Composting is the biological decomposition of the organic matter under controlled aerobic conditions. The basic composting process is shown in figure 10. The main factors affecting the aerobic decomposition of organic matter by microorganisms are oxygen, moisture and nutrients. Carbon and nitrogen are essential for microbial growth and activity. Carbon is the principal source of energy, and nitrogen is needed for cell synthesis. Other important factors that can slow down the composting process are temperature and pH. Temperature is the result of the microbial activity as well as the influence of the surrounding temperature, mass of the composting materials and heat loss.

Figure 10: The composting process (Source: Epstein, 1997)

In the process of composting, microorganisms break down organic matter and produce carbon dioxide, water, heat, and compost. In addition, ammonia and other volatile compounds are emitted to the atmosphere. However, in comparison to CO₂ and H₂O these represent a very small amount.

All organic material will eventually decompose. The rate at which it decomposes depends on the physical as well as chemical factors. The optimal condition for composting is shown in table 2.

Tabelle 2: Overview of optimal condition for composting