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Hi John, I have been in touch with Roger Haug, the author of The Practical Handbook of Compost Engineering, who has offered the following advice:

There are two potential problems in cold weather composting. One is thermodynamic and one is kinetic. Let’s start with the thermodynamic issue. While you don’t see it, the major mass moving through a composting system usually is air. When the air is cold it takes a great deal of energy to warm it. Further, when you warm it up it then carries more water vapor and the composting material must also supply the heat of evaporation. So the substrate needs to contain a high level of biodegradable material so that sufficient heat is released. Human faeces is highly biodegradable so the question is what other amendments are they adding to the mix. They are probably adding other amendments to produce a friable mixture that allows air flow. I have no idea what they’re using. In general, hardwood sawdust is more biodegradable than softwood sawdust. I once had a composting facility that switched to hardwood sawdust to improve their energy balance.

If the starting substrates are reasonably degradable, then we need to look at controlling the air flow through the composting mixture. Obviously, less air flow means less heat removed with the off gases. Then we need to control loses to the surroundings. If this is a low-tech windrow system, then larger windrows will be better than smaller ones.

Now let’s talk about kinetics. I once visited a biosolids composting facility in New England. It was winter and the piles would not heat up. It appeared that the starting mixture was energy “rich”, but the piles just sat there. Temperatures were near freezing and the starting substrates were all near freezing. Mind you, the biosolids were anaerobically digested at 95 0F but were allowed to sit around and lose all that heat. I reminded them that that is why we put perishables in a refrigerator…to decrease their rates of reaction. Their problem was that the kinetics were so slow at 0 deg F that nothing would get started. If the substrates retained their temperature, the kinetics might be fast enough to “light the biological fire”. If all else fails, temporarily heating the incoming air might be enough to get things started.

There is a type of composter that is particularly well suited for cold climates. There are several names: box composter, tunnel composter, Herhof Box. Picture a square shaped tunnel, perhaps 8-ft by 8-ft and say 100-ft long. The length is variable depending on the amount of composter material. It has a floor, two walls, and a top. One end is closed, while the other has doors that can be opened and closed. All sides are insulated. With the doors open, a front end loader loads a batch of composting substrate into the tunnel, filling it to about 80% of its height. The doors are closed and air begins to be supplied at a low rate. The outlet gases are collected at the top, but instead of being discharged they are recycled back into the feed air to conserve their heat. This can continue until the oxygen content drops to something in the range of 5 to 10% (compared to 21% in air). At this point, the fresh air supply can be controlled to maintain a set point O2 content. The air that is finally discharged to atmosphere can even be directed to a heat exchanger to preheat the inlet air. It’s the ultimate for conserving the heat released during composting. I once visited a facility like this in Maine during winter and temperatures were very thermophilic.

Of course, there are always tradeoffs. It’s a mechanical system and, therefore, more expensive than an open windrow type system. It also requires more controls and operator expertise, but it’s nothing a wastewater operator can’t handle. If you google “images of tunnel composter” you will see some good photos and diagrams.

Happy to contribute more if you send me more specifics.

Roger