Understanding Moisture Loss During Composting

For┬ácomposters in many parts of the world,┬ácomposting is a drying process, and many are more concerned about adding additional moisture than having compost that is too wet. For some of us in cooler and wetter climates, our compost doesn’t seem to dry rapidly enough. Also, for some of us thinking about composting indoors to control odor or meet regulatory requirements, we need to consider the moisture equation.

Lets start with a batch of 1000 tonnes of composting material at a moisture content of 65%. This means that this compost contains 650 tonnes of water. The rule of thumb for the composting process is that we reduce the weight by 50% during the composting process and we end up with a moisture content of about 50%. This means that we now have 500 tonnes of compost at a moisture content of 50%, which means that we now have 250 tonnes of water remaining. This means that somehow we have lost 400 tonnes of water from that 1000 tonne batch of compost? That is 89,000 imperial gallons or 105,000 US gallons of water. Where is it going?

During outdoor composting, the moisture leaving the composting material simply goes to the air. If the composting process is indoors, particularly in cold climates, this water may condense in the air, or in exhaust ducting. How much of this will freeze inside ducting, or result in snow falling inside the building in really cold climates?!

This principle is also important to understand in cases where we are considering down draft or negative aeration. With each 1000 tonnes of composting material, we now have up to 100,000 gallons of largely uncontaminated condensate blending with organic material or leachate, which means that we may have a much larger volume of “leachate” to consider.

We need to consider the obvious in our compost system designs – except that it is not always so obvious until we actually think about it!

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