Composting is usually divided into aerobic composting and anaerobic composting. Modern composting processes are basically aerobic composting, because aerobic composting has the advantages of high temperature, more complete substrate decomposition, shorter composting cycle, less odor and large-scale mechanical treatment. Anaerobic composting uses anaerobic microorganisms to complete the decomposition reaction. The compost is isolated from the air, the temperature is low, the process is simpler, and the nitrogen content in the product is preserved. However, the composting cycle is too long, and there is a strong odor. The product contains impurities that are not particularly well decomposed.
The aerobic composting process is generally divided into three stages.
1. Mesophilic stage. The mesophilic stage is also called the heating stage. It refers to the early stage of the composting process. The pile is basically at a medium temperature of 15 to 45°C. Thermophilic microorganisms are relatively active and use the soluble organic matter in the compost for vigorous life activities. These thermophilic microorganisms include fungi, bacteria and actinomycetes, mainly using sugars and starch as substrates.
2. High temperature stage. When the compost temperature rises above 45°C, it enters the high temperature stage. In this stage, thermophilic microorganisms are inhibited or even die and are replaced by thermophilic microorganisms. The residual and newly generated soluble organic matter in the compost continues to be oxidized and decomposed, and complex organic matter such as hemicellulose, cellulose and protein in the compost also begins to decompose strongly.
3. Cooling stage. In the later stage of fermentation, only some difficult-to-decompose organic matter and newly generated humus remain. At this time, the activity of microorganisms decreases, heat production decreases, the temperature decreases, thermophilic microorganisms dominate, and the remaining more difficult-to-decompose organic matter is further decomposed. Humus continues to increase and stabilizes. The compost enters the decomposition stage, the oxygen demand is greatly reduced, the water content is reduced, the porosity of the compost increases, and the oxygen diffusion capacity is enhanced. At this time, natural ventilation is the only option.
The principle of anaerobic composting. Anaerobic composting is the process of using anaerobic microorganisms to ferment under anoxic conditions. In addition to carbon dioxide and water, the final products include ammonia, hydrogen sulfide, methane and other organic acids. Among them, ammonia and hydrogen sulfide have a foul smell. In addition, anaerobic composting takes a long time, and it often takes several months to fully mature. Traditional farmyard composting is anaerobic composting, but the nutrient loss is relatively large.
The anaerobic composting process is divided into two main stages.
The first stage is the acid production stage, in which acid-producing bacteria decompose large molecular organic matter into small molecular organic acids and acetic acid, propanol and other substances;
The second stage is the methanogenic stage, in which methanogens continue to decompose organic acids into methane gas. The anaerobic process does not involve oxygen, and the acidification process produces less energy. Most of the energy is retained in the organic acid molecules and released by methanogens in the form of methane gas. The characteristics of anaerobic composting are that there are many reaction steps, slow speed and long duration. Considering environmental production and processing costs, tank aerobic fermentation means that it can be produced on a large scale, and it is easy to master and has low manual operation technology difficulty. Therefore, we believe that aerobic fermentation is more suitable for the current actual situation in rural China, and the equipment investment cost is relatively low.
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