Precipitation of sparingly soluble salts is one of the main factors limiting the recovery in reverse osmosis of brackish water sources. Recoveries can be increased and antiscalant usage reduced or curtailed by applying flow reversal to RO trains. Flow reversal works by changing the place of the entrance and exit of the 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 induction clock”.

The concept was successfully demonstrated in a laboratory simulation using calcium sulfate solutions and silica with bulk supersaturation factors reaching 300% in the presence and absence of scale inhibitors.

In a pilot laboratory unit operating with calcium sulfate solutions, recoveries of 82% were maintained by reversing flow every half hour (concentrate bulk Saturation Index = 257%) whereas induction time was less than 90 minutes without flow reversal. Evidence was also found for its efficacy in preventing calcium carbonate scale.

Ideas will also be presented on how to use this for enhanced recovery and decreased energy consumption in desalination processes.