Composting means the autothermic (self heating) and thermophilic biological decomposition and stabilisation of biodegradable waste under controlled conditions that result in a stable sanitised material that can be applied to land for the benefit of agriculture, horticulture or ecological improvement.
Composting can range from simple outdoor windrows (large heaps) to more mechanised and, at least partially, computer controlled systems that enclose all or some phases of composting. In-vessel composting (IVC) is a commonly used term for composting systems that enclose at least one phase of composting, which is usually the initial, highest temperature, intensively monitored phase of fast-decomposition composting. ‘housed windrows’ inside a building and ‘enclosed’ composting are terms for other composting systems operated.
The most common form of material used for composting is biodegradable waste arising from either domestic households or from commercial and industrial sources. The former may be either garden wastes typically lignin based in origin such as, grass cuttings, branches, discarded plants and this may also include food waste where it is commingled with the 'green waste'. In some instances food waste is collected separately and used as a feedstock for AD plants (see AD technology section for further information)
The initial treatment of the feedstock once arriving at a composting facility is shredding, blending and moisture addition. Shredding reduces the particle size and increases the surface area to facilitate biodegradation.
Blending of materials is essential to ensure that the optimal Carbon to Nitrogen ratio (C:N ratio) is achieved by mixing a range of feedstock’s together such as grass and which is very high in nitrogen to more woody lower nitrogen material such as woodchip. Feedstock blending is crucial in order to promote optimal conditions for biodegradation and reducing any potential odour emission issues at this stage.
Moisture addition likewise is essential in ensuring that the correct moisture of 50-60% is achieved through the addition of water or recirculated production leachate. Failure to achieve the correct moisture early in the process will have an adverse effect on the process at subsequent stages.
Composting is a process of controlled biological decomposition of biodegradable materials under managed, aerobic conditions in which thermophilic temperatures (between 40 to 80 degrees Celsius) develop as a result of microbes generating heat as they decompose the material. These temperatures reduce pathogen levels and sanitise the substrate.
The pasteurisation process may typically last for 2-3 weeks, although in reality compost will maintain temperatures in excess of 60 degrees Celsius for many more weeks than this.
This is the name given to the stage of the process whereby biological conditions within the composting mass give rise to stable compost. Compost is said to be stable when microbial respiration will not significantly resurge under altered conditions e.g. changes of moisture, oxygen levels or temperature. The rate of carbon dioxide and heat release decreases with increased stability.
Screening and blending
At the stage that the compost is deemed to be sufficiently stabilised to meet the requirements of the end user (this will vary from agriculture use or for bagging for retail outlets), the compost will be screened to remove physical contaminants such as plastic and stones to produce a uniform particle size of output. This output will vary from 0-10mm material for bagging and the retail trade through to 0-40mm for use as an agricultural soil amendment.
The brown, soil-like compost produced is a rich source of organic matter and provides useful amounts of the key nutrients and trace elements needed by plants. It is also a good source of beneficial, naturally occurring microbes.