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.

Scale reduction and cleaning techniques were investigated for direct contact membrane distillation (DCMD) during processing of seawater reverse osmosis (SWRO) brine at feed and distillate temperature of 50 °C and 25 °C, respectively. The results showed that MD achieved distillate electrical conductivity (EC) of less than 5 μS/cm and good flux (above 20 L/m2/h) prior to rapid flux reduction at a brine feed EC of approximately 160 mS/cm. Analysis by inductively coupled plasma (ICP) and scanning electron microscopy coupled with an energy dispersive spectroscopy (SEM–EDS) revealed that flux reduction was primarily caused by precipitation of CaCO3 and CaSO4. Titration results showed that concentrating brine feed water pH was linked to distillate flux, and further revealed the precipitation process of CaCO3. Implementation of a 0.45 μm cartridge filter at the brine feed inlet extended water recovery from 45% to 60% due to the removal of precipitating salts. Simple chemical free membrane cleaning regime involving distillate flushing restored MD initial flux to 98%. Periodic raw SWRO brine feed flushing also restored membrane initial flux to 84% over 133 h of operation. Results also showed that possible organic fouling developed over long-term MD operation with constant EC of SWRO brine at feed.

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