Lime Softening
Resources and Materials
Students should review and utilize the following on-line resources:
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http://www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/lime_softening_DWFSOM41.pdf
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http://www.lime.org/documents/publications/free_downloads/fact-properties2007rev.pdf
http://www.sciencestuff.com/msds/C1450.html
http://www.sciencestuff.com/msds/C2549.html
http://www.sciencestuff.com/msds/C2648.html
http://www.epa.gov/safewater/mdbp/coaguide.pdf
http://www.lime.org/documents/publications/free_downloads/acid-neut-final-2000.pdf
https://www.nemi.gov/apex/f?p=237:38:3327616939156383::::P38_METHOD_ID:4679
https://www.nemi.gov/apex/f?p=237:38:3327616939156383::::P38_METHOD_ID:5701
‘Hard’ water
No, “Hard” water is not physically solid
(like ice).
“Hard” water contains positively charged ions
(cations) with an electrical charge
of 2 + or
greater. This includes the ions of most common
metals, as well as calcium and magnesium.
Potential issues from scaling
• Customer complaints
• Increased pumping costs
Customer complaints
Residential customers:
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Visible scale on coffeemakers, aerators, etc.
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Spots on glassware
Commercial customers:
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Reduced efficiency of boilers, increased expenses
for boiler operation
Increased costs for water system operation
Scale deposits roughen
the interior surface of pipes.
Increased friction between
Water and pipe walls must
be overcome with additional
pumping energy.
In some cases, the buildup is so large that the interior diameter of the pipe
is reduced by up to 50% or more, resulting in a loss of carrying capacity.
Self-Check Activity:
The following are common minerals found in the earth’s crust. Identify all of
the minerals which have a high potential to add hardness to groundwater in
contact with the mineral for an extended period of time:
Quartz:
Calcite:
Sand:
Dolomite:
Rock Salt:
Silicon Oxide
Calcium Carbonate
Silicon Dioxide
Calcium Magnesium Carbonate
Sodium Chloride
Calcium and Magnesium create most water
hardness:
The following are common minerals found in the earth’s crust. Identify all of
the minerals which have the potential to add hardness to groundwater in
contact with the mineral for an extended period of time:
Quartz:
Calcite:
Sand:
Dolomite:
Rock Salt:
Silicon Oxide
Calcium Carbonate
Silicon Dioxide
Calcium Magnesium Carbonate
Sodium Chloride
Calcium Carbonate Equivalent
Calcium Carbonate is a component of limestone,
an extremely common mineral in bedrock across
the world. For this reason, hardness caused by
any mineral (including magnesium, iron, etc.) is
commonly expressed in terms of an equivalent
amount of calcium carbonate hardness.
Total Hardness
Since calcium and magnesium are the most
common minerals creating hardness in water, water
treatment professionals use the following
assumption:
Calcium hardness + Magnesium hardness = Total hardness
Calcium and magnesium are removed at different
rates by treatment processes. The treatment facility
must identify the types of hardness involved to
design effective softening strategies
Total Hardness
Calcium and magnesium can form compounds such as
carbonates, sulfates, etc. The chemical properties of calcium
sulfate, for example, are different from the chemical
properties of calcium carbonate. Each compound contains
calcium (and therefore contributes hardness to water) but
carbonates and sulfates require different types of softening
techniques.
Carbonate hardness + Noncarbonate hardness = Total hardness
Carbonate hardness is sometimes referred to as ‘temporary
hardness’ because the carbonate is degraded to CO2 and
driven off by boiling. The calcium and magnesium will remain
in the boiling vessel as scale after the carbonate is boiled off.
Removal of Hardness through Precipitation
At high pH, the ions which create hardness
(calcium and magnesium) become insoluble in
water. These ions will precipitate out of the
water in solid form. This process is similar to
sedimentation in the conventional treatment
process: chemical feed systems, clarifiers and
sludge removal equipment are required.
Treatment Chemicals used to raise pH
Common treatment schemes for softening
include the addition of these chemicals:
• Lime (quicklime or slaked lime)
• Lime and Soda Ash (sodium carbonate)
• Caustic Soda (sodium hydroxide)
Review of Chemical Names
Many chemicals have one or more “common
names” (such as ‘salt’) in addition to their formal
chemical names (‘sodium chloride’).
Water treatment professionals may use any or all of the following terms:
HYDRATED LIME….LIME SLURRY…..
CALCIUM OXIDE….SODIUM HYDROXIDE……..SODA ASH
Common Name
Chemical Name
CAUSTIC SODA
SODIUM HYDROXIDE
QUICKLIME
CALCIUM OXIDE
MILK OF LIME
LIME SLURRY
SODA ASH
SODIUM CARBONATE
SLAKED LIME
HYDRATED LIME
Processes which take place after the water
has been softened:
• Ph adjustment
– The high pH’s required for softening are too
high for customer use
• Settling and removal of lime floc particles
– Lime particles give water a cloudy
appearance and can overload filters quickly
pH adjustment
After the softening process, pH must be lowered
to a level acceptable for consumption by
customers. Carbon dioxide is often used for this
adjustment (recarbonation). Carbon dioxide can
be purchased or generated on-site.
CO2 is heavier than air, and can displace
oxygen. Air monitoring is essential to maintain
safe operation when feeding carbon dioxide.
pH must be monitored for process control.
Settling and removal of lime particles
Review pages 6-20 through 6-22 of the following
document:
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http://www.epa.gov/safewater/mdbp/coaguide.pdf
Be familiar with the impact of lime floc carryover on
downstream treatment processes
Softened sludge
After removal from basins, sludge must be stored
or disposed of in some manner. To reduce the
final volume for storage or disposal, water
systems normally use one or more of the
following de-watering methods:
Sedimentation in sludge lagoons
Sand drying beds
Mechanical dewatering (filter press, centrifuge, etc)
Lagoons and drying beds
Both methods rely on natural processes with low energy
and equipment costs; however, both require a relatively
large land area.
LAGOONS: Natural sedimentation with discharge of
clarified supernatant to sewer or receiving stream.
Accumulated solids must be removed when lagoon
volume is significantly reduced.
DRYING BEDS: Perform best in dry sunny areas; must be
scraped clean after sludge has dried
Mechanical dewatering equipment
Usually requires pre-treatment with a sludge
thickener (settling device with draw-off of thicker
sludge from the bottom)
Vacuum filters, filter presses, or centrifuges are
used to further de-water the sludge to a dried
caked solid material. This solid material is
usually landfilled or applied to land as a soil
conditioner
Process Control Testing for Softening
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Hardness
pH
Alkalinity
Jar testing
Langelier Saturation Index
Carbon Dioxide
Safety
Review the MSDS for lime, caustic soda, and soda ash from
the sites listed below:
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http://www.sciencestuff.com/msds/C1450.html
http://www.sciencestuff.com/msds/C2549.html
http://www.sciencestuff.com/msds/C2648.html
1.
2.
3.
4.
Which of these has the lowest overall health hazard rating?
Which is a liquid?
Which should not be stored with permanganates?
Which, if any, require mechanical ventilation in the storage
area?
Safety
ANSWERS:
1.
2.
3.
4.
Soda ash (a.k.a. sodium carbonate)
Caustic soda a.k.a. sodium hydroxide
Lime
None of them