Water and Wastewater Engineering
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13-48 WATER AND WASTEWATER ENGINEERING
16. Given a set of data of contact times and corresponding chlorine doses to achieve a
given percent inactivation, determine a Ct value.
17. Select a primary and a secondary disinfectant for a given set of water quality data.
18. Calculate the weight percent of a chlorine compound.
19. Design a contact chamber for disinfection by chlorine compounds given the required Ct.
20. Design a contact chamber for ozone disinfection given the required Ct.
21. Determine the appropriate Ct value required for a given water quality, predisinfection
treatment, and disinfectant.
22. Determine the appropriate fluoride dose for a given tem
perature.
23. Calculate the feed rate of a given fluoride compound to achieve a given fluoride dose in
a given water flow rate.
13-7 PROBLEMS
13-1. What is the pH of a water at 25 C that contains 0.50 mg/L of hypochlorous acid? As-
sume equilibrium has been achieved. Neglect the dissociation of water. Although it
may not be justified by the data available, report the answer to two decimal places.
13-2. If the pH in Problem 13-1 is adjusted to 7.00, what would be the OCl
concentration
in mg/L?
13-3. Estimate the mass feed rate (g/min) of HOCl and of NH
3
to achieve a monochlora-
mine residual of 1.8 mg/L in a flow rate of 38,000 m
3
/ d.
13-4. Estimate the mass feed rate (g/min) of Cl
2
and of NH
3
to achieve a monochloramine
residual of 2.0 mg/L in a flow rate of 1,700 m
3
/ d.
13-5. Estimate the percent available chlorine in chlorinated lime (CaOCl
2
).
13-6. Estimate the percent available chlorine in monochloramine.
13-7. Estimate the percent available chlorine in dichloramine.
13-8. Based on a hydraulic analysis, the town of Longview has determined that the travel time
for water to be carried to the most distant customer is 26 hours. A laboratory study of the
decay of
chlorine in the filtered water yielded the results shown below. What dose of chlo-
rine is required to maintain a residual of 0.5 mg/L of chlorine at the most distant custom-
er’s tap? Use a spreadsheet program you have written to determine the decay constant.
Time, h
Chlorine
residual, mg/L
0 1.1
1 1.02
3 0.90
6 0.76
Laboratory study
DISINFECTION AND FLUORIDATION 13-49
13-9. Because of high TOC in the raw water, the town of Nome has asked your firm to
evaluate the feasibility of using chlorine dioxide as a primary disinfectant. The con-
cern of the utility is the potential for odor complaints that are reported to occur at
concentrations greater than 0.1 mg/L (Hoehn et al., 2003). The following data have
been provided by the utility. What is the max
imum dose they can apply without caus-
ing odor complaints? Use a spreadsheet program you have written to determine the
decay constant and use it to estimate the maximum dose. Ignore the initial ClO
2
de-
mand in determining the decay constant.
Contact chamber detention times
100 min at Q
min
42 min at Q
max
Time, min
Chlorine dioxide
residual, mg/L
15 1.15
30 0.79
60 0.41
90 0.24
Laboratory study
Initial dose 3.0 mg/L
13-10. Two options are available in the design of the storage tank for sodium hypochlorite:
one tank to hold a 60-day supply or two tanks, each to hold a 30-day supply. The
room housing the tanks will have a maximum temperature of 25 C in the summer.
The supplier will provide either 15.89% NaOCl or 7.93% NaOCl. The unit cost
($/Mg of available chlorine) of the more dil
ute solution is higher than that of the
more concentrated solution. The client has indicated that, from an operational point
of view, a 60-day resupply schedule is preferable. The decay data for the two solu-
tions has been prepared by a reputable independent laboratory. It is shown below.
Use a spreadsheet program you have written to determine the rate c
onstants, and
determine the concentration of each solution at 60 days. Analyze the data and make
a design recommendation to your client.
Time, d 15.89% 7.93%
01.589 10
5
7.930 10
4
10 1.40 10
5
7.75 10
4
20 1.25 10
5
7.58 10
4
40 1.03 10
5
7.26 10
4
A dapted from Gordon et al., 1995.
NaOCl decay data in mg/L at 25 C
13-50 WATER AND WASTEWATER ENGINEERING
13-11. Using the U.S. EPA’s Ct tables, plot a Ct line on a copy of Figure 13-5 for the fol-
lowing conditions: Giardia inactivation by free chlorine, 99.9% inactivation, 10 C,
pH 7.0.
13-12. The Code of Federal Regulations provides tables of Ct values for 99.9% inactivation
at various temperatures and pH values (10 CFR 141.74 and 10 CFR 141.720). To
deter
mine the Ct value for another inactivation percentage, the following equation is
suggested in the Guidance Manual (U.S. EPA, 1991):
Ct required
log inactivation required
log
3
⎛
⎝⎝
⎜
⎞
⎠
⎟
()Ct
99 9.
Another method to determine log inactivation credits at intermediate or extrapolated
values is with the use of the Chick-Watson law. Using a spreadsheet you have written
and the following data from the Guidance Manual, determine the inactivation rate
constant and determine the percent error in estimating Ct for a 94.38% inactivation of
Giardia with chlorine at a temperature of 10
C and a pH of 7.0. The following data
were extracted from the Guidance Manual at a temperature of 10 C, a pH of 7.0, and
a chlorine dose of 1.6 mg/L.
Log
inactivation
Ct,
mg-min/L
0.5 20
1.0 40
1.5 60
2.0 79
2.5 99
3.0 119
Giardia inactivation
13-13. The Lakeview water treatment plant is considering the use of chlorine dioxide as
a primary disinfectant because of high TOC in the raw water. The plant requires
a log credit of 1.0 to meet the Cryptosporidium inactivation requirements. To
avoid taste and odor problems, the dose of chlorine must not exceed 2.3 mg/L.
The ClO
2
will be dosed at the head end of the plant to achieve a detention time of
150 min.
Your boss has asked you to check the ClO
2
dose at the average raw water tem-
perature of 10 C, and at the minimum winter temperature of 1.5 C, and advise him
on the potential for odor problems. Assume the flow rate and detention time is the
same at both temperatures.
13-14. Select the primary disinfectant for the village of Sleepy Hollow, which uses ground-
water for its water supply. The de
sign flow rate is 7,600 m
3
/ d. The water is softened
DISINFECTION AND FLUORIDATION 13-51
by ion exchange. The time for water to reach the most distant customer at the mini-
mum demand flow rate is six hours. The groundwater analysis is shown below.
13-15. Select the primary and secondary disinfectants for the town of Hard Times, which
uses the Verde River for its water supply. The design flow rate is 30,600 m
3
/ d. The
water is treated by conventional coagulation, sedimentation, and filtration. The time
for water to reach the most distant customer at the minimum demand flow rate is
62 hours. The Verde River water analysis is shown below.
Constituent Concentration
TOC 10–20 mg/L
Bromide 0.010 mg/L
Turbidity 10–500 NTU
Total coliforms 100–1,000/100 mL
Giardia cysts
5/100 L
Virus 2/100 L
Cryptosporidium oocysts
3.0/L
Verde River water analysis
The potential AOC and DBPFP are rated high.
13-16. Design a longitudinal-serpentine chlorine contact chamber for a design flow of
38,400 m
3
/ d. The required t
10
to achieve a Ct of 165 is 82.8 min. Assume that
Figure 13-12 applies and that the design criterion is a performance t
10
/ t
0
0.8.
Use H 3 W for the height estimate.
13-17. Rework Problem 13-16 with the assumption that the chlorine contact chamber is fol-
lowed by a distribution pipe that can be used as a contact chamber. It is a straight
pipe 300 mm in diameter, and it has no service connections for 1.0 km. Assu
me
t
10
/ t
0
1.0 for the pipe.
Constituent Concentration
Raw water
TOC 0.2 mg/L
Bromide Not detected
Turbidity 2 NTU
Giardia cysts
1/100 L
Virus 1/100 L
Cryptosporidium oocysts
0.075/L
Treated water
Turbidity 0.3 NTU
Sleepy Hollow groundwater analysis
13-52 WATER AND WASTEWATER ENGINEERING
13-18. Design a cross-baffled serpentine ozone contact chamber for a design flow of
38,400 m
3
/ d. The required t
10
to achieve a Ct of 30 is 15 min, and the second order
rate constant was determined to be 3.5 L/mol · s. Assume that Figure 13-12 applies
and that the design criterion is a performance t
10
/ t
0
0.8. Also assume a depth of
6.0 m and H 4 L.
13-19. Using fluoride as a tracer, the following data were gathered to determine t
10
for the
clearwell storage that follows a treatment plant. The raw water concentration of
fluoride was 0.2 mg/L. The fluoride dose for the tracer study was 2.0 mg/L. Correct
the measured fluoride tracer concentrations to account for the raw water concentra-
tion, and calculate the ratio C / C
0
for each data point. Using a spreadsheet program,
plot C / C
0
versus time, and determine t
10
by reading the graph at C / C
0
0.10.
Time, minutes
Measured F
concentration, mg/L
0 0.20
3 0.20
6 0.20
9 0.20
12 0.29
15 0.67
18 0.94
21 1.04
24 1.44
27 1.55
301.52
33 1.73
36 1.93
39 1.85
42 1.92
45 2.02
48 1.97
51 1.84
54 2.06
13-20. The village of Match Box is using a slow sand filter followed by chlorination. The
filter effluent has a turbidity in the range 0.4 to 0.6 NTU. Chlorine is added to a
265 m
3
clearwell. The 100 mm diameter pipeline to the first customer is 500 m
long. The residual chlorine concentrations in the clearwell and the pipeline are
1.6 and 1.0 mg/L, respectively. The clearwell is considered to have a very poor
performance rating because it lacks baffling. The pipeline is considered to have a
nearly perfect performance rating. Determine the G
iardia log-inactivation at a
temperature of 10 C and a pH of 7.5. The peak flow rate is 0.38 m
3
/ min (adapted
from Lin, 2001).
DISINFECTION AND FLUORIDATION 13-53
13-21. The town of Wallowa has asked your firm to evaluate their water treatment facil-
ity for compliance with the requirements of the rules for surface water treatment
(LT2ESWTR) for Cryptosporidium, Giardia, and viruses. The plant design capacity
is 3,800 m
3
/ d. The winter conditions have been selected as critical. From plant data,
the pH is 8.0 and the temperature is 5 C. The water is treated at the intake structure
with chorine dioxide to oxidize organic matter. There is no residual chlorine dioxide
after the demand is satisfied. Gaseo
us chlorine is added at the intake and at the efflu-
ent line from filtration before the water enters the clearwell. Ammonia is added at the
inlet to the clearwell to form chloramine. The first service connection is immediately
adjacent to the plant. The following data are representative of the raw water quality
for the microorganisms of concern:
Wallowa River water analysis
Microorganism Concentration
Cryptosporidium
0.075 oocysts/L
Giardia
1/100 mL
Virus 1/100 mL
The following data were obtained from a survey of the plant:
Wallowa water treatment plant
Treatment unit Volume, m
3
Outlet residual
Cl
2
, mg/L Baffling conditions
Rapid mix 0.91 0.4 Baffled at inlet and outlet
Flocculator 68.1 0.3 Baffled at inlet and outlet
with horizontal paddles
Clarifier 568 0.2 Baffled only at outlet
Filters 25.0 0.1 Assumed performance
rating 0.5
Clearwell 1,817 0.3 free No baffling
1.6 combined
A dapted from Lin, 2001.
13-22. The village of Sleepy Hollow ( Problem 13-14 ) has selected sodium hypochlorite as
its disinfectant. The regulating agency has given them credit for 3-log Cryptospo-
ridium oocyst, 2-log Giardia cyst, and 2-log virus removal by the treatment process.
The NaOCl is to be added prior to a 300 m
3
clearwell. The clearwell is unbaffled. The
treated water has a pH of 7.5 and a winter temperature of 5 C. The first service connec-
tion is adjacent to the plant. Design a disinfection system for Sleepy Hollow. Assume
a chlorine dose of 1.2 mg/L and a client preference for a contact chamber with superior
performance. Provide the following for the des
ign: (1) additional Ct required to meet
LT2ESWTR, (2) contact chamber design to achieve Ct, ( 3) plan view with dimensions.
13-54 WATER AND WASTEWATER ENGINEERING
13-23. The town of Hard Times ( Problem 13-15 ) has selected ozone as the primary disin-
fectant and chloramine (NaOCl NH
3
) as the secondary disinfectant. The pH of
the water entering the contact chamber will be 7.0, and the winter water temperature
will be 5 C. Design a disinfection system for Hard Times. Assume a trial ozone dose
of 2.5 mg/L and a second order rate constant of 6.0 10
3
L/mg · min and a client
preference for a contact chamber with superior performance. Provide the following
for the design: (1) additional Ct required to meet LT2ESWTR, (2) contact chamber
design to achieve Ct, (3) plan view with dimensions, and (4) feed rate of NaOCL and
ammonia (g/min) to achieve a dichloramine dose of 1.6 mg/L.
13-24. The city
of Ten Sleep proposes to use Crater Lake for its water supply. The plan is to
treat 3,500 m
3
/ d by direct filtration. The first customer is adjacent to the plant. The
raw water analysis is shown below. The city council is concerned about safety and
the lack of skilled operating personnel. The available space at the site is restricted.
1. Determine the total removal/inactivation required.
2. Determine the credits for ph
ysical removal.
3. Determine the credits required for inactivation by disinfection.
4. Select the primary disinfectant.
5. Determine the required Ct to achieve the required inactivation for the design con-
ditions (pH, temperature).
Crater Lake water analysis
Constituent Concentration
Raw Water
TOC 1.0 mg/L
Bromide Not detected
Turbidity 5 NTU
Giardia cysts
1/100 L
Virus 1/100 L
Cryptosporidium oocysts
0.075/L
Treated Water
Turbidity 0.3 NTU
pH 7.5
Temperature 5C
13-25. A 76,000 m
3
/ d direct filtration plant is applying free chlorine as a disinfectant at a
dose of 2.0 mg/L. Their low flow is 19,000 m
3
/ d. Their water analysis is the same
as that for Crater Lake ( Problem 13-24 ). To save money, they would like to reduce
the chlorine dose at low flow. The t
10
of their contact chamber is 27.5 minutes under
peak flow conditions (pH 7.0 and 5 C). Check the plant operation to confirm that
they are meeting the LT2ESWTR disinfection requirements, and determine what
dose they should use at low flow.
DISINFECTION AND FLUORIDATION 13-55
13-26. Determine the chemical feed rate for fluorosilicic acid in mL/min for the following
conditions:
Average maximum daily air 10 C
Naturally occurring fluoride concentration 0.1 mg/L
Flow rate 794 m
3
/h
Assume the specific gravity of fluorosilicic acid is 1.27.
13-27. Determine the chemical feed rate for sodium fluoride from a saturator in mL/min for
the following conditions:
Average maximum daily air temperature 25 C
Naturally occurring fluoride
concentration 0.2 mg/L
Flow rate 318 m
3
/h
Assume the sodium fluoride solubility is 42 g/L and the purity is 93%.
13-8 DISCUSSION QUESTIONS
13-1. Using the EPA’s Ct tables at a temperature of 10 C and a pH of 7.0, discuss the rela-
tive effectiveness of chlorine, chloramines, chlorine dioxide, and ozone as a Giardia
disinfectant.
13-2. One proposal for treating water in developing countries is for individual families to
use a small, home-made slow sand filter followe
d by storage of the filtered water in a
clear plastic bottle exposed to the sun for a day. Explain the purpose of the sand filter
and the mechanism that disinfects the water.
13-3. Based on reactor mass balance principles, explain why plug flow is preferred over a
completely mixed reactor for disinfe
ction.
13-4. A new employee has proposed using a vertical serpentine flow chlorine disinfection
chamber rather then a longitudinal-flow serpentine chlorine chamber in order to save
space. Do you agree with this proposal? Explain your reasoning.
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