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re: composting toilets/outhouses in remote areas
3 jun 1998
pa seo rich  wrote:

>either a composting or incinerating toilet will work...

i can imagine an "incinerating" toilet with a tank
8' above the ground, and a sunspace below to dessicate it.

i've read that some earthships and western park visitor centers have
sun powered toilets. the humanure handbook says average adults produce 
40 ounces of feces and urine per day, ie about 2.5 pounds. completely
evaporating that requires at most 2500 btu, or 10k btu per day for a
family of 4, about 30 cents worth of electrical energy at 10 cents/kwh,
or about the same as the amount of sun that falls on 10 square feet of
vertical south glazing on an average january day near philadelphia.

assuring complete pathogen death by exposure to say, 122 f for 24 hours
(one point on the safety zone curve on page 133 of the humanure handbook)
takes even less energy and glazing area.

a 2nd floor toilet on the south side of a house might have a shallow
sunspace below to collect heat for evaporation (like window seat toilets
in old castles, but glazed below), and a couple of pans under the toilet
that can be emptied of dry feces from the outside, with the water vapor
condensing on some surface that gives some of the solar heat back to the
house in the winter. those pans need a large surface. but why evaporate
all the water, if low-temperature pasteurization would do? these days, a
tiny computer might keep a temperature record to make sure it's working,
and automatically drain disinfected septage into some sort of reed bed.

a very small holding tank might be electrically heated to 145 f for 1 hour
every day at 3 am, then automatically flushed. with good insulation,
that takes about 10lb(145f-70f) = 750 btu, about 2 cents worth of
electricity per day. this could be done more quickly and often in
an insulated low-flush toilet trap.

a continuous system might use a 12v marine-style macerator ("blender")
and counterflow heat exchanger. the ashrae handbook of fundamentals says
a couple of 10' pvc pipes, say 4" and 8" in diameter, with a heater and
lots of insulation at one end, and incoming and outgoing septage slurries
at the other, would have a "number of transfer units" ntu = au/cmin
= 10ftxpix4"/12x10btu/h-f-ft^2/(10lb/24h) = 240, and an "effectiveness"
e = ntu/(ntu+1) = 0.996, so maintaining 122 f makes the outgoing flow
temperature 122-e(122-70) = 70.2 f, and the heat exchange only needs
10lb(70.2-70) = 2 btu/day, like 2 kitchen matches. surrounding the pipe
with 6" of fiberglass insulation makes the daily exterior heat loss about
24h(96f-70f)28ft^2/r19 = 920 btu, ie 3 cents worth of electricity.

alternatively, a 1'x4'x1' thick box with an internal horizontal partition
made from 8 layers of 1.5" thick styrofoam cut from a single 4x8' sheet,
held together with silicone caulk and deck screws would have... au/cmin
= 4ft^2x10btu/h-f-ft^2/(10lb/24h) = 96, e = 0.99, tho = 70.5, and an
exterior heat loss of about 24h(96-70)18ft^2/r22.5 = 500 btu/day.

i wonder if people make systems like these, and how their cost compares
to those using chlorine or ozone for disinfection.


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