1. 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

2. Water Hardness
Hardness is the sum of calcium and magnesium concentrations,
expressed as mg/l as CaCO3 equivalent in waterworks practice.

3. Hardness Values mg CaCO3 equivalent / liter
City Source Raw Softened
Kansas City 75% MO River, 25% alluvial wells
St. Louis 66% MS River, 34% MO River
Columbia Alluvial wells, MO R. flood plain
Chicago Lake Michigan, South Dist. Plant
Highland Silver Lake
Normal 14 Wells
Bloomington Lakes Bloomington, Evergreen

4. 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)

5. 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

6. Cation Exchangers Throughput set for Ammonium Ion Removal
Morton, Illinois

7. NaCl + H2O Na+ + Cl– (brine)
SALT STORAGE PIT
NaCl ~ $27 / ton (2007)
NaCl + H2O Na+ + Cl– (brine)

8. Brine Transfer Pump Brine ~11% NaCl (1 lb/gal)
Corrosive to metal, concrete

9. Spent Brine Disposal
Sewer
Evaporation
Injection
Ocean
Deicing

10. Lime Precipitation versus Cation Exchange Softening
1 meq/ l = 50 mg CaCO3 eq./ l

11. 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.

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

17. Lime Contact Clarifier
Lime Feed
CaO(s) + H2O ^ Ca(OH)2 (s)

18. Recarbonation Basin

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

20. Lime Hopper and Slaking Facilities

21. Lime Auger

22. Lime Feed Line

23. Dorr-Oliver, Permutit and and CBI-Walker Upflow Contact Clarifiers
Bloomington, Illinois, Water Treatment Plant

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

25. Dorr-Oliver Softener drained for maintenance
Influent pipe, lime feed and mixing

26. Dorr-Oliver Softener: Detail of dewatered peripheral effluent channel

27. Permutit Softener Conical reaction well
Solids contact with slurry recirculation

28. Permutit Softener Settling compartment with radial launders
Submerged orifices minimize variations in effluent discharge

29. Permutit Softener
Two effluent launders convey softened water to recarbonation basin

30. Sludge Rake
steel truss with squeegees to plow solids to center hopper

31. 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

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

33. 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.

34. Removal of Softening Sludge from Storage Lagoon
Champaign, Illinois

35. Sidewall Sludge Cutting Device
Champaign, Illinois

36. Utilization of Reclaimed Sludge
Champaign, Illinois
Sludge pumped to tanker truck for application to farmland as ‘soil sweetener’.

37. Contact Clarifier - Floating Solids Missouri Lime Softening Plant

38. Floating Solids

39. Micrograph of Floating Solids
Green crystals = calcium carbonate
Orange strands = algal filaments

40. Lime Softening Solids Calcium carbonate crystals (green)
Algal cells (orange)

41. Jar Testing EVALUATE Chemical dosages PAC, polymer feeds
Flocculation time and paddle speed
Floc size, density
Floc settling rates
Temperature effects
Organism removals

42. Why lime softening also removes trace metals.