How to Generate Power from Garbage
(Page 2 of 12)
May/June 1970
By the Mother Earth News editors
SINGLE OR MULTIPLE FAMILY INSTALLATIONS
RELATED CONTENT
Either single or multiple family installations can be built, the choice depending on whether the single family has sufficient manure and other wastes to operate a unit. A minimum single family installation should normally include a digester tank of about 4-5 m 3 capacity and a gasholder of at least 2 m 3 capacity. Two or more digesters are desirable so that there will not be an interruption of gas production and so that one tank may be loaded while the other is digesting. A single gas-holder can serve more than one digester unit.
If two or more neighboring families have only one farm animal each, it may be advantageous to combine their wastes in one digester installation from which the gas can be distributed to each dwelling. The plant can be located to minimize transportation of wastes and to provide latrine facilities for the co-operating families. This arrangement would permit the use of more than one digestion compartment with a resulting, more uniform gas production.
The cost of the mutual installation, per ton of manure decomposed, will be less for multiple family plants than for single plants. However, a multiple family plant serving more than two families may require such excessive piping of gas and transportation of organic matter as to make individual plants generally more economical.
AVAILABLE WASTES
Horses and cows each produce from 10 to 16 metric tons of manure per year, depending upon stabling conditions and the amounts of organic litter used for bedding. To this may be added garbage, waste straw, cane stalks, or any other organic material. Where night-soil is used as a fertilizer, it should be digested with other organic wastes before application to the land, in order to prevent the spread of faecal-borne diseases.
While human excretement does not add much weight to the digester (30-60 pounds per person per year), it does provide appreciable quantities of nitrogen and phosphorus. These elements are necessary for biological digestion and methane production from cellulose and other materials with a high carbon content. The sanitary treatment of night-soil for the reclamation of nutrients is most important. It should be further mentioned that when night-soil and animal manure containing large quantities of nitrogen and phosphorus are digested, large amounts of waste materials such as straw, cane stalks, and sawdust, can be added to increase gas production. When night-soil is digested with other wastes, a digestion period of three months or more is desirable in order to ensure adequate destruction of pathogenic organisms and parasites.
Stable manure and mixed organic refuse weighs from less than one-half to as much as one metric ton per cubic meter, depending upon the amount of moisture and the degree of compaction.
GAS PRODUCTION
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