Aerobic decomposition of organic wastes I. Stoichiometric calculation of air change

Abstract

The objectives for aerobic composting are to convert organic wastes into a valuable compost, with a low content of heavy metals, suitable for use in agriculture, horticulture and forestry. In order to avoid competition between plants and microbes for oxygen and nitrate in the soil, the most readily decomposable compounds ought to be decomposed before mixing them into a soil. On the other hand, structuralizing compounds are valuable both during decomposition and for protection of the soil surface. A well accompliced decomposition removing carbon dioxide and excess water vapour demands a good aeration. Accumulation of carbon dioxide involves a decreased decomposition of water soluble fatty acids formed by decomposition of sugars. Precipitation of water vapour decreases aeration, and carbon dioxide dissolves in the water with a risk of getting anaerobic conditions. A decomposition of protein and amino acids forms ammonia and hydrogen carbonate. Then the pH increases and ammonia volatilization starts. Composting in a reactor renders collection of ammonia possible, which is a benefit to the environment. Removal of carbon dioxide and excess water vapour is often a speed regulating factor. Calculations show that by composting one tonne of wastes with 25% of dry matter, more than 100 m 3 of carbon dioxide and nearly 600 m 3 of water vapour had to be removed. The necessary amount of air to collect and remove the water vapour is dependent on the temperature. At 70°C, it is 1900 m 3; at lower temperatures, it increases exponentially. In a compost heap, the outer layers become too dry and the deeper layers too wet. In order to get better composting results, the heap has to be digged over. Even then, it is difficult or maybe impossible to obtain optimal conditions. By decomposition in a reactor it is possible to increase the speed and to obtain a more uniform compost.