William, my comments only relate to potential potability of desalinated water. From a practical point of view, you wouldn't choose highly polluted water to desalinate. Algae and pollutants will tend to clog the membrane. Also, the brine will have pollutants concentrated and will need to be disposed of. The most efficient way of managing water pollutants is to reduce their input into the system at the source.
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"How many legs does a dog have if you call the tail a leg? Four; calling a tail a leg doesn't make it a leg." Abraham Lincoln
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