Water Softening by Precipitation with Lime at the Bloomington, Illinois Water Treatment Plant Dr. John T. O’Connor, PE Tom O’Connor, PE
Water Hardness Hardness is the sum of calcium and magnesium concentrations, expressed as mg/l as CaCO3 equivalent in waterworks practice.
Hardness Valuesmg CaCO3 equivalent/ liter City Source Raw Softened Kansas City 75% MO River, 25% alluvial wells 218 85 St. Louis 66% MS River, 34% MO River 208 107 Columbia Alluvial wells, MO R. flood plain 350 155 Chicago Lake Michigan, South Dist. Plant 128 128 Highland Silver Lake 104 141 Normal 14 Wells 419 108 Bloomington Lakes Bloomington, Evergreen 195 111
Benefits 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 Exchangeas alternative to lime precipitationWashington. Illinois • Sodium ion exchanged for calcium and magnesium ions • Increases in Total Dissolved Solids (TDS), conductivity • Regeneration with NaCl (brine); brine disposal required • Potential adverse health effects of sodium ion increase
Cation Exchangers Throughput set for Ammonium Ion Removal Morton, Illinois
SALT STORAGE PIT NaCl ~ $27 / ton (2007) NaCl + H2O Na+ + Cl–(brine)
Brine Transfer Pump Brine ~11% NaCl (1 lb/gal) Corrosive to metal, concrete
Spent Brine Disposal • Sewer • Evaporation • Injection • Ocean • Deicing
Lime Precipitation versus Cation Exchange Softening 1 meq/ l = 50 mg CaCO3 eq./ l
Lime Softening - Precipitation Single-stage—when source water has high calcium and low magnesium hardness - CaCO3 Excess lime—source water is high in both calcium and magnesium hardness - CaCO3 + Mg(OH)2 Mg(OH)2 may be recovered at lowered pH for use as a ‘recycled’ coagulant.
‘Overfeed’ of Lime leads to post-precipitation upon Recarbonation
Lime Contact Clarifier Lime Feed CaO(s) + H2O ^ Ca(OH)2 (s)
Dorr-Oliver, Permutit and and CBI-Walker Upflow 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 Softener Two effluent launders convey softened water to recarbonation basin
Sludge Rake steel truss with squeegees to plow solids to center hopper
Stabilization Carbon dioxide (CO2 or H2CO3) - Recarbonation lowers pH; alkalinity unchanged Sulfuric Acidlowers pH and alkalinity; adds sulfate ion (microbial activity may lead to H2S formation) Chlorineproduces hydrochloric acid (HCl); lowers pH
Recarbonation Basin Settling Compartment CO2 was Injected 20 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 Lagoon Champaign, Illinois
Sidewall Sludge Cutting Device Champaign, Illinois
Utilization of Reclaimed Sludge Champaign, Illinois Sludge pumped to tanker truck for application to farmland as ‘soil sweetener’.
Micrograph of Floating Solids Green crystals = calcium carbonate Orange strands = algal filaments
Lime Softening Solids Calcium carbonate crystals (green) Algal cells (orange)
Jar Testing • EVALUATE • Chemical dosages • PAC, polymer feeds • Flocculation time and paddle speed • Floc size, density • Floc settling rates • Temperature effects • Organism removals