Andrey Repin
Composting specialist, consultant.
There are two methods for compost preparation: aerobic (with the involvement of oxygen) and anaerobic (without oxygen). The anaerobic method is also called decay and occurs with the help of putrefactive bacteria. This method takes considerable time, sometimes more than a year (especially for hard-to-decompose waste). The aerobic composting method is faster, often taking just a few months, weeks, or even days. It primarily involves bacteria and fungi, generating heat during the process. To ensure uniformity and speed, the compost must be periodically moistened and mixed. This maintains favorable conditions for microorganisms and ensures adequate oxygen supply. Without sufficient oxygen, composting slows down.
For producing compost used in growing Agaricus mushrooms, the aerobic method is used. Many compost yards use specialized facilities (such as aerated floors, bunkers, and tunnels). The composting time is shortened. This method is often called the accelerated aerobic method because specialized ventilation systems are used to raise compost temperatures above 80°C, speeding up the process significantly. These ventilation or climate control systems maintain the necessary oxygen levels in the compost to achieve the required temperatures. Thus, measuring oxygen in modern compost production is really important.
Oxygen levels are important at every stage of compost production. If we can measure oxygen levels, we can control them. We need to measure oxygen levels directly or indirectly, not only in the compost itself but also in the recirculated water.
Recirculated water is very important at the early stages of compost production. It contains thousands of bacteria needed for fermentation. These bacteria help remove the waxy layer from straw. Oxygen is needed for the bacteria to function properly. Aerating recirculated water should thus be a key consideration when designing compost production facilities.
To check if there’s enough oxygen in the recirculated water, you can:
- Measure the water’s pH. With adequate oxygen, pH ranges between 7.0 and 8.0. Insufficient oxygen lowers pH below 7.0.
- Use pH meters capable of measuring the oxidation-reduction potential, indicating oxygen saturation levels.
- Rely on a technologist’s visual assessment, though subjective, it helps validate measurements.
Signs of oxygen deficiency in recirculated water include unpleasant odors, dark sludge islands floating on the surface, and gas bubbles rising from the bottom.
After soaking, maintaining oxygen levels in the straw is important. Natural ventilation helps achieve this, provided bales (1.2 m x 0.9 m in size) are stacked no more than four high. Overstacking compresses the straw at the bottom , creating anaerobic conditions and disrupting fermentation. This causes uneven compost.
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After mixing and placing compost on aerated floors or in bunkers, oxygen control is also important. Many facilities use oxygen sensors in bunkers, which, based on experience, are helpful but prone to malfunctions. It’s advisable to use oxygen sensors periodically, especially when production schedules, bunker loads, or compost formulas change, or when there are doubts about ventilation adequacy.
If using an oxygen sensor, aim for oxygen levels of 2% to start ventilation and 9% to stop. Maintaining adequate oxygen levels promotes compost heating, with a standard heating rate of about 1°C per hour. Insufficient oxygen or excessive ventilation slows down heating. Anaerobic conditions in Phase 1 composting can later cause issues, such as “gypsum spots,” on mushroom farm shelves. Though not severely damaging, gypsum spots indicate ventilation issues in Phase 1.
Controlling oxygen levels during pasteurization is also important, during phases: leveling and preheating for pasteurization phases. During leveling, rapid temperature fluctuations in compost and incoming air can occur. During preheating, fresh air intake is restricted to raise air temperature, potentially reducing oxygen levels. Low oxygen reduces compost activity, causing temperature drops and preventing compost from reaching pasteurization temperatures. Anaerobic conditions in Phase 2 delay or prevent ammonia removal, disrupting production schedules and delaying mycelium seeding. These conditions also encourage competitive molds later.
Oxygen sensors are useful in Phase 3, especially at the beginning. During tunnel loading, in the cold seasons, compost activity is low, and minimal fresh air intake can lead to oxygen deficiency.
Measuring and controlling oxygen levels during compost production is thus necessary at all stages. Modern tools like oxygen sensors make the process manageable. However, even without sensors, indirect indicators (e.g., pH levels, water condition, compost appearance) allow for adjustments. While this method is less convenient and requires constant attention, technological expertise, and potentially additional staff, it remains effective.
In my opinion, all available tools and methods should be utilized to maximize control over the process. The more we control the process, the fewer questions we’ll have about unexpected outcomes.
