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Solid Waste Recycling Composting |
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Soild
Waste
Recycling
Composting- The utilisation of human and animal solid wastes is of great importance both from the public health and agricultural point of view. Composting is the conversion of the organic fraction of solid waste materials through microbiological processes leading to the production of a humus-like end product that is used primarily as a soil conditioner.
The art of composting is very old but in recent years progress has been made in understanding this biological process. These studies have enabled the production of a finished compost which is both valuable and safe from the view point of public health.
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There are basically two methods of composting; in the first method, compostable material is placed in pits or in stacks on the ground and the material is turned over at intervals to allow adequate aeration and mixing. This method has proved most satisfactory.
In the second method, the waste is filled in pits or trenches in layers and allowed to decompose without being turned over during the composting period of 4.6 months.
In big cities where the amount of waste is large, highly mechanized composting plants which handle large quantities of heterogenous wastes containing both compost able and non compostable materials exist. In these plants, garbage is first sorted out, the organics are then crushed, sprayed with sewage and heaped to allow decomposition.
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The quality, composition and nature of the wastes available for composting vary widely with the season and region. In villages, the type of waste depends on the type of agriculture, size of the farm, the animals used and the economic condition of the area. In cities, the quantity and quality of waste is determined by the waste products of industry, street sweepings, ash and garbage.
There are several factors which affect the composting process since these affect the microbial activity during decomposition. These include the quality of waste, carbon to nitrogen ratio (C/N ratio), moisture content, pH, temperature, aeration, and climatic conditions.
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Segregation of waste into compost able and non-compostable materials is often desirable specially with town wastes which contain metal cans, glass, china pieces, concrete, plastics etc. Metal cans and other ferrous metals can be separated by magnetic separators, while other objects can be mechanically removed by screening or by hand picking.
Shredding or grinding of organic material for composting is beneficial since shredding makes the material more accessible to microbial attack through increase in the surface area. Decomposition of such shredded material occurs at a faster rate.
The decomposition of organic matter is brought about by microorganisms which utilize carbon and nitrogen in the waste for synthesizing cellular constituents. The C/N ratio is, therefore, an important factor determining the rate and extent of decomposition. When excess of carbon is available (C/N ratio is high), the rate of decomposition is slow. Generally, a C/N ratio of 30-40 is considered as optimum.
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During
decomposition,
a
part
of
the
carbon
is
assimilated
and
a
part
is
oxidized
to
carbon
dioxide.
As
a
consequence,
there
is
a
decrease
in
the
C/N
ratio
and
this
decrease
in
the
C/N
ratio
is
an
useful
index
of
composting.
Moisture
is
an
important
factor
that
determines
successful
composting.
For
satisfactory
aerobic
composting
a
moisture
content
of
40-60
percent
is
desirable.
If
the
moisture
content
is
40
percent
or
lower,
water
can
be
sprayed
during
turning.
In
anaerobic
composting
(as
in
the
pit
method),
the
maintenance
of
moisture
content
is
not
necessary
since
there
is
not
much
loss
from
the
initial
moisture
level.
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During decomposition, due to microbial activity, a large amount of heat is generated. Normally, a temperature of 45-50°C is reached in the first 24 hours of composting and temperatures of 60-70°C are obtained in 2-5 days.
The decline in temperature is gradual and the high temperature does not persist if the material becomes anaerobic. The creation of aerobic conditions is, therefore, essential to maintain high temperature during the early course of decomposition.
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