Environmental Engineering Reference
In-Depth Information
becomes Eq. (9.1) for CBOD and the following for the DO deficit:
LK
0
d
Kt
K t
K t
D
e
e
D
e
(9.21)
11
a
a
t
0
KK
a
11
Rather than use a negative sedimentation rate, in some models resuspension, resulting from scour or
from lifting by rising gases, is viewed as a distributed source of CBOD. This distributed source may be
summed with overland runoff, baseflow, and contributions from minor tributaries as a line source of
CBOD,
L
rd
. The effect of this CBOD source on DO can be computed as follows:
KL
L K
K t
Kt
K t
K t
D
d d
(1
e
)
d d
e
e
D
e
(9.22)
a
11
a
a
rdt
0
KK
(
K
K K
)
a
11
a
11
11
where
D
rdt
= equals to the dissolved oxygen deficit resulting from a distributed source of CBOD. Again
the results of Eqs. (9.5), (9.20), and (9.22) may be summed to determine the deficit resulting from the
combined effects of CBOD, NBOD, and a distributed source of CBOD.
Combining Eqs. (9.22) and (9.1) allows for settling and resuspension to be modeled as occurring
simultaneously in a reach, and, in reality, both removal and addition of CBOD to the flow can occur in a
heavily polluted stretch of river that includes sludge deposits. However, depending principally on the
velocity and temperature, either removal or addition will dominate. Further, in practical application of
stream models to a reach where
L
rd
and
K
3
are determined from in-stream CBOD measurements, where
L
m
,x
is the measured CBOD at point
x
, and
K
d
has been taken from a “bottle” estimate, either
L
rd
or
K
3
will be set equal to zero as follows:
!
K t
e
x
LLL
then CBOD is being added to the water column and
L
rd
will be determined such that the measured CBOD
concentrations are matched and
K
3
= 0 and
K
11
=
K
d
. Whereas
if
m,
0
x
LLL
then CBOD is being removed faster than by the oxidation process and
L
rd
= 0 and
K
3
equals the
increment beyond
K
d
needed to match the measured CBOD concentrations. Thus, selection of computational
reaches should consider which process dominates each section of the river.
Sediment oxygen demand (SOD)—
SOD represents the combined effects of the oxygen diffusion
into the benthic layer to satisfy the oxygen demand in the aerobic zone of this layer and the removal of
oxygen by purging action of gases rising from the benthic layer. Di Toro (2001) has shown that the SOD
is not restricted to just times when aerobic conditions are present in the surface sediments. Di Toro (2001,
p. 167) proposed a model of SOD that dispenses with the complexity of the various processes that affect
SOD by relating SOD to the flux of oxygen equivalents of all reduced substances in the benthic pore
water without specific regard to the causing processes. Thus, SOD can be computed from a mass balance
model of the oxygen equivalents in the sediment. The organic carbon and settled algae in the benthic layer
are mineralized anaerobically. Both reactions are sinks for oxygen and quickly drive the oxygen in the
benthic top layer negative. This negative concentration indicates a redox state that in the benthic layer that
is reduced rather than oxidized. The calculated negative concentration is taken as the oxygen equivalence
of the reduced intermediate products yielded by the mineralization reaction. The reduced carbon
intermediates (expressed as oxygen equivalents) are assumed to be transported across the sediment
interface and are oxidized to CO
2
and H
2
O in the overlying water column. In this approach, the SOD is
computed as the transport of oxygen and oxygen equivalents across the sediment water interface and is
controlled by the decomposition of organic carbon in the sediment and the DO concentration in the
overlying water (DUFLOW, 2000).
if
K t
e
m,
0
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