Water and Wastewater Engineering
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8-20 WATER AND WASTEWATER ENGINEERING
Solution:
a . The mass of resin is divided equally between the three columns.
b. Using the total m eq from step h in Example 8-2 , the meq of hardness to be removed in
one column at bed exhaustion is
1 3710
3
4 5710
6
5
.
.
meq
meq
c. The time for one bed to become exhausted at a flow rate of 208.5 m
3
/ d (step f in
E x ample 8-2 ) is
4 5710
6 572 208 05 100
5
3
.
..,
meq
meq/L m /d()()(
00
0 33 8
3
L/m
d or h
)
.
d. If breakthrough is 67% of the exchange capacity, the time to breakthrough is
()( )
()(
067 45710
6 572 208 05
5
3
..
..
meq
meq/L m
//d L/m
d or h
)( )1 000
022 5 4
3
,
..
e. The third column in the series can be regenerated in two hours. Therefore, this design
will work because the third column can be brought on-line before breakthrough occurs
in the second column.
Vessel Design
While pressurized tanks are used for small ion exchange columns, gravity flow is used in larger
applications. F iberglass and steel are the usual tank materials. Fiberglass tanks are generally
limited to about 2.5 m in diameter, pressures under 1,000 kPa, and temperatures und er 40 C.
Prefabricated steel tanks may be up to 3.6 m in diameter. Steel tanks must be lined and often are
more expensive than fiberglass.
Column heights are generally less than 4 m. Mu ltiple columns in series are used if greater
height is required. The height of the bed to diameter ratio is usually in the range of 0.2:1 to 3:1
(Clifford, 1999; Reynolds an
d Richards, 1996). Resin bed depths of less than 0.9 m are not recom-
mended to avoid premature breakthrough due to nonideal resin bed behavior (GLUMRB, 2003).
Resin bed depths greater than 4 m are generally avoided because of pressure drop limitations of
the resin. The column height m ust be sufficient for expansion of the bed
during backwashing.
The dimensional limits of prefabricated u nits are usually governed by roadway constraints for
underpasses and turning radii.
Table 8-3 summarizes the typical design criteria used in sizing the tank. Example 8-4 illus-
trates the design of the vess
el.
ION EXCHANGE 8-21
Example 8-4. Des ign the tank for the Hard Times ( Examples 8-2 and 8 -3 ) ion exc hange col-
umn. The following design parameters have been established for the initial trial design:
Resin shipping weight moist kg/m()800
3
Solution:
a . The Solver* program in a spreadsheet was used to perform the iterations for solution of
this problem. The spreadsheet cells are shown in Figure 8-7 . The values shown in the
figure are the final solver solution. The cell locations used in the figure are identified
by brackets [ ] in the discussion below.
b . Begin with the input data by setting the following:
[ B 5] Q 275m
3
/ d (from Example 8-2 )
[B6] Resin mass 750 kg (on moist basis from Example 8-2 )
[B7] Shipping weight 800 kg/m
3
(on moist basis given in the problem statement)
[B8] Surface loading rate 408 m
3
/ d · m
2
(Any rate between 400 and 800 may be se-
lected as the starting point. The value shown here is the first guess. The value shown
in the spreadsheet is the final Solver value.)
c. Compute the volume of the media in cell [B13] as
[]
[]
B
B
kg
kg/m
m
6
7
750
800
094
3
3
.
TABLE 8-3
Typical range of design criteria used in sizing ion exchange columns and vessels
Parameter Range of values Comment
Pressure drop 35–70 kPa 135 kPa maximum
Diameter (D)
2.5 mFiberglass tanks
3.6 m Steel tanks
Height (H) of resin
0.9 m To avoid premature breakthrough
and 4 m To limit pressure drop
H:D of resin bed
1.5:1 to 3:1
Expansion of resin bed 100% T
ypically 50%
Height of column height 4 m Use columns in series for greater height
Sources: GLUMRB, 2003; Gottlieb, 2005; Reynolds and Richards, 1996.
*Solver is a “tool” in Excel
®
. Other spreadsheets may have a different name for this program.
8-22
Example 5-3 IX column dimensions
1
ABCDEFG
Input Data
Criteria
Q 275 m
3
/d
Resin mass
750
kg on moist basis
Shipping weight 800
kg/m
3
on moist basis
SLR 400 m
3
/d - m
2
400 and 800
Compute volume of media
V = 0.94
m
3
Compute diameter of column
Area = 0.69
m
2
Diameter =
0.94 m < 3.6 m for steel tank
Compute height of column
H =
1.36 m >0.9 m and <4.0 m
Check ratio
H:D = 1.46 >0.2 and < 3
Solver parameters
Target cell
B27
Equal to
min
By changing B8
Subject to constraints
B8
400
B8
800
B19
3.6
B23
4.0
B23
0.9
B27
3
B27
0.2
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
Solver parameters
Set target cell: B27 Solve
Close
OptionsGuess
Add
Change
Delete
Reset all
Help
Equal to:
By changing cells:
B8
Subject to the constraints:
B19 3.6
Max value of:Min
B23 4.0
B23 0.9
B27 3.0
B27 0.2
B8 800
B8 400
FIGURE 8-7
E xample 8-4 spread sheet with
Solver dialog box.
ION EXCHANGE 8-23
d. Compute the the area in [B17].
[]
[]
B
B
m /d
m /dm
m
5
8
275
408
067
3
3 2
2
.
e. And then the diameter of the column in [B19].
[]B
m
17 4
3 1416
093
0 5
∗
⎛
⎝
⎜
⎞
⎠
⎟
.
.
.∧
f. Compute the height of the column in cell [B23].
[]
[]
B
B
m
m
m
13
17
094
067
1
3
2
.
.
.40
g. Check the H: D ratio.
[]
[]
B
B
m
m
23
19
140
093
1 50
.
.
.
h. Activate the dialog box for Solver and designate the target cell [B27], that is, the one for
the height to diameter ratio ( H: D ).
i. Set Equal to to “Min.”
j. Set By changing to the cell containing the surface loading rate (SLR), that is, [B8].
k. Add the following four constraints in the dialog box:
(1) SLR
[]
[]
B
B
8 800
8 400
(2) Diameter
[]B m19
3 6 .
(3) Height
[]
[]
B
B
23 40
23 09
.
.
(4) H: D ratio
[]
[]
B
B
27 3
27 0 2
.
l. Execute solve to find the SLR 400 m
3
/ d · m
2
.
m. With resin column volume, check the EBCT.
EBCT
m
m /d
d or or
094
275
000344924
3
3
.
...99 min
8-24 WATER AND WASTEWATER ENGINEERING
This is within the typical range of 1.5 to 7.5 min.
n. Check the SFR
SFR
m /d
m
or m /dm
275
094
292 55 290
3
3
33
.
. of water
This is within the typical range of 200 to 1,000 m
3
/ d · m
3
.
o. Check the pressure drop. Using Figure 8-6 and the operating temperature of 10 C from
E xample 8-2 , the pressure drop at a surface loading rate of 400 m
3
/ d · m
2
i s about 37 kPa/m
and the total pressure drop is (1.36 m)(37 kPa/m) 50.3 kPa. This is within the typical
range of 35 to 70 kPa and is less than the maximum of 140 kPa.
p. Allowing for 50% expansion of the resin bed, the tank height will be
Height of tank m or m()( )1 5 1 3620421.. . .
This is less than the maximum of 3.6 m for prefabricated tanks and is acceptable.
Comment: Normally, one would not check both the EBCT and SFR as they are reciprocal of
each other. The check is performed in this example for academic demonstration and to provide
some experien
ce with the the magnitude of the numbers and their relationship to one another.
Piping
PVC piping is most common for smaller systems. Either plastic-lined or stainless steel is used for
larger systems.
Brine Disposal
Historically, brine has been disposed to the sanitary sewer system. For small household units, the
impact on the wastewater treatment plant will not be s ignificant. For larger municipal systems
the problem is much more significant. Those communities near the ocean have it as an option for
disposal. Deep well injection into a s
aline aquifer is another alternative as is evaporation. All of
these options are highly site specific.
Because the issue of brine disposal is significant, the examination of the option of ion
exchange as a treatment technology should consider the disposal alternatives and costs very early
in the design process
.
8-5 OPERATION AND MAINTENANCE
The major operational requirements in addition to routine monitoring are preparation of the
regenerant and conducting the regeneration. Often these are automated.
In addition to routine maintenance of valves and pumps, periodic resin cleaning and/or
replacement will be required. Resin cleaning may require eight or more hours of out-of-service
time for the column. Extraor
dinary regeneration of the resin may be required after cleaning. The
cleaning agents are hazardous and may be incompatible with the column materials.
Resin life for cation resins is on the order of 10 years for softening and 3 to 5 years for anion
exchanger resins. Shorter resin life can be expected where the water fouls the resin.
ION EXCHANGE 8-25
8-6 CHAPTER REVIEW
When you have completed studying this chapter, you should be able to do the following without the
aid of your textbooks or notes:
1 . Explain the difference between a strong cation exchange resin and an weak cation exchange.
2. Given the separation factors for several cations, identify those that will be adsorbed in
preference with respect to the others.
3. Explain why high pressure drops through the resin bed are detrimental to ion exchange
perform
ance and operating life.
4. Explain why multiple columns in series are preferred over a single column with the
same resin mass.
5. Sketch a three-column “merry-go-round” bed arrangement and explain how it works to
a client.
6. Select a softening ion exchange resin from breakthrough curves for several resin
s treat-
ing the same raw water source.
7. Define the following abbreviations: EBCT, SFR, SLR.
8. Explain why ion exchange may be of benefit in removing constituents of concern other
than calcium and magnesium.
W ith the use of this text, you should be able to do the following:
9. Given a table of screen sizes and a manufacturer’s specification, i
dentify the largest and
smallest diameter resin particle that is being marketed.
10. Estimate the maximum volume of water per liter of resin that can be treated given the
total capacity of the resin and the concentration of the ions to be treated.
11. Calculate the fraction of the “split” for an ion exchange softening system.
12. Calculate EBCT, SFR, and SLR given appropriate data.
13. Design an ion exchange column including estimating the resin volume to treat a water
given the breakthrough data and the volume of water to be treated or the flow rate and
cycle time for regeneration.
14. Determine the number of columns and sequence of their use given the cycle time of a
single resin column.
8-7 PROBLEMS
8-1. E stimate the maximum volume of water per liter of resin that can be treated by a
strong acid exchange resin in the hydrogen form if the resin has total capacity of 2.0
eq/L, the magnesium concentration is 1.4 meq/L, and the sodium concentration is 2.6
meq/L. Assume no other cations are in the solution.
Visit the text website at www.mhprofessional.com/wwe for supplementary materials
and a gallery of photos.
8-26 WATER AND WASTEWATER ENGINEERING
8-2. A pilot scale ion exchange column was used to develop design data for a softening
system for the Village of Calcite. The data are shown below. The design flow rate is
4,000 m
3
/ d. The design final hardness is 120 mg/L as CaCO
3
. Regeneration is to be
once per day. The manufacturer’s operating capacity to breakthrough is 60%.
Assume a two-column system: one in service and one in regeneration or standby.
Also assume that there is no leakage before breakthrough. Use the mass balance tech-
nique to determine the mass of resin that will be required.
Raw water data:
C a
2
444.5 mg/L as CaCO
3
Mg
2
66.7 mg/L as CaCO
3
Operating temperature 10 C
Iron and turbidity concentrations are negligible
Pilot scale column data:
C o l umn diameter 10.0 cm
Column height 150.0 cm
R e sin density 0.70 g/cm
3
on a moist basis
Moisture content 48%
Flow rate through the column 0.25 m
3
/h
Pilot scale breakthrough data for Calcite raw water
Volume, m
3
Effluent concentration,
meq/L
0.5 0
1.0 0
1.5 0
2.0 0
2.1 0.05
3.0 0.46
3.5 1.05
4.0 2.00
4.53.20
5.0 4.90
5.35 6.00
5.45 6.40
5.5 6.70
5.6 7.00
5.7 7.40
5.8 7.80
5.9 8.20
6.0 8.50
6.4 9.00
7.0 9.70
7.5 10.0
ION EXCHANGE 8-27
8-3. Repeat Problem 8-2 assuming column leakage is 5.0 mg/L as CaCO
3
.
8-4. Repeat Problem 8-2 assuming a three column system in series and operation of the “lead”
column until the bed is exhausted. Assume the total out-of-service time is two hours.
8-5. A pilot scale ion exchange column was used to develop design data for a softening
system for the Village of Dolomite. The d
ata are shown below. The design flow rate
is 7,000 m
3
/ d. The design final hardness is 120 mg/L as CaCO
3
. Regeneration is to be
once per day. The manufacturer’s operating capacity to breakthrough is 50%.
Assume a two-column system and that there is no leakage before breakthrough. Use
the mass balance technique to determine the mass of resin that will be required.
Raw water data:
C a
2
111.0 mg/L as CaCO
3
Mg
2
131.8 mg/L as CaCO
3
Operating temperature 5 C
Iron and turbidity concentrations are negligible
Pilot scale column data:
C o l umn diameter 10.0 cm
Column height 150.0 cm.
R e sin density 0.80 g/cm
3
on a moist basis
Moisture content 70%
Flow rate through the column 0.25 m
3
/h
Pilot scale breakthrough data for Dolomite raw water
Volume, m
3
Effluent concentration,
meq/L
0.0 0.0
0.1 0.0
0.25 0.0
0.50 0.0
1.0 0.0
2.0 0.0
2.25 0.0
2.5 0.0
3.0 0.1
3.3 0.6
3.4 1.05
3.5 1.58
3.6 2.37
3.7 3.09
3.8 3.49
4.0 4.33
8-6. Repeat Problem 8-5 assuming column leakage is 10.0 mg/L as CaCO
3
.
8-28 WATER AND WASTEWATER ENGINEERING
8-7. Repeat Problem 8-5 assuming a three-column system in series and operation of the
“lead” column until the bed is exhausted. Assume the total out-of-service time is
three hours.
8-8. Continuing the design of the ion exchange column for the Village of Calcite, provide
the following for the two-bed system that will operate in parallel with staggered
regeneration cycles:
a. Surface loading rate (m
3
/ d · m
2
)
b. Height and diameter of the resin bed
c. Empty bed contact time (min)
d. Service fl ow rate (m
3
/ d · m
3
)
e. Height and diameter of the column vessel
f. Pressure drop
Assume the density of the resin is 830 kg/m
3
(moist), a resin height less than or equal
to 4.0 m, and that 50% expansion of the bed will be used during backwash. The pres-
sure drop across the bed should be less than 140 kPa.
8-9. Repeat Problem 8-8 using the three bed system designed in Problem 8-4 .
8 -10. Continuing the design of the ion exchange column for the Village of Dolom
ite, pro-
vide the following for the two-bed system that will operate in parallel with staggered
regeneration cycles:
a. Surface loading rate (m
3
/ d · m
2
)
b. Height and diameter of the resin bed
c. EBCT
d. Service fl ow rate (m
3
/ d · m
3
)
e. Height and diameter of the column vessel
f. Pressure drop
Assume the density of the resin is 850 kg/m
3
(moist), a resin height less than or equal
to 4.0 m, and that 50% expansion of the bed will be used during backwash. The pres-
sure drop across the bed should be less than 140 kPa.
8-11. Repeat Problem 8-10 using the three-bed system designed in Problem 8-7 .
8-8 DISCUSSION QUESTION
8-1. Two pilot ion exchange columns treating the same raw water in a parallel opera-
tion yielded the breakthrough curves shown below. Assuming the resins are equally
priced, which resin would you choose? Explain your choice.
ION EXCHANGE 8-29
8-9 REFERENCES
Brown, C. J. (1998) “Ion Exchange,” in H. M. Freeman (ed.) Standard Handbook of Hazardous Waste
Treatment and Disposal, McGraw-Hill, New York, pp. 6.57–6.85.
Clifford, D. A. (1999) “Ion Exchange and Inorganic Adsorption,” in R. D. Letterman (ed.) Water Quality
and Treatment, 5th ed., American Water Works Association, McGraw-Hill, New York, pp. 9.1–9.91.
GLUMRB (2003) Recommended Standards for Water Works, Great Lakes–Upper Mississippi River
Board of State and Provincial Public Health and Environmental Managers, Health Education
Services, Albany, New York, pp. 35–38, 66–80.
Gottlieb, M. C. (2005) “Ion Exchange Applications in Water Treatment,” in E. E. Baruth (ed.), Water
Treatment Plant Design, McGraw-Hill, New York, pp. 12.1–12.61.
Helfferich, F. (1962) Ion Exchange, McGraw-Hill, New York.
MWH (2005) Water Treatment: Principles and Design, John Wiley & Sons, Hoboken, New Jersey,
pp. 957, 1371, 1403, 1431, 1445, 1472, 1564.
Reynolds, T. D. and P. A. Richards (1996) Unit Operations and Processes in Environmental Engineering,
PWS Publishing, Boston, pp. 379–3
80.
Treybal, R. E. (1968) Mass Transfer Operations, McGraw-Hill, New York, pp. 253, 535.
U.S. EPA (1981) Development Document for Effluent Limitations: Guideline and Standards for the
Metal Finishing Point Source Category, U.S. Environmental Protection Agency Publication no.
EPA/440/1-83-091, Washington, D.C.
0
2
(a)
4
6
8
10
12
0123 4 5 678
Volume, m
3
C, meq/L
0
1
(b)
2
3
4
5
6
0123 4 5 6
Volume, m
3
C, meq/L