Treatment of water

A process to treat saline water which includes Ca and sulphate, the process including

• adding a salt-forming base to the saline water thereby to form a Na, K, Mg or NH4 sulphate salt, to increase the pH of the water and to form calcium carbonate;
• precipitating and removing calcium carbonate to avoid the possibility of gypsum fouling of membranes of a membrane separation stage employed to desalinate the saline water;
• treating the saline water, at a temperature T1 which is above the saturation temperature of the saline water, in a first membrane separation stage to provide clean water and a first brine, the salinity of the first brine being higher than the salinity of the saline water;
• cooling the first brine to a temperature T2 to precipitate some of the sulphate salt from the first brine and separating the precipitated sulphate salt from the first brine producing a second brine, the temperature T2 being below the temperature T1 but above the freezing temperature of the first brine;
• and
• treating the second brine at a temperature T3 above the saturation temperature of the second brine in a second membrane separation stage to provide clean water and a third brine,
• wherein the salt-forming base, said temperature T1 and said temperature T2 are selected so that the sulphate salt which is formed in the saline water has a solubility in water at said temperature T1 which is at least 1.5 times the solubility of said sulphate salt in water at said temperature T2.