Microbes in Energy Generation: Biogas

Biogas: A Renewable Fuel From Microbial Digestion

Biogas is a mixture of gases -- predominantly methane, along with carbon dioxide, hydrogen and hydrogen sulphide -- produced when organic waste is broken down by microorganisms in the absence of oxygen. Because it can be generated continuously from waste materials that would otherwise need disposal, and because it burns cleanly as a fuel for cooking, lighting and small-scale power generation, biogas has become an important source of decentralised, renewable rural energy, particularly in India, where it is also commonly called gobar gas.

Methanogens: The Microbes Behind Biogas

The organisms directly responsible for producing methane, the fuel-value component of biogas, are a specialised group of anaerobic microorganisms called methanogens, classified among the Archaea. A well-known example is the genus Methanobacterium. Methanogens are naturally found wherever organic matter decomposes in the total absence of oxygen -- including marshy and waterlogged soils, and, notably, the rumen (the large fermentation chamber that forms part of the stomach) of cattle and other ruminant animals. In the rumen, methanogens and other symbiotic microbes help the animal digest cellulose from the plant material it eats, and methane is a by-product of this digestion, which is why cattle dung is so rich in these organisms and so effective as a biogas feedstock -- it already carries a dense, ready-made anaerobic microbial community capable of continuing the same digestive fermentation outside the animal.

The Basic Design of a Biogas Plant

A typical rural biogas plant consists of a few simple, interconnected parts. Dung (usually mixed with water to form a slurry) is introduced through an inlet chamber into a large underground digester tank, which is sealed to exclude oxygen. Inside this tank, over a period of days to weeks, the resident anaerobic bacterial community -- including methanogens -- ferments and digests the organic matter in the slurry, releasing biogas as they do so. This gas collects at the top of the digester, in many designs pushing up a floating dome or gas-holder that rises as gas accumulates and provides a simple built-in pressure system for delivering the gas through a pipe to wherever it is needed, such as a kitchen stove. Once the readily digestible organic matter has been broken down, the spent, nutrient-rich slurry is pushed out through a separate outlet, from which it can be collected and used directly as a high-quality organic fertiliser for fields -- meaning a biogas plant produces two useful outputs simultaneously: a fuel gas, and an enriched soil amendment.

Why Cattle Dung Works So Well

Cattle dung slurry is a particularly suitable raw material for rural Indian biogas plants for a combination of practical reasons: it is abundantly and cheaply available in agricultural and pastoral communities that keep livestock; it already carries a dense population of the very anaerobic bacteria (including methanogens from the animal's own rumen) needed to drive digestion, so the plant needs little or no separate inoculation; and it has a favourable balance of carbon- and nitrogen-containing compounds for efficient anaerobic fermentation.

Advantages of Biogas as a Fuel …