Application of fluidized pellet reactors for high recovery brackish water reverse osmosis : a step towards zero liquid discharge

The application of reverse osmosis (RO) membranes in the desalination of brackish water is increasing worldwide. The operation of these membranes is being challenged and limited by the scaling of sparingly soluble salts (SSS). Most common among those are: calcium carbonate, barium and strontium sulphate, silica and calcium fluoride. The scaling of these salts lower the recovery of the membranes and increase the volume of liquid discharge. In this paper, a new unit operation - Fluidized bed reactor or pellet reactor (PR)- is being investigated for its use to remove sparingly soluble salts from the membranes feed and reject streams. Analysis of field data from several Dutch softening plants reveal the following: pellet reactors are effective in removing di-valent cations: calcium, barium and strontium at 74%, 60% and 47% respectively. Fluoride and silica on the other hand are poorly removed at 10% and 8% respectively. Temperature seems to have an effect on the removal of calcium. Calcium is better removed at elevated temperature values. The removal mechanism for calcium was through precipitation mainly as calcium carbonate. Barium and fluoride are removed at higher efficiency in lime-based softening. The removal mechanism for barium and strontium on the other hand is thought to be incorporation into calcium carbonate crystal lattice where it is thought to be adsorption onto calcite crystals for fluoride. This is supported by the fact that all sparingly soluble compounds of these ions are under saturated in pellet reactors' feed. Also, better removal was witnessed in lime softening more calcium carbonates crystals- and in winter time where more lime was added. Silica removal was not affected by temperature or pH changes and was low in both caustic soda and lime based softening