Two types of bacteria—called mesophilic and thermophilic—are usually present in organic waste. Mesophilic bacteria grow best in moderate temperatures, typically between 20 °C (68 °F) and 45 °C (113 °F), while thermophilic bacteria grow best in higher temperatures, usually between 40 °C (104 °F) and 65 °C (150 °F).
At the beginning of the process, mesophilic bacteria predominate. As they break down chemical compounds in waste, they generate heat and the compost pile becomes increasingly warmer. One reaction that takes place during aerobic composting is the oxidation of glucose (C6H12O6), a simple sugar present in plants, as follows:
C6H12O6 + 6 O2 ➞ 6 CO2 + 6 H2O + heat energy
Thermophilic bacteria take over when the temperature of the compost exceeds 40 °C (104 °F). As the reactions progress and the organic materials are consumed, the reactions slow, the thermophilic bacteria die off, and the mesophilic bacteria predominate again. Finally, the waste becomes a stable end product called mature compost.
For this process to happen, the compost pile has to support growth and reproduction of both types of bacteria. In addition to the right temperature range, access to oxygen is necessary for a successful compost. To ensure a good supply of oxygen throughout the organic material and to prevent anaerobic processes from kicking in, the compost pile needs to be turned over and fluffed, which also helps release heat. Up to a certain temperature, the heat generated through the bacterial reaction is beneficial, and at about 55 °C (131°F) it is intense enough to kill disease-causing bacteria.
At a temperature of 63 °C (145 °F), the composting material is hot enough to destroy weed seeds. Beyond that temperature, it is too hot for bacteria to survive, and they either die or become dormant spores that can withstand hostile conditions. High temperatures cause water to evaporate from the waste more quickly, leaving the pile dry so it’s unable to support bacteria growth, and it introduces the risk of spontaneous combustion and fire.
Another important factor is having the right amounts of carbon and nitrogen in the system. In general, biological organisms need about 25 times as much carbon (C) as nitrogen (N). Fruit waste, such as an apple, has a carbon-to-nitrogen ratio of 35:1. In this case, there is much more carbon than nitrogen, and the composting process slows. To speed it up, the high C:N ratio of the apple needs to be offset. For example, coffee grounds, which have a C:N ratio of 20:1, could be added to lower the overall ratio of carbon.
Adding too much nitrogen would be a mistake because it can lead to the production of ammonia and other nitrogenrich products, which would cause the composting material to smell bad. Ideally, the C:N ratio should be between 20:1 and 25:1. Bacterial growth doesn’t stop if the ratio is imprecise; it simply slows down.
With bacterial growth nurtured by appropriate oxygen levels, at the right carbon-to-nitrogen ratio, and within a reasonable temperature range, the organic composting material converts into CO2, water, and mature compost fairly quickly. In summer, warm temperatures encourage bacterial activity, so with frequent turning, compost can be ready in about three months. The mature compost can then be spread over soil, so plants can grow and soil can hold carbon. This capability has an underappreciated role in mitigating climate change. Reducing the emissions of methane, a greenhouse gas, is often considered critical, but it is only one advantage—drawing excess carbon out of the atmosphere is the other. Not to mention reducing the amount of organic material discarded in landfills!