WATER SOFTENING removal of hardness How is Softening done?... Hardness is?... How is Softening done?... primarily Ca, Mg, plus Fe, Mn, St, Al Precipitation of Ca and Mg, or Ion exchange of Ca / Mg with ion such as Na

Why bother? Hardness in 300-500 mg/l as CaCO3 range considered excessive Even > 150 mg/l may result in consumer objection 60-120 mg/l as CaCO3 is considered a moderate amount high soap consumption scaling in heating vessels and pipes

Formation of Hardness Precipitation Topsoil Subsoil CO2 + H2O  H2CO3 Limestone CaCO3(s) + H2CO3  Ca(HCO3)2 MgCO3(s) + H2CO3  Mg(HCO3)2 Precipitation Topsoil

Hardness Carbonate Hardness Often called "temporary hardness" because heating the water will remove it. When the water is heated, the insoluble carbonates will precipitate and tend to form bottom deposits in water heaters. Ca2+, Mg2+ associated with HCO3-, CO32- CH = TH or Total alkalinity, whichever is less

Hardness Non-Carbonate Hardness Called permanent hardness because it is not removed when the water is heated. It is much more expensive to remove non-carbonate hardness than carbonate hardness. Ca2+, Mg2+ associated with other ions, Cl-, NO3-, SO42- NCH = TH - CH If Alkalinity  Total hardness, then NCH = 0

Hardness Units milligrams per liter (mg/L) as calcium carbonate (most common) parts per million (ppm) as calcium carbonate grains per gallon of hardness (to convert from grains per gallon to mg/L, multiply by 17.1) equivalents/liter (eq/L)

LIME - SODA ASH SOFTENING Addition of lime, Ca(OH)2, & soda ash, Na2CO3 causes precipitation of Ca, Mg Lime often added as CaO, quick lime CaO + H20 --> Ca(OH)2 Three basic processes Excess lime treatment Selective calcium removal Split treatment

Stoichiometry CO2 + Ca(OH)2 --> CaCO3 + H2O Ca(HCO3)2 + Ca(OH)2 --> 2 CaCO3 + 2 H20 Mg(HCO3)2 + Ca(OH)2 --> CaCO3 + MgCO3 + 2H20 Mg(CO3) + Ca(OH)2 --> Mg(OH)2 + CaCO3 MgS04 + Ca(OH)2 --> Mg(OH)2 + CaS04 MgCl2 + Ca(OH)2 --> Mg(OH)2 + CaCl2 CaS04 + Na2CO3 --> CaC03 + Na2SO4 CaCl2 + Na2CO3 --> CaC03 + 2NaCl

Solubilities Ca(OH)2 is very soluble, Mg(OH)2 is not MgCO3 is very soluble, CaCO3 is not CaCO3 and Mg(OH)2 are relatively insoluble CaCO3: ~ 30 mg/l as CaCO3 0.6 meq/l Mg(OH)2: ~ 10 mg/l as CaCO3 0.2 meq/l MW is ?... EW is?... mg/l is ?... 58 mg/mmol 29 mg/meq 5.8 mg/l as Mg(OH)2

Removal by precipitation Is complete removal possible?... No, lime-soda ash softening cannot remove all hardness What about CO2? CO2 + Ca(OH)2 --> CaCO3 + H2O CO2 must be considered because it consumes lime

Effectiveness 80-100 mg/l as CaCO3 is usually considered acceptable result of lime-soda ash softening, as long as Mg is < 40 mg/l as CaCO3 any more causes scaling in heating vessels

Stoichiometry Table meq of lime and soda ash to remove a meq of X initially present X Lime Soda Ash CO2 Ca(HCO3)2 Mg(HCO3)2 MgCO3 MgSO4 CaSO4 1 0 2 0 1 1 0 1