated on the Millennium Development Goals target “to halve by 2015, the number of people without effective access to modern cooking fuels, and to make improved cooking stoves widely available.”
Estimates show that 1 m3 of biogas derived from 13 kg of animal dung is equivalent to 3.47 kg of fire wood, 0.62 liter of kerosene, 0.61 liter of diesel, 1.5 kg of charcoal, 1.25 kWh of electricity, 0.45 kg of LPG gas, and 0.5 kg of butane. The discussions, from five small-sized restaurant owners (popularly called bukha), indicated that they are willing to adopt the technology as long as it is cheap and does not pose a health problem through food contamination.
The activated biogas digester reported here is flexible in design, and the size can be manipulated to meet the needs of individual household and community energy needs. Use of biogas also solves the energy needs in rural areas where people traditionally forage for fuel wood in the forest. A 10 m3 digester in rural areas can save 2,000 kg of fuel wood, which is equivalent to reforesting 0.26 to 4.0 hectares. The spent slurry, which is free of pathogens, is an excellent source of mineral nutrients that can be used as organic fertilizer, livestock feed, fish feed and for mushroom cultivation.

The authors are with Division of Environmental Health Sciences, Faculty of Public Health, College of Medicine, University of Ibadan in Ibadan, Nigeria. M.K.C. Sridhar, the corresponding author, can be reached at

Sign up