Water Softening Dr. John T. O’Connor, PE Tom O’Connor, PE 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 Values mg 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 Exchange as alternative to lime precipitation Washington Cation Exchange as alternative to lime precipitation Washington. 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

NaCl + H2O Na+ + Cl– (brine) 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)

Recarbonation Basin

Chlorine Cl2 + H2O HCl + HOCl HCl H+ + Cl-

Lime Hopper and Slaking Facilities

Lime Auger

Lime Feed Line

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 Acid lowers pH and alkalinity; adds sulfate ion (microbial activity may lead to H2S formation) Chlorine produces hydrochloric acid (HCl); lowers pH

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

Clear water indicates no post-precipitation of calcium carbonate. 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’.

Contact Clarifier - Floating Solids Missouri Lime Softening Plant

Floating Solids

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

Why lime softening also removes trace metals.