Process for sterilization and removal of inorganic salts from a water stream

A process for the desalination of unpotable feedwater comprising the steps of:

• (a) decarbonating said feedwater by adding sodium hydroxide to said feedwater in a quantity sufficient to raise the pH level to at least about 9.5 but no greater than 11 to thereby convert substantially bicarbonate to carbonate and to precipitate said carbonate substantially as calcium carbonate in a sludge and a supernatant saline stream;
• (b) clarifying said supernatant saline stream by filtration to remove suspended solids and form a filtrate;
• (c) adding acid to the filtrate to lower its pH level;
• (d) subjecting the acidulated filtrate to first stage reverse osmosis across a membrane to produce a first stage permeate stream and a first stage brine stream whereby multivalent cations at least are substantially rejected into the latter and a portion of the monovalent cations and of the water from the saline feed are preferentially retained as first stage permeate;
• (e) subjecting at least some of the first stage permeate stream to second stage reverse osmosis, to form a second stage permeate stream of potable water and a second stage chloride brine stream;
• (f) separating the second stage sodium chloride brine stream into a more concentrated sodium chloride stream and a less concentrated sodium chloride stream;
• and
• (g) recovering as purified water the said less concentrated stream from step (f) and the second stage from step (e).