Page 289 - Water and wastewater engineering
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7-6   WATER AND WASTEWATER ENGINEERING


                                           Total hardness (TH)
                                                           Noncarbonate
                                     Carbonate hardness (CH)  hardness (NCH)
                             0                                     250
                                         Ca 2               Mg 2

                                          HCO   3             Cl
                             0                            200      250
                                                (a)

                                           Total hardness (TH)
                                          Carbonate hardness (CH)
                             0                                     250   275
                                          2                   2
                                        Ca                  Mg        Na
                                                  HCO
                                                     3
                             0                                           275
                                                   (b)

                            FIGURE 7-4
                            Relationships between total hardness, carbonate hardness, and noncarbonate
                            hardness. (Source: Davis and Cornwell, 2008.)


                              7-2   LIME-SODA SOFTENING
                              Objectives
                              Prior to the mid-twentieth century, the primary purpose of lime-soda softening by municipal
                            water treatment systems was to satisfy domestic consumer desire to reduce the aesthetic and
                            economic impact of soap precipitation. The importance of this objective has been reduced by the
                            introduction of synthetic detergents and home water softeners. Other benefits of lime-soda soft-
                            ening systems have been shown to be quite substantial. These include removal of heavy metals,
                            NOM, turbidity, and pathogens as well as improving the water quality that reduces costs for
                            distribution system corrosion, boiler and cooling water feed, and home water heater systems.
                            The concurrent removal of arsenic, chromium, iron, lead, manganese, and mercury provides an
                            additional benefit to the removal of hardness and, in some cases, may be the overriding reason for
                            selection of the technology (Kawamura, 2000).

                              Lime-Soda Softening Chemistry
                              Solubility Product.  Because all solids are soluble to some degree, there is an equilibrium
                            between the ions in solution and the solid. This equilibrium can be expressed as

                                                                       b
                                                          A B ()s    aA   bB a                           (7-5)
                                                           ab
                            where (s)     solid precipitate.
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