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re: composting toilets
1 dec 1997
i live in a semi-rural part of the northeastern us, near philadelphia, pa.
my neighbor has about 50 sheep, 30 chickens, 2 dozen goats, 9 llamas, 2 dogs
and a cat. most of these creatures are not house trained :-) 

next to him are 147 new houses on 1/2 acre lots, with sewers. i live in an
old solar heated stone farmhouse on 5 acres, with about 1200 ft^2 of plastic
film solar glazing, and a minimally-functioning grandfathered septic system.
i would like to build another solar house or three on this property, but the
drainage is very poor, almost zero perc, with a seasonal high water table in
a flattish partial flood plain. there are no plans for sewers here.
perhaps my experiments will help bring them.

most of the new houses send their sewage to a treatment plant 10 miles away,
using overgrown blenders and lots of pumps and pipes. on-site treatment makes
more sense to me, but most of the available building land in pa is unsuitable
for a conventional septic system, or even a sand mound, these days. years ago,
i made a low transparent cover for my cesspool (a 6' diameter x 2' deep partly
collapsed pit lined with stones on my front lawn, about 30' from the road,
connected to the house by about 50' of 5" well casing butted together, buried
6" underground.) i was surprised to see lots of green plants growing under
the cover in the winter, hugging the glazing, seeking the sun.

i recently built a new geodesic cover made from 25 8' wood 2x3s, some 2" x 5"
thin metal mending plates, and 11 bolts. it's now covered with $20 worth of
0.006" uv greenhouse polyethylene plastic film with a 3 year guarantee and
a us r-value of about 1 ft^2-f-h/btu, with about 90% solar and 80% longwave
ir transmittance. i wonder what will eventually end up growing in there.

this "cover" is about 12' in diameter and 11' high, bolted to 5 used tires
full of gravel that sit on the ground, with a poly skirt to make it fairly 
airtight and contain anaerobic odors. i hope it will end up like a terrarium,
a not-very-constructed wetland with tall accidental self-selected plant life
inside which consumes c02 and makes oxygen, then dies to become compost that
consumes oxygen and makes c02. some sort of equilibrium, which might even
involve animals, eg stray cats and mice. depending on how things work out
in the spring, i may build an 17' diameter x 15' tall dome out of 25 2" x 10'
pieces of pvc drainpipe, cover it with $100 worth of clearer bayer urethane
greenhouse film with a 10 year guarantee, run some nylon ropes up to the peak
to help plants climb, and perhaps plant runner beans, cucumbers or tomatoes
inside. and add a picnic table, and another low cover over the cesspool. 

pa already has one approved evapotranspirative ("sundrive") alternative
septic system, a sort of indoor reed bed with an impervious bottom that
puts water vapor out into the atmosphere in the winter, using solar power,
and more recently, a dehumidifier. i suppose it wasn't very hard to get
approved, since it has no liquid discharge. but those greenhouses have to be
very large and expensive. it seems to me that a condensing system could be
smaller and more efficient, ie it could evaporate water faster, since the
latent heat of condensation would keep the greenhouse warmer. is that so?
i guess a system like this might even distill water for reuse.

the energy for the distillation, ie evaporation from the pond and plant
surfaces and condensation on the inside of the glazing, would come from the
sun (about 3 kwh/m^2-day on an average december day here, when the average
outdoor temperature is about 2 c) and the heat of the ground, kept moist
and unfrozen (i hope) by the condensed water that drips from the glazing
onto the ground. the soil here is mostly clay, and the average yearly
temperature is about 18 c. my ashrae hof shows moist clay having a thermal
conductance for downward heatflow of about 10 btu-in/h-ft^2-f, but as i
understand it, the main mechanism for upward underground heatflow is
evaporation of water from lower layers of soil and recondensation above.

a 10x10' pool sitting on top of the ground with 55 f soil 6' underneath
might have a thermal conductance of 10/72x100 = 14 btu/h-f, allowing
24h(55f-30f)14 = 8k btu of heat to flow upwards on a 30 f day, enough to
evaporate about 1 gallon of water. recessing the pool in the ground would
reduce the thermal resistance and increase this rate of evaporation. as
400 gallons of water cools from 70 to 30 f it loses 128k btu, enough heat
to evaporate 128 pounds or 16 gallons of water. about 375k btu of sun would
fall on a 16' cover on an average phila december day, enough to evaporate
another 47 gallons of water. plants with 1 ft^2 of 55 f moist leaves per
ft^3 of internal dome volume might lose 24h(55-30)2400ft^2x1.5 = 2 million
btu/day of sensible heat, enough to evaporate 270 gallons of water, if the
dome were filled with 30 f moist air. an outdoor air film moving at 10 mph
might have a conductance of about 2+10/2 = 7 btu/h-f-ft^2, limiting daily
heat transfer to about 24h(55f-30f)7x700ft^2 = 3 million btu, enough to
evaporate 370 gallons of water per day.

a system like this would have a critical freezing threshold, and hysteresis:
it might work well unless and until the condensation begins to freeze up
on the inside of the glazing to the point where things snap, when the frozen 
condensation reflects enough of the sun back out, keeping it out of the
enclosure, that the temperature drops and the greenhouse dries up and 
heatflow from the ground slows and the plants freeze and die and cease to
transpire. (i saw this happen in a sunspace with no plants, and too much
moisture. bright sun, frozen sunspace. it heated up dramatically after i
put a vapor barrier on the ground.)

if a pond has an initial temperature of 55 f, it needs a rc time constant
of at least 128 hours to end up at 40 f after 5 days without sun when it's
30 f outdoors, so a 16' diameter pond with an effective us r1 cover needs
to be about 2' deep to stay 40 f. if plant transpiration stops for 5 days,
it needs to store something on the order of 2,000 extra gallons, adding
another 15" of depth (and more thermal capacitance.)

i wonder if this sort of thing has been tried before, and what the results
were, and how it might be improved, and how to model it, and how to get it
approved, and what diameter and depth is required for a pond with an
impervious bottom, and how large and well-insulated the surrounding enclosure
has to be to evaporate and condense an average of 400 gallons of water per
day, which seems to be the local standard estimate for wastewater volume
for a single family.


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