(290x) Prevention of Precipitation Fouling in Ro by Reverse Flow Operation

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?.

Reversing the flow in a series of RO elements was simulated in laboratory experiments by running a flat sheet test cell with two solutions, one supersaturated and one undersaturated, that were periodically switched. The supersaturated solution simulated the concentrate stream at the exit of an RO pressure vessel. The undersaturated solution simulated the feed stream into an RO pressure vessel. In the lab experiments, the flat sheet test cell was run for 30-90 minutes on the supersaturated solution and then 10 minutes on the undersaturated solution. The feasibility of flow reversal was successfully demonstrated with supersaturated calcium sulfate solution (2.0 saturation index) formed from calcium chloride and sodium sulfate and 300 mg/L SiO₂ in the presence of 300 mg/L CaCl₂ and 125 mg/L MgCl₂. Membranes which scaled in 170-180 minutes without switching were run for three times that time without scaling when the above switching technique was employed.