Difference between aerobic composting and anaerobic fermentation

1. Aerobic Composting

The aerobic decomposition process generally occurs in the presence of oxygen and water. Its formation is as follows:

Organic matter + aerobic bacteria + oxygen + water → carbon dioxide + water (steam state) + nitrate + sulfate + oxide

2. Anaerobic fermentation

Anaerobic processes generally occur in the absence of oxygen, and their formation is as follows:

Organic matter + anaerobic bacteria + carbon dioxide + water → gaseous methane (biogas) + ammonia + final product

Aerobic composting and anaerobic fermentation are both processes of degradation of organic matter under the action of microorganisms. They have similarities and differences. This article will explain in detail the principles, process flow, fermentation stages, influencing factors, etc.

The similarities between aerobic composting and anaerobic fermentation

They are all organic matter degradation processes under the action of microorganisms. They require conditions for microbial culture, including reasonable distribution of nutrients, temperature, pH, etc.; they degrade organic pollutants, kill pathogens, increase the ratio of N and P, and turn raw fertilizer into more plant-friendly fertilizers. Easily absorbed cooked fertilizer.

The difference between aerobic composting and anaerobic fermentation

Different principles

Aerobic composting is where aerobic bacteria absorb, oxidize, and decompose waste under aerobic conditions. Through their own life activities, microorganisms oxidize part of the absorbed organic matter into simple inorganic substances, and at the same time release the energy required for microbial growth activities, while other part of the organic matter is synthesized into new cytoplasm, allowing the microorganisms to continue to grow and reproduce. The process of producing more organisms. Anaerobic fermentation is the stabilization of waste through the metabolic activities of microorganisms under anaerobic conditions, accompanied by the production of methane and CO2.

Different Process

1) Aerobic composting

The main process flow is: food waste pre-treatment → main fermentation → post-fermentation → post-treatment → storage

①Pretreatment of raw materials

Including food waste sorting, food waste crushing and adjustment of moisture content and carbon-nitrogen ratio. First, remove impurities such as metal, glass, plastic, and wood from the waste and crush it to a particle size of about 40 mm. Then select compost raw materials for batching to adjust the moisture and carbon-nitrogen ratio. You can use pure garbage. The ratio of garbage to feces is 7:3 or the ratio of garbage to sludge is 7:3 for mixed composting.

② Fermentation stage of raw materials

Most of our countries use the primary fermentation method, with a cycle of up to 30 days. Currently, the secondary fermentation method is used, with the cycle generally taking 20 days. Primary fermentation is a microbial metabolic process in two stages of aerobic composting: medium temperature and high temperature. Specifically, the entire process starts from fermentation, passes through medium temperature, high temperature and then reaches the point where the temperature starts to drop. It generally takes 10-12 days, and the high temperature stage lasts longer.

Secondary fermentation means that after primary fermentation, some of the materials still contain easily decomposed and large amounts of difficult-to-decompose organic matter, which needs to be sent to the post-fermentation room and piled into 1-2 meter high stacks for secondary fermentation and decomposition. When the temperature stabilizes at around 40°C, it will be fully decomposed, which usually takes 20-30 days.

③Post-processing stage

It is to process the fermented and mature compost to further remove the impurities in the compost that were not removed during the pre-treatment process and perform the necessary crushing process. The water content of the refined compost obtained after treatment is about 30%, and the carbon-nitrogen ratio is 15-20.

④Storage stage

Storage means that compost must be stored and managed before processing. Generally, it can be stored directly or in bags. However, pay attention to keeping it dry and ventilated during storage to prevent air tightness and moisture. It is divided into three stages: initial stage, high temperature stage and maturation stage.

2) Anaerobic fermentation

The first stage is the hydrolysis and fermentation stage, which refers to the hydrolysis and fermentation of responsible organic matter under the action of microbial extracellular enzymes, breaking the chains of large molecular substances into small molecular substances such as: monosaccharides, amino acids, etc. to prepare for the next stage.

The second stage is the hydrogen production and acetate production stage. In this stage, the small molecule substances produced in the previous stage are decomposed under the action of acid-producing bacteria such as acetic acid bacteria and some Clostridium species to generate acetic acid and hydrogen. At this stage, the acid production rate is very fast, causing the pH value of the feed liquid to drop rapidly, giving the feed liquid a putrid smell.

The third stage is the methanogenesis stage, in which organic acids and soluble nitrogen-containing compounds decompose into ammonia, amines, carbonates and carbon dioxide, methane, nitrogen, hydrogen, etc. Methanogens decompose acetic acid to produce methane and carbon dioxide, and use hydrogen to reduce carbon dioxide to methane. At this stage, the pH value rises.

3) Different influencing factors

① Factors affecting the composting process

The amount of oxygen supply must be appropriate. The actual amount of air required should be 2-10 times the theoretical amount of air. The water content should be 50%-60%, and 55% is ideal. At this time, the decomposition rate of microorganisms is the fastest, and the role of water is Two: The first is to dissolve organic matter and participate in the metabolism of microorganisms; the second is to adjust the temperature of the compost.

When the temperature is too high, part of the heat is taken away through the evaporation of water: the carbon-nitrogen ratio should be appropriate, generally considered to be between 20-35 for municipal waste; Phosphorus ratio is 75-150; pH value, when organic sludge is used as compost raw material, pH adjustment is required. At the beginning of the composting process, due to the action of acid bacteria, the pH is 5.5-6.0. After composting, the pH is 8.5-9.0.

② Anaerobic fermentation raw material ratio

The appropriate carbon-nitrogen ratio for anaerobic fermentation is 20-30. When the carbon-nitrogen ratio is 35, the gas production decreases significantly; the appropriate temperature is 30-40°C; the pH value is absolutely necessary for methane bacteria to maintain a weakly alkaline environment. Yes, its optimal pH range is 6.8-7.5. The pH value is low, which greatly increases CO2 and produces a large amount of water-soluble organic matter and H2S. The increase in sulfide content inhibits the growth of methanogens.

Lime can be added to adjust the pH, but The best way to adjust the pH is to adjust the carbon-nitrogen ratio of the raw material, because the alkalinity used to neutralize acid in the substrate is mainly ammonia nitrogen. The higher the nitrogen content of the substrate, the greater the alkalinity. When VFA (volatile fatty acids) When >3000, the reaction will stop.

In Conclusion

Both aerobic composting and anaerobic fermentation have their advantages and disadvantages. They have both similarities and differences. Regarding the processing needs of organic waste, users can choose the corresponding processing method according to the actual situation and process the organic waste to a high level. Value-based, pollution-free treatment.