Lime Softening
Noncarbonate hardness
"permanent hardness". caused by calcium and magnesium salts other than carbonate and bicarbonate salts CaSO4 MgSO4 CaCl2 MgCl2
Carbonate hardness
"temporary hardness". hardness caused by calcium and magnesium bicarbonates. Calcium and magnesium combined with carbonate also contribute to the total hardness. CaCO3 MgCO3 Ca(HCO3)2 Mg(HCO3)2
What are the chemical storage characteristics of the lime-soda ash process?
-CALCIUM OXIDE -usually ordered in pebble form -shipped in bulk by rail or truck -solids form is not corrosive -stored in steel or concrete bins -CALCIUM HYDROXIDE -shipped in 50 and 100 lb bags -bags should remain sealed to prevent absorption of moisture -SODA ASH -shipped in bulk, bags, or barrels -solid form is not corrosive -stored in stell or concrete bins
What causes sludge solids carryover?
-improper hydraulic conditions in the sedimentation basin or solids-contact clarifier -sudden changes in water quality
What are the LCR requirements of the softening process?
-soft water can be aggressive and cause lead or copper in household plumbing to leach into the water -Systems must be careful to not remove all hardness from the water to avoid elevated levels of lead and copper in their customer's water supply
Paste type (lime slaker)
-water to lime ratio of 2 or 2.5:1 by weight -operates near boiling point -takes 10-15 minutes -slaked lime is diluted to create a 10% "milk of lime" slurry which can be fed to the system -calcium hydroxide and soda ash are fed into a chemical solution tank using a dry chemical, gravimetric feeder to create the lime slurry that is fed to the system
Detention type (lime slaker)
-water to lime ratio of 4 or 5:1 by weight -takes 20-30 minutes -heat generated is transferred to incoming water -temperature maintained at greater than or equal to 160 degees F
Water that has a hardness concentration ranging from ___ to ____ mg/L as CaCO3 is considered soft.
0 60
Softening facilities process
1. rapid mixing 2. flocculation 3. sedimentation basins 4. solids-contact basins 5. sludge collection and disposal equipment 6. recarbonation basins 7. fitlers
Water that has a hardness concentration ranging from ____ to ____ mg/L as CaCO3 is considered hard.
121 180
The slaking process should be maintained at _______ degrees F or higher for best results
160
Water that has a hardness concentration > ____ mg/L as CaCO3 is considered very hard.
180
It takes about _____ to _____ minutes to slake lime in a detention-time lime slaker
20 30
The sludge blanket in a softening plant's contact clarifier should have a solids concentration in the range of _____ to ______% total solids.
5 15
Water that has a hardness concentration ranging from ___ to ___ mg/L as CaCO3 is considered moderately hard.
61 120
Calcium carbonate equilibrium
A condition in which the water will neither dissolve nor precipitate calcium carbonate. The water is considered stable
Supersaturated
An unstable condition of a solution in which the solution contains a substance at a concentration above the saturation point
There are two types of dry lime. What are they?
CALCIUM HYDROXIDE hydrate lime slaked lime -Used by smaller plants CALCIUM OXIDE quicklime unslaked lime -requires special feed equipment -less expensive -used by large plants
Slaked lime
Calcium oxide that has been mixed with water (calcium hydroxide)
How does the softening process affect taste and odor removal?
Coagulation of organics that cause poor taste and odor occurs better at lower pH
What forms does soda ash (Na2CO3) come in?
Either powder or granular
When is water considered unstable?
If it deposits scale or is corrosive.
Chemical exposure first aid
If soda ash or a lime slurry gets into the eyes, they should be flushed for at least 15 minutes with water and then consult a doctor for an evaluation. If the skin is contacted with lime or soda ash, flush the skin with large volumes of water. If inhaled or ingested, contact a physician immediately
How to fix unstable water problems?
If water tends to deposit scale, recarbonation can reduce pH and eliminate this problem. If water is corrosive, pH may need to be raised to the point of stability
What are the additional contaminants removed as a result of water softening.
Iron manganese heavy metals radionuclides dissolved organics viruses
What are the safety precautions of lime handling?
LIme creates dust whn handled and the operator should wear appropriate PPE. It can burn the skin or eyes when combined with moisture. Calcium oxide and water react to generate a large amount of heat -- enough to start a fire if flammable materials are nearby
What are the safety precautions of soda ash?
PPE is similar to that used for lime handling. Skin that cannot be covered with protective clothing can be covered with a protective cream or petroleum jelly to minimize harm to the operator. Soda ash should be stored in a cool dry location to prevent moisture absorption and caking which could cause a hazardous condition.
Another name for noncarbonated hardness is _____________.
Permanent hardness
____________ is required to stabilize the water following the lime softening process to prevent CaCO3 from precipitating out on filters.
Recarbonation
What are the two main objections to water containing excessive hardness?
Scale formation and reaction with soap
________ _______ will produce a cake that is both difficult and hazardous to handle when it comes into contact with moisture.
Soda ash
What are concentration that classify water as "hard"?
Soft= 0-60 mg/L as CaCO3 moderate = 61-120 mg/L CaCO3 hard = 121-180 mg/L CaCO3 Very hard = > 180 mg/L CaCO3 Target = 50-85 mg/L CaCO3
Trihalomethane control of softening process
THM form more quickly as pH rises. Softening process increases pH which causes elevated levels of THMs. solutions: remove THM precursors, use alternate disinfectant, remove THMs after softening
Slake
The addition of water to quicklime to form calcium hydroxide which is used in the water softening and stabilization processes.
Total hardness
The combination of calcium and magnesium hardness. Also the combination of carbonate and noncarbonate hardness.
Saturation
The point at which a solution can no longer dissolve any more of a particular chemical
Recarbonation
The reintroduction of carbon dioxide into water following the softening process. This lowers the pH of the water
Precipitate
The separation from solution of an insoluble substance
What occurs after chemical precipitation?
The water is stabilized by using carbon dioxide and bringing the pH up to around 8.6 pH units.
What are the operational control tests performed with the softening process?
alkalinity total hardness magnesium carbon dioxide pH jar testing langelier saturation index coupon testing
What types of coagulants are used?
alum ferric chloride ferric sulfate polymers
Quicklime
another name for calcium oxide (CaO). It is used for water softening and water stabilization
_______ and _______ are the two main constituents of water hardness
calcium magnesium
lime is added and forms calcium carbonate when it reacts with which chemicals?
calcium bicarbonate magnesium bicarbonate magnesium carbonate
Where do calcium and magnesium come from in geologic formations?
calcium comes from limestone and magnesium primarily comes from dolomite.
What are the corrective actions for excess calcium carbonate?
correct "caused by" problems or add sodium hexametaphosphate to hold the calcium carbonate in solution.
TOC removal compliance of softening plants
if a softening plant cannot meet the TOC removal requirements of the D/DBPR, alternative compliance criteria may be used. -softening resulting in alkalinity reduction of treated water to < 60 mg/L as CaCO3 -Softening resulting in the removal of at least 10 mg/L as CaCO3 of magnesium hardness
What is excess calcium carbonate caused by?
inadequate mixing short detention time interference from organics
How does the softening process affect the disinfection process?
increased pH during the softening process can cause the disinfection process to be less effective.
How does the softening process affect THM formation?
increased pH during the softening process causes THMs to form more rapidly.
the lime softening processes can cause the formation of trihalomethanes (THMs) due to the high pH levels required since THM formation _________ as pH ________.
increases increases
Raw water requires both _____ and _____ _____ when there are nominal amounts of magnesium hardness in the water.
lime soda ash
What are the variations of water softening using chemical precipitation?
lime softening lime-soda ash softening Excess-lime treatment split treatment caustic-soda treatment * most expensive - produces less sludge
The double-stage process in excess-lime softening is designed to achieve greater ________ removal.
magnesium
Raw water can usually be softened by only using lime when it contains little or no ________ hardness
noncarbonate
When softening water using the lime-soda ash softening process, soda ash dosage is base don the amount of _____ ______ to be removed.
noncarbonate hardness
What are the negatives of soft water?
poor taste increased sodium in water corrosive to plumbing leaves skin feeling slimy
What are the safety precautions of Carbon dioxide handling?
presents an asphyxiation hazard as it is a colorless, odorless gas that accumulates in low lying areas because it is heavier than air. Adequate ventilation in occupied areas is necessary to prevent the accumulation of carbon dioxide to unsafe levels. Carbon monoxide is also produced by on-site CO2 generators and poses the same risk as carbon dioxide.
Hydrated lime
quicklime that has been combined with water. Also called slaked lime or calcium hydroxide
What factors must be considered when calculating lime and soda ash dose required for softening?
raw water hardness carbonate + non-carbonate hardness desired finished water hardness amount of CO2 ini the raw water Jar testing should be used to verify the calculated dose as it takes into account additional factors not calculated
How to prevent after precipitation
reduce pH by adding CO2 to bring the pH to 8.6-8.7 range.
How can sludge solids carryover problems be resolved?
reducing hydraulic load
incomplete settling of coagulants results in?
shorter filter runs poor turbidity removal scale formation on filter media
Water that is too soft will cause your skin to feel slippery after washing with soap due to a _______ ________ feeling on your skin
soap scum
Chemical storage of softening facilities
softening plants should have a 30 days supply of chemicals on hand.
Softening is similar to conventional treatment but has special equipment. What are these pieces of equipment?
solids-contact clarifiers pellet reactors
insoluble
something than cannot be dissolved in a solution
Ion exchange softening processes can be used for direct groundwater treatment as long as _________ and _______ levels are not too high.
turbidity iron
