A Bucket Of Poop – A.K.A. The Humanure Bioreactor

This is all about a bucket of poop.  Several actually.  I mean, as long as everybody keeps pooping, and I don’t see that ceasing anytime soon, then we’ll have to put it somewhere.  Mine?  It goes in the bucket.  Not urine though.  That’s disgusting.  Yep, just the poop.  I have to save it; it’s still alive when it comes out of me.  Not in the the jars of fingernails and balls of hair kind of saving it, but saving the microbial cousins that used to be part of my digestive tract, which are set to continue the digestive process long after I’ve extracted all of the useful nutrition my physiology is capable of absorbing from it.  I’ve heard it said that the contents of one’s colon can themselves be considered an organ, just as important as any other.  Vital really.  That is, until it leaves my body.  Then, it’s still alive, it’s still digesting, but not so vital to me individually.  More like vital to the health of the entire biosphere which ultimately, loops right on back to me.  So, continuing the digestion process, managing the nutrient cycle is pretty damned important.

That’s why I’m excited about a bucket of poop.  Read:  Proud Pooper

Managing your “waste” requires that you first can collect it.  Defecating into potable water and collecting it in a septic tank does not count.  In this case, as in most, one should look toward nature for guidance.  Nature hasn’t had five gallon plastic buckets for long so we have to be creative.  Continuing digestion outside the body requires the right conditions.  The type of bacteria that thrive in the composting process breathe oxygen, just like us and they respire carbon dioxide too.  Oxygen is in short supply in a sewer resulting in the proliferation of anaerobic bacteria, all manner of toxins and foul odors.  That won’t do here.  Composting also requires the right amount of moisture.  Not so much that the little critters drown, deprived of oxygen, and not to dry either.  Just moist.  The food they eat should be a balanced diet of 30 parts carbon to 1 part nitrogen.  Given suitable conditions, nature does it’s thing, turning feces and carbonaceous material into nutrient dense fertilizer.

What I’m fascinated by about humanure is not the collection process.  That’s the easy part.  Sprinkle some sawdust in a bucket, do your thing, and cover it with more sawdust.  Fill the bucket and replace with empty.  It’s a little more involved than that, but the composting toilet is not where the real magic happens, it’s just the collection.

No, the good stuff happens in the compost pile.  If managed relatively well, an outside humanure compost pile will breakdown in a matter of months to a year or so.  A hot composting process will do this even faster, killing harmful pathogens by raising the temperature in the core of the pile by the accumulated metabolism of trillions of bacteria.  As the temperature rises, these bacteria begin to die from the heat stress while a different group that thrive at these temperatures flourish, generating yet more heat and higher temperatures until they too die, relinquishing the food supply and their bodies to others still.  The pile gets hot enough to kill pathogens like viruses, protozoa, parasites and harmful bacteria.  All of this activity breaks down the pile quickly into rich hummus.

That’s how to handle poop.

The Humanure Bioreactor

The conditions of my planned living arrangements do not warrant the construction of an outdoor compost pile.  See:  Dreams With Wheels.  Instead, I want to compost on the go.  It will have to be a small, self contained unit which provides all the conditions to bring about fast, effective hot composting.  This means regulating oxygen supply, managing moisture content and controlling the temperature.  I’m excited just thinking about it!  I’ve built a few hot compost piles, and in order to get the temperature to build, the pile has to be at least a cubic meter or so.  This is because the outer layers will help insulate the core where the temperatures will climb the highest.  Essentially, the outer layers lose heat faster than they generate it.  They’re cold, so they compost slower than the core where most of the action is happening.  Periodically, you have to turn the pile to give all the material a chance to compost at higher temperatures.  Why not insulate the thing?

Insulation

I certainly would not be the first to insulate a compost pile, people do it with straw bales and household insulation to keep core temperatures up during the cold months.  What I want to do is compost in a small vessel say, less than 30 gallons.  It would be a bucket or barrel with a LOT of household insulation surrounding it, including the lid.  I’d like to keep the vessel small and manageable so that turning it would require only a compost aerator rather than a shovel and pitchfork.  I don’t know if a batch this small can generate the heat to raise the temperature, but I don’t see why it wouldn’t if it were retained by sufficient insulation.

Oxygen

Additionally, air supply is crucial, so I think maybe a perforated plastic pipe inserted in the center could serve this purpose with exhaust gases drawn off the top.  This would probably have to be an active system with an electric fan.  A solar chimney may provide the necessary air flow when the conditions for it’s operation are met.  An aquarium air pump might also work, but I wouldn’t count on it.  The biggest issue I see with the air supply is providing the needed oxygen without losing heat and moisture in the batch.  That might be remedied with a heat exchanger, warming the incoming fresh air with the outgoing exhaust air.  Coupling the humanure digester with a terrarium containing plants that love moisture and heat, sized correctly would create a neat symbiosis between the bacteria and plants.  They could breathe the same air, the bacteria producing the CO2 the plants need, and the plants would in turn supply the bacteria with oxygen.  The warm, moist air would be more tightly controlled then.  Just a thought.  Of course the plants would grow in a medium largely composed of rich compost from the digester.

Preferably, the vessel I compost in will the be the same as the receptacle used in the toilet.  A five gallon bucket.  Rather than dumping the material back and forth or worse yet, shoveling it, I’d like to take the bucket from the toilet and insert it directly into an insulated “well” where it can go through the hot compost process.  Maybe a specially fixed lid with a thermometer probe and ventilation fixtures would be used.  I may find it useful to add material to help “fuel the fire” or otherwise want to disturb the contents of the bucket in which case I’ll use an aerator to manually mix the compost.

Cooling

Ideally, the temperature would rise over a few days to the target level of 155 to 165 degrees Fahrenheit and stay there for however long it takes to make the material safe.  I’m sure I’ll notice it plateau and fall.  It’ll be interesting to track this progression and note how agitation and air supply affect the process.  I must be careful not to exceed 165 degrees or so, to avoid killing the thermophilic bacteria.  If this happens I’ll need to dissipate the excess heat energy somehow.  One could increase the air flow, or if there were a great deal of heat available, a water jacket or water tubing coil wrapped around the bucket inside the insulating barrier could carry the heat away to a radiator or water tank.  There is possibly the opportunity to heat domestic water with this energy or at least preheat the supply to a water heating system.

One advantage of a simple hot compost pile is that it has so much mass that the temperature won’t fluctuate as quickly as a small bucket of compost.  One way to regulate the temperature swings would be a water jacket that changes in temperature more slowly than without the water mass.  Another, would be a a vessel jacket composed of a Phase Change Material like paraffin wax that melts at or about 130 degrees Fahrenheit.  See:  An Outlook On Phase Change Materials

Safety and Timing

I would like to see this work out so the feces becomes becomes relatively benign after two or three weeks.  I’ll likely have the material tested for dangerous pathogens and adjust the process accordingly.  I’m planning on filling a five gallon bucket in the toilet every month or so.  Preferably the hot composting could be done in less than three weeks.  I may find it useful to date and shelve the bucket after hot composting, where they can continue composting at a slower rate before discarding it.

A Shit Tree

It will be a good while before I’m comfortable using the compsoted materials for vegetable plants.  I’d rather not just discard the humanure I worked so hard to make, so instead, I think I’ll grow some potted plants like flowers and grasses that are low maintenance and maybe be cut off periodically to add back to the compost digester.  Maybe, I’ll grow small potted trees and plant them around the countryside when they get big enough.  I can be Johnny Shit Tree!  Okay, okay, too far.

The Whole Picture

This is all really just about taking responsibility for my life and my interaction with the rest of the universe.  Yeah, I said universe.  This whole biological life thing here on Earth is a natural consequence of a fundamental force that seeks to maximize entropy.  That is, to minimize potential energy, disperse collections of matter (very dense energy) evenly throughout the universe.  Life seems to be an organized phenomenon that does this quite effectively and has been doing so for billions of years.  As citizens of the green Earth, we have the responsibility to care for it and be good stewards of the organism which has birthed us and for which, in it’s way, found a usefulness in human beings to take part.  This form of waste management is certainly not the best way to handle human metabolism, but it is one conscious being’s effort to mimic the metabolism of the mother organism, with all it’s wisdom originating from a 3.8 billion year research and development project.

-M.C. Pletcher