Composting is the controlled biological decomposition and pasteurization of organic materials under aerobic conditions— it involves the action of mesophilic microorganisms followed by thermophilic microorganisms that thrive under increased (more than 50 °C) temperature conditions and if correctly managed, can destroy disease-causing organisms, even weed seeds. Biodegradable organic matter is mineralized while carbon dioxide (CO2), water and heat are liberated, and the residual organic components are stabilized mainly to humic acids. There are various ways of composting—aerobic static piles (non-interventionary), aerobic windrows (interventionary), using worms (vermicomposting, which is in fact a different type of process that does not involve the thermophilic stages), etc. The choice of the method is usually based on the objective. There are many different technologies named after the origin of the process among windrows and static systems, such as Rutgers Strategy, Indore process or Camby process.
Composting is a way of achieving the natural microbial humification process that converts the material into another form—the main conversion taking place being that of carbon and nitrogen compounds. The process of composting, which is an aerobic process, creates heat and converts solid wastes into compost that can be used as a fertilizer. More scientifically, in bioremediation technologies (sometimes known as microbiological engineering) the natural ability of certain organisms to degrade organic chemicals is used to contain contamination. The desired end results of the active bioremediation processes are carbon dioxide, water and cell biomass; and the process is termed composting. The process of composting is mostly biological in nature, achieved by the actions of different organisms. Composting involves the interactions of different organisms at various levels of the food chain in the different nutrient cycles in the substrate being degraded. The various organisms involved in the process consume the waste materials and other organisms that are lower in the food chain at different rates, and a basic understanding of their biology is necessary to design a composting system and its mode of operation.
A compostable material is a material which undergoes physical, chemical, thermal and/or biological degradation in a composting facility such that it enters into and is physically indistinguishable from the finished compost and which ultimately mineralizes (biodegrades to carbon dioxide, water and biomass as new microorganisms) at a rate like that of known compostable materials in solid waste such as paper and yard waste. A compost compatible material is a material that disintegrates and becomes indistinguishable from the final compost, and is either biodegradable or inert in the environment. A removable material is a material that can be removed (not to be composted) by existing technologies in municipal solid waste (MSW) composting (such as plastic, stone, glass, etc.)
For the compost process to be successful, it requires a "recipe" be followed to sustain the desired biological activity. The amount or ratio of carbon and nitrogen in the original material is highly important as a good balance is required for the process to proceed. Oxygen is a major factor controlling the composting process. For solid wastes, aeration and adequate supply of oxygen is important; for liquid wastes, dissolved oxygen level is important. Composting, being an aerobic microbial process, demands a continuous and good supply of oxygen for the microbes and other organisms such as worms, beetles and nematodes. There are other important factors that can make or break the process such as temperature, moisture content, removal of heat and moisture, material composition, etc.
Generally, there are four indicators considered for process performance and product quality, namely, volatile solids, respiration rate, germination tests and pathogen indicators. Composting as an engineered process is not set up to fully decompose all degradable organic materials, but to degrade putrescibles that would otherwise cause odors by anaerobic degradation.
Composting has been used as one form of waste recycling—generally in the agricultural sector and for the biodegradable part of municipal solid waste (MSW). Composting may also be used to convert industrial waste, particularly in industries with substantial biological waste materials such as food and paper processing. Composting is perhaps best-known as a household or on-farm activity where food and yard or farm wastes (such as bedding straw from barns or field stubble) are transformed into benign organic material through the composting process.
See composting tips.