Biogas, also known as sewage gas, is a by-product of the anaerobic digestion of organic wastes such as plants, crop residues, and human and animal wastes, and consists mainly of methane. The potential of biogas as an energy source has continued to widen, not only in industrialized countries, but also in developing countries. It can be used for lighting and domestic cooking, running internal-combustion engines, to pump water, or to generate electricity.

According to a survey conducted by the Resource Inventory Management Company of Lagos in 1992, there are many millions of chickens, goats, cattle, sheep, and pigs in Nigeria presenting potential but untapped sources of feedstocks for biomass digesters. The effective calorific value of biogas compared with some other fuels is presented below in Table 1.

Table 1 :The effective calorific value of biogas compared with some other fuels.

Before biogas can be used effectively in cottage industries, however, there is the need to purify it (to remove the corrosive H2S and noncombustible CO2) and to increase its heating value. To achieve this objective, a low cost biogas plant for cottage industry in rural areas has been designed, fabricated and tested (see diagram).

The other product of biogas technology sludge - has been claimed to be a high quality fertilizer. Agriculture is the main industry in the rural areas and about 90% of the food production in Nigeria comes from there. Over the years, synthetic fertilizers have played a major role in food production in Nigeria. The huge spending of foreign exchange on fertilizer importation (reaching 2 billion Naira (US$140 million) in 1992) and the associated problems of distribution, inflation of prices by middlemen and late arrival for usage have prompted the Federal Government to remove the 91% subsidy on fertilizers. Sludge is a viable and cheaper alternative/complement to synthetic fertilizer.

It is estimated that the waste material from two adult cattle will supply the gas required for cooking and lighting for a family of four for a day. (FAO 1978).

Plant description

The diagram illustrates a digester with a 24-litre capacity 3mm-gauge mild steel drum. There is an inlet and outlet at the top for the substrate and raw biomass respectively. The outlet is connected via a rubber hose into the H2S purifier which is a 5-litre mild steel drum with an airtight cover. There is a control valve between the H2S purifier and the digester. Two 12.5mm-diameter galvanized pipes welded at the side and at the centre of the cover serve as an inlet from the digester and outlet into the CO2 purifier via a flexible rubber hose. The CO2 purifier consists of two cylindrical mild steel drums. One serves as the KOH solution bath and a smaller one inverted as a gas holder to put the gas under pressure. The holder is connected to the KOH bath by a threaded iron rod welded on the sides with the ability to move up and down with the aid of a bolt and nut arrangement on the rod. A 12.5mm diameter galvanized pipe serves as an outlet from the holder into a storage tank via a rubber hose and there is a control valve between the holder and the storage tank.

The total cost of labour and materials used for the construction of the biogas plant is N2800 (US$200).