Compost Toilet

A compost toilet is a fixture that uses no water. Instead of flushing waste to a septic tank or a sewer, it collects it in a chamber where it breaks down into a soil amendment. The nutrients that would otherwise be diluted and buried stay available: one person passes roughly 4.5 kg (10 lb) of nitrogen, 0.5 kg of phosphorus and 1.2 kg of potassium in a year.

It works by air rather than by heat. Composting hot enough to kill disease organisms, 50 to 65 degrees C (120 to 150 F), does not happen in a household-sized pile unless it is heated, so safety here is bought with time instead. A chamber is left at least four months before it is emptied, longer in cold conditions, and some methods ask for two further years of composting in a pile. That means two chambers or two piles, one filling while the other cures. A handful of sawdust or similar dry, fine, carbon-rich material goes in at every use; it is not there to mask a smell but to supply carbon and the air spaces the process needs. A screened vent draws air out and keeps insects from getting in.

Cover material is a permanent dependency: a household with no source of sawdust, shavings, chaff, dry leaves or shredded paper buys it forever. The commonest failure is a chamber that gets too wet, where organisms that work without air take over and the contents turn foul; the answer is dry carbon, not drainage. Flies follow a lid left open or cover applied too thinly. The most dangerous failure is emptying early, because material that is not finished looks finished. And the fixture removes only the toilet: sink, shower and laundry water keep flowing at full volume and still need somewhere to go, so it reduces a household's wastewater rather than closing it. It suits a temporary or off-grid dwelling especially well, since nothing has to be buried first.