The composting principle is based on the controlled aerobic decomposition of organic matter by micro-organisms.

Composting takes place in two main stages: a fermentation phase and a maturation phase.

>> the mesophilic phase
This is the phase during which composting begins. The non-specific mesophilic micro-organisms (mainly bacteria and fungi) present in organic waste multiply, and break down simple compounds (such as sugars, lipids and amino acids) and some polymers (proteins, nucleic acids and starches).
This aerobic fermentation generates heat, causing the temperature to rise to 30 - 40 °C in a few hours or days.

>> the thermophilic phase
As the temperature increases, the quantity of micro-organisms decreases and the proportion of fungi also decreases as the number of bacteria increases. When the temperature reaches 60 - 70°C, only thermophilic bacteria and actinomycetes survive.

By not exceeding 60°C, a wide variety of microbial species can thrive. Click here to find out more about the role that temperature plays in compost.

The thermophilic phase hygienises the compost (eliminating pathogens, parasites and seeds).

 

The intense activity of micro-organisms exhausts the oxygen in the composting mass, making it anaerobic. The temperature will then fall, because the metabolism of anaerobic micro-organisms generates little heat. Anaerobic micro-organisms produce disagreeable odours, such as methane, ammonia and hydrogen sulphide. It is therefore vital to compensate for the lack of oxygen by ensuring good aeration.

 

>> the cooling phase

When the easily degradable substrates have been broken down, the intensity of the reactions decreases and the temperature drops to under 50°C.

The environment may be colonised again by mesophilic micro-organisms that are different from those active in the first phase and capable of slowly breaking down cellulose, hemicellulose and lignin. Fungi play an important role in this phase.

It is during this phase that humus begins to form and nitrogen is incorporated into complex molecules.

During the maturation phase, the microbial biomass (bacteria and fungi) grows and the macrofauna, including earthworms, colonise the environment. Organic matter is stabilized in comparison with the raw materials which were composted and converted into humus.