Controlling Aeration of a Compost Pile by Oxygen or by Temperature

Two years ago, I had a fascinating debate with someone in South America who insisted that controlling the aeration of compost using oxygen was better and less costly than timer/temperature feed back control.

This was the argument for oxgyen control of the composting material:

The operational cost with a composting system that is controlled by oxygen is lower because the system supplies only the oxygen than the microorganisms require. “If the oxygen content is continuously regulated and kept on the upper level of saturation, temperatures which are too high will loose the described negative effects. Odour and decomposition results remain untouched. This astonishing result is confirmed by the quality monitoring of the German Federal Compost Quality Assurance Organisation (BGK).”

This was my argument for controlling aeration based on temperature feedback:

Continuous oxygen measurement sounds like a great idea, but it is cumbersome, expensive to install, and has a high failure rate. You can ask any operator in North America that uses oxygen for aeration control. Some have installed a oxygen control system, but are now only using the timer function of their control system. Operators have found a high failure rate and expensive maintenance with continuous oxygen measurement systems, particularly as the ammonia concentration increases in the composting mass. We use wireless temperature probes, but recommend having a hand held oxygen probe to verify the oxygen status periodically. Its much less cumbersome and costly.

The statement that oxygen control of the composting process reduces the aeration cost of composting is not logical because measuring oxygen does not change the microbiology of the pile, nor the oxygen content of the incoming air. The cost of aeration is based on the amount of time that the blower is operating.

It is possible to say that controlling a compost pile using oxygen MAY reduce operational costs because the blower does not operate as much and that the composting mass is allowed to reach very high temperatures. This is based on the fact that the amount of air required to remove heat from a compost pile is much greater than the amount of air to supply the stoichiometric oxygen demands for the microbes – so the more air we provide, the higher the cost, the lower the temperature.

Increasing setpoint temperatures from 55 C to 80 C on a temperature controlled aeration system would also reduce operational costs simply because the blower will not be operating as much to remove heat.

We also need to consider the microbes in the composting material.  When we operate our composting process at 75 or 80 C, we are limiting the diversity of microorganisms in the composting material, which may affect the overall decomposition rate.

There is another really practical consideration with oxygen control. The variation in temperature within a composting pile is much lower than the varation in oxygen concentration.  This means that a temperature probe is more likely to be a consistent indicator of the temperature of the entire mass of compost, than a single oxygen probe placed in a windrow. With an aerated static pile or windrow, when I begin to aerated, the composting mass begins to develop some preferential air pathways, because air finds the path of least resistance, then as the air goes through this path, it slowly begins to dry the material around it (air at high temperature holds much more water than air at low temperature). So within hours or days I begin to develop “volcanoes”, or preferential air pathways. So, where do I measure oxygen content? In the air pathway, where the oxygen concentration may be 18%,  or a centimeter beside it where the oxygen concentration may be zero?

In summary, continuous oxygen control of my compost pile sounds like a great idea, but has no practical advantage over a system that is controlled using a simple wireless temperature probe!

 

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