Geoscience Reference
In-Depth Information
resource is exhausted and the phytoplankton begins to use the available nitrate. For
nitrate uptake, the initial step is a reduction to ammonia that produces oxygen. Thus,
the total rate of oxygen production rate due to phytoplankton growth is (Wool
et al
.,
1995)
K
Phy
32
C
Phy
48
14
a
NC
(
=
12
+
−
p
NH
3
)
P
1
(12.72)
where
p
NH
3
is the preference factor for ammonia uptake, determined by Eq. (12.81);
and
a
NC
is the phytoplankton nitrogen-carbon ratio. The stoichiometric constant
32/12 arises because 32/12 g of oxygen corresponds to1gofphytoplankton pro-
duced by the growth, and the constant 48/14 arises because 48/14 g of oxygen is
produced for 1 g of phytoplankton nitrate reduced. Note that the
P
determined by
Eq. (12.72) is substituted into Eq. (12.53).
Oxygen is diminished in the water column as a result of phytoplankton respiration,
which is basically the reverse process of photosynthesis. Thus, the rate of oxygen loss
due to phytoplankton respiration is
32
12
K
M
C
Phy
R
=
(12.73)
which is subsitiuted into Eq. (12.53).
In addition, phytoplankton may be predated by zooplankton. This can be modeled
by considering the predator-prey relation and the nutrients/food chain interaction. The
details can be found in Chapra (1997).
Carbonaceous BOD
CBOD represents the oxygen demand by bacteria in the oxidation of organic (carbona-
ceous) matters present in a waste. CBOD is exerted by the presence of heterotrophic
organisms that are capable of deriving the energy for oxidation from an organic carbon
substrate. A large number of these heterotrophic organisms are contained in municipal
sewage as well as most rivers, estuaries, and lakes.
CBOD may be particulate or dissolved in the water. The change in concentration of
CBOD usually results from both settling of the particulate CBOD and oxidation of the
dissolved CBOD. The oxidation of the dissolved CBOD can be represented by a first-
order kinetic process. In addition, CBOD is generated as a result of phytoplankton
death and loss due to denitrification reaction under low DO conditions. Thus, the
kinetic rate of CBOD can be determined by
DC
CBOD
Dt
32
12
K
M
C
Phy
−
ω
CBOD
h
=
(
1
−
f
CBOD
,
d
)
C
CBOD
5
4
32
14
K
NO
3
C
NO
3
K
d
f
CBOD
,
d
C
CBOD
−
−
(12.74)
where
f
CBOD
,
d
and 1
f
CBOD
,
d
are the fractions of the dissolved and particulate CBOD
in the total CBOD, respectively;
−
ω
CBOD
is the settling velocity of particulate CBOD;
