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greenhouse on-site septic systems
29 may 1998
kelly mashford writes:
>...i am a final year environmental engineering student at deakin
>university in melbourne, australia. my major project focusses on what
>onsite wastewater treatment and disposal systems could be applicable on
>isolated rural communities in australia - particularly for central
>australian (desert/ semiarid) aboriginal communities.
sounds like water conservation and reuse and irrigation would be useful.
>in australia, disposal guidelines are currently being reassesed by several
>state regulatory bodies to incorporate 'new' designs which will allow for
>greater flexibility in how guidelines are met. i am very interested to
>know about what systems (apart from septic tanks) are gaining/ have gained
>approval from regulatory bodies and how they are actually performing
>once installed - particularly in isolated areas.
pennsylvania (ne usa) has an old reliable approved alternative "sundrive"
evapotranspirative system which uses reeds (or tomato plants) inside a
greenhouse with an impervious bottom to evaporate wastewater from a nearby
house and discharge the vapor into the atmosphere. i think some newer
versions use mechanical dehumidifiers. having no liquid discharge makes
regulators more comfortable, but this isn't a very energy-efficient system,
since the vapor isn't condensed inside the greenhouse, so the latent heat
is wasted: about 1000 btu/lb of water, about the same as the amount of sun
that falls on a square foot of south wall in the winter here.
condensing the vapor by moving the moist air between the two inflated
polyethylene film layers that make the north (south in au) greenhouse
("polytunnel") wall might be a first step in raising the efficiency and
decreasing the needed greenhouse size. if the condensing vapor could
somehow give up its heat to incoming wastewater, so much the better.
"multiple-effect distillation" is a lot more energy-efficient than
simple distillation. it usually involves boiling water at higher than
atmospheric pressure, then condensing it on some surface with water at
lesser pressure on the other side, to make that water boil, and so on...
here's an interesting variation:
hodges, c n, thomson, t l, groh, j r [who also worked on insulating
greenhouses by filling the poly pillows with soap bubbles], and
frieling, d h, (1966), "solar distillation utilizing multiple effect
humidification," environmental research laboratory, university of
arizona, tucson az, a 159 page report.
a partial description from meinel and meinel's "applied solar energy":
an efficient multiple-effect solar distillation system was designed by
hodges et al (1966)... a pilot plant constructed at puerto penasco,
sonora, mexico, produced 19,000 liters/day (5000 gal/day) from a simple
solar collector of 1000 m^2 (10,400 ft^2). the solar collector [was]
a series of long shallow ponds of water covered by a single layer of
plastic [with another floating layer]... stretched tight because the
air pressure inside the collector was slightly above atmospheric...
[ie the top layer was statically inflated with a small blower. the bottom
layer was black polyethylene plastic, lying on leveled desert sand.]
the multiple effect in this system was obtained not by pressure gradients
but by simply reusing the airstream that was dehumidified by the cooling
water [incoming sea brine, which was heated by outgoing condensate.] after
a first pass through the hot water in the evaporating tower, the water
vapor was condensed out by the cooling water system, this "dried" air
being again passed through the warm water in a lower section of the tower
and rehumidified for subsequent condensation in the cooling portion of
the system. the net result was a multiple-effect system that effectively
used one-eighth of the heat of vaporization to distill one gram of water.
the mexican system used a 50' tall tower packed with 3" plastic pall rings
for the evaporator, and a tower full of fin-tube pipe next to it as the
condenser, in a counterflow system with multiple horizontal "bleed through"
air ducts at 5 different heights connecting the two towers. can this be
done with an inexpensive plastic film greenhouse with some wastewater ponds
covered with plastic film on the sunny side, and vegetable plants or reeds
for the evaporating and condensing surfaces, and a fan blowing counterflow
air in a horizontal loop, with the "bleed through ducts" being holes in a
lengthwise vertical plastic film wall dividing the north and south halves
of the greenhouse? the greenhouse might also be used for composting, or
rainwater collection and storage, with a "roof washer" made from a u-shaped
poly film fold near the ground with a few small holes in the bottom.
nick
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