Geoscience Reference
In-Depth Information
K
d
is the CBOD decay rate coefficient; and
K
NO
3
is the denitrification rate coefficient
of nitrate (day
−
1
), determined by Eq. (12.84).
The terms on the right-hand side of Eq. (12.74) represent the production of CBOD
due to phytoplankton death, settling of the particulate CBOD, oxidation of the
dissolved CBOD, and sink of CBOD due to denitrification, respectively. The stoichio-
metric constant 32/12 arises from the conversion between oxygen and phytoplankton
(as carbon) concentration. The constant (5/4)
·
(32/14) appears because for each g
of nitrate nitrogen reduced, (5/4)
·
(12/14) g of carbon are consumed, which reduces
CBOD by (5/4)
(32/12) g.
Note that the settling term in Eq. (12.74) is valid only for the 1-D and depth-averaged
2-D models. In the 3-D and width-averaged 2-D models, the settling process should
be represented by the settling term similar to that in Eq. (12.63).
The effective deoxygenation coefficient
K
d
is usually used to replace
K
d
f
CBOD
,
d
.
Typical values of
K
d
are in the range from 0.1 to 4.0 day
−
1
, with larger values for
untreated wastewater and smaller values for treated wastewater and natural waters.
Wright and McDonnell (1979) suggested the following relationship for
K
d
(day
−
1
)at
20
◦
C:
·
(12/14)
·
1.80
Q
−
0.49
K
d
(
20
)
=
(12.75)
where
Q
is the flow discharge (m
3
s
−
1
).
The deoxygenation coefficient
K
d
is also affected by water temperature and DO
concentration. These effects are considered by
C
DO
k
BOD
,1
/
2
T
−
20
K
d
=
K
d
(
20
)θ
(12.76)
d
+
C
DO
where
θ
d
is the temperature coefficient for deoxygenation, with a value of about
1.047; and
k
BOD
,1
/
2
is the Michaelis-Menten half-saturation DO concentration for
deoxygenation, with a value of about 0.5 gO
2
m
−
3
.
·
Nitrogen cycle
In natural aerobic waters, the nitrogen cycle consists of several steps of transformation.
Organic nitrogen (ON, contained in organic wastes and algae) is converted first to
ammonia nitrogen (NH
3
_N), which is then oxidized to nitrite (NO
2
_N) and nitrate
(NO
3
_N). Oxygen is required for the oxidation of ammonia to nitrite and then to
nitrate. In addition, phytoplankton utilize ammonia and nitrate nitrogen, and recycling
occurs to the organic forms as they die.
The sum of organic nitrogen and ammonia nitrogen is called the total Kjeldahl
nitrogen (TKN) in a laboratory analysis procedure. Most of the NBOD is due to TKN,
because the nitrite concentration in most wastewater streams and ambient waters is
very low, less than 0.1 mg
l
−
1
. Early models simulate TKN collectively (Thomann
and Mueller, 1987; Huber, 1993); however, many recent models, such as QUAL2E
and WASP, compute the nitrogen components individually. Introduced below is a
modeling framework for the nitrogen cycle, which is essentially similar to the WASP
·
