How Do Biodigestors Function As Methane Gas Generators?

Methane gas, CH4, is sometimes referred to as biogas, because it is produced by the deterioration of biological matter in an anaerobic environment. This means that in order to produce this gas, the organic material being digested by bacteria must be isolated from the oxygen in the air, either inside a sealed chamber or, as in the case of buried waste matter, under layers of soil or trash. In an uncontrolled environment, methane is released into the atmosphere wherever plant or animal residues are in the process of decaying on the forest floor, in garbage dumps and manure piles, or inside the intestines of humans and animals. Methane is a greenhouse gas, with a very high GWP (Global Warming Potential) over 20 years, methane traps 72 times more solar heat in the planet's atmosphere than carbon dioxide. Ideally, the release of this dangerous substance into the air should be diminished, to reduce its negative contribution to the over-heating of the Earth.

Fortunately, methane has some positive characteristics that facilitate the reduction of its emissions and impacts, besides making it useful in other ways. It is highly flammable, and when contained and canalized, can be used like propane or butane to feed flames for heating water or air, cooking, illumination (in gas lanterns), drying clothes, or dehydrating food products. It can also be used to fuel internal combustion engines, which may operate moving vehicles, pumps and other machinery, or electrical generators. The burning of methane gas transforms it into carbon dioxide, a chemical compound which is much less harmful to the planet.

One of the easiest techniques for producing methane under controlled conditions, thus permitting its capture and utilization as fuel, is the biodigestor a sealed tank where bacteria consume organic material without the presence of air, and expel the biogas. The material used to feed these bacteria can range from solid plant and animal residues, such as manure, leaves and grass cuttings, to liquid sewage. In rural applications, where animals are constantly furnishing excrement, this material can be used in the biodigestor to generate methane for the varied uses mentioned. In this case, the manure serves a double function after it has been used in the gas generation and transformed into sludge, which accumulates in the bottom of the chamber, this sludge can be used as fertilizer for some plant cultures.

In general, there are two forms of using the biodigestor either as a closed gas generating system, or as a constant flow sewage treatment system, which of course also generates gas as a highly beneficially side effect. In the first example, the biodigestor has a single hatch, and is filled with the organic mixture desired, then sealed and left to digest this material and fabricate methane. The resulting methane is burned until the bacteria have exhausted their source of food, the organic material in the tank. When no more gas is produced, the biodigestor is opened, emptied and cleaned, then refilled to repeat the process.

In the second case of the constant flow system, liquid sewage is continuously entering one side of the biodigestor and partially treated water is leaving the other side. In this type of application, very practical for small urban or rural communities where more ample sewage collecting networks are not available, the biodigestor serves as a septic tank, decomposing the solid waste in the sewage, as well as a methane generator. The liquid effluent can be further treated at the outlet from the biodigestor by small settling ponds or plant filters. These filters are basically small tanks, open at the top and filled with soil, where plants are used to consume the remaining nutrients in the effluent, thus leaving the water which finally leaves the system in a much cleaner state.

There are two basic models of biodigestors being used in many countries, copied from China or India. In the Chinese model, which consists of a more or less spherical sealed chamber of brick or concrete, there is an entrance for the organic material, an outlet for the water (if it is a constant flow system), and a tube which conducts the methane to its destination. The gas is pressurized by the inflexible structure of the tank body, much as in the case of bottled gas, only here the bottle is made of masonry. The other, the Indian model, is made up of a vertical, tubular tank filled with the organic mixture to be digested, and covered by a floating dome that fits inside the tank. The weight of the dome pressurizes the gas, and raises or lowers its elevation depending on the volume of gas inside the tank. Once again, this can have an inlet and an outlet for the constant flow of liquid sewage. The biodigestor has proved to be very practical in many situations, and its benefits for health, sanitation, the economy and the climate make this form of technology an alternative to be widely applied all over the world.

Author: David Michael (Teres�polis, Brazil)

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Edited by: Rajesh Bihani ( Find me on Google+ )

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