Composting is not more than the optimization of the natural process by which the organic remains returns to the soil closing the cycle of matter. The knowledge of this process to obtain an humus or mulch allows that what it even takes months and years to happen in forests could be obtained in much less time.

From the current point of view of the biowastes management and its problematic we could considerer the composting like:

“An exothermic aerobic biological process, susceptible to be applied to any biodegradable solid organic remain and from which are obtained intermediate products like CO2, water vapour and other metabolites; and like final product an stabilized organic matter, free of pathogens and polluting elements, being beneficial its application to the soil.”

Imagine a particle of the biowaste to be treated formed by: an easily degradable organic matter, a fraction of organic matter with a partial resistance to be biologically degraded and a mineral fraction that will be unaltered by the biological process. The fraction of easily degradable organic matter is mainly composed of short chain carbohydrates, basically monosaccharide and oligosaccharides, simple lipids, proteins and some types of polysaccharides like the hemicelluloses. The most resistant fraction to the degradation will vary according to the origin of the organic matter, but it is mainly composed by celluloses and lignins as well as with complex lipids.

The presents micro organisms in the material begin their superficial degradative activity on the organic matter by the extra cellular enzyme liberation, so certain material surface moisture is needed. The aerobic micro fauna has a higher degradative rate, so according to their increases activity, the oxygen consumption will be increased, being able to be created an anaerobic environment if the physical conditions of the material are not the adequate ones or there is not a control of the process.
If the conditions are the adequate ones, the fraction of easily degradable organic matter will be first in being attacked by the micro organisms, obtaining as final metabolites of their decomposition CO2, H2O and very often NH3, that would be mainly liberated in gaseous form. This metabolic activity causes a fast mass self heating by the liberation of the energy contained in the chemical bonds of degraded molecules in form of heat. This increase of the temperature will be the principal marker of the different phases of the process, favouring or inhibiting different groups of micro biota according to the reached values at each moment of the process. The degradation of the more resistant fraction of organic matter in the first stages of the process will not be more than partial, although it will contribute to the heat generation and it will mainly occur a loss of CO2 and H2O, being very low, if there is it, the NH3 production.
If the adequate environment conditions are kept, it would be a moment in which the labile organic fraction will have been large extent degraded, so there will not be resources to maintain all the new generated microbial population as well as its biological activity rate. The nutrients and energy sources will have to be looked for in the fraction of more resistant organic matter, which brings about a generalized reduction of the biological activity and therefore of the temperature. The environmental conditions change and new micro organisms, more adapted to the decomposition of the more resistant organic matter, will be those that predominate in the stage of the process, being their lower degradative rate reason why also the oxygen demand descends.
Finally, if the conditions have been the appropriate ones for the micro fauna, it will pass to a maturation stage of the organic matter that has been degraded towards more stable forms. As we are referring us to a process that lasts between several weeks and some months following the waste, conditions, place, etc…, it would not be correct to speak about humification or mineralization of the organic matter, because they are processes that need much longer periods of time, but it would be possible to be considered that a beginning of them takes place. The organic matter already is in stable forms and if the conditions of the process have been fulfilled it would have to be free of pathogens. The final resulting product, denominated compost, it can be used as like fertilizer or organic amendment to the soil.
Depending on the composting system that has been used and the kind of composted waste, there will have been necessary throughout the process conditioning the waste with bulking, periodically turning or ventilation (either passive or forced form), a final phase of screening, separation of improper elements,…