Oxygen supply is very important during the aerobic fermentation phase. Proper aeration prevents the onset of anaerobic fermentation and the production of odiferous compounds.

The moisture of the compost influences temperature and aeration, as well as the types and activity of micro-organisms. Optimal moisture conditions are around 60% (wet weight). If the level is below 40%, the activity of micro-organisms falls sharply and the organic matter does not compost quickly.
If the moisture content is too high, anaerobic conditions may set in.

During the composting process, the pH level will fluctuate according to the different reactions produced by the micro-organisms. For example, the pH will fall when the thermophilic phase begins, because the micro-organisms produce CO2 and lactic acids. It subsequently rises again to neutral or alkaline.
In practice, this factor is not a cause for concern because it stays within acceptable limits for the development of micro-organisms (a pH level of approximately 6 to 7.5).

Micro-organisms consume between 15 and 30 times more carbon than nitrogen during the composting process. They use carbon as a source of energy, while nitrogen is used to produce the protein they need to develop.

>> If the C/N ratio is too high, the composting time increases because the available nitrogen is used up quickly and the micro-organisms must find it elsewhere - in the decomposition of dead micro-organisms, for example.

>> If the C/N ratio is too low, the nitrogen is released as gas, causing an unpleasant odour of ammonia.

>> The C/N ratio decreases during the composting process because carbon is consumed more quickly and is lost as CO2.

Generally, it has often been shown that green, moist matter contains a high level of nitrogen, while brown, dry matter tend to contain more carbon.
Particle size affects the availability of carbon and nitrogen. Thus, larger elements give the windrow a more ventilated structure and thus improve oxygenation, but they provide less carbon for micro-organisms in the compost mass.
When particles are smaller, the decomposition rate increases, but there is also a risk of anaerobiosis due to poor air circulation.

The optimal C/N ration would be 35 for green waste and 25 for mixed urban waste.