1 WATER SOFTENING l removal of hardness »Hardness is?... l 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

2 Why bother? l Hardness in mg/l as CaCO 3 range considered excessive l Even > 150 mg/l may result in consumer objection l mg/l as CaCO 3 is considered a moderate amount high soap consumption scaling in heating vessels and pipes

3 Formation of Hardness CO 2 + H 2 O  H 2 CO3 Subsoil Limestone CaCO 3(s) + H 2 CO 3  Ca(HCO 3 ) 2 MgCO 3(s) + H 2 CO 3  Mg(HCO 3 ) 2 Precipitation Topsoil

4 Hardness l 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. »Ca 2+, Mg 2+ associated with HCO 3 -, CO 3 2- »CH = TH or Total alkalinity, whichever is less

5 Hardness l 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. »Ca 2+, Mg 2+ associated with other ions, Cl -, NO 3 -, SO 4 2- »NCH = TH - CH »If Alkalinity  Total hardness, then NCH = 0

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

7 LIME - SODA ASH SOFTENING l Addition of lime, Ca(OH) 2, & soda ash, Na 2 CO 3 causes precipitation of Ca, Mg l Lime often added as CaO, quick lime »CaO + H > Ca(OH) 2 l Three basic processes »Excess lime treatment »Selective calcium removal »Split treatment

8 Stoichiometry CO 2 + Ca(OH) 2 --> CaCO 3 + H 2 O Ca(HCO 3 ) 2 + Ca(OH) 2 --> 2 CaCO H 2 0 Mg(HCO 3 ) 2 + Ca(OH) 2 --> CaCO 3 + MgCO 3 + 2H 2 0 Mg(CO 3 ) + Ca(OH) 2 --> Mg(OH) 2 + CaCO 3 MgS0 4 + Ca(OH) 2 --> Mg(OH) 2 + CaS0 4 MgCl 2 + Ca(OH) 2 --> Mg(OH) 2 + CaCl 2 CaS0 4 + Na 2 CO 3 --> CaC0 3 + Na 2 SO 4 CaCl 2 + Na 2 CO 3 --> CaC NaCl

9 Solubilities l Ca(OH) 2 is very soluble, Mg(OH) 2 is not l MgCO 3 is very soluble, CaCO 3 is not »CaCO 3 and Mg(OH) 2 are relatively insoluble ü CaCO 3 : ~ 30 mg/l as CaCO 3 l 0.6 meq/l ü Mg(OH) 2 : ~ 10 mg/l as CaCO 3 l 0.2 meq/l l MW is ?... l EW is?... l mg/l is ? mg/l as Mg(OH) 2 29 mg/meq 58 mg/mmol

10 Removal by precipitation l Is complete removal possible?... No, lime-soda ash softening cannot remove all hardness What about CO 2 ? l CO 2 + Ca(OH) 2 --> CaCO 3 + H 2 O CO 2 must be considered because it consumes lime

11 Effectiveness l mg/l as CaCO 3 is usually considered acceptable result of lime- soda ash softening, »as long as Mg is < 40 mg/l as CaCO 3 üany more causes scaling in heating vessels

12 Stoichiometry Table l meq of lime and soda ash to remove a meq of X initially present X LimeSoda Ash CO 2 Ca(HCO 3 ) 2 Mg(HCO 3 ) 2 MgCO 3 MgSO 4 CaSO