Environmental Engineering Reference
In-Depth Information
extensively in Chapter 7), nitrogen can also have a
direct impact on the level of Do in a river via the
oxygen demand of the nitrification process. This impact
of excessive nitrogen levels is particularly prevalent
downstream of discharges of treated domestic wastewa-
ter where levels of organic- and ammonia-nitrogen are
high.
Modeling the fate and transport of nitrogen in
streams is more commonly done than for phosphorus,
and the essentials nitrogen models are described below.
0.25 m/d (Brigand et al., 2007). Lower values of nitrogen
removal rates, several-fold to more than an order of
magnitude lower, are typically found in laboratory-scale
studies compared with field-scale studies in streams.
EXAMPLE 4.21
nitrogen removal in a slow-moving stream occurs pri-
marily by denitrification at the sediment-water inter-
face with a denitrification rate constant of 0.16 m/d. If
the stream has a mean velocity of 3 cm/s and a mean
depth of 3.5 m, estimate the distance required for 50%
of the nitrogen to be removed by denitrification. If the
nitrate concentration at a particular location is 8 mg/L,
estimate the rate of removal of nitrate from the stream
at this location.
Nitrogen Models
Removal processes of nitrogen in streams are com-
monly associated with: (1) plant uptake; (2) storage and
burial in the sediment; and (3) denitrification. Denitrifi-
cation is commonly considered to be the dominant
process, although this is not always the case. Denitrifica-
tion is the reduction by anaerobic bacteria of one or
both of the ionic nitrogen oxides (
NO
−
and
NO
−
) to
gaseous oxides (no and n
2
o), which may be further
reduced to n
2
. Denitrification will not occur in streams
unless anaerobic conditions exist in either the water
column or in the sediment. Most denitrification occurs
at the sediment-water interface, denitrification in
flowing water is negligible in streams, and denitrification
in biofilms in the water column is potentially important.
At the sediment-water interface, nitrates are generated
by the nitrification process, and the generated nitrate
(along with nitrate in the water column) typically dif-
fuses directly from the water column to denitrifying
sites within the sediment. This is commonly referred to
as the
nitrification-denitrification process
. According to
Seitzinger (1988), nitrification is usually the major or
sole source of denitrification in most aquatic sediments,
however, the concentration of nitrate in the water
column can also be a controlling factor, particularly
when advective transport in the hyporheic zone is an
important transport process to denitrifying sites
(Brigand et al., 2007). In most cases, nitrogen removal
rates in streams are expressed as a mass of nitrogen
removed per unit area of projected stream bottom per
unit time, typically mg n/m
2
·d. This constant removal
rate does not consider the influence that nitrogen con-
centration has on the removal rates, and Brigand et al.
(2007) proposed the first order relation
Solution
From the given data:
k
n
= 0.16 m/d,
V
= 3 cm/s =
2592 m/d,
d
= 3 . m
, and
c
0
= 8 mg/L. Assuming that
denitrification is described by Equation (4.148), the con-
centration,
c
, as a function of time,
t
, can be expressed
as
k
d
t
n
c
=
c
0
exp
−
where
c
0
is the concentration at
t
= 0. If
t
50
is the time
for 50% removal of no
3
-n, then
k
d
t
n
0.5
0
c
=
c
exp
−
0
50
0.16
3.5
0.5
=
exp
−
t
50
which yields
t
50
= 15.16 d. If the travel distance for 50%
removal is denoted by
x
50
, then
x
=
Vt
=
(2592)(15.16)
=
39 300
,
m
50
50
Therefore, 50% removal of no
3
-n occurs over 39.30 km.
The removal rate of no
3
-n at the location where
c
= 8 mg/L is given by Equation (4.148) as
dc
dt
k
d
c
dc
dt
k
d
c
0.16
3.5
(8)
n
(4.148)
= −
n
= −
= −
=
0.366
mg/(L d
mg/(m d)
⋅
)
where
c
is the nitrate concentration in the water column
(ML
−3
),
t
is trav
e
l time (T),
k
n
is a mass-transfer coeffi-
cient (T
−1
), and
d
is the average depth of the stream (L).
Typical nitrogen removal rates in streams vary in the
range of 350-1250 mg n/m
2
·d and mass transfer coeffi-
cients can be expected to vary in the range of 0.07-
=
366
3
⋅
Since the depth of the stream is 3.5 m, the no
3
-n
removal rate per unit surface area of the stream can be
estimated as 366/3.5 = 10.3 mg/(m
2
·d).
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