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258       Practical Design Calculations for Groundwater and Soil Remediation



                2.  Despite the relatively low oxygen-transfer efficiency in this exa-
                   mple, the air sparging still adds a significant amount of oxygen
                   to the aquifer. With regard to oxygen addition, an air injection
                   rate of 5 ft /min with an oxygen-transfer efficiency of 10% is
                              3
                   equivalent to reinjection of air-saturated water at 116.6 gpm.

           6.7.3  Injection Pressure of Air Sparging

           Injection pressure is an important component for design of an air-sparging
           system. The applied air injection pressure should overcome at least (1) the
           hydrostatic pressure corresponding to the water-column height above the
           injection point and (2) the air-entry pressure, which is equivalent to the cap-
           illary pressure necessary to induce air into the saturated media.

                                  P injection  = P hydrostatic  + P capillary  (6.31)
           Reported values of injection pressures range from 1 to 8 psig [5].
             The following procedure can be used to determine the minimum air injec-
           tion pressure:

              Step 1:  Determine the water-column height above the injection point.
                    Convert the water-column height to pressure units by using the
                    following formula:


                                    P hydrostatic  =ρg h hydrostatic      (6.32)
                      where ρ is the mass density of water and g is the gravitational
                    constant (32.2 ft/s ).
                                    2
              Step 2:  Use Table 2.2 to estimate pore radius of the aquifer media and
                    then use Equation (2.11) to determine height of capillary rise (or
                    obtain the capillary height from Table 2.2 directly). Convert the
                    capillary height to the capillary pressure by using the follow-
                    ing formula:


                                     P capillary  =ρg h capillary         (6.33)

              Step 3:  The minimum air injection pressure is the sum of these two pres-
                    sure components (P hydrostatic  + P capillary ) as stated in Equation (6.31).

             Information needed for this calculation:

              •  Depth of the injection point, h hydrostatic
              •  Mass density of water, ρ
              •  Geology of the aquifer material or the pore size of the matrix
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