October 16, 2004

CATTLE dung plays a central part in everyday life in rural area of country. Dung is used primarily for three purposes — as building material and plasterwork undertaken by women, as cooking fuel, and as fertilizer. A woman normally spend up to two hours a day making dung-cakes, depending on how much dung she has access to and the amount of cooking fuel required.

Dung and dung-cakes are produced largely for home use. Limited sale of dung-cakes takes place in the local town. Dung collection is considered as exclusively women's work. Men normally not handle it and are ridiculed if mentioned in connection with such work.

A UNDP report says that two billion people rely on traditional fuels such as wood, dung and agricultural residues to meet their heating and cooking needs. In rural area, women cook on biomass fires inside poorly ventilated huts without chimneys.

As a result of the household use of unprocessed biomass fuels, concentrations of health-damaging air pollutants tend to be highest indoors, where biomass fuels such as wood, animal dung, and crop residues are burned by many households for cooking and heating.

According to World Health Organization (WHO) some 3.5 billion people are exposed to high levels of air pollutants at their homes, mostly in the rural areas of developing countries. Because women in rural areas particularly among rural poor households, who rely on dung for cooking, tend to be more malnourished effects of indoor air pollution are likely to be stronger.

Young children are often carried by mothers or kept in the kitchen area during cooking, exposing them to high levels of smoke. Because women do most of the cooking and spend more time indoors, they are exposed more to the pollutants and are believed to have greater adverse health impacts. Young children who usually stay with their mothers indoors also have elevated exposures.

Indoor air pollution from cooking and heating with traditional fuels has been designated by the World Bank as one of the four most critical environmental problems in developing countries.

Biomass smoke contains many noxious components, including respirable particulates, carbon monoxide, nitrogen oxides, formaldehyde, and polyaromatic hydrocarbons. High exposures to these air pollutants have been shown to cause serious health problems, such as acute respiratory infections, chronic obstructive lung disease, tuberculosis, blindness and lung cancer.

Energy conservation, coupled with concern for the management of livestock wastes, has revived an interest in generating methane from livestock manures. Converting organic materials, such as animal wastes, to an easily used form of energy can be accomplished by several methods. The process with the greatest potential is anaerobic fermentation or digestion.

Anaerobic fermentation, or digestion, is the most promising process for converting organic materials to methane and other gases. Biogas usually contains about 60 to 70 percent methane, 30 to 40 percent carbon dioxide, and other gases, including ammonia, hydrogen sulfide, mercaptans and other noxious gases. It also is saturated with water vapor. The heat value of the raw gas is about 400 to 600 British thermal units (BTU) per cubic foot. In comparison, natural gas has a heat value of 850 BTU per cubic foot and gasoline contains approximately 120,000 BTU per gallon. Partial removal of the impurities are required that complicate the system to some extent.

Anaerobic digestion is a two-part process and each part is performed by a specific group of organisms. The first part is the breakdown of organic matter (manure) into simple organic compounds by acid-forming bacteria. The second group of microorganisms, the methane-formers, breaks down the acids into methane and carbon dioxide.

In a properly functioning digester, the two groups of bacteria balance so that the methane-formers use just the acids produced by the acid-formers. These bacteria's are anaerobes in nature and operate only in free oxygen environments. Constant temperature, pH and fresh organic matter promote maximum methane production.

A simple sequence of equipments can produce biogas. The amount of the gas and the reliability desired has a great influence on the cost and complexity of the system.

A simple batch-loaded digester requires an oxygen-free container, relatively constant temperature, a means of collecting gas, and some mixing. Because methane gas is explosive, appropriate safety precautions are needed. The number, size and type of animals served, dilution water added, and detention time control tank size. The factor that can be most easily changed with regard to tank size is detention time. Ten days is the minimum, but a longer period can be used. The longer the detention time, the larger the tank must be. Longer detention times allow more complete decomposition of the wastes.

Fifteen days is a frequently used detention time. Because the processed manure still contains most of the original nitrogen, phosphorus and potassium, and is still highly polluted, the waste should not be allowed into water a stream after it leaves the digester. Lagoons are commonly used to hold the waste until either hauling or pumping onto agricultural land as fertilizer.

The volume of effluent actually may be greater than the volume of manure prior to digestion. This increase is due to the dilution water added to liquefy the manure to the desired solid content for the digester.

There is no increase in the amount of nitrogen, phosphorus or potassium in this material, although it may be in a more available form. A small portion of the nitrogen may be lost to the gas portion of the system, thus reducing the amount of nitrogen initially available.

The conventional Gobar unit is constructed by First digging a pit around ten feet deep. A watertight cement cylinder (with brick or gravel) is constructed in the pit. Next, a wall is built across the middle, extending up from the bottom, not quite to the top and intake and outlet pipes are installed. A large metal bell-like cover tops the whole unit. It acts as gasholder and made up and down with the help of pulleys. The whole unit should be watertight.

The conventional unit is operated by mixing manure with water in the Intake basin to make slurry, which then goes down the pipe to the bottom of the one side of cemented cylinder. Waste fed into the digester should contain 90 to 95 percent water. One side of the cylinder gradually fills and overflows to the other side. When both sides of the cylinder are full, the effluent flows out from the bottom of the other side each time more raw manure is added. The refused manure come out from the plant is of more value as fertilizer than the raw manure. The formation of gas starts in week.

As the gas start collecting in the drum, the drum begins to rise and float. The gas is devoid of its condensed moisture and used as fuel. The gas builds pressure, and can be taken off through a rubber tube to a gas stove in a kitchen.

The biogas production varies depending on the organic material digested, the digester-loading rate, and the environmental conditions in the digester. It has been estimated that about 60kg of dung per day is required to produce 2.8 m3 of gas, which is sufficient for one family. The gas can be used for domestic lighting and small pump running. The table-1 shows the relationship between plant size, manure requirement and amount of gas generated.

The plant should be located away from the drinking water supply, in the sun, close to the source of the manure, close to a source of water, and close to the point where the gas will be used.

A plant that produces three cubic meters of methane per day requires an area approximately 2x3 metres. If a larger plant is required, figure space needs accordingly. Plant also requires arrangement of water for mixing with the manure and space for storage of manure. Although the gas plant itself takes up a very small area, the slurry pits should be large and expandable. The site should be open and exposed to the sun. The digester operates best and gives better gas production at high temperatures 35°C (85°F).

For gas utilization, a compressor and storage tank are used, along with the hardware to provide flame traps, regulators, pressure gauges, hydrogen sulphide scrubber, carbondioxide removal and pressure relief valves. A common facility for gas storage is the floating cover that floats upward while maintaining essentially constant pressure. Most applications are somewhat intermittent; therefore, storage is required. The amount of storage depends on the storage time and pressure.

Methane in a concentration of 6 to 15 percent with air is an explosive mixture. It is relatively odorless and detection may be difficult. Extreme caution and special safety features are necessary in the digester design and storage tank, especially if the gas is compressed.

Access to modern energy services is an essential pre-requisite for increasing productivity and improving people's livelihoods, new approaches are needed to deliver such energy services to meet the needs of the poor and support sustainable development.