Softening of Drinking Waters at Midwestern U.S. Water Utilities

Water SofteningWater Softeningby Precipitation with Lime

at the Bloomington, Illinois Water Treatment Plant

Dr. John T. O’Connor, PETom O’Connor, PE

Water HardnessWater HardnessHardness is the sum of calcium and magnesium concentrations,

expressed as mg/l as CaCO3 equivalent in waterworks practice.

Hardness ValuesHardness Valuesmg CaCO3 equivalent / liter

City Source Raw SoftenedKansas City 75% MO River, 25% alluvial wells 218 85St. Louis 66% MS River, 34% MO River 208 107Columbia Alluvial wells, MO R. flood plain 350 155Chicago Lake Michigan, South Dist. Plant 128 128Highland Silver Lake 104 141Normal 14 Wells 419 108Bloomington Lakes Bloomington, Evergreen 195 111

Benefits of SofteningBenefits of Softening• Reduced film formation on household fixtures,

glassware, pots and pans, silverware

• Reduced scale formation in pipes, hot water heaters and plumbing fixtures (if stabilized)

• Removal of metals (Fe, Mn, Cu, Pb, Cd, Zn, Ni)

• Removal of radionuclides (radium, uranium)

• Disinfection (due to high pH of lime)

• Marginal removal of TOC (lime co-precipitation)

Cation ExchangeCation Exchangeas alternative to lime precipitationas alternative to lime precipitation

Washington. Illinois

• Sodium ion exchanged for calcium and magnesium ionsSodium ion exchanged for calcium and magnesium ions• Increases in Total Dissolved Solids (TDS), conductivityIncreases in Total Dissolved Solids (TDS), conductivity• Regeneration with NaCl (brine); brine disposal requiredRegeneration with NaCl (brine); brine disposal required• Potential adverse health effects of sodium ion increasePotential adverse health effects of sodium ion increase

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Throughput set forAmmonium Ion Removal

Morton, Illinois

SSAALLTT

SSTTOORRAAGGEE PPIITT

NaCl ~ $27 / ton (2007)NaCl + H2O Na+ + Cl– (brine)

Brine Transfer PumpBrine Transfer Pump

Brine ~11% NaCl (1 lb/gal)

Corrosive to metal, concrete

Spent Spent Brine Brine

DisposalDisposal

•Sewer•Evaporation• Injection•Ocean•Deicing

Lime

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1 meq/ l = 50 mg CaCO3 eq./ l

Lime Softening - PrecipitationLime Softening - Precipitation

Single-stageSingle-stage—when source water has high calcium —when source water has high calcium and low magnesium hardness - and low magnesium hardness - CaCOCaCO3 3

Excess limeExcess lime—source water is high in both calcium —source water is high in both calcium and magnesium hardness - and magnesium hardness - CaCOCaCO3 3 + Mg(OH) + Mg(OH)22

Mg(OH)Mg(OH)2 2 may be recovered at lowered pH for use as a ‘recycled’ coagulant.may be recovered at lowered pH for use as a ‘recycled’ coagulant.

‘Overfeed’ of Lime leads to post-precipitation upon Recarbonation

Lime Contact ClarifierLime Contact Clarifier

Lime FeedCaO(s) + H2O Ca(OH)2 (s)

Recarbonation BasinRecarbonation Basin

Lime Hopper and Slaking FacilitiesLime Hopper and Slaking Facilities

Lime AugerLime Auger

Lime Feed LineLime Feed Line

Dorr-Oliver, Permutit and and CBI-WalkerDorr-Oliver, Permutit and and CBI-WalkerUpflow Contact ClarifiersUpflow Contact Clarifiers

Bloomington, Illinois, Water Treatment Plant

Dorr-Oliver Softener with V-notch overflow weirs;effluent channels in center and perimeter of tank

Dorr-Oliver Softener drained for

maintenance

Influent pipe, lime feed and

mixing

Dorr-Oliver Softener: Detail of dewatered peripheral effluent

channel

Permutit Softener•Conical reaction well

•Solids contact with slurry recirculation

Permutit Softener•Settling compartment with radial launders

•Submerged orifices minimize variations in effluent discharge

Permutit SoftenerTwo effluent launders convey softened water to recarbonation basin

Sludge Rake

steel truss with squeegees to plow solids to center hopper

Carbon dioxide (COCarbon dioxide (CO22 or H or H22COCO33) -) - Recarbonation Recarbonation lowers pH; alkalinity unchangedlowers pH; alkalinity unchanged

Sulfuric AcidSulfuric Acid lowers pH and alkalinity; adds sulfate ionlowers pH and alkalinity; adds sulfate ion (microbial activity may lead to H(microbial activity may lead to H22S formation)S formation)

ChlorineChlorine produces hydrochloric acid (HCl); lowers pHproduces hydrochloric acid (HCl); lowers pH

StabilizationStabilization

Bloomington, Illinois’ ‘Old’ Plant (1929, 1956, 1966)

Recarbonation BasinRecarbonation Basin Settling CompartmentSettling CompartmentCO2 was Injected20 feet below

surface; no

bubbles evident; almost total

absorption of applied

gas.Clear water indicates no

post-precipitation of calcium carbonate.

Removal of Softening Sludge from Storage LagoonRemoval of Softening Sludge from Storage LagoonChampaign, Illinois

Sidewall Sludge Cutting DeviceSidewall Sludge Cutting DeviceChampaign, Illinois

Utilization of Utilization of Reclaimed Reclaimed

SludgeSludgeChampaign, Illinois

Sludge pumped to tanker truck for

application to farmland as ‘soil sweetener’.

Contact Clarifier - Floating SolidsContact Clarifier - Floating SolidsMissouri Lime Softening Plant

Floating SolidsFloating Solids

Micrograph of Floating SolidsMicrograph of Floating Solids

Green crystals = calcium carbonateOrange strands = algal filaments

Lime Softening SolidsLime Softening Solids

Calcium carbonate crystals (green)Algal cells (orange)

EVALUATE

• Chemical dosages

• PAC, polymer feeds

• Flocculation time and paddle speed

• Floc size, density

• Floc settling rates

• Temperature effects

• Organism removals

Jar TestingJar Testing

Why lime softening also removes trace metals.