Home composting bins
Practical Action
The essential elements required by the composting microorganisms are Carbon, Nitrogen
(represent by C: N ratio), Oxygen and Moisture. If any of these elements are lacking, or if they are
not provided in the proper proportion, the microorganisms will not flourish and will not provide
adequate heat. A composting process that operates at optimum performance will convert organic
matter into stable compost that is odour and pathogen free, and a poor breeding substrate for
flies. In addition, it will significantly reduce the volume and weight of organic waste as the
composting process converts much of the biodegradable component to gaseous carbon dioxide
and water. Composting is relatively simple to manage and can be carried out on a wide range of
scales in almost any indoor or outdoor environment and in almost any geographic location. It has
the potential to manage most of the organic material in the waste stream including kitchen waste,
leaves and yard wastes, farm waste, animal manure, paper products, sewage sludge etc.
So composting has been a vital component in many integrated waste management plans that is
developing worldwide.
Optimum conditions for composting
Oxygen / aeration
If there is insufficient oxygen, a different set of anaerobic microorganism dominates the
degradation process and produce odorous intermediate products such as methane, organic acids
and hydrogen sulphide. A constant supply of oxygen will give the aerobic microorganisms an
advantage over the anaerobic micro-organisms. Approximately a 5% minimum concentration of
oxygen is required within the pore spaces in the media.
Aeration is the process of providing oxygen into the composting material. This will also provide a
platform to remove water vapour, gases and excess heat trapped within the material. Aeration is
common practice with high rate large scale composting facilities.
Moisture content
Moisture supports the metabolic processes of the micro-organisms. Water is the medium for
chemical reactions. Biological activity ceases below 15% moisture content and in theory activity
is optimal when materials are saturated. Generally moisture content of between 40% and 65%
should be maintained. At moisture content of below 40%, micro-organism activity will continue
but at a slower rate and above 65% water will displace much of the air in the pore spaces of the
composting material. This will limit the movement of air and lead to anaerobic conditions.
Temperature
Composting takes place within two temperature ranges known as mesophilic (100 – 400 C) and
thermophilic (over 420C). It is generally accepted that maintaining temperatures between 430C
and 650C allows for effective composting. The thermophilic temperatures are favoured in the
composting materials, because they destroy more pathogens, weed seeds and fly larvae. In some
composting processes, Temperatures can continue to rise above 700C due to insulation effects
and on-going microbial activity. At these temperatures many micro-organisms die or become
dormant and the process effectively stops until the micro-organisms can recover.
Nutrients and the Carbon Nitrogen (C:N) ratio
The microbes involved in composting use carbon for energy and nitrogen for proteinsynthesis. The
proportion of these two elements required by the microbes averages about 30 parts carbon to 1
part nitrogen. Accordingly, the ideal ratio of Carbon to Nitrogen (C: N) is 30 to 1 (measured on a
dry weight basis). This ratio governs the speed at which the microbes decompose organic waste.
Most organic materials do not have this ratio and, to accelerate the composting process, it may be
necessary to balance the numbers by mixing different substrates. (eg.. kitchen waste is rich in
nitrogen while garden waste is in poor)
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