Water Treatment Plant Optimization Study Snyder and AccociatesWATER TREATMENT PLANT
OPTIMIZATION STUDY
City of Blair, Nebraska
112.0314
Prepared by:
Dr. L.D. McMullen, Ph.D., P.E.
Water Resources Practice Leader
SNYDER & ASSOCIATES, INC.
2727 SW Snyder Blvd.
Ankeny, Iowa 50023
(515) 964-2020
June 14, 2012
JA2012_Projects\112.0314\Correspondence\Reports\optimization study.doe
r
Water Treatment Plant — Optimization Study
- City of Blair, Nebraska
TABLE OF CONTENTS
INTRODUCTION.....................................................................................................Page 1
PROJECTSCOPE.....................................................................................................Page 1
RESULTS — LIME SOFTENING/RECARBONATION..........................................Page 2
RESULTS — DISINFECTION BYPRODUCT .........................................................Page 5
CONCLUSIONS........................................................................................................Page 7
APPENDIX
LABORATORY RESULTS
JA2012_Projects\112.0314\Correspondence\Report
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Snyder & Associates, Inc.
Water Treatment Plant — Optimization Study
City of Blair, Nebraska
INTRODUCTION
The City of Blair, Nebraska operates a surface water lime softening water treatment plant located
along the Missouri River in the southeastern part of the City. The treatment plant has a rated
capacity of 17 million gallons per day with a daily average of 10 million gallons. Recently the
City Council authorized the expansion of the water plant to a rated capacity of 20 million gallons
per day.
Source water for the water treatment plant comes from a river intake on the Missouri River.
Water quality in the Missouri River is generally stable due to the large reservoirs upstream on the
river. Spring runoff is normally a period when river water turbidity spikes resulting in an
operational challenging period.
The treatment plant consists of a river intake, pre -sedimentation to remove turbidity, lime
softening to reduce hardness, filtration for water clarity and high service pumpage to the
distribution system. Chemicals used are lime, alum, chlorine and a polymer for clarification.
The biggest customer is Cargill, which has a large corn processing complex near the water
treatment plant. On an average day, Cargill uses 8.5 million gallons per day and the City of Blair
uses 1.5 million gallons per day. The authorized expansion is to satisfy the anticipated increased
demand for water by Cargill. It is the goal of the City of Blair to provide safe, high quality water
at affordable prices to all of its customers.
PROJECT SCOPE
Treatment plant efficiency is always a part of plant operations. The appropriate staffing, power
and chemical use are the three major variables impacting cost. For lime softening water
treatment, lime and carbon dioxide are two major costs that need to be optimized in an effort to
be cost efficient. With the anticipation of an expansion to the treatment plant, the City of Blair
determined it was desirable to conduct an optimization study of the lime softening/recarbonation
systems and investigate the appropriate process to control disinfection byproducts.
The optimization process included a laboratory study to determine the optimum pH to reduce
hardness in Missouri River water and computer modeling to determine the optimum
recarbonation pH. Water samples were collected on April 25, 2012 and transported to Des
Moines Water Works Laboratory for testing.
The disinfection byproduct testing was completed on the same water sample that was used for
the lime softening. A disinfection byproduct potential test was conducted on three samples: 1)
water from the presedimentation basin, 2) softened water with a pH near where the existing plant
is operating, and 3) near the optimum pH for softening.
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Water Treatment Plant — Optimization Study
City of Blair, Nebraska
RESULTS — LIME SOFTENING/RECARBONATION
Figure 1 presents the water hardness as a function of pH. As can be seen, the total hardness
decreases to around 180 mg/L for a pH range of 10 to 11. Both low pH and high pH values
result in higher values of total hardness. Figures 2 and 3 present similar information, but for
calcium hardness and magnesium hardness. These curves show calcium hardness reduced to
below 100 in the pH 10 range but increase significantly at higher pH values due to the excess
lime needed for the higher pH. Magnesium hardness decreases with increased pH to near zero at
a pH of 11.5
Figure 1 - Total Hardness
350
300
rn 250
U
Um 200
N
150
o\o
E 100
50
0
8 8.5 9 9.5 10 10.5 11 11.5 12
pH
Figure 2 - Calcium Hardness
300
250
M
O 200
III
�a
150
J
100
50
0
8 8.5 9 9.5 10 10.5 11 11.5 12
pH
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Water Treatment Plant — Optimization Study
City of Blair, Nebraska
Figure 3 - Magnesium Hardness
140
120
O 100
U
80
60
E 40
20
0
8 8.5 9 9.5 10 10.5 11 11.5 12
pH
In summary, the total hardness reduces to a pH of 10 and is relatively stable to a pH of 11. As
the pH increases above 10, magnesium hardness reduction is traded for higher calcium hardness
with no real change in total hardness until a pH of 11. Above a pH 11 total hardness increases
with excess lime used for magnesium hardness removal.
However, hardness reduction is not the only factor to consider. Turbidity reduction and total
organic carbon (TOC) removal is also important. Figure 4 presents turbidity versus pH. As can
be seen, the lower turbidity readings occur at the higher pH which results in longer filter runs.
This is due to the formation of magnesium hydroxide at a higher pH which is an excellent
coagulant. TOC removal follows turbidity removal due to the additional coagulation of TOC by
magnesium hydroxide as seen in Figure 5.
70
60
50
40
H
Z 30
20
10
f n
Figure 4 - Turbidity
10.5 11 11.5
pH
Inti mizatio
12
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-3- Snyder & Associates, Inc.
Water Treatment Plant - Optimization Study
City of Blair, Nebraska
Figure 5- Total Organic Carbon
4.5
4
3.5
3
- 2.5
E 2
1.5
1
0.5
0
8 8.5 9 9.5 10 10.5 11 11.5 12
pH
The second chemical to optimize in a lime softening water treatment plant is carbon dioxide
which is used for recarbonation. Using the data from the optimum pH, a computer model was
used to calculate the amount of carbon dioxide that would be needed to stabilize the water. The
target for proper pH was to have a calcium carbonate precipitation potential slightly under 10.
At this value, a light deposit of calcium carbonate is applied to the interior of the water
distribution piping to protect it from corrosion. The results of the modeling showed that a final
pH in the range of 9.1 would be the appropriate value.
Considering all factors, the optimum pH for quality water and cost effective treatment is in the
range of 10.6 to 10.8 for the softeners and 9.1 for the recarbonated water after softening.
Table 1 presents a comparison of the current operation to the optimum operation.
Table 1
Parameter
Current
Optimum
Total Hardness (mg/L as CaCO3)
180
180
Calcium Hardness (mg/L as CaCO3)
50
90
H
11.3
10.8
Turbidity (NTU)
2.0
3.2
TOC Removal (%)
2.8
8.9
Lime Dosage (mg/L)
179
137
Final pH
7.5
9.1
Carbon Dioxide for (mg/L) Recarbonation
68
16
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Water Treatment Plant — Optimization Study
City of Blair, Nebraska
As can be seen, the lower softener pH results in a shift in hardness type from calcium to
magnesium. It also results in a higher turbidity and lower removal of TOC which should not
result in significant operational issues. Chemicals used for both lime and carbon dioxide are
reduced. For an average flow of 10 million gallons per day, the reduction in chemical use and
cost would be:
Lime
(179 — 137) mg/L x 8.34 x 10 mgd = 3503 lbs/ day or 639.3 tons/year
At a cost of $190.45/ton delivered:
639.3 tons x $190.45/ton = $121,750/year
Carbon Dioxide
(68-16) mg/L x 8.34 x 10 mgd = 4339 lbs/day or 791.4 tons/year
At a cost of $23.22/ton delivered:
791.4 tons x $23.22/ton = $18,376/year
FOR A TOTAL SAVINGS OF $140,130/year
RESULTS — DISINFECTION BYPRODUCT
The water treatment plant for Blair, Nebraska uses free chlorine for disinfection. In the process
of chlorine reacting with natural organics in the Missouri River water, disinfection byproducts
are formed. USEPA has established allowable standards for two groups of byproducts,
trihalomethanes with a standard of 80 µg/L and haloacetic acids of 60 µg/L. Water sample
testing for disinfection byproducts within the City of Blair has shown low values for haloacetic
acids, but close to the standard for trihalomethanes. However, water samples collected in the
Village of Kennard and the Papio-Missouri River Natural Resources District Washington County
Rural Water Number 2 have tested, at times, above the USEPA standard for trihalomethanes.
The Village of Kennard, Nebraska purchases bulk water from the City of Blair and adds
additional chlorine at the point of connection to maintain adequate disinfection residual within
their distribution system. This additional chlorine and contact time seems to be responsible for
the formation of additional byproducts such that the Village of Kennard violated the 'drinking
water standard for trihalomethanes and has been ordered by the Nebraska Department of Health
and Human Services to correct the violations.
PMRNRD Washington County Rural Water System Number 2 also purchases bulk water from
the City of Blair. Like Kennard, they also add additional chlorine to the water, and like many
rural water systems, have areas in their distribution system with high water age. In 2009, the
system violated the USEPA total trihalomethane standard and required notification of the
violation to its customers. Since disinfectant byproducts formation is a function of disinfectant
JA2012_Projects\112.0314\Correspondence\Reports\optimization -5-
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371
iter, Inc.
Water Treatment Plant - Optimization Study
City of Blair, Nebraska
concentration and time, the long resident time and added chlorine are most likely the cause of the
elevated concentrations of trihalomethane.
To determine the appropriate approach to minimize the formation of disinfection byproducts, a
series of disinfection byproducts formation potential tests were conducted in the laboratory on
three different water samples: 1) Blair clarifier effluent water, 2) Blair clarifier effluent water
adjusted to pH 10.5 and 3) Blair clarifier effluent water adjusted to pH 11.4. The results of the
testing are presented in Table 2.
Table 2- Trihalomethanes Formation
Potential Results (mg/1L)
This test can be used as an indicator of the maximum concentration of trihalomethanes in a given
water using chlorine as the disinfectant. The results show that high pH results in lower
formation potential. This is likely due to the removal of organics and/or the form of chlorine at
the high pH. However, all samples indicate that the potential of exceeding the drinking water
standard is likely. Thus, optimization of the softening/recarbonation systems will not resolve the
problem of elevated trihalomethanes.
The formation of trihalomethanes is dependent on disinfectant type and concentration along with
the reaction time and the natural organics in the treated water. Since reaction time and organics
concentration are not easily changed, a change in disinfectant is the likely solution. If Blair
would convert to a chloramination process rather than free chlorine, the trihalomethanes would
reduce due to the slower reaction rate for combined chlorine versus free chlorine. This reduction
would also provide assistance to the Village of Kennard and PMRNRD Washington County
Rural Water System Number 2. Once converted, both systems should come into compliance
with the trihalomethane standard and may not have to add additional chlorine to maintain
adequate disinfection residual in their distribution system.
If the City of Blair converts to chloramination, an educational program for the customers is very
important prior to the conversion. Customers that have specialized water treatment needs, such
as dialysis or aquariums, will need to make arrangements to reduce the chloramines
concentration to prevent damage to equipment or fish. This reduction of chloramines is not
difficult and can be easily done at the point of use.
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Total
Chloroform
Bromodichlormethane
Dibromochloromthane
Bromoform
Trihalomethane
Blair raw
225
40.5
6.3
0.2
272
Blair adjusted to
246
37.1
8.5
0.7
292.3
p1110.5
Blair adjusted to pH
166.5
24.7
7.4
1.5
200.1
11.4
This test can be used as an indicator of the maximum concentration of trihalomethanes in a given
water using chlorine as the disinfectant. The results show that high pH results in lower
formation potential. This is likely due to the removal of organics and/or the form of chlorine at
the high pH. However, all samples indicate that the potential of exceeding the drinking water
standard is likely. Thus, optimization of the softening/recarbonation systems will not resolve the
problem of elevated trihalomethanes.
The formation of trihalomethanes is dependent on disinfectant type and concentration along with
the reaction time and the natural organics in the treated water. Since reaction time and organics
concentration are not easily changed, a change in disinfectant is the likely solution. If Blair
would convert to a chloramination process rather than free chlorine, the trihalomethanes would
reduce due to the slower reaction rate for combined chlorine versus free chlorine. This reduction
would also provide assistance to the Village of Kennard and PMRNRD Washington County
Rural Water System Number 2. Once converted, both systems should come into compliance
with the trihalomethane standard and may not have to add additional chlorine to maintain
adequate disinfection residual in their distribution system.
If the City of Blair converts to chloramination, an educational program for the customers is very
important prior to the conversion. Customers that have specialized water treatment needs, such
as dialysis or aquariums, will need to make arrangements to reduce the chloramines
concentration to prevent damage to equipment or fish. This reduction of chloramines is not
difficult and can be easily done at the point of use.
JA2012_Projects\112.0314\Correspondence\Reports\optimization _6_ Snyder &Associates,Inc.
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M
The study concluded that the lime 'softerung/recarnonauon sy5Lc111h UL L11C Dldll VV MCI I IUaL111G11L
Plant could be optimized by changing the pH in the softening process to 10.8 and the
recarbonation process to 9.1. While the optimized pH would save $140,000/year, it would result
in additional turbidity having to be removed in the filters, which should not cause any
operational problems. In 'addition, there would be a decrease in the TOC removal in the
softening process which also should not have a negative impact on plant processes.
The study also concluded that the treated Missouri River water has a high potential for forming
trihalomethanes when free chlorine is used for disinfection. This potential can be reduced by
switching to a chloramination process. This should reduce the trihalomethane concentration in
Blair, as well as for the Village of Kennard and PMRNRD Washington County Rural Water
System Number 2. Additionally, both communities should not have to add additional chlorine at
their point of connection with the City of Blair.
JA2012_Projects\112.0314\Correspondence\Reports\optimization _']_ Snyder &Associates, Inc.
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APPENDIX A
LABORATORY RESULTS
JA2012_Projects\112.0314\Correspondence\Reports\optimization Snyder & Associates, Inc.
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