Lime Softening

Resources and Materials

Students should review and utilize the following on-line resources:

• http://www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/lime_softening_DWFSOM41.pdf• 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:• Visible scale on coffeemakers, aerators, etc.

• Spots on glassware

Commercial customers:• Reduced efficiency of boilers, increased expenses

for boiler operation

Increased costs for water system operation

Scale deposits roughenthe interior surface of pipes.Increased friction between Water and pipe walls must be overcome with additionalpumping 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: Silicon Oxide

Calcite: Calcium Carbonate

Sand: Silicon Dioxide

Dolomite: Calcium Magnesium Carbonate

Rock Salt: 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: Silicon Oxide

Calcite: Calcium Carbonate

Sand: Silicon Dioxide

Dolomite: Calcium Magnesium Carbonate

Rock Salt: 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’).

Common Name Chemical Name

CAUSTIC SODA SODIUM HYDROXIDE

QUICKLIME CALCIUM OXIDE

MILK OF LIME LIME SLURRY

SODA ASH SODIUM CARBONATE

SLAKED LIME HYDRATED LIME

Water treatment professionals may use any or all of the following terms:

HYDRATED LIME….LIME SLURRY…..

CALCIUM OXIDE….SODIUM HYDROXIDE……..SODA ASH

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:

• 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

• 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:

• http://www.sciencestuff.com/msds/C1450.html• http://www.sciencestuff.com/msds/C2549.html• http://www.sciencestuff.com/msds/C2648.html

1. Which of these has the lowest overall health hazard rating?

2. Which is a liquid?

3. Which should not be stored with permanganates?

4. Which, if any, require mechanical ventilation in the storage area?

Safety

ANSWERS:1. Soda ash (a.k.a. sodium carbonate)

2. Caustic soda a.k.a. sodium hydroxide

3. Lime

4. None of them