Environmental Engineering Reference
In-Depth Information
(a) How far downstream of the discharge loca-
tion will the wastewater be well mixed across
the river? Call this location X .
(b) If the rate constants for BoD and reaeration
are 0.1 and 0.7 d −1 , respectively, it is assumed
that the wastewater is well mixed across the
river at the outfall location, and BoD removal
by sedimentation is negligible, estimate the
5-day BoD and Do at location X .
(c) What conclusions can you draw from the
results of this problem?
river is 0.6 d −1 at 25°C, the velocity is 25 cm/s, and
the depth is 178 cm.
(a) What is the ultimate BoD 24 km downstream
from the outfall?
(b) What is the Do deficit 24 km downstream of
the effluent discharge?
(c) At the location 15 km downstream of the
effluent discharge, how much oxygen (kg/d)
would have to be added to have the Do at
7.0 mg/L?
(d) Is it possible to reduce the BoD of the efflu-
ent to maintain the Do above 7 mg/L at all
places in the river? If so, what is the target
BoD of the effluent?
4.29. A dairy processing plant is planning to discharge
wastewater at a rate of 5 m 3 /s into a river that has
a 7 Q 10 discharge of 30 m 3 /s. Available data indi-
cate that the river has a bottom width of 20 m, side
slopes of 2:1 (H :v), depth of flow of 3 m at a dis-
charge of 35 m 3 /s, average roughness height of
1.5 cm, Do concentration of 8.5 mg/L, 5-day BoD
of 5 mg/L, temperature of 22°C, and average
annual wind speed over the river of 5 m/s. The
milk processing waste will have negligible Do and
be at approximately the same temperature as the
river. Laboratory tests on the combined river
water and wastewater indicate that the BoD
decay factor of the mixed water is 0.15 d −1 at 22°C,
and BoD removal due to sedimentation is negli-
gible. The dairy plant is to be located in a state
where the Do in the stream is required by law to
be greater than or equal to 5.0 mg/L, and the
boundary of the mixing zone is located 800 m
downstream of the wastewater discharge.
4.31. A tannery discharges wastewater into a river
under the following conditions:
Parameter
Wastewater
River
Flow (m 3 /s)
1.148
7.222
BoD at 15°C (mg/L)
-
7.66
Do (mg/L)
1.00
6.00
Temperature, °C
15.0
15.0
Speed (m/s)
-
0.300
Depth (m)
-
2.92
The BoD rate constant, k d , in the river/wastewater
mixture is measured as 0.379 d −1 at 20°C. Deter-
mine the ultimate BoD (in kg/d) that the tannery
may discharge and still keep the Do in the
minimum Do in the river above 5 mg/L.
4.32. A river has a flow velocity of 3 cm/s, an aeration
coefficient of 0.5 d −1 , and a temperature of 20°C.
After initial mixing with a wastewater, the river
has a Do concentration of 8.0 mg/L, an ultimate
BoD of 25 mg/L, and a BoD decay constant of
0.8 d −1 . neglecting the effect of sedimentation on
BoD removal, determine the distance over which
the Do is less than or equal to 7 mg/L.
(a) Calculate the required length of the diffuser
such that complete cross-sectional mixing
occurs at approximately 100 m downstream
of the diffuser. Does significant decay in BoD
occur during cross-sectional mixing?
(b) Using the calculated diffuser length, estimate
the Do in the river 100 m downstream of the
diffuser, and find the maximum 5-day BoD of
the dairy wastewater that will cause the river
to meet the Do criteria on the boundary of
the mixing zone.
4.33. Treated domestic wastewater is discharged into a
river with an average flow velocity of 20 cm/s and
an estimated reaeration coefficient of 0.4 d −1 . The
TKn of the mixed wastewater discharge is
0.9 mg/L, and the nitrification rate constant is esti-
mated to be 0.5 d −1 .
4.30. A river has an upstream flow of 2.27 m 3 /s, an ulti-
mate first-stage BoD of 5 mg/L, and a Do of
6 mg/L. An effluent enters the river with a flow of
0.283 m 3 /s, a 5-day BoD of 20 mg/L, and a Do of
zero. A long-term CBoD test on the effluent has
indicated a BoD decay constant of 0.25 d −1 , and
there is negligible BoD removal by sedimenta-
tion. Do saturation in the river is 8.26 mg/L at a
temperature of 25°C. The CBoD decay rate in the
(a) Use the nBoD model to estimate the oxygen
deficit due to nitrification 10 km downstream
of the wastewater discharge.
(b) If the theoretical oxygen deficit neglecting
nitrification is 5.5 mg/L, what is the percent
error incurred by neglecting nitrification?
 
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