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.

Permeate flux of five repetitive DCMD tests and the corresponding CaSO4 concentration in the feed solution. The membrane was flushed with MilliQ water at the end of each test and a fresh 2 g/L CaSO4 solution was used for the subsequent test. The DCMD set-up: (1) Cooling units; (2) distillate overflow collector; (3) distillate reservoir; (4) recirculating pumps; (5) membrane cell; (6) feed reservoir (with encased heating element).Figure optionsDownload as PowerPoint slideHighlights
► Membrane scaling caused by CaSO4 was more severe than that by CaCO3 or silicate.
► CaSO4 scaling could only occur after a sufficiently long induction time.
► The induction period decreased and the size of the CaSO4 crystals increased as the feed temperature increased.
► A simple operational regime involving regular membrane flushing to reset the induction period was developed.
► The proposed approach was proven to be effective for controlling CaSO4 scaling.