Reverse osmosis permeate post-treatment by upflow calcite contactors: Dissolution and mass transport
Reverse osmosis (RO) desalination may be used for desalination of brackish and saline waters, but the product water may have low concentrations of hardness and alkalinity, which may corrode infrastructure. Adjustment of pH and addition hardness and alkalinity may be required to meet potable water guidelines. Upflow calcite contactors may be used instead of lime or caustic addition for post-treatment of RO permeate to deliver non-blended, stable, non-corrosive finished water. The first objective of this research was to experimentally determine the sensitivity of performance of upflow calcite contactors with respect to several design and operational parameters such as: feed pH, overflow rate, empty bed contact time, and calcite particle size. The second objective of this research was mathematical modeling of calcite dissolution as a function of hydrodynamic conditions, such as empty bed contact time, Reynolds number, and filtration rate. A TOMCO2 carbon dioxide delivery unit was used to adjust the RO permeate before contact with the calcite. Four parallel upflow calcite columns were constructed of four-inch diameter transparent PVC pipe for comparison of experimental variables. The calcite beds were initially 30 inches with particle sizes of 0.2–2.0 mm, and the bed height was measured periodically in order to calculate the calcite dissolution. The range of loading rates and empty bed contact times were 1.9–17 gallons per minute per square feet (gpm/ft2) and 0.23–9.8 minutes, respectively. The feed pH ranged from 5.5 to 6.5. The rate of calcite dissolution ranged from 5–100 mg/L of CaCO3, and the product water pH ranged from 7.5–9.0. Of all the tested calcite products, the one with a nominal particle size of 1-mm, and 95 percent purity produced the best results for post-treatment of the KBH desalination plant permeate. The optimal feed pH for calcium and alkalinity dissolution was determined to be less than 5.5 with an overflow rate of 9.5 gpm/ft2. For a calcite upflow contactor operating at steady-state, the calcite dissolution was calculated using the model developed by Letterman; mass transfer in this study was determined to behave according to the following relationship between Sherwood, Reynolds, and Schmidt numbers: Sh=(Re') 1.0 (Sc)0.5.
Maldonado-Castaneda, Luis D, "Reverse osmosis permeate post-treatment by upflow calcite contactors: Dissolution and mass transport" (2012). ETD Collection for University of Texas, El Paso. AAI1533237.