Recycling of organic waste
Practical Action
In composting, provided the right conditions are present, the natural process of decay is speeded up.
This involves controlling the composting environment and obtaining the following conditions:
The correct ratio of carbon to nitrogen. The correct ratio is in the range of 25 to 30 parts
carbon to 1 part nitrogen (25:1 to 30:1). This is because the bacteria which carry out the
composting process digest carbon twenty five to thirty times faster than they digest
nitrogen. This is often seen as being a roughly equal amounts of "greens" and "browns".
Carbon to nitrogen ratio will be referred to hereafter as the C:N ratio. The C:N ratio can be
adjusted by mixing together organic materials with suitable contents.
The correct amount of water. Plants have a liquid rather than a solid diet and therefore the
compost pile should be kept moist at all times. On the other hand, a wet compost pile will
produce only a soggy, smelly mess.
Sufficient oxygen. A compost pile should be turned often to allow all parts of the pile to
receive oxygen.
The optimum pH level of the compost is between 5.5 and 8.
In these conditions, bacteria and fungi feed and multiply, giving off a great deal of heat. In well
managed heaps, this temperature can reach as high as 60 C, which is sufficient to kill weed seeds
and organisms that cause disease in plants and animals. While the temperature remains high,
invertebrates are not present in compost heaps, but when the temperature drops, the invertebrates
enter the heap from the surrounding soil and complete the process of decomposition.
Forms of decomposition
Anaerobic. In anaerobic decomposition, the breakdown of the organic material is caused by
bacteria and fungi that thrive in low or no-oxygen conditions. It is the type of
decomposition that takes place in closed containers. This type of system is more complex
and difficult to control and requires complex equipment for larger scale composting (see
Box 4).
Aerobic. In aerobic decomposition, bacteria and fungi which thrive in high oxygen
conditions are responsible for the decomposition. This form of decomposition occurs in
open heaps and containers that allow air to enter. With open heaps and more ventilated
containers, compost can be formed in a matter of a few months, and even faster if the
organic material is turned regularly. In heaps or bins where aerobic decomposition is
occurring, there should be no unpleasant odours.
Some methods of composting
Composting systems can be opened or closed, that is the organic matter will either be placed in
open piles or rows or in a closed container or reactor. The open system is rarely used in low-income
countries due to its technical complexity, so we look at some of the open systems in use.
Backyard composting at the household level is a simple technique. It requires only suitable
organic waste, space to construct the heap and time to carry out the necessary work. The
waste can be placed in a pit (say 2m x 2m x 1m deep) and left to decompose for 2 – 3
months. Alternatively, the waste can be piled up within an enclosure of 4 poles and
surrounded by boards or chicken wire and left for a similar period. This produces a rich
compost which can be used as a fertiliser on fields or gardens.
Neighbourhood composting. A commonly used technique for neighbourhood composting is
the use of windrows. Here waste is simply laid out in long rows and turned occasionally.
Another method is the rotating bin method which uses a series of closed, aerated bins (see
Lardinois3).
Co-composting is technique whereby organic food waste is mixed with human or animal
excreta and composted Similar techniques are used to those described above. See Box 3
for an example of co-composting. There are many examples of successful co-composting
systems throughout the world (see Lardinois3).
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