Prevention of Scaling of Reverse Osmosis Membranes by “Zeroing” the Elapsed Nucleation Time. Part I. Calcium Sulfate

Abstract

Precipitation of sparingly soluble salts is one of the main factors limiting the recovery in reverse osmosis (RO) of brackish water sources. Recoveries can be increased and antiscalant usage eliminated or reduced by applying flow reversal to RO process trains. Flow reversal works by changing the place of the entrance and exit of the pressurized feed before the induction time of the supersaturated solution along the membrane wall runs out and precipitation occurs. Reversing the flow before the induction time of the system is reached replaces the supersaturated brine at the exit with the unsaturated feed flow and thus “zeroes the elapsed nucleation time”, thereby resetting the induction clock. Laboratory experiments successfully demonstrated the technical feasibility of this concept by periodically exposing RO membranes to undersaturated solution after exposure to supersaturated calcium sulfate solution (bulk saturation index up to 3.0) formed from calcium chloride and sodium sulfate. The induction time to precipitation fouling without periodical switching of solutions was 150−270 min for stirred solutions. Periodic switching of solutions prevented precipitation for >480 min from stirred solutions. In follow-up experiments, a small scale pilot unit containing 2.5 in. diameter spiral RO elements was run continuously for 22 h with a calcium sulfate supersaturation index of 5.4 on the wall of the last element, when operated under reverse flow conditions. Without reverse flow the last RO element began to scale within 1 h.