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 which is rich in humus and 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 for gardening and agricultural purposes. 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. Normally the temperature increases fast in the initial days up to 45-60°C, and stay in this interval for several weeks, this is the active composting phase. When the readily available compounds become depleted, the metabolization speed slows down and the temperature decreases slowly till ambient air temperature.

After the active composting fase, there is a maturation period. During this period, the composting process goes on, but at a much slower rate. Due this slower rate, the oxygen consumption decreases, and the temperatures stay lower. While this process continues, the amount of humus increase, and nutrients (like nitrogen) are stored within stable organic compounds. This reduces the immediate availability of nutrients to the plants and allows them to be released at a more gradual rate. After the active composting phase, most nitrogen is readily available in the form of ammonium (NH₄⁺). Concentrated amounts of ammonium can cause damage to several plants. Some of the processes occur only at low temperatures or in well-decomposed organic matter, like this conversion (oxydation) of ammonium to nitrate (NO₃^-), which is less harmful to plants. Because the composting process does not stop at a particular point, the process can go on till only "energy-exhausted" organics and inorganics remain. However, compost becomes relatively stable and useful long before this point. An immature compost continues to consume oxygen after application to the field and thereby reduces the availability of oxygen in the soil to the roots of plants, and diminishing their growth. An immature compost can also contain high levels of organic acids (low pH) or have a high C/N ratio (carbon/nitrogen). This can cause competition between the plants that need the nitrogen for growth and the nitrogen needed for the stabilization of the organic matter. Immature compost can thus cause damage to crops and plants. Compost is judged to be stable by characteristics such as C/N ratio, oxygen demand, temperature and odor.

Recommended conditions for rapid composting

Condition	Reasonable range	Preferred range
C/N ratio	20/1 - 40/1	25/1 - 30/1
Moisture content	40-60%	50-60%
Oxygen concentration	>5%	>>5%
Particle size (diameter in cm)	0.3-5	varies
pH	5.5-9.0	6.5-8.0
Temperature (°C)	40-65	50-60

Factors affecting the composting process

Oxygen and aeration

The major oxygen supply for the microbial population in the compost is the air present in the pore space of the composting mass (the interstices). The amount of pore space depends on the size, size variability, form and hardness of the particles, and the amount of water present.

There are 2 important processes to provide the micro-organisms with oxygen:

Bulk movement: this is the movement of air in the pores due to mechanical forces. This movement can have different causes: density difference between the warm (lighter) air in the pile and the cold (heavier) surrounding air so that the warm air will lift and cold air will move into the pile at the bottom; wind that strikes a composting pile moves air in the pores; an electric blower or turning the pile (forced aeration). The aeration not only provides fresh air to the composting mass, but also removes heat, water vapor and other gases trapped in the composting mass. When the aeration is too excessive, the pile can cool down and eventually dry out when there is more heat and water vapor removed than there is produced by the composting process.

Diffusive movement: diffusion is the movement of atoms or molecules (for example: O₂) from higher concentration to lower concentration: the driving force is the gradient (concentration difference) between 2 places.

In composting piles there are 3 oxygen gradients:

1. between the surrounding air and the air in the pores of the pile
2. between the air in the pores of the pile and the liquid layer around the composting particles
3. between the liquid layer around the composting particles and the micro-organisms at the interception between the the liquid film and the particle.

Because the oxygen concentration is decreased after each step, only a small concentration of oxygen is available for the micro-organisms. When the aeration is not sufficient, oxygen can be a restricting compound in the composting process.

Moisture

Water provides the medium for chemical reactions, nutrient transport and allows the micro-organisms to move. Water is thus necessary to support the metabolic process of the micro-organisms: a moisture content under 40% inhibit the composting process. However, a moisture content above 65% displace much air in the pore space and leads to diminished oxygen supply. During the composting process, a considerable amount of water is produced by the organisms as a result of the degradation of organic matter. Besides the water production, water is also lost due to the aeration and the heat production. Generally the moisture content tends to decrease as the composting proceeds. As a rule of thumb, the materials are too wet if water can be squeezed out a handful and too dry if the handful does not feel moist to the touch.

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Temperature

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Nutrients

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pH

The preferred range for composting is 6.5-8 because most enzymes work optimal near neutral pH (pH = 7), although the composting is relatively insensitive to the pH because of the broad spectrum of microorganisms that are involved. When the composting process starts, some organic acids are formed (acetic acid for example) and this may lower the pH in the early stage of the composting process. Also the CO2, produced when organic materials are mineralized, can lower the pH when dissolved in the water present in the compost:

CO₂ + H₂O ${\overrightarrow {\longleftarrow }}$ H₂CO₃ ${\overrightarrow {\longleftarrow }}$ H⁺ + HCO₃^-

Raw materials

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Time

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Organism active during composting

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Process design

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Applications

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