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10.38                       CHAPTER TEN

                    Chlorine
                    dioxide
                    solution
                   to process   Ejector


                                                     Ejector
                                                      water
                                                     supply
                                   Reaction zone

                           m
                              Feed rate
                        ~      meters



                               I
                             Chlorite
                                               ]  Chlorine gas
                             supply               supply
                   FIGURE  10.16  Gas  chlorine-sodium chlorite system schematic.
                   (Source: M. Aieta and J. Berg, "A Review of Chlorine Dioxide in Drink-
                   ing Water Treatment,"  Journal AWWA, vol. 78, no. 6, June 1986, p. 62.)


         gas  (H2), are produced at the cathode.  The overall reaction is shown by the following
         formula:

                         2NaCIO2 +  2H20 ~  2C102 +  2NaOH +  H2
           The caustic solution is dilute and can be used as a feedstock  chemical for pH adjust-
         ment in the  process  water,  and the  H2, while extremely explosive, poses  no particular
         problem because it is diluted to low concentrations (one-fourth the explosion limit) with
         air and vented to the atmosphere (Gates,  1998). The C102 is removed from the anode por-
         tion of the reactor to avoid further oxidation to chlorate ion (CIO3-) and, possibly,  per-
         chlorate  ion (C104-) (Gordon, 2001).  One commercial electrochemical system separates
         the C102 from the anolyte stream by stripping it from solution with air while another sys-
         tem separates  it by a patented membrane process  called perstraction  (Figure 10.17). Gor-
         don (2001) described perstraction as a membrane  distillation  process  whereby  "a gas-per-
         meable bydrophobic membrane is used to  separate  a donor solution (reaction  chamber)
         from an acceptor  solution (water  flow)."  Additional details  regarding the membrane and
         the perstraction process  are provided by Cowley et al.  (1996).
           Commercial electrochemical generators  used by the paper pulp industry produce  sev-
         eral hundred pounds of C102 per day, but those used to treat drinking water are typically
         much smaller, most commonly producing less than 50 to  150 lb (23 to 68 kg) per day.


         Sodium  Chlorite
         Sodium chlorite is available as a solid, orange-colored powder that is about 80% NaC102
         by weight and in the form of liquid solutions that are 25% to 32% NaC102 by weight. If
         granular sodium chlorite  is used, it should be stored  in a separate  building equipped with
         sloped  floors,  drains, and facilities for hosing down spills.  The building should be con-
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