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

Abstract

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 5μS/cm and good flux (above 20L/m2/h) prior to rapid flux reduction at a brine feed EC of approximately 160mS/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 0.45μm 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 133h of operation. Results also showed that possible organic fouling developed over long-term MD operation with constant EC of SWRO brine at feed.