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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
Catio
n Ex
chan
gers
Catio
n Ex
chan
gers
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
Prec
ipita
tion
vers
usLim
e Pr
ecip
itatio
n ve
rsus
Catio
n Ex
chan
ge S
ofte
ning
Catio
n Ex
chan
ge S
ofte
ning
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.