Lecture 9: Lime-Soda Ash Softening (Part 2)

Figure by MIT OCW. Adapted from: Viessman, W., Jr., and M. J. Hammer. Water Supply and Pollution Control. 7th ed. Upper Saddle River, NJ: Pearson Education, Inc., 2005, p. 446. Before treatment: 0.4 0 3.5 2+ Ca CO2 HCO3 4.3 2+ Mg – SO4 2– 4.6 + Na Cl– 2.3 1.2 meq CaSO4 Non-carb. hardness 2.3 meq Ca(HCO3)2 Carbonate hardness 0.4 CO2 0.8 meq MgSO4 NCH After treatment with lime Ca(OH)2 and intermediate reaction to remove carbonate hardness: (chemical equations 1, 2, & 3) 0 1.25 1.2 Ca2+ Ca2+ 2.0 Mg2+ 2.3 Na+ OH– SO42– Cl– 1.25 meq excess lime 2.0 meq NCH 0.3 NaCl After treatment with lime and intermediate reaction to remove noncarbonate Mg hardness: (chemical equations 4 & 5) 0 1.25 1.8 2.0 Ca2+ Ca2+ 2.3 Mg Na+ OH– SO42– Cl– 1.25 meq excess lime 2.0 meq NCH 0.3 NaCl After treatment with soda ash Na2CO3: (chemical equations 8 & 9) 0 1.25 2+ Ca OH CO3 2.8 2– Na SO4 residual 0.8 meq hardness 3.1 + Mg Ca – 1.25 meq excess lime 0.6 0.8 2+ 2– Cl– 2.0 meq added soda ash 0.3 NaCl After recarbonation: (chemical equations 6 & 7) 0 0.6 0.8 2+ Mg Ca HCO3 2.8 – residual 0.8 meq hardness 3.1 + Na SO4 2– 2.0 meq added soda ash Cl– 0.3 NaCl 0.3 NaCl Carbonate system equilibrium 1.E+00 1.E-01 1.E-02 1.E-03 D0, D1, D2 D1 - [HCO3–] 1.E-04 D0 - [H2CO3*] 1.E-05 D2 - [CO32–] 1.E-06 1.E-07 1.E-08 1.E-09 0 1 2 3 4 5 6 7 pH 8 9 10 11 12 13 14 Summary of chemical dosage calculations required for lime & lime-soda ash softening* REQUIRED CHEMICAL DOSAGE CALCULATIONS PROCESS Single-Stage Lime: Lime addition for softening: For waters with high calcium, low magnesium, & carbonate hardness CaO = {carbonic acid concentration} + { calcium carbonate hardness} Soda ash addition for softening: Na2CO3 = none Carbon dioxide for pH adjustment after softening: CO2 = + Excess Lime: For waters with high calcium, high magnesium, and carbonate hardness; process may be one or two stages { estimated carbonate alkalinity of softened water { estimated residual calcium hardness of softened water { { = source water alkalinity { { - source water calcium hardness { { Lime addition for softening: { CaO = carbonic acid concentration { { + total alkalinity {{ + magnesium hardness { { + excess lime dose { Soda ash addition for softening: Na2CO3 = none Carbon dioxide for pH adjustment after softening: CO2 = +2 { { - estimated residual source water - excess lime + calcium hardness total hardness dose of softened water + estimated residual magnesium hardness of softened water {{ {{ source water alkalinity excess lime dose {{ Single-Stage Lime Soda Ash: Lime addition for softening: For water with high calcium, low magnesium, & carbonate and noncarbonte hardness Soda ash addition for softening: {{ { { CaO = {carbonic acid concentration} + { calcium carbonate hardness} Na2CO3 = {calcium noncarbonate hardness} and /or {magnesium noncarbonate hardness} Carbon dioxide for pH adjustment after softening: CO2 = { source water alkalinity {{ + {{ soda ash dose source water estimated residual calcium + calcium hardness hardness of softened water Excess Lime - Soda Ash: Lime addition for softening: For waters with high calcium, high magnesium, and carbonate and noncarbonate hardness; process may be one or two stages CaO = concentration + concentration + { { carbonic acid excess lime requirement {{ { {{ calcium carbonate { magnesium magnesium + 2 carbonate + noncarbonate hardness hardness { { {{ { Soda ash addition for softening: Na2CO3 = { calcium noncarbonate hardness magnesium + noncarbonate hardness {{ { Carbon dioxide for pH adjustment after softening: CO2, first stage = { estimated hydroxide excess lime alkalinity of softened = dose water CO2, second stage = + { { {{ {{ { estimated residual + magnesium hardness of softened water {{ {{ { source estimated hydroxide soda ash source water - water total + dose alkalinity of softened = alkalinity hardness water {{ { estimated residual hardness of softened water * All quantities are expressed as mg/L as CaCO3 Figure by MIT OCW. Adapted from: MWH, J. C. Crittenden, R. R. Trussell, D. W. Hand, K. J. Howe, and G. Tchobanoglous. Water Treatment: Principles and Design. 2nd ed. Hoboken, NJ: John Wiley & Sons, 2005, pp. 1610-1611.