<P> The Reverse Osmosis process is not maintenance free . Various factors interfere with efficiency: ionic contamination (calcium, magnesium etc .); DOC; bacteria; viruses; colloids & insoluble particulates; biofouling and scaling . In extreme cases the RO membranes are destroyed . To mitigate damage, various pretreatment stages are introduced . Anti-scaling inhibitors include acids and other agents like the organic polymers Polyacrylamide and Polymaleic Acid), Phosphonates and Polyphosphates . Inhibitors for fouling are biocides (as oxidants against bacteria and viruses), like chlorine, ozone, sodium or calcium hypochlorite . At regular intervals, depending on the membrane contamination; fluctuating seawater conditions; or when prompted by monitoring processes, the membranes need to be cleaned, known as emergency or shock - flushing . Flushing is done with inhibitors in a fresh water solution and the system must go offline . This procedure is environmental risky, since contaminated water is diverted into the ocean without treatment . Sensitive marine habitats can be irreversibly damaged . </P> <P> Freeze - thaw desalination uses freezing to remove fresh water from salt water . Salt water is sprayed during freezing conditions into a pad where an ice - pile builds up . When seasonal conditions warm, naturally desalinated melt water is recovered . This technique relies on extended periods of natural sub-freezing conditions . </P> <P> A different freeze - thaw method, not weather dependent and invented by Alexander Zarchin, freezes seawater in a vacuum . Under vacuum conditions the ice, desalinated, is melted and diverted for collection and the salt is collected . </P> <P> Solar evaporation mimics the natural water cycle, in which the sun heats the sea water enough for evaporation to occur . After evaporation, the water vapor is condensed onto a cool surface . </P>

Why is a large quantity of water used in industrial processes in the u.s. purified by desalinization