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8.19 What percent of particles with diameter of 100 µm and a particle density of 2,650 kg/m3 are
removed in a 1,500 m2 rectangular sedimentation basin of that contains water at 10°C? Assume
the plant flow rate is 1.26×106 m3/day .
Solution:
8.20 A treatment plant has a horizontal flow sedimentation basin with a depth of 4 m, width of 6
m, and length of 36 m. It has a process flow rate of 400 m3/hr. What is the percent removal for
particles entering this basin, assuming they all have a diameter of 0.0029 cm, a particle density
of 2,650 kg/m3, and they are in water at a temperature of 10°C?
Solution:
8.21 Research the use of a filtration method that provides household (point-of-use) treatment in
the developing world. Write a one-page report that is clearly referenced. In your report, describe
the technology and address these issues: Is the technology affordable to the local population?
Does it use local materials and local labor for its construction? What are the observed health
improvements after implementation of the treatment system? What specific training do you
believe is required to ensure proper operation of the technology?
Solution:
Students’ responses will vary.
8.22 Research the global arsenic problem in the United States and in Bangladesh. In a two-page
essay, identify, compare, and contrast the extent of the problem (spatially and in terms of
population affected). What are the current methods of treatment employed to remove the arsenic
in both countries? What is the current regulatory standard for arsenic set by the EPA and the
guideline suggested by the World Health Organization?
Solution:
Students’ responses will vary.
8.23 A 1,000-g sample of naturally occurring sand was sifted through a stack of sieves and the
weight retained on each sieve is recorded as shown in the table below. Determine the effective
size and uniformity coefficient for the media.
Sieve
Designation
Sieve opening,
mm
Weight of
retained media,
g
10
2.000
140
14
1.400
160
18
1.000
170
20
0.850
100
25
0.710
90
30
0.600
85
35
0.500
80
40
0.425
70
45
0.355
65
Solution:
0
0.5
1
1.5
2
2.5
020 40 60 80 100
Percent of media passing sieve
Sieve size, mm
8.24 When Cl2 gas is added to water during the disinfection of drinking water, it hydrolyzes with
the water to form HOCl (Equation 8.13). Assume that the disinfection power of the acid HOCl is
88 times greater than the conjugate base, OCl-. The pKa for HOCl is 7.5. (a) What percentage of
the total disinfection power measured as free chlorine (HOCl + OCl-) exists in the acid form at
pH = 6? (b) At pH = 7?
Solution:
8.25 Given the following data, graph the data for the Poliomyelitis virus, using hypobromite as a
disinfectant. Determine the Chick’s law rate constant and the time required for 99.99 percent (4
log removals) inactivation of this virus.
Time
N
(s)
(# org/L)
0.0
1,000
2.0
350
4.0
78
6.0
20
8.0
6
10.0
2
12.0
1
Solution:
ln 0.0001 0.6092
15 s
t
t
=−×
=
8.26 Graph the following data for the inactivation of a virus using hypochlorous acid (HOCl).
Determine the coefficient of specific lethality and the time required to obtain 99.99 percent
inactivation using 1.0 mg/L of HOCl.
Time
log[N/N0]
1.0
-0.08
3.0
-0.64
5.0
-1.05
9.0
-1.87
15.0
-3.23
Solution:
K10 = -0.2127
For 99.99 % removal
log 0.0001 0.2127
18.8 s 0.31min
t
t
=−×
= =
Use the Chick-Watson equation to determine the specific lethality (
λ
).
ln / , min
NCt C mg L t
No
mg
λ
=−==
8.27 (a) Define the meaning of Ct product. (b) In addition to C and t, what factors influence the
rate of chemical disinfection? (c) What kind of microorganism is most readily inactivated by free
chlorine? (d) What kind is most difficult to inactivate?
Solution :
8.28 During drinking water treatment, 17 lb. of chlorine are added daily to disinfect 5 million
gallons of water. (a) What is the aqueous concentration of chlorine in mg/L? (b) The chlorine
demand is the concentration of chlorine used during disinfection. The chlorine residual is the
concentration of chlorine that remains after treatment so the water maintains its disinfecting
power in the distribution system. If the chlorine residual is 0.20 mg/L, what is the chlorine
demand in mg/L?
Solution:
8.29 Visit the Web site for the World Health Organization (www.who.org). Write a referenced
essay of up to two pages about a particular pathogen and associated disease that is transmitted
through contaminated water. What is the global extent of the public health crisis in terms of
spatial and population effects? Does the disease affect wealthy or poor communities? What are
the social and engineered barriers that can be used to reduce human exposure to the specific
pathogen?
Solution:
Students’ responses will vary.
8.30 Research the professional engineering society Water for People that works on water supply
in the developing world. Determine how you could contribute to such a professional society as a
student and after graduation. Provide specific detail about membership requirements, costs, and
how you might be involved.
Solution:
