How to Generate Power from Garbage
(Page 9 of 12)
May/June 1970
By the Mother Earth News editors
This method may be improved by heating the digester's liquid through the heat generated in an outside manure pile and recirculating it by means of pipes installed as shown in Figure 10. The heated liquid enters the tank at the top, the cold layers at the same time being drawn into the lower branches of the pipes. In this way, constant recirculation is ensured.
RELATED CONTENT
In large installations some of the gas can be used to heat water in a boiler; the hot water is then circulated through heat-exchange coils in the digester.
MATERIALS AND INITIAL LOADING
Considerable care should be exercised in putting a plant into operation. Until the conditions have become satisfactory for the growth of large numbers of the organisms necessary for good anaerobic decomposition and methane production, there is a danger of acid formation, which will retard digestion and inhibit gas production. In first starting a digester, material which has been partly decomposed by aerobic fermentation for a period of 1-2 weeks should be introduced. This initial aerobic fermentation will eliminate some of the components which may cause production of acids. After loading, the material should be allowed to ferment aerobically for a further period of about three days to develop a high temperature. If available, it is best to add some digested humus and liquid from another plant which has been in operation and producing gas for some time. The remaining volume of the digester is then filled with water.
After the tank has been sealed to provide anaerobic conditions, the material will undergo a maturing period of several days before gas production starts. If the initial material is not satisfactorily decomposed and seeding from another tank is not possible, acid conditions can develop and it may be a month or more before the conditions become favorable for gas production. The addition of lime or of an alkali or ammonium phosphate will help to correct an acid condition and facilitate earlier gas production. Horse and cow manures are more alkaline, and usually less difficulty will be experienced if the digester is started with these materials after partial aerobic decomposition.
The desirable pH is in the range of 6.8 to 7.5, with an optimum value above 7.0. If by some rare circumstance the initial material is highly alkaline (i.e., pH up to 9.5), the pH will soon be lowered by the CO 2 produced by decomposition. Therefore, the need for correction of the initial pH by the addition of acid is extremely unusual. In no case should sulfuric acid be added since it would contribute to the production of hydrogen sulfide.
The conditions for good decomposition and gas production will be established more rapidly if the temperature can be maintained near 35° (95° F). While it is not necessary to have this high temperature for developing digestion, it is most desirable to maintain a temperature of above 20°C (68° F) during the initial stages.
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