Scale reduction and cleaning techniques during direct contact membrane distillation of seawater reverse osmosis brine

• Reduction of distillate flux is primarily caused by precipitation of CaCO3 and CaSO4. • Changing of pH in hot bulk brine provides early indication of scaling formation. • Inline cartridge filtration improves DCMD operational time and water recovery. • Antiscalant prolongs DCMD operational time by mitigating salts precipitation. • SWRO brine and distillate flushings are effective in membrane cleaning.

(1) A bench scale DCMD test achieved 60% water recovery to treat SWRO brine of 68 g/L TDS. It was found that distillate flux reduction was mainly caused by precipitation of CaCO3 and CaSO4 in the bulk brine solution. (2) 5 mg/L of antiscalant in 120 mS/cm brine feed effectively maintained CaCO3 at supersaturated condition (with S&DSI = 1.24) for 50 h of continuous mode operation. Possible organic fouling on membrane surface caused flux decline. The membrane provided greater than 99.9% rejection of salt, and membrane pore wetting was not observed. (3) Salt precipitation occurred when brine TDS concentration reaches a critical EC level of approximately 160 mS/cm even in the presence of antiscalant. The rapid formation of crystal deposits was correlated to the pH variation in brine solution, where pH was an early indicator of rapid flux decline. (4) Inlet cartridge filter effectively removed salt crystals formed in brine solution and improved SWRO brine water recovery from 45% to 60%. (5) Intermittent membrane flushing with distillate water and raw SWRO brine restored distillate flux where distillate flushing restored near-complete original flux conditions. (6) Future study is needed on MD full hot cycle system scale removal.