Aerobic fermentation is the core biotransformation step in organic fertilizer production. Essentially, it utilizes the metabolic activities of aerobic microorganisms to degrade organic waste such as livestock manure and crop straw into stable humic substances. In this process, aeration and oxygen supply are not only prerequisites for maintaining microbial activity but also key variables determining fermentation efficiency, product quality, and operating costs. Understanding the underlying principles of aerobic fermentation and scientifically designing a supporting aeration system is the technological foundation for achieving efficient, stable, and clean production in organic fertilizer plants.
Biological Principles and Stage Characteristics of Aerobic Fermentation
Aerobic composting is a biochemical process that relies on the action of obligate and facultative aerobic bacteria to degrade organic matter. Through their own life activities, microorganisms oxidize some of the absorbed organic matter into carbon dioxide, water, and heat, releasing energy for their own growth; another portion of the organic matter is synthesized into new cellular material, driving the continuous proliferation of the microbial population.
From the temperature change curve, aerobic fermentation typically goes through three stages:
Heating stage (mesothermal period). In the initial stage of composting, mesophilic aerobic bacteria rapidly proliferate, decomposing soluble organic matter (such as sugars and starches), and the pile temperature rises from ambient temperature to around 45℃ within 1 to 3 days.
High-temperature stage (thermophilic period): When the pile temperature rises above 45℃, thermophilic microorganisms become the dominant flora, continuously decomposing complex organic matter such as cellulose and hemicellulose. The temperature at the center of the pile can reach 55 to 65℃. This temperature range is not only the most active stage of organic matter degradation but also a crucial window for killing pathogens, insect eggs, and weed seeds. Forced ventilation static stacking systems should ideally control the highest temperature at the center of the pile to around 60℃.
Cooling and maturation stage: When easily degradable organic matter is almost completely consumed, microbial activity weakens, and the pile temperature gradually drops to ambient temperature. At this point, the compost products tend to stabilize, the humus content increases, and the seed germination index rises, indicating that the compost has reached the maturation standard.
Core process parameters affecting fermentation efficiency: Aerobic fermentation is the result of the synergistic effect of multiple factors, among which the following parameter is the most critical: Carbon-to-nitrogen ratio (C/N). Microbial growth and reproduction require carbon as an energy source and nitrogen as a raw material for protein synthesis. The C/N ratio of compost materials should ideally be controlled between 25:1 and 35:1. If the C/N ratio is too high, microbial growth is hindered due to nitrogen deficiency, and the degradation rate decreases; if the C/N ratio is too low, nitrogen is easily lost through volatilization as ammonia, resulting in nutrient loss and unpleasant odor.
Moisture content. Moisture is an indispensable medium for microbial metabolic activities. The initial moisture content of the compost should ideally be controlled between 45% and 65%. If the moisture content is too high, the material pores are filled with water, hindering oxygen transport and reducing the oxygen consumption rate; if the moisture content is too low, microbial activity is inhibited, and the fermentation process slows down.
Oxygen concentration and porosity. Aerobic microorganisms require a sufficient oxygen supply. A porosity below 30% will hinder oxygen transport, while a porosity above 60% will cause excessive heat loss. In a closed composting system, the oxygen volume fraction between the compost layers should ideally be maintained between 15% and 20%.
Design Considerations for the Ventilation System The ventilation system is the “respiratory system” of aerobic fermentation engineering, and its design must comprehensively consider the three functions of oxygen supply, heat dissipation, and moisture removal.
Ventilation Method Selection. There are three main oxygen supply methods in industrial composting: mechanical turning (natural ventilation), forced ventilation (positive pressure blowing or negative pressure exhaust), and a combination of turning and forced ventilation. Forced ventilation static systems are currently the mainstream choice for large-scale organic fertilizer plants, with advantages including strong controllability of oxygen supply and good uniformity of pile temperature. Positive pressure blowing is beneficial for moisture removal, but the temperature at the bottom of the pile is relatively low; negative pressure exhaust provides a relatively uniform temperature distribution in the pile, but the moisture removal effect is slightly inferior. In actual engineering, the selection should be based on a comprehensive consideration of material characteristics and climatic conditions.
Ventilation Volume Calculation and Fan Selection. The determination of the ventilation volume must simultaneously meet the needs of both “metabolic air” (the amount of oxygen required for microbial respiration) and “cooling air” (the amount of heat dissipation required to prevent overheating of the pile). The ventilation volume calculation formula proposed by the School of Environment at Tsinghua University is: Q = (0.05 × M × T) / (ρ × Cp × Δt), where M is the mass of the compost pile and T is the temperature gradient. During forced ventilation composting, the ventilation rate is generally controlled between 0.1 and 0.3 m³/(min·m³). In addition to meeting the air volume and pressure requirements, the selection of fans should maximize operational efficiency. The rated air volume of the fan needs to be determined by multiplying the calculated total air volume by a ventilation coefficient of approximately 1.1. Centralized and distributed fans each have their advantages and disadvantages—centralized fans can be designed around the average required air volume and have lower installed power; distributed fans need to be selected based on the peak air volume, but offer more flexible control.
Ventilation control strategies. Traditional ventilation control often uses a fixed-time aeration method with scheduled start and stop. Modern composting projects tend to use intelligent control based on temperature feedback—when the temperature at the center of the compost pile reaches a set threshold (e.g., 60℃), the ventilation volume is automatically adjusted to ensure sufficient oxygen supply while avoiding excessive ventilation that leads to heat loss. Time-temperature feedback control systems are currently the most economical and suitable control solution for composting plants in China.
Conclusion Aerobic fermentation is the biological basis of organic fertilizer production, while the aeration system is the core technological carrier that transforms this biological process into a controllable engineering practice. From the precise adjustment of the carbon-nitrogen ratio to the scientific control of moisture content, from the rational selection of ventilation methods to the optimized calculation of fan parameters—the scientific design of each link directly affects the length of the composting cycle, the quality of the product, and the operating costs. In today’s organic fertilizer industry, which is developing towards large-scale and standardized production, a deep understanding of the principles of aerobic fermentation and mastery of the engineering design methods for aeration systems are necessary prerequisites for achieving efficient, stable, and green production.
