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re: building cheap water collection surface?
18 jun 2004
brent geery   wrote:
 
>>i can get some cheap desert land in an area i really like, but the
>>problem is access to water... we are talking an arid region, with
>>about 4" average yearly precipitation...

i wonder where...

>if there's lots of sun, why not distill the water you drink in
>a greenhouse and filter the rest through a high-temp wetland
>in the greenhouse that serves as the evaporator?

take phoenix, please. nrel says 1020 btu/ft^2 of sun falls on the ground
and 1550 falls on a south wall on an average 53.6 f january (the worst-
case month for solar heating) day, so a 16'x16'x8' tall greenhouse would
collect about 0.8(256x1020+128x1550) = 368k btu of sun. distilling 10
gallons of potable water at 1,000 btu/lb would leave 368k-83k = 284k btu.
with 600 ft^2 of r1.2 surface, the greenhouse thermal conductance would
be 500 btu/h-f, so 284k btu could keep it 53.6+284k/(24hx500) = 80 f (27 c)
on an average day.

you might put a outer layer of plastic film over 4 16' long x 8' tall
outer bows and another layer over 4 inner bows, with a slot at the top
to allow warm moist air to enter the spaces between the glazings and
condense water into east and west troughs on the ground made from a fold 
in the outer layer, with a slot in the inner layer above the trough to
allow the air to recirculate. the troughs might also collect rainwater.
the greenhouse might have a gravel bed over an epdm layer to make an
artificial wetland.

metcalf and eddy say an average family produces 400 gallons per day of
wastewater with a 200 mg/l bod concentration, ie about 0.7 lb/day, and
it needs about 175 ft^2 of surface for treatment. sherwood reed says
blackwater contains more than 75% of the nitrogen in only 40% of the
flow. eliminate that with a composting toilet, and you might only treat
240 gallons per day (0.908 m^3/day) with 0.175 lb of bod, ie 86 mg/l.

can we purify this to 10 mg/l (a tertiary sewage treatment standard)
and recycle it for non-potable uses? pe sherwood reed's natural systems
for waste management and treatment book (mcgraw hill, 2nd edition, 1995)
has a detailed design procedure for constructed wetlands, aka "attached-
growth biological reactors." they work well at higher temperatures.

equation 6.38 on page 226 of reed's book says the rate constant for
bod removal in subsurface-flow wetlands at 20 c is k20 = 1.104d^-1.
equation 6.34 makes k27 = 1.104x1.06^(27-20) = 1.66d^-1 at 27 c. we
need to reduce the bod from co = 86 to ce = 10 mg/l with 256 ft^2
(23.8 m^2) of surface. if the bed is "medium gravel" (d10 = 32 mm)
with a 0.38 porosity, it needs to be 0.908ln(co/ce)/(k27x23.8x0.38)
= 0.13 meters deep, ie 5.1 inches. we might add gravel or divide
the bed into 3 series cells for better plug flow and performance.

what will grow in this rainforest? will it need summertime shading?

nick

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