During a lively twitter discussion about composting toilets, Nick Grant of Elemental Solutions, a British passivhaus consultancy, posted a remarkable drawing. It's from Goodbye to the flush toilet, edited by Carol Hupping Stoner and published by Rodale Press in 1977. There was an explosion of interest in alternative building technologies at the time and many alternative toilet and gray water systems are shown here, but this one is particularly interesting: it's not just a toilet, it's a furnace.
Long before passive houses existed, builders were experimenting with super-insulated houses and the 12" of insulation in the walls plus 16" in the ceiling, not to mention the quadruple-paned windows come pretty close to what one would call passive house today.
But what's most fascinating is the heating system: it's the toilet. The composting process generates heat, and is working here as a "biological furnace." Carl Lindstrom describes the system designed by Swedish physicist Mats Wolgast:
It has been calculated that the wast in the Clivus Mutrum [composting toilet] has an average heating value that is about the same as wood. The aerobic decomposition process gives off the same amount of BTUs that would be generated if the material was burned instead, but at a much lower temperature.... The compost process gives off an average of 1kwh/person/day. In the Wolgast house with five people, this means about 1,800 kwh/year from the composting wastes.
The supply of air for the composter is drawn from the toilet and the kitchen exhaust, then fed up through a heat exchanger (what today would be called a heat recovery ventilator or HRV, which transfers the heat energy in the smelly toilet exhaust to fresh incoming air.
This makes so much more sense than just flushing the stuff away. Put your poop to work!