Composting is a complex process involving a wide range of chemical substances which are capable of undergoing a wide range of chemical, biological and physical transformations under environmental conditions which are continually changing as the composting process proceeds. The objective of the commercial compost operator is to control these processes and the composition of the feed material to produce a quality compost in the most cost-effective and efficient manner, while ensuring that established guidelines and regulations are adhered to. While "know how" is an essential tool for all commercial compost operators, "know why" can be indispensable when determining what has gone wrong and determining what to do when external changes are imposed. This gaper presents information on some of the fundamentals of the chemical transformations taking place when an organic waste material is converted into a compost. The importance of these various chemical, physical and biological processes on the formation and release of odourous chemical species is presented. While this paper identifies some of the more odourous compounds that are found at a commercial composting operation and the importance of odour thresholds and vapour pressures, the main focus is on the importance of aeration. In this respect, the oxidation and reduction processes taking place with typical compost feed materials are examined and the importance of these reactions in the production of malodourous chemicals are presented. By understanding both the oxidation and reduction reactions of the carbon, nitrogen, and sulphurous compounds in a composting operation, a greater understanding of odour formation is obtained. This information will enable the commercial compost operator to better understand the importance of proper feed material control and aeration, if offensive odour emissions sire to be avoided.
