Ca(HCO₃)₂ Ca(OH)₂ 2CaCO₃ 2 H₂O Mg(HCO₃)₂ 2Ca(OH)₂ Mg(OH)₂ 2CaCO₃ 2 H₂O MgSO₄ Ca(OH)₂ Mg(OH)₂ CaSO₄ CaSO₄ Na₂CO₃ CaCO₃ Na₂SO₄ MgCl₂ Ca(OH)₂ Mg(OH)₂ CaCl₂ CaCl₂ Na₂CO₃ CaCO₃ 2NaCl DOE-HDBK-1015/2-93 DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL Principles of Water Treatment CH-04 Rev. 0 Page 20 Removal of Suspended Solids Referring back to Figure 5 and examining the effluent of the softener, we find that both sodium salts and precipitates are present.  These substances result from reactions that typically occur based  on  the  presence  of  Ca      and  Mg      salts.    The  chemicals  most  commonly  used  for ++ ++ softening are soda ash or sodium carbonate (Na  CO  ) and hydrated lime (Ca(OH)  ).  Hard 2 3 2 water (water containing Ca     and Mg     salts) contains calcium and magnesium bicarbonates ++ ++ (Ca(HCO  )  ) and (Mg(HCO  )  ), as well as calcium sulfate (CaSO  ) and magnesium chloride 3 2 3 2 4 (MgCl  ).  These impurities produce the following reactions. 2 (4-18) (4-19) (4-20) (4-21) (4-22) (4-23) As  evident  from  the  above  reactions,  although  Ca      and  Mg      ions  can  be  removed  from ++ ++ solution, soluble sodium salts are formed.  Consequently the total dissolved solid content is essentially unchanged.  CaCO  and Mg(OH)   are in precipitate form and must also be removed 3 2 from the solution.  One method of doing this is by filtration.  Filtration is the process in which insoluble solids are removed from the water by passing them through a filter medium consisting of some type of porous material.  This process will remove suspended solids and precipitates, but has no effect on dissolved solids.  Numerous materials are used as filter media and include sand, activated charcoal, anthracite, diatomaceous earth, and to some extent resin in an ion exchanger.  Sand is not normally used in nuclear applications because of the silicate ion (SiO    ) 3 = associated.  Silicate ions are undesirable because they hydrolyze in water and form a weak acid, which tends to increase corrosion.  Activated charcoal is often used following a chlorinator in a  water treatment system because it removes excess residual chlorine as well as suspended matter. There are two types of mechanical filters in use, gravity and pressure.  Pressure filters are the most widely used because they can be installed in a pressurized system, thereby eliminating the need for additional pumps (gravity filters require pumps to provide a motive force).  In addition, in a pressurized filter system, flow rate and other associated parameters can be better controlled.