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Thermodynamic (heat) Stages of Composting and the Microorganisms Responsible

Updated: Nov 16, 2021

In our first blog post we talked about the role of microorganisms in the composting process and how they promote soil health. This post aims to explain the thermodynamic (fancy term for heat) stages of the composting process and the types of microorganisms that come into play during the different stages.

A compost pile breaks down largely because of the microorganisms that eat the organic waste. As we have already learned these microorganisms also produce heat that helps speed up the decomposition process. The generation of this heat creates “high temperatures [that] accelerate the breakdown of proteins, fats, and complex carbohydrates like cellulose and hemicellulose, the major structural molecules in plants.” This is what we mean by the sometimes mystifying and intimidating term thermodynamics - it is simply the study of energy and its effects on matter. The energy we are concerned with investigating is the heat that microorganisms produce that ultimately speeds up decomposition during the composting process.

When a compost pile is first put together the temperature will range between 0-55°C (32-131°F). Before the pile starts to noticeably heat up there is a lag faze where it seems like nothing is happening because the microorganism communities must first build themselves up within the pile. The first thermodynamic compost stage begins when the pile begins to heat up; microorganisms known as mesophilic and psychrophilic bacteria dominate the pile. The word mesophilic simply refers to organisms that survive in moderate temperatures that aren't too hot or too cold. These are the most common types of microorganisms found in the top soil we find in our yards, woods, and farms. The word psychrophilic refers to organisms that survive at low temperatures.

In the second thermodynamic composting stage the pile heats up to about 55-66°C (131-151°F). When this happens the mesophilic and psychrophilic bacteria start to overheat and die allowing for thermophilic bacteria to thrive and take over the compost pile. The term thermophilic refers to organisms that survive at high temperatures.

Once the organic matter is all eaten up by the microorganisms the pile enters the third thermodynamic composting stage where the pile "cools down." At this point the pile returns to around the temperature of whatever it is outside and becomes recolonized by the mesophilic microorganisms and some psychrophilic microorganisms. These same microorganisms are the ones that are beneficial to the regeneration of the soil and to the health of plant roots.

Graph of the thermodynamic stages of a compost pile and the microorganisms responsible

In our next post we will explain the specific names of the microorganism life forms that all play different key roles in the composting process. Understanding the principles discussed in this post is essential to creating an ideal compost pile. Our aim here at the Kaimuki Compost Collective is to collect food waste and convert it to compost to prevent the emissions our waste ultimately produces and regenerate the health of Hawaii's environment.

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