The Owner Built Home & Homestead
(Page 21 of 23)
The primary key to good compost-building is to establish
correct proportioning and blending of the raw materials. In
essence the problem is one of determining carbon and
nitrogen ratios (C/N), along with the correct amount of
moisture and aeration. Cellulose organic matter like straw
or sawdust is rich in carbon, and excreta are rich in
nitrogen. It has been found desirable to keep the C/N ratio
above 30 to 1 when excreta is used; excreta should equal
10% to 25% of the total weight. Urine contains considerable
larger amounts of nitrogen than do feces. Raw garbage has a
C/N ratio of 25 to 1; sawdust, 511 to 1; farmyard manure;
14 to 1.
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Aeration helps maintain the required high temperatures in
an aerobic composting condition. Turning the compost pile
at frequent intervals has been a traditional method of
achieving aeration. Yet, turning adversely effects nitrogen
conservation. Ammonia readily escapes to the atmosphere
when the material is disturbed and exposed.
As one becomes more familiar with the whole process of
aerobic composting, the design of an appropriate facility
falls into place. The facility design can be likened to
that of a furnace: Material (fuel) is placed in a
combustion chamber; a vent stack (chimney) is provided to
carry off gases that are produced from the decomposition
(methane, carbon-dioxide, ammonia); and, finally, a storage
compartment must be supplied for the end product (humus, or
ash, in the case of a furnace).
The size of the facility depends, of course, upon the
number of people using it. About 2 pounds of excreta per
person per day, or 1 1/2 cubic feet per person per year, is
used as a design figure.
If the initial C/N ratio is 30 to 1, it takes about 10 days
composting time; a 78 to 1 ratio takes 20 days. Using the 1
to 5 volume ratio of excreta to refuse, and figuring that a
family of five will produce about 1 cubic yard of partly
digested excreta in four years, a compartment size of 1
cubic yard would fill in about 9 months. It must be
remembered, however, that decomposition into gases and
soluble materials reduces volume and mass as much as 80%.
The Indian Council of Agricultural Research at Bangalore
developed extensive composting programs based on the
compost privy principle.
They built an experimental "double vault" latrine. During
the time that one compartment was being used, compost
material in the adjacent compartment was ripening. A period
of 6 months lapsed between clean-outs, This two-compartment
system appears to be superior to others. However by
incorporating a simple damper mechanism, only one squatting
plate need be provided.
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