RIVERS AND STREAMS - Water Quality Engineering in Natural Systems - page 138

4.34. After initial mixing with a wastewater discharge,
the concentration of Do is 6 mg/L, the concentra-
tion of organic nitrogen is 4 mg/L, and the concen-
tration of ammonia nitrogen is 5 mg/L. The river
has a temperature of 20°C, an average velocity of
3 cm/s, and an aeration rate constant of 0.6 d −1 .
The rate constants in the nitrification process
are k oa = 0.3 d −1 , k ai = 0.2 d −1 , and k in = 0.4 d −1 .
Determine the minimum Do in the stream and
how the minimum oxygen concentration would be
affected if nitrification is inhibited. In the absence
of inhibition, what is the ammonia concentration
at the location where the Do concentration is a
minimum?

0.8 d −1 , a TKn of 3 mg/L, and a nitrification con-
stant of 0.5 d −1 . There is negligible BoD removal
by sedimentation. The chlorophyll a concentration
in the river is 10 µ g/L. Estimate the magnitude
and location of the minimum Do concentration
in the river.
4.39. Distributed oxygen sources and sinks collectively
produce a net oxygen demand of 1.00 mg/(L·d) in
a stream that moves with an average velocity of
3 cm/s and has an aeration rate coefficient of
0.50 d −1 . What is the ultimate oxygen deficit caused
by these processes? How far downstream from the
beginning of these oxygen sources and sinks would
the induced deficit be 95% of its ultimate value?
4.40. A waste effluent is discharged into a river that has
a mean velocity of 5 cm/s and a temperature of
20°C. After initial mixing across the river, the Do
concentration of the mixed river water is 5 mg/L,
the ultimate BoD is 25 mg/L, the laboratory rate
constant for BoD decay in the river water is
0.4 d −1 , and the reaeration constant is estimated to
be 0.8 d −1 . Lateral distributed BoD sources extend
5 km downstream of the discharge location, and
the distributed BoD sources have an average ulti-
mate BoD of 4 mg/L. BoD removal by sedimen-
tation is estimated to increase the BoD decay rate
by 15%. Assess the impact of the distributed BoD
input on the oxygen concentration 5 km down-
stream of the discharge location.
4.41. A waste effluent is discharged into a river that has
a mean velocity of 6 cm/s. Analysis of the waste
discharge and distributed BoD input over a 1-km
section of the river indicates that the Do 1 km
downstream of the waste outfall is expected to be
8 mg/L, where it is assumed that the reaeration
constant is equal to 0.8 d −1 , and the average tem-
perature of the stream is 20°C. Photosynthesis
within the river section is expected to generate
7 mg/L·d, plant respiration 4 mg/L·d, and sediment
oxygen demand 2 mg/L·d. Estimate the fluctuation
in Do expected 1 km downstream of the outfall.
4.42. A section of a slow-moving river has diurnal Do
variations where the minimum Do deficit occurs
3 hours after solar noon, Do that varies over a
range of 7.5 mg/L, and a daily mean oxygen deficit
of 1.5 mg/L. If the photoperiod is 12 hours, use the
delta method to estimate the reaeration constant,
mean photosynthesis rate, and mean respiration
rate. How do these results compare with those
obtained using the approximate delta method?

4.35. A nitrification model estimates the ammonia-
nitrogen concentration at a particular location in
a stream as 3.77 mg/L. If the pH at this location is
8.5 and the temperature is 15°C, what is the con-
centration of undissociated ammonia?

4.36. Repeat Problem 4.25, accounting for a respiration
oxygen demand of 3 g/m 2 ·d and a benthic oxygen
demand of 5 g/m 2 ·d. Determine the initial Do
deficit that will cause the critical oxygen level to
occur at the outfall.

4.37. A wastewater treatment plant is being designed to
discharge into a 10-m-wide stream that has an
approximately rectangular cross section, a longi-
tudinal slope of 0.05%, and a Manning's n of 0.01.
Under design conditions, the flow is 1.75 m 3 /s, the
Do concentration is 7 mg/L, the BoD is 2 mg/L,
and the temperature of the stream is 25°C. It is
anticipated that the plant discharge will have a
BoD of 30 mg/L and will contain no Do. Field
measurements have indicated that downstream of
the proposed discharge location, the sediment
oxygen demand is 5 g/m 2 ·d, and the chlorophyll a
concentration is 20 µ g/L. It is further estimated
that when the plant discharge is operational, the
mixed stream/wastewater will have a BoD decay
constant of 0.1 d −1 , and the reaeration constant
can be adequately approximated by the o'Connor
and Dobbins, (1958) formula. Water-quality regu-
lations require that the Do concentration in the
stream be maintained at a level greater than or
equal to 5 mg/L under design conditions. What is
the maximum plant discharge that will meet this
regulatory requirement?

4.38. A wastewater is discharged into a river that has a
mean velocity of 5 cm/s and a flow depth of 2.8 m.
After initial mixing, the river has a temperature
of 20°C, an oxygen concentration of 7 mg/L, a
CBoD 5 of 10 mg/L, a CBoD rate constant of

4.43. A stream is estimated to have a denitrification rate
constant of 0.25 m/d, a mean velocity of 2 cm/s,