So Mathew and I are back with a new and improved composter for our backyard, a plan we sketched out while recovering from a flu picked up in San Francisco. After the cable to the sensors got eaten by some fuzzy creature last week, I went out and bought 22′ by 4′ wide 1/4″ galvanized mesh to build a new and improved composter. Mathew suspected it was a bird, but a burrow hole confirmed it was a rat. If rats want to get at our delicious composter tonight, they’ll have to chew their way through metal, 13″ of wood chips followed by more metal. Take that rats! Raccoons don’t even try!
The whole set up looks like it might have landed in our yard by a distressed starship. We rigged it together from salvaged materials from around the house and our local salvaged building supply store for a total of $80 not including the soon to be installed temperature monitoring system. $60 was spent on the mesh, which definitely was a bit overpriced at our local hardware store.
How it works: The solar air collector warms air to roughly 20 degrees hotter than ambient. This pre-heated air is blown in to the bottom of the compost pile for 6 minutes each hour during hours of solar gain from 7 am to 7 pm. I built this from a pallet we had on hand, ducting and insulated it with a terribly unattractive wool skirt. We’ll be monitoring the temperature starting later this week to determine the optimum aeration schedule. For now this aeration schedule is based on research of the New Alchemists’ composting greenhouse which used 17.8 cubic feet of air per square foot of compost per day (Fulford 9). Our fan is a tiny little 12V 0.22 amp fan used for cooling computers which blows approximately 1 cubic foot per minute. The goal is to aerate our pile without cooling. Maintaining thermophilic temperature without manual stirring. The compost container is surrounded by 13″ of wood chips plus a wood chip cap on top for insulation and odor prevention (God forbid).
The bacteria present in a well insulated pile with enough oxygen will theoretically raise the temperature of the pile to a thermophilic bacteria friendly temperature range of 113 F to 150 F. The New Alchemists found in their study that composting horse manure generated 103 to 177 BTUs per cubic foot of compost (Marshall et al. 5). We’ll see if our alpaca manure and table scraps will generate as much heat.
I’m so glad this composter is up and running and I hope that now that we have a heated aeration system I will stop dreaming that I am a compost turner.
Fulford, Bruce. The Composting Greenhouse at New Alchemy Institute: A Report on Two Years of Operation and Monitoring. New Alchemy Institute. 1986.
Marshall et al. Improved Composting Greenhouse Designs Based on Energy, Nitrogen, and Materials Handling Research. New Alchemy Institute. 1987.