How to Generate Power from Garbage
(Page 3 of 12)
May/June 1970
By the Mother Earth News editors
In practice, about 50% of the carbon theoretically available for gas production is converted into gas. A metric ton of waste will normally yield about 50-70 m 3 of gas per digestion cycle, depending upon the proportion of organic matter and the carbon content of the waste.
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The digestion cycle will be shorter at high temperatures than at low temperatures, and the daily yield per ton of material will be greater. Considerably greater digestercapacity is required to produce a fixed amount of gas at a temperature of about 20°C. than at a temperature of 30° 35°C. The following estimates are for gas production per ton of manure for different digestion periods at different temperatures:
Similar data on gas production and digestion time for sewage sludge at different temperatures may be found in books on sewage treatment. The amount of gas produced and the rate of digestion at different temperatures are the important factors in determining the size of digestion tanks to be used.
NUMBER AND SIZE OF DIGESTERS
The number and size of the individual digestion tanks of a plant will vary with the amount of decomposable material available and the amount of gas desired. A minimum of two, and preferably three, tanks is recommended to maintain more uniform gas production. A digestion compartment of 8 m 3 capacity (2m x 2m x 2m) has been found to be an efficient size.
In small installations, where sufficient materials are not available to supply two digesters of this size, smaller digesters could be used, but the saving in cost of construction would be small. For larger installations, up to six compartments of about 8-12 m 3 each may be used. It is doubtful whether more than six or, possibly, eight compartments would be economical, and for very large plants the size - rather than number - of individual compartments should be increased.
LOCATION OF DIGESTERS
The digesters should be located near the source of manure and waste material to avoid excessive handling and transportation. Also, it is desirable to place them so as to minimize the amount of gas piping required. In Europe I have seen composters built into homes and barns. Usually this was done when livestock was housed beneath the living quarters of the farm family.
It is important to orient a digester so that it will receive the maximum amount of sunlight to help maintain higher digestion temperatures. Greater heating from the sun can be obtained when the tank is placed on top of the ground, but this involved lifting the materials higher when loading.
If tanks are used only for manure and litter, it is common practice to place them partly below and partly above the ground. This arrangement also permits the placement of compost around the tank for heating. If a latrine is incorporated in the digester, the top might be as much as 0.7 m above the ground without necessitating too many steps.
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