Desalination of thermally softened water by membrane distillation process

The water containing up to 12 g/L of soluble salts was successfully desalinated in membrane distillation (MD) for 250 h using polypropylene membranes. However, the MD process was affected by bicarbonates dissolved in water, which undergo a decomposition reaction when feed was heated, and the precipitation of carbonates on the membranes surface was observed. Scaling was responsible for a decrease in the MD efficiency and thermal water pretreatment was used to prevent it. The amount of HCO3− ions was lowered 2-3 times by keeping water at the boiling point for 15 min. This operation was beneficial only for underground waters with high hardness. In this case thermal pretreatment significantly lowered the amount of precipitate formed on the membrane surface. Surface waters (i.e. rivers and lakes) have a relatively low concentration of HCO3− ions, and a small amount of these ions was to be found after the thermal pretreatment. However, this low concentration of HCO3− ions caused the formation an amorphous, non-porous precipitate on the membrane surface. This hinders transport of water to membranes wall, what decreased the MD efficiency. This phenomenon was eliminated by adding to thermally softened water a small amount of HCI acid (to pH = 4).