Last week, I had a call from a client in upstate New York, who was using the agitated bed composting system that we designed in 2006 for recycling separated dairy solids to produce animal bedding. He is processing up to 180 cubic yards per day of separated solids through his agitated bed system inside a double poly greenhouse. He has been doing this successfully since March, using a 10 day composting process with 2 mixes per day. The moisture content decreases from 65% moisture to less than 55%.
He asked me if there any magic to getting the compost drier, now that the outside temperatures were getting cooler. The compost was no longer drying like it did during the summer. Based on my discussions in the last few blogs, it could have been predicted.
I explained how moisture removal from composting is dependent on the temperature of the air above the composting material, as predicted by the psychometric curve. This is illustrated below.
I suggested that there were two solutions. The first was to somehow enclose the top of the composting bed so that the air would remain warm above the compost, and ventilate this air to remove it from the top of the compost while it held more moisture. The second was to utilize some of the methane that he was producing from his covered manure lagoons to heat the incoming air that was blown into the base of the composting material. Although some of the moisture would still condense on top of the composting material, there would be a much greater volume of air supplied by the blowers. Normally, the volume of air is restricted by the ability of the microorganisms in the composting material to heat it, but with an external heat source, the air volume would be increased, and more drying would occur.
This observation of less drying in the winter is very important for two reasons:
1. it demonstrates how evaporation or moisture loss during composting is dependent on the air temperature above the compost as outlined in previous blogs.
2. the rotary drum type of composters for composting separated dairy solids to produce animal bedding will achieve more drying than an agitated bed system because the air is removed while it is hot. The challenge is the cost of this system that could process up to 180 yds per day.