CE 428 Water and Wastewater Treatment Design
Problem Set 4
Due Date: April 25
Anaerobic Digestion
1.
Solve Problem 14-5 (page 1624)
Water Treatment Design
2.
A water sample is to be treated using alum or ferric chloride. The water has the following analysis.
Determine whether the water analysis is complete. Use the principle of electroneutrality.
(mg/L)
(mg/L)
Ca2+
37
HCO3119
Na+
6.5
SO4222
Mg2+
8.1
Cl20
K+
1.2
NO30.1
Fe3+
1.0
3.
For the water above, what will be the remaining bicarbonate alkalinity if a dose of 20 mg/L of (added
separately):
(1) Al2(SO4)3●14.2 H20
(2) FeCl3
4.
Design a rapid mixing system consisting of two tanks for a water treatment plant. The design flow is 30 x
103 m3/d. The design velocity gradient is 1000 s-1 and detention time is 30 seconds. The water temperature
ranges from 15 to 20 oC. Assume that the depth to width ratio is 1:25:1. Use a vane disk impeller with six
flat blades (Power number = 6.3) and the diameter is 40% of the basin width. The tank is not baffled.
Report the following:
(1) basin dimensions
(2) input power required
(3) impeller speed
5.
For the flow above, design a flocculator basin that can be connected to the rapid mix tank. The basin will
have horizontal shafts with paddle wheel as shown below. The detention time is 36 minutes. Assume
three compartments with average G value of 40 s -1 at 15o C. The slates of each paddle wheel should have a
width of 15 cm and length of 2.5 m. The outside blades should be clear the floor or wall by 0.30 m and be
0.3 m below the surface of the water. Clear space between adjacent paddle wheels, if any, should be
between 0.6 to 0.9 m.
Determine:
(1) basin dimensions
(2) paddle wheel design (there can be more than one paddle wheel for a given shaft)
(3) Power requirements for each compartment (if more than one)
(4) Rotational speed of the paddles
Provide a drawing (with dimensions) on how the rapid mix tank is connected to the flocculator.
2.5 m
6.
For the above design flow, estimate the number and dimensions of single media filter(s) needed. Specify
the number, and dimensions of the filter (length, width and depth), and depth of the sand media.
If the sand has the following sieve analysis, determine the clean water head loss. Porosity for the filter is
0.39 and the sphericity of the particles is 0.92.
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Sieve number
% (by weight) of sand retained
Mean size (mm)
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16 - 18
1.6
1.09
18 - 20
5.3
0.92
20 - 25
12.5
0.77
25 - 30
25.1
0.65
30 - 35
23.4
0.54
40 - 45
15.1
0.46
45 - 50
13.8
0.38
45 - 50
2.2
0.32
50 - 100
0.8
0.27
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