Humic acid fouling in the membrane distillation process

This work investigates the extent of humic acid fouling during the membrane distillation process for water treatment. The effects of pH, ionic strength, and divalent ion on fouling were studied. The experiments were performed with a 0.22-μm PVDF flat-sheet membrane in a direct contact membrane distillation unit. Flux declines were negligible (less than 6%) for the ranges of humic acid concentration, ionic strength, and pH studied. The examination of the membrane surface by SEM revealed a thin deposit layer. The addition of divalent cations (Ca2+) into the solution considerably reduced flux when Ca2+ concentration exceeded the critical coagulation concentration. Ca2+ affected flux by forming complexes with humic acids and resulted in coagulation on the membrane surface. The normalized flux, J/J0, was 0.57 after 18 h of operation when the CaCl2 concentration was 3.775 mM. However, the deposit of humic acid coagulate on the membrane surface was loosely packed, and was rather easily removed. Rinsing of the fouled membrane with clean water and a 0.1 M NaOH solution gave 100% of flux recovery.