A scaling mitigation approach during direct contact membrane distillation
Membrane scaling during the treatment of aqueous solutions containing sparingly soluble salts by direct contact membrane distillation (DCMD) was investigated. The results reveal that membrane scaling caused by CaSO4 was more severe than that by CaCO3 or silicate. However, under the experimental condition used in this study and at feed and distillate temperature of 20 °C and 40 °C, respectively, CaSO4 scaling occurred only after a sufficiently long induction time of up to 25 h (corresponding to a saturation index of up to 1.5). The induction period decreased and the size of the CaSO4 crystals increased as the feed temperature increased. SEM analysis reveals that prior to the onset of CaSO4 scaling, the membrane surface was relatively clean and was completely free of any large crystals. Subsequently, a simple operational regime involving regular membrane flushing to reset the induction period was developed and was proven to be effective in controlling CaSO4 scaling. At a low system recovery, the permeate flux was constant despite the fact that the feed solution was always at a super saturation condition. Results reported here also confirm the interplay between induction time and the saturation index.