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re: reed bed septic
13 jun 1997
george brothers   wrote:

>anybody have any information on reed bed septic systems as an alternative 
>to a conventional septic bed -

check out the book natural systems for waste management and treatment, mcgraw 
hill, 2nd edition, 1995, isbn 0-07-060982-9, by sherwood reed, pe, among others
who also wrote a couple of epa handbooks on reed bed design, available from
the small flows clearinghouse in west virginia.

>i believe they incorporate some solar power to help aeorate (sp?)the sludge -
>or i may have two different types confused

aeration helps. reed's book doesn't talk much explicity about solar power, but
it's full of temperature-dependent equations like this one for the wetland
area required to reduce biological oxygen demand ("bod," a measure of the
harmful tendency of wastewater to suck oxygen out of streams, which depends
on the nutrients left over after wastewater treatment.)

   as = q(ln(co)-ln(ce)+ln(a))/(kt(y)(n)), where

   as = surface area of wetland, m^2
   co = influent bod, mg/l
   ce = effluent bod, mg/l
   a = fraction of bod not removed as settleable solids near the headworks
       of the system, 0.75 for secondary effluent
   q = average flow in the system, m^3
   kt = k20^(1.06(t-20)), t in degrees c
   k20 = 0.28/day
   y = average water depth in the system, m
   n = porosity of system (fraction of space available for water to flow),
       0.65 for dense mature vegetation

as the system temperature goes up, kt increases, and the required wetland area
goes down. putting a reed bed inside a greenhouse may keep it a lot warmer if
water vapor evaporated by the sun condenses on greenhouse walls instead of
blowing away as vapor in the wind, so the latent heat of about 1000 btu/lb
is reclaimed. 

page 72 of reed's book, has an equation for reducing fecal coli concentration
in a series of ponds:

  cf/ci = 1/(1+tkt)^n, where

  ci = influent fecal coli concentration, #/100 ml
  cf = effluent fecal coli concentration, #/100 ml
  t = actual detention time in a cell, in days
  n = the number of cells in series
  kt = temperature-dependent rate constant = 2.6(1.19)^(tw-20)/day
  tw = mean water temperature in cell, c.

so, it looks like reducing the coliform content of 400 gallons per day of
wastewater from a typical value of 10^7/100 ml to 0.1/100 ml using 4 ponds
in series with some airflow above them requires 1+t(kt) = 100, or with
t = 400/v, individual pond volumes

v = 400x99/(1.19^(tw-20)),

eg 13,000 gallons at 20 c, or 500 gallons at 40 c (104 f), eg 4 4' plywood
cubes lined with epdm rubber, the last of which might also serve as a hot tub,
for brave of foolish people like howard reichmuth, pe, while also preheating
water for showers via a simple heat exchanger. this bodes well for combining
higher temperature sewage treatment with multiple-day thermal storage for
solar house heating, using a few small insulated septic tanks in a basement
or sunspace, with an air inlet piped to a rooftop vent. 

page 98 of reed's book has a similar equation for bod reduction in wastewater
stabilization ponds:

  cn/co = (1+tkc)^n, where

  cn = effluent bod5 concentration, mg/l
  co = influent bod5 concentration, mg/l
  kc = complete mix first order reaction rate = 1.2(1.085)^(t-35) per day
  t = hydraulic residence time in each cell, days, and 
  n = number of cells in series

it looks like the system above would reduce bod from a typical 200 mg/l
to an acceptable value of 200/(1+400/500x1.2(1.085)^(40-35))^4 = 6 mg/l,
well below the minimum national standard of 30 mg/l.

freezing reed beds are also used for sludge dewatering... 

nick




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