Biomedical Engineering Reference
In-Depth Information
Figure7.1
Schematichydrauliccontainment
Another approach sometimes encountered is the production of vegetative caps,
which has found favour as a means of finishing off some American landfill sites.
The principle involves planting to preventing the downward percolation of rain-
water into the landfill and thus minimising leachate production while at the same
time reducing erosion from the surface. The method seems to be successful as a
living alternative to an impermeable clay or geopolymer barrier. The vegetative
cap has also been promoted for its abilities to enhance the biological breakdown
of the underlying refuse. In this respect, it may be seen as an applied form of rhi-
zodegradation or even, arguably, of phytodegradation. How effective it is likely
to be in this role, however, given the great depths involved in most landfills and
the functionally anoxic conditions within them, appears uncertain.
To understand the overall phytoremediation effect of hydraulic containment, it
is important to realise that contaminating organics are actually taken up by the
plant at lower concentration than they are found
in situ
, in part due to membrane
barriers at the root hairs. In order to include this in a predictive mathematical
model, the idea of a transpiration stream concentration factor (TSCF) for given
contaminants has been developed, defined as TSCF
=
0
.
75 exp
{−
[
(
log K
ow
−
2
.
50
)
2
/
2
.
4]
, (Burken and Schnoor, 1998) where K
ow
is the octanol-water par-
tition coefficients. These latter are a measure of the hydrophobia or hydrophilia
of a given organic chemical; a log K
ow
below 1 characterises the fairly soluble,
while above 3.5 indicates highly hydrophobic substances.
Thus the uptake rate (U in mg/day) is given by the following equation
}
U
=
(TSCF)TC
Where,
TSCF
=
transpiration stream concentration factor, as defined
T
=
transpiration rate of vegetation, l/day
C
=
concentration in site water, mg/l.
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