Water and Wastewater Engineering
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LIME–SODA SOFTENING 7-27
Mn(OH)
2
(s)
MnOH
Mn
2
14
12
10
8
6
4
2
0
02468101214
pH
log[species]
Mn(OH)
3
FIGURE 7-12
Solubility diagram for Mn(OH)
2
.
Fe(OH)
2
(s)
FeOH
Fe
2
14
12
10
8
6
4
2
0
02468101214
pH
log[species]
Fe(OH)
3
FIGURE 7-11
Solubility diagram for Fe(OH)
2
.
Turbidity
Although it is generally not of c oncern for groundwater sources, turbidity removal is a major
objective in treating surface water. In those cases where the surface water is hard, s oftening by
chemical precipitation will concurrently remove turbidity by sweep coagulation.
Other Contaminants
Sorg et al. (1977) identified a number of other contaminants that are removed concurrently by
lime-soda softening or coagulation. These are listed in Table 7-3 .
7 -6 PROCESS CONFIGURATIONS AND DESIGN CRITERIA
The process flow diagrams for three common softening treatment schemes are shown in Figure 7-13
on page 7-29 . Many other variations are possible including, for example, the use of blended raw
water CO
2
to recarbonate and the use of coagulation and flocculation after the precipitation pro-
cess to reduce the solids load to the filters. In the case of coagulation/flocculation, the high pH of
the water coming from the precipitation process favors the use of ferric chloride as the coagulant
because it is less soluble at higher pH than alum (see, for example, Fig
ure 6-9).
7-28 WATER AND WASTEWATER ENGINEERING
Conventional Softening Basins
These basins are similar to conventional basins used for coagulation and flocculation. Rapid
mixing either with paddle mixers or vertical turbines is followed with paddle mixers or vertical
turbines for the reaction basin (also called the flocculation basin because of its analogy to the
coagulation/flocculation pro
cess). When conventional basins are used, a portion of the precipitate
is recycled to the head end of the process. Recycling accelerates the precipitation reactions, and
the process more closely approaches true solubility limits when mixed with the previously formed
precipitate crystals (Horsle
y et al., 2005).
Because they are similar to coagulation/flocculation systems, the design criteria are specified
in a similar fashion. Flash mixing is generally provided by radial flow impellers though older
plants may be using paddle wheels. The flash mixing velocity gradient ( G
) should be in the range
300 to 700 s
1
and the minimum mixing time should be 10 to 30 s. With cold water found in
most groundwater, rapid mixing for as long as 5 to 10 minutes may be required for dissolution
TABLE 7-3
Effectiveness of lime softening and coagulation in removing inorganic contaminants
a
No full scale evidence.
(Source: Benefield et al., 1982; Sorg et al., 1977)
Contaminant Method Removal, %
Arsenic
As
3
Oxidation to As
5
required 90
As
5
Ferric sulfate coagulation, pH 6–8 90
Alum coagulation, pH 6–7 90
Lime softening, pH 11 90
Barium Lime softening, pH 10–11 90
Cadmium
a
Ferric sulfate coagulation, pH 8 90
Lime softening, pH 8.5 95
Chromium
a
Cr
3
Ferric sulfate coagulation, pH 6–9 95
Alum coagulation, pH 7–9 90
Lime softening, pH 10 95
Cr
6
Ferrous sulfate coagulation, pH 6.5–9 95
(pH may have to be adjusted after
coagulation to allow reduction
to Cr
3
)
FluorideLime-soda softening, pH 11–12 55
Lead
a
Ferric sulfate coagulation, pH 6–9 95
Alum coagulation, pH 6–9 95
Lime softening, pH 7–8.5 95
Mercury
a
Ferric sulfate coagulation, pH 7–8 60
Selenium
a
(Se
4
)
Ferric sulfate coagulation, pH 6–7 70–80
Silver
a
Ferric sulfate coagulation, pH 7–9 70–80
Alum coagulation, pH 6–8 70–80
Lime softening, pH 7–9 70–80
7-29
(a)
(b)
(c)
Lime
Recarbonation
Granular
media filtration
Filter washwater
Waste washwater
to water recovery
system or disposal
Softened water to
disinfection, storage, and
distribution system
Softened water to
disinfection, storage, and
distribution system
Softened water to
disinfection, storage, and
distribution system
Solids
to
dewatering
Raw water
CaCO
3
sludge
CO
2
Granular
media filtration
Filter washwater
Waste washwater
to water recovery
system or disposal
Lime
Raw water
CaCO
3
sludge
Soda
Recarbonation
Solids to
dewatering
CaCO
3
sludge
CO
2
Granular
media filtration
Filter washwater
Waste washwater
to water recovery
system or disposal
Lime
Recarbonation
Solids to
dewatering
Raw water
CaCO
3
sludge
CO
2
Soda
Recarbonation
Solids to
dewatering
CaCO
3
sludge
CO
2
FIGURE 7-13
Process flow diagrams of common softening treatment techniques: (a) single-stage lime treatment; (b) two-stage excess lime-soda treatment; (c) split-flow lime
treatment.
7-30 WATER AND WASTEWATER ENGINEERING
Discharge channel
Discharge
channel
and launder
Lime slurry feed
Launder
Mixer
“Hood” or
“skirt”
Launders
Sludge
scraper
Sludge
circulation port
Raw water
Sludge hopper
Sludge
withdrawal
Mass effluent
launder
Chennal feed
Separation zones
Sludge
blanke
t
Flocculation
FIGURE 7-14
Upflow solids contact basin.
of the lime slurry. Flocculation mixing is provided by axial flow impellers or paddle wheels. The
flocculation mixing velocity gradient is between 130 and 300 s
1
with a mixing time of 30 to
45 minutes, to achieve Gt values ranging from 200,000 to 400,000. The velocity through ports in
the flocculation basin is in the range 0.15 to 0.36 m/s (Davis and Cornwell, 2008; Horsley et al.,
2005; Kawamura, 2000).
The design process follows that shown in the Chapter 6 examples for vertical turbine and
paddle mixers
.
Upflow Solids Contact Basins
Conventional softening basins are mostly found at older facilities. They provide a high degree
of process stability , but the size and number of basins result in a high capital cost. Since the
late 1960s most new plants have been built with upflow solids contact basins as shown in
Figure 7-14 .
LIME–SODA SOFTENING 7-31
Although rapid mixing may be provided ahead of the solids contact unit, generally the soft-
ening chemicals are applied to the mixing zone of the contact unit. The mixing and recirculation
zone is separated from the s edimentation zone by a conical baffle identified as the “hood” in
Figure 7-14 .
Effe
ctive solids contact units draw the settled prec ipitate from near the floor at the cen-
ter of the basin with a large diameter turbine or impeller and recirc ulate it with the incoming
water. When multiple units are used, the precipitate may be recirc ulated from one unit to
another.
Because the units are sold as proprietary manufa
ctured items, the designs are established by
the manufacturer. Whenever possible, design requirements should be based on successful plants
using the same or similar source water. The following information from Horsley et al. (2005) is
provided for guidan
ce in evaluating proprietary designs:
• The maximum recirculation rate is typically 10:1 based on the incoming raw water flow
rate. The mixer is provided with a variable-speed drive to allow the operator to adjus t the
recirculation rate.
• The side water depth of the tank generally varies from 4.3 to 5.5 m. Contact time in the
mixing zone is
typically measured by the volume of water within and directly under the
baffle cone.
• Overflow rate (that is, flow rate divided by the surface area, Q / A
s
) is generally measured
0.6 m below the water surface, based on the surface area between the baffle wall and the
basin wall.
GLUMRB (2003) recommends the following design criteria:
1 . Flocculation and mixing period should not be less than 30 minutes.
2. Detention time should be two to four hours for solids
contact clarifiers and softeners
treating surface water.
3. Detention time should be one to two hours for solids contact clarifiers treating only
groundwater.
4. Upflow rate (overflow rate) shall not exceed 2.4 cubic meters of flow per square meter
of surface area per hour (m
3
/h · m
2
or m/h) at the slurry separation line for units used for
clarifiers.
5. Upflow rate (overflow rate) shall not exceed 4.2 m
3
/h · m
2
or m/h at the slurry separa-
tion line for units used for softeners.
6. Weir loading shall not exceed 0.120 m
3
/ min · m of weir length for units used as clarifiers.
7. Weir loading shall not exceed 0.240 m
3
/ min · m of weir length for units used as
softeners.
Table 7-4 provides information similar to that provided by manufacturers for selection of an
upflow solids contact unit.
7-32 WATER AND WASTEWATER ENGINEERING
TABLE 7-4
Representative dimensions for upflow solids contact basin
a
SWD side water depth. This depth is measured at the wall of the basin.
Weirs extend radially from the column to wall of the basin. Both sides are used in evaluating weir length.
Note: These basins are representative but do not represent actual choices. Actual manufacturer’s data must be used for real-world design.
Nominal
diameter, m
Nominal
SWD,
a
m
Nominal
volume, m
3
Cone dimensions, m
Separation
zone area, m
2
Column
diameter,
m
Motor
power,
kW
Number
of
weirshr
1
r
2
6 3.7 109 2.53.8 1.2 25 0.6 0.5 8
94.3 288 3.1 5.3 1.5 60 1 1.5 8
12 4.8 580 3.6 6.5 2 100 1.3 28
155.3 970 4.1 7.5 2.7 160 1.7 3.5 8
18 5.5 1500 4.3 8.53.3 23025.5 10
21 5.6 2140 4.4 9.5 4.2 300 2.3 7.5 10
24 5.8 2850 4.6 10 5390 2.6 10 10
27 5.9 3730 4.7 10.5 6 480 3 15 10
30 6 4760 4.8 11.5 6.8 590 3.2 15
11
Tank diameter
Weir length
r
2
r
1
h
Column
diameter
SWD
Example 7-7. Select and evaluate an upflow solids contact unit for the Sweetwater softening
plant. The design flow rate is 7,400 m
3
/ d. The data in Table 7-4 have been provided by the manu-
facturer. The tank is provided with eight radial weirs that are equal to the radius of the tank minus
the radius of the top of the truncated cone ( r
2
).
Solution:
a . Make a trial selection of the 12 m diameter tank and check the overflow rate using the
separation zone area.
v
Q
A
s
7 400
100
74 3 08
3
2
,
.
m /d
m
m/d or m/h
LIME–SODA SOFTENING 7-33
The GLUMRB design guidance is less than 4.2 m/h. Therefore, the unit is acceptable for
overflow rate.
b. Next check the detention time for softening using the volume of the basin.
t
Q
580 m
7,400 m /
0.078 d or 1.88 h
3
3
d
V
The GLUMRB design guidance is one to two hours. Therefore, the unit is acceptable for
detention time.
c. The unit is next checked for mixing time by calculating the volume of the truncated cone
and calculating the detention time in the cone. The volume of a truncated cone is
h
3
1
2
12 2
2
[()()()]rrrr
V
where h height of the truncated cone
r
1
radius at bottom of cone
r
2
radius at top of cone
For the trial tank the values of h, r
1
, and r
2
are taken from Table 7-4 .
()
[( ) ( )( ) ( ) ]
3 6
3
6 5 6 5 22
223
22
.
..
m
mm m
.. 4
3
m
V
The mixing time in the cone is
t
223 4
7 400
003043 46
3
3
.
,
..
m
m /d
d or min
The GLUMRB design guidance is a minimum of 30 minutes. Therefore, the unit is ac-
ceptable for mixing time.
d. The weir length is checked by computing the length of the weir taking into account that
there are eight weirs and that water enters both sides of each weir.
L
()( )82
12 1 3
weirss
ides/weir
mdiameter .
mmdiameter column
m
2
85 6
⎛
⎝
⎜
⎞
⎠
⎟
.
With a flow rate of 7,400 m
3
/ d, the weir hydraulic loading rate is
WL
7 400
85 6
86 45 006
3
3
,
.
..
m /d
m
m /dmor 00
3
m /min m
7-34 WATER AND WASTEWATER ENGINEERING
The GLUMRB design guidance is a maximum of 0.240 m
3
/ min · m of weir length.
Therefore the unit is acceptable for weir loading.
Because the trial unit meets all the criteria, it may be considered in the design evaluation.
Comment. If the trial selection did not meet the criteria, then iterative trials would be evaluated
to see if any of this manufacturer’s units is acceptable for the design.
7-7 OPERATION AND MAINTENANCE
The most important operation and maintenance task in softening is the selection of the appropri-
ate chemicals and adjustment of the dose to changing raw water quality and plant flow. Monitor-
ing of the chemical feed system to detect c logging of the lines and maintenance of the mixers
ranks second in the need for clos
e O&M oversight. Encrustation is a significant problem. Annual
removal of calcium carbonate build-up during seasonal low-demand periods is customary.
Hints from the Field. Experience suggests the following:
• Open flumes are preferred over pipelines.
• Design of pipelines and flumes shoul
d include additional capacity for encrustation.
• Sludge scraper mechanisms should be kept in operation during low-flow periods when por-
tions of the plant are off-line but are not to be drained. This prevents the sludge blanket
from settling and “freezing” the scraper so that it cannot start moving again because the
settled sludge is too dense for the torque that c
an be applied.
• S l udge withd rawal (called blowdown ) must o ccur regularly to keep the draft tube in an
upflow clarifier open so that recirculation occurs in the mixing portion of the clarifier.
7-8 STABILIZATION
A stable water is one that exhibits neither scale forming nor corrosion properties. In the water
industry, a stable water is considered to be one that will neither dissolve nor deposit calcium. The
original objective of water stabilization was to adjust the pH of the treated water to prevent cor-
rosion of the water distribution system pipes by depositing a thin film of calcium c
arbonate as a
protective coating. Numerous investigations have revealed that although the Langelier index (a
method of calculating stability) is a reasonable predictor of the potential for CaCO
3
to precipitate
or dissolve, it does not predict how much CaCO
3
( s) will precipitate or whether its structure will
provide resistance to corrosion (Schock, 1999).
Although its validity as a method of corrosion protection is limited, the Langelier index is still
useful in predicting the potential for CaCO
3
to precipitate or dissolve. This is particularly valu-
able in designing the processes to reduce precipitation of CaCO
3
in the rapid sand filters and pipe
network as well as those processes used to reduce the corrosivity of reverse osmosis/nanofiltration
(RO/NF) treated water.
The Langelier Saturation Index (LSI)
Langelier (1936) developed the following relationship to predict whether or not a given water
will deposit or dissolve CaCO
3
:
LSI pH pH
s
(7-35)
LIME–SODA SOFTENING 7-35
where pH is in the actual hydrogen ion concentration and pH
s
i s the pH at saturation. pH
s
i s fur-
ther defined as
pH pCa pAlk C
s
2
(7-36)
where pCa
2
negative logarithm of the calcium ion concentration, moles/L
pAlk negative logarithm of the total alkalinity, equiv/L
C an empirical constant to correct for ionic strength and the temperature depen-
dence of the solubility of CaCO
3
( s)
The value of the constant for various ionic strengths (or total dissolved solids (TDS) concentra-
tions) and temperatures is given in Table 7-5 .
The state of saturation with respect to CaCO
3
depends on the LSI:
• If the LSI 0, then the solution is undersaturated and CaCO
3
will dissolve.
• If the LSI 0, the solution is at equilibrium.
• If the LSI 0, then the solution is supersaturated and CaCO
3
will precipitate.
TABLE 7-5
Values of C for various ionic strengths and temperatures
Source: T. E. Larson and A. M. Buswell, “Calcium Carbonate Saturation Index and Alkalinity Interpretations,” J. Am. Water Works Assoc. 34 (1942):
1667. Copyright 1942 by the American Water Works Association, Inc.
C
Ionic
strength
Total dissolved
solids, mg/L 0C10C20C 30C40C 50C60C70C80C90C
0.000 2.45 2.23 2.02 1.86 1.68 1.521.36 1.23 1.08 0.95
0.001 40 2.582.36 2.15 1.99 1.81 1.65 1.49 1.36 1.21 1.08
0.002 80 2.62 2.40 2.19 2.03 1.85 1.69 1.53 1.40 1.25 1.12
0.003 120 2.66 2.44 2.23 2.07 1.89 1.73 1.57 1.44 1.29 1.16
0.004 160 2.68 2.46 2.25 2.09 1.91 1.75 1.59 1.46 1.31 1.18
0.005 200 2.71 2.49 2.28 2.12 1.94 1.78 1.62 1.49 1.34 1.21
0.006 240 2.74 2.522.
31 2.15 1.97 1.81 1.65 1.521.37 1.24
0.007 280 2.76 2.542.33 2.17 1.99 1.83 1.67 1.541.39 1.26
0.008 320 2.78 2.562.35 2.19 2.01 1.85 1.69 1.56 1.41 1.28
0.009 360 2.79 2.572.36 2.20 2.02 1.86 1.70 1.57 1.42 1.29
0.010 400 2.81 2.592.38 2.22 2.04 1.88 1.72 1.59 1.44 1.31
0.011 440 2.83 2.61 2.40 2.24 2.06 1.90 1.74 1.61 1.46 1.33
0.012 480 2.84 2.62 2.41 2.25 2.07 1.91 1.75 1.62 1.47 1.34
0.013520 2.86 2.64 2.43 2.27 2.09 1.93 1.77 1.64 1.49 1.36
0.014 560 2.87 2.65 2.44 2.28 2.10 1.94 1.78 1.65 1.501.37
0.015 600 2.88 2.66 2.45 2.29 2.11 1.95 1.79 1.66 1.511.38
0.016 640 2.90 2.68 2.47 2.31 2.13 1.97 1.81 1.68 1.53 1.40
0.017 680 2.91 2.69 2.48 2.3
2 2.14 1.98 1.82 1.69 1.54 1.41
0.018 720 2.92 2.70 2.49 2.33 2.15 1.99 1.83 1.70 1.55 1.42
0.019 760 2.92 2.70 2.49 2.33 2.15 1.99 1.83 1.70 1.55 1.42
0.020 800 2.93 2.71 2.502.34 2.16 2.00 1.84 1.71 1.56 1.43
7-36 WATER AND WASTEWATER ENGINEERING
Many modifications have been made to the basic equation to account for other alkalinity-
contributing species, activity coefficients, and so forth. These are discussed in detail in Schock
(1999). The basic equation will be used for the purpose of illustration of its use in design.
Stabilization Design for Lime-Soda Softened Water
A s noted earlier in this chapter, recarbonation is the last step in the softening process. The pur-
pose of the recarbonation step is to “stop” the precipitation reaction by lowering the pH. Carbon
dioxide addition has been the method of choice for lowering the pH. Other chemicals may be
used (for example, H
2
SO
4
) based on economic and operational considerations. Furthermore, split
treatment of groundwater may bring sufficient CO
2
in the bypassed water to lower the pH with-
out the addition of purchased chemicals.
The objective in stabilization is to achieve an LSI 0. The reactions are given in Equa-
tions 7-27 , 7-28 , 7-29 and 7-30. The equilibrium equations for carbonic acid (7-31 and 7-32)
are used to estimate the concentration of CO
2
that must be added. The estimation of the CO
2
dose is illustrated in the following example.
Example 7-8. Estimate the dose of CO
2
in mg/L to stabilize the water from split treatment
softening. The bypass water has 1.37 meq/L of CO
2
. The estimated constituents and parameters
of interest in the blended water are listed in the table below.
Constituent
or parameter
Concentration,
meq/L as CaCO
3
or units as shown
Ca
2
1.38
Mg
2
0.80
0.50
1.20
pH 9.95 units
TDS 320 mg/L
Temperature 10C
CO
3
2
HCO
3
Solution:
a . Calculate the LSI.
Converting Ca
2
meq/L to moles /L,
()()1 3820
40 000
690
.
,
.
meq/L mg/meq
mg/mole
110
4
moles/L,
where 20 mg/meq is the equivalent weight of Ca
2
pCa log moles/L
()690 10 3 16
4
..
The alkalinity is the sum of
CO
3
2
and HCO
3
. The units are given in meq/L. They must
be in equiv/L
pAlk log
()0 50 10 1 20 10 2 77
33
.. .