Discussion: Pilot Testing a Limestone Contactor in British Columbia

FEBRUARY 1993 In their article in the May 1992 issue "Pilot Testing A Limesto e Contactor in British Columbia" Benjamin et al showed that a limestone contactor could be used o effectively treat the corrosive water at their study site. At superficial velocities on the order of 2 gpm/sq ft, flow through 5 ft or more of limestone bed raised the pH of the water from below 6 to above 9. Benjamin et al state on page 76 of their article, "for the range of flows tested and for the size and type of limestone used, the rate of CaC03 dissolution is mainly a function of contact time . . . (i.e., varying the flow rate does not influence the rate of CaC03 dissolution)." This author agrees that for the range of superficial velocities and other experimental conditions used by Benjamin et al (and with their measurement errors), the superficial velocity did not have an obvious effect on the effluent quality-empty bed contact time (EBCT) * relationship. The results obtained by Benjamin et al have, however, been compared with the predictions of the DESCON program, and this analysis shows that the superficial velocity is an important parameter and should be considered in limestone contactor design. The DESCON program1,2 t uses a kinetic model of calcium carbonate dissolution described in an article by Letterman et al.3 This model assumes that the rate of dissolution is controlled by two resistances that act in series a surface reaction that controls the release of calcium from the solid and a mass transfer resistance that controls the rate of calcium transport between the solid surface and the bulk solution. DESCON calculates the EBCT, the depth of the limestone bed, or both that