Theoretically, the end products of aerobic decomposition are carbon dioxide and water and a small quantity of salts. In reality, there is a significant fraction of all organic matter that is refractory to oxidation. This is the organic residual found in topsoil and stable forms of humus. Since the normal aerobic process is never continued for a sufficient period to accomplish complete oxidation and because of the presence of the refractory component, there is always a residual called sludge that must be disposed. The stability of this sludge depends on the conditioning process employed