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In-Vessel Composting

The Composting Process

Decay of organic materials under normal environmental conditions is sometimes slow, but eventually most organics ultimately break down to humus that helps to build topsoil. In much the same way that livestock producers improve animal growth rates by creating a favorable environment (adequate food, water, and shelter), organic decomposition rates can be accelerated by providing favorable conditions for bacterial growth and reproduction. The process of accelerating biological decomposition by controlling moisture content, temperature, oxygen, and carbon to nitrogen ratios (C:N) is called composting.

Water (moisture content)

Many factors affect the composting process, but moisture content is often the most crucial. Wastes that are too dry (less than 40% moisture content) decay slowly because they lack sufficient water for the survival of the bacteria. At moisture levels above 60%, small pore spaces that allow oxygen to move into the compost become filled with water, causing the process to become anaerobic.

Temperature

Heat is an important byproduct of bacterial activity. Internal temperatures within properly sized composting operations often reach 60-75°C. This temperature range stimulates rapid growth of thermophilic (heat-loving) bacteria that promote decay.

As an added benefit, exposure to high temperatures helps to kill disease-causing microorganisms (pathogens), thereby reducing the risks of disease transmission from infected materials.

Ventilation (oxygen availability)

Although organic decay readily occurs under anaerobic (no oxygen present) conditions, the term "composting" typically refers to aerobic (oxygen-using) decay processes. In most cases, aerobic microbial activity is considered more desirable than anaerobic decay because the by-products of aerobic processes (water, carbon dioxide, and heat) are not offensive and produce a heat-treated product that is low in viable pathogens or weed seeds. Anaerobic decomposition, by contrast, produces little heat and generates highly odorous products such as hydrogen sulphide and organic acids.

To keep a composting process sufficiently aerobic, oxygen concentrations of at least 5% within the compost pile are typically recommended. Since highly active aerobic micro-organisms use oxygen rapidly, maintaining aerobic conditions throughout a compost pile — particularly at the core — requires constant monitoring, plus fan-powered aeration or frequent mechanical turning.

Nutrition (carbon:nitrogen ratio)

Like all living things, the bacteria and fungi that decompose organic materials need carbon and nitrogen to grow and reproduce. Bacteria grow quickly, and breakdown organics most rapidly when their food source contains about 25 times as much carbon as nitrogen. De-composition will occur (although more slowly) when carbon-to-nitrogen (C:N) ratios are as low as 10:1, or as high as 50:1.

The C:N availability directly affects the speed of the composting process. Usually when a C:N ratio is high, it means there is not enough nitrogen. Too little nitrogen slows the process because the organisms need the nitrogen. Too much nitrogen, on the other hand, doesn't really slow the process, but there is an excess of nitrogen waiting for the carbon to become available. In the meantime, the excess nitrogen forms ammonia. This can cause odours depending upon how much ammonia is being volatilized. The loss of nitrogen also is usually a negative in regard to the quality of the compost.

Equally important to the amount of carbon in the mix is how much carbon is available — which relates directly to the degradability of the material. Carbon that is locked up in organic matter is going to take a long time to decompose (degrade), which means it won't be available to the organisms. For example, the carbon in woody materials is locked up within the chemical structure of the wood and is particularly tough to release. Corn husks, on the other hand, decompose relatively quickly, making the carbon available to the organisms.

Reducing the particle size can speed up carbon release to a certain degree, e.g. using wood shavings instead of wood chips, or wood chips instead of chunks. But even in a chipped form, wood passes through the composting process relatively unchanged. Some break down, of course, occurs, but the slow decomposition of woody feedstocks has to be recognized when trying to balance the C:N ratio.

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