Processes and apparatus are disclosed for producing clear reverse osmosis retentates having silica concentrations which are substantially supersaturated in silica from feedwaters having silica concentrations, without substantial formation of alkali-soluble scale having substantial silica content in the associated reverse osmosis apparatus, by adjusting the pH of such feedwaters to an acidic pH range prior to reverse osmosis in accordance with this invention. Also disclosed are related processes and apparatus for periodically removing minor amounts of alkali-soluble scale having substantial silica content from the apparatus.