Recycling of organic waste
Practical Action
is merely a by-product.
during the digestion process pathogens in the manure are killed, which is a great benefit to
environmental health.
Figure 3: a typical floating cover biogas digester
Two popular simple designs of digester have been developed for use in developing countries; the
Indian ‘floating cover’ biogas digester (see figure 3 above) and the Chinese fixed dome digester.
Both operate in the same way but the storage chambers have a slightly different design.
The residual slurry is removed at the outlet and can be used as a fertiliser. Biogas can be used for a
number of applications, including lighting, cooking, electricity generation and as a replacement for
diesel in diesel engines. Some countries have initiated large-scale biogas programmes, Tanzania
being an example. The Tanzanian model is based on integrated resource recovery from municipal
and industrial waste for grid-based electricity and fertiliser production (Karekezi 1997).
Waste Material
C:N Ratio
Gas yield (litres per
kg)
Human excreta
6 – 10
-
Cow dung (up to 12kg per cow per day)
18
90 – 300
Pig manure (up to 2.5kg per pig per day)
-
370 – 500
Chicken manure
7 300
Grass (hay)
12 Not suitable alone
Grass with chicken manure
- 350
Paper
- Not suitable alone
Paper with chicken manure
- 400 – 500
Sewage sludge
- 600
Wheat straw
150 Not suitable alone
Bagasse (sugar cane waste)
150 Not suitable alone
Sawdust
200 – 500
Not suitable alone
A gas-cooking burner needs 300 – 600 litres of gas per hour.
A peasant family uses 4000 – 5000 litres per month per person.
The ideal C:N ratio is between 25:1 and 30:1.
Table 1: Ability of waste to produce methane (Source: Vogler, Work from Waste)
Composting toilet
There are a number of methods commonly used for home composting (or small-scale community
composting) of human excreta. The simplest and most elegant solution is the composting toilet.
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