Yikes! This really took off! First, my sincere thanks to Pearl for making this a thread! I probably would never have done it on my own and I really appreciate having a place that this can be discussed. I'll try to catch up with some random responses, but let me reiterate, I'm no expert. Just a curious experimenter so I'm looking for input just like everyone else.
1) Yes, Jay Angler, sawdust and urine works! My original intent for the barrel in a box was to make a pellet stove without combustion to circumvent code and insurance problems. I thought it would be cool if a renter or apartment dwellers could use one as a portable appliance that wouldn't jeopardize their lease. Wood pellets are basically sawdust and once you get them wet so bacteria can digest them, they revert to wet sawdust. I used urine as a handy source of nitrogen to get things going... so to speak. The smaller the particle size the faster it digests, so sawdust gets things hot quickly (like hours not days) and require feeding the unit more frequently. The result is a lot of heat quickly from a small unit. For this reason I figured wood pellets would be an ideal material for a small high performance unit.
2) Matthew - to clarify, I used a small fan to circulate air through the barrel to both to introduce oxygen and to exhaust CO2 and water vapor. The flow rate is low and there is no reason it can't be dumped into the room if things are running properly. If things don't run right and the little critters don't get enough oxygen, things can become anaerobic and smelly. I vented outdoors primarily because I have a humidity problem. The bacteria doesn't need trace elements to survive. They eat glucose from the organic matter and water and poop out CO2 and heat. Most trace elements in organic matter are fine, but some of course are toxic. Most common yard and forest materials are fine. Sorry, sadly no pictures. Victims of the great back-up drive crash in 2018.
Some general notes:
Bacteria are tough little critters but they are living creatures and need care and feeding to prosper. Too hot, too cold, not enough oxygen or water, and they die. If we make sure these factors stay within proper bounds, they live and make heat. Stray outside the limits because they are suddenly chilled because a lot of cold fresh material is dumped into their cozy little lair too quickly, and they die... quickly.
Digesting one pound of material releases about 8,000-9,000 Btu. To digest 1 lb of material per hour, air flow should be about 7 CFM. Only about 1.2 CFM in needed to provide adequate O2, but exhausting CO2 and water vapor required the full 7 CFM. Hence, by making sure that adequate flow is available to exhaust byproducts, plenty of O2 is available.
If fresh wet material is used very little water needs to be added. If dry material is used, about 1/2 lb of water needs to be added to every pound of dry matter.
Mesophilic bacteria will bring temperatures up to about 120 deg-F. After that they begin to die and thermophilic bacteria take over and generate heat at a much higher rate. Somewhere beyond 150 deg-F they die and if they get colder than about 120 deg-F they also die. Its important to keep them in the sweet zone of about 125 to 150 deg-F.
Such devices are nearly 100% efficient. A couple percent is lost in the biological processes, and a couple percent is lost if it is vented outside. If it is located indoors, all the rest goes into the space being heated.
Digesters aren't easily throttled. The oxygen supply can be restricted somehat to slow things down, but generally they are either on or off, and if they are off, they usually need a couple days to get back up to temperature.
These things are basically wood stoves that use biological decomposition to make heat and CO2. This means lugging many pounds of material into the house to keep it warm, be it fire wood, wood pellets, or yard waste. Plan accordingly.