Composting

Composting is the aerobic (=oxygen using) decomposition and stabilization of solid organic materials by aerobic organisms. This process can be done under controlled conditions (=optimal conditions and environmentally safe) but also occurs in almost every environment where solid organic materials are supplied with oxygen, moist and the right temperature. The created compost should be a stable and hygienic substance witch is rich in humus en looks like soil.

Purpose

The purpose of the composting process is the removal of the biodegradable part of the organic materials, reducing volume, mass, particle size and humidity of the original waste. This process transforms the waste into a valuable soil conditioner that can be used in gardening and agricultural goals. The biodegradable part consists of saccharides (glucose, fructose, lactose, sucrose, starch), proteins and most fats. In the case that there is no oxygen available, the composting process is impossible and an anaerobic digestion takes place. This causes production of several gases (methane, small amounts of hydrogen sulfide and hydrogen gas...), resulting in bad smells.

The conversion process

The active composting process takes place at the surface of the composting particles. Every particle consists of an anaerobic inner core, a partially aerobic layer below the particle surface, an outer aerobic surface layer and an aerobic liquid film surrounding the particle. The microbial community lives in the surrounding liquid layer, so while the composting proceeds, the particles shrink till the original raw materials are discernible. To stay alive, reproduce and regulate itself, every living being needs energy sources (light or oxido-reduction-reactions) and nutrient sources (carbon, nitrogen, minerals, water...). In this case the energy (in the form of electrons from oxido-reduction-reactions) is coming from the oxidation of organic matter with oxygen (chemo-organotrophic), and the carbon source is the organic matter (heterotrophic), so this organisms are called hetero-chemo-organotrophics. The micro-organisms produce enzymes to do the job, these are proteins acting as a catalyst in the oxydation of the organic waste and in producing microbial biomass. This is the overall conversion during the composting:

C₆H₁₂O₆ + 6O₂ => 6CO₂ + 6H₂O + heat + new microbial biomass

Since the release of heat is directly related to the microbial activity, temperature can be used as an important process indicator. During the initial days, the readily degradable compounds are metabolized and the temperature rises fast, depending on the conditions the temperature can rise well above 60°C. This high temperatures causes the weed seeds and pathogens to be killed, but also the desirable composting micro-organisms begin to die or at least slow down.