Biogas
Practical Action
Compact biogas digester using waste foodstuffs
For those without cattle or within urban centres, a conventional digester may not be appropriate.
The Indian Appropriate Rural Technology Institute (ARTI) has introduced a small biogas digester
that uses starchy or sugary wastes as feedstock, including waste flour, vegetable
residues,waste food, fruit peelings, rotten fruit, oil cake, rhizomes of banana, canna (a plant
similar to a lily but rich in starch), and non-edible seeds.
The compact plants are made from cut-down high-density polythene (HDPE) water tanks, which
are adapted using a heat gun and standard HDPE piping. The standard plant uses two tanks, with
volumes of typically 0.75 m3 and 1 m3. The smaller tank is the gas holder and is inverted over the
larger one which holds the mixture
of decomposing feedstock and
water (slurry).
Gas outlet
The feedstock must be blended so
that it is smooth using a blender
powered by electricity or by hand.
Two kilograms of such feedstock
produces approximately 500 g of
methane, and the reaction is
completed with 24 hours.
Gas holder
Inlet
An inlet is provided for adding
feedstock, and an overflow for
removing the digested residue. The
digester is set up in a sunny place
close to the kitchen, and a pipe
takes the biogas to the kitchen.
(ARTI, 2006)
Outlet
Fermenter
Figure 3: Compact biogas (Neil Noble/Practical Action)
Larger-scale biogas plants
Industrialised countries commonly use biogas digesters where animal dung, and increasingly fuel
crops, are used as feedstock for large-scale biogas digesters. Brazil and the Philippines lead the
world in crop-based digesters using sugar-cane residues as feedstock.
Interest and public support in biogas has been growing in most of the European countries. After a
period of stagnation, caused by technical and economic difficulties, the environmental benefits
and increasing price of fossil fuel have improved the competitiveness of biogas as an energy fuel.
This has been seen in both small and large scale plants in Denmark, Germany (with over 3000
plants producing 500MW electricity and 1000MW of heat) and Switzerland, and as a transport
fuel in Sweden (where vehicles using biomass were voted environmental cars of the year in
2005). There have been interesting biogas projects in the UK, Ireland, and the Netherlands.
Despite this, the use of biogas in Europe is modest in relation to the raw-material potential, and
biogas produces only a very small share of the total energy supply.
Several countries are experimenting with dedicated biogas energy crops, such as newly bred grass
varieties (Sudan grass and tropical grass hybrids) or biogas ‘super maize’ developed in France.
The crops are developed in such a way that they ferment easily and yield enough gas when used
as a single substrate. Biogas crops can be used whole, which allows for the use of far more
biomass per hectare.
When produced on a large scale, biogas can be fed into the natural gas grid and enter the energy
mix without consumers being aware of the change. A select number of European firms have
already begun doing so, while in Germany, farmers who generate excess biogas on their farms,
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