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close this bookBiogas Plants (GTZ, 1988)
View the document(introduction...)
View the documentAcknowledgements
View the documentPreface
View the document0. Biogas as appropriate technology
View the document1. Benefits and costs of a biogas plant
Open this folder and view contents2. The dgestion process
Open this folder and view contents3. Biogas plants
Open this folder and view contents4. Scaling of biogas plants
Open this folder and view contents5. Design of biogas plants
Open this folder and view contents6. Biogas utilization
View the document7. Planning, design and construction
View the document8. Appendix
View the documentBibliography
View the documentList of figures

1. Benefits and costs of a biogas plant

A biogas plant supplies energy and fertilizer. It improves hygiene and protects the environment. A biogas plant lightens the burden on the State budget and improves working conditions for the housewife. A biogas plant is a modern energy source. A biogas plant improves life in the country.

A biogas plant can satisfy these high expectations only if it is well designed.

A biogas plant supplies energy. However, a biogas plant also consumes energy. Energy is already consumed in the production of the construction material:

- for 1 m³ of masonry, about 1000 kWh or 180 m³ of biogas,
- for 100 kg of steel, about 800 kWh or 150 m³ of biogas,
- for 1 kg of oil paint, about 170 kWh or 28 m³ of biogas.

Energy is consumed in transporting the materials of a biogas plant. Construction and maintenance also consume energy:

- for 1 km of transport by lorry, about 1.5 kWh or 1.05 m³ of biogas
- for 1 km of transport by car, about 0.5 kWh or 0.35 m³ of biogas.

A biogas plant must operate for one or two years before the energy put into it is recovered.

The degree of digestion (see page 17) increases with the retention time. Long retention times save energy. The net energy gain is smaller with short retention times: if the retention time for 50 kg of cattle dung is reduced from 90 to 45 days, some 790 kWh or 240 m³ of biogas per year is lost.

A biogas plant eases the work of the housewife. However, a biogas plant also creates additional work for the housewife: dung and mixing water have to be supplied to it. The fermentation slurry has to be mixed. Long retention times help the housewife. Biogas plants with short retention times need more labour: To replace 20 kg of firewood by biogas, a housewife must supply 121 kg of dung and 121 litres of water if the retention period is 45 days. For a 90-day retention period, only 84 litres of dung and of water are required. This represents a difference of nearly 9 kg of dung and nearly 9 litres of water per m³ of gas per day.

If the plant is filled only every other day, working time is saved - because of the saving of preparation time.

If the biogas plant is too far from the source of water or from the animal housing, the housewife must perform additional work: the housewife's workload is lightened by a biogas plant only if the distance to the water source and that to the byre together are less than a quarter of the distance to the wood collection point.

The least amount of work results from locating the biogas plant directly beside the animal shelter (byre), which should have a paved floor. This makes it easy to sweep urine and dung into the plant's inlet pipe. Often enough, no extra mixing water is needed' and the gas yield is considerably higher.

The designer decides in whose interests the biogas plant is economic: a biogas plant for short retention times is economic for a farmer with many animals and cheap labour.

A plant with long retention times is beneficial to:

- a farmer with few animals,
- the housewife,
- the national economy.

The personal benefit of a biogas plant to the owner depends on how he previously met his energy and fertilizer requirements: the benefit is greater the more energy had to be bought in (diesel oil, coal, wood) and the higher the cost of that energy. However, there is always a close relationship between energy costs and those of construction.

Energy costs are set out in the tables in Fig. 38 (page 55).

Example:
Previous wood consumption say 200 kg/ month,
Biogas equivalent (Fig. 38): 0.18 m³/kg, Comparable biogas volume: 0.18 x 200= 36 m³,
Required daily biogas volume: 36/30= 1.20 m³.

If daily gas production is at least 1.20 m³, all fuel costs are saved. The excess is available free of charge; The excess can be counted on the credit side only if practical use is made of it.

The benefit of the fertilizer depends primarily on how well the farmer knows how to use it. Assuming that the digested slurry is immediately utilized - and properly applied - as fertilizer, each daily kg can be expected to yield roughly 0.5 kg extra nitrogen, as compared with fresh manure. If the slurry is first left to dry and/or improperly applied, the nitrogen yield will be considerably lower.

If parasitic diseases had previously been common, the improvement in hygiene also has economic benefits (reduced working time). The more fully the sludge is digested, the more pathogens are killed. High temperatures and long retention times are more hygienic.

The following are the principal organisms killed in biogas plants: Typhoid, paratyphoid, cholera and dysentery bacteria (in one or two weeks), hookworm and bilharzia (in three weeks).

Tapeworm and roundworm die completely only when the fermented slurry is dried in the sun.