Unlock the Heat: Composting Thermodynamics and Kinetics at Work

Did you know composting is only possible because of key physical and chemical reactions that break down the organic matter within a compost pile? These reactions create temperature changes within the compost, which is why you can see steam coming off of some types of outdoor compost bins on a chilly day! Learn more about the main temperature stages of composting and how they occur in this quick read. 

Heating Up: The Mesophilic Stage

As compost initially begins to break down (the mesophilic stage), communities of microorganisms (primarily made up of mesophilic and psychrophilic bacteria) start to build within the organic matter. This causes the compost pile to rapidly heat up, resulting from the chemical energy stored in the organic matter. For an indoor compost system less than 10 gallons, the materials can heat up to 100-120ºF in just a few days. For larger or commercial compost systems, the temperature can get up to around 170ºF!

Breaking Down: The Thermophilic Stage

Once the compost bin reaches higher temperatures, the thermophilic stage starts. Thermophilic bacteria thrive in very warm environments, and, because of the higher temperature, they are able to break down the organic matter quickly. The temperature stays high as the bacteria degrades the starches, sugar, and fats in addition to proteins and cellulose. This stage can last weeks to months depending on the system. Many facilities opt to aerate their compost during this stage because many beneficial microbes cannot survive at temperatures higher than ~170ºF. This aeration of the degraded organic matter transitions the compost bin into the cooling stage. 

Cooling It Down, Letting the Compost Mature

In the cooling stage, the lack of degradable materials and recent aeration leads to declining microbial activity. When the microbes stop degrading material, the heating process stops and the compost pile will drop to near its original temperature. The cooling stage leads into the maturation stage, and these two combined can last weeks to months. In the maturation stage, the temperature very slowly goes down to almost its original temperature. 

Maintaining the Compost Process in Your Own Compost Bin

While this article outlines the general temperature and time measurements of the four stages of composting, these are not uniform across all compost bins or methods of composting. Register for an upcoming workshop or webinar with Let's Go Compost, or check out our blog, to learn more about the composting process.