Optimising dose of antiscalant in membrane filtration systems using a desupersaturation unit

The strategy behind the desupersaturation unit (DU) is rapid desupersaturation of the membrane concentrate stream by forced precipitation of sparingly soluble inorganic compounds on seed crystals. The efficiency of deposition will depend on the supersaturation and the condition of seeds, i.e. their reactivity. Poisoning of seeds by adsorption of natural organic matter and/or added commercial antiscalant will hinder the desupersaturation efficiency by blocking centres available for crystallisation. Barium sulphate precipitation in a laboratory scale DU was examined from natural reverse osmosis (RO) concentrates from surface water treatment pilot plant. Tests indicated similar effects on poisoning the barite seed grains by commercial antiscalant and natural organic matter from surface water (the Rhine River) treatment RO pilot plant (80% conversion) while no effect was observed from supersaturated synthetic (no organic matter) concentrate. An attempt was made to relate the effects of organic matter and the RO system conversion (50-85%). The rate of barite seed grain poisoning was attributed to the relative barium supersaturation in the concentrate. The efficiency of natural organic compounds started to decline above the supersaturation ratio of 3.3. Addition of commercial antiscalant enhanced the effect of natural organic matter. If the effects of organic matter in membrane systems are quantified and related to the RO system conversion, a laboratory DU can practically be used to control scaling.