Biofilters
The flow through a submerged filter (that is, a filter which is fully submerged in the
water) can, with reasonable approximation, be considered to be plug flow. Fig 5.11
shows a submerged filter. For a infinitesimal section, the following balance can be
set up under stationary conditions:
in - removed =
out
as
QS - rv:s Ady = Q (S+-a )dy
'
y
where Q
is the flow through the filter
S
is the concentration
A
is the cross sectional area of the filter
rv;s is the volumetric removal rate of the filter
as
A
-=-rvs- ay
, Q
or dimensionless:
s=: 1 , TJ=R, HA=V, 8=~
~=- rv,s e
~
51
where e is the hydraulic retention time (corresponding to an empty filter).
(5.36)
(5.37)
(5.38)
Here rv;s = ro · rA,S can be substituted where ro is the surface area of the filter
medium per unit volume of the filter. For rA,S the three cases observed during the
biofilm diffusion can be substituted:
a_. I
....__A--+
QS
t
rv,s
r t
Q(S + ~ ~ dy)
H
1
T
y
-*d
..:Ly
AH
1-Q
-+
zero order reaction
s
Fig 5.11 Mass balance for a submerged filter in which a heterogeneous process takes place, and
the concentration distribution in the filter if a zero order reaction occurs.
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