Low‐pressure reverse osmosis desalination with improved cellulose acetate membranes

The advantages of carrying out reverse osmosis desalination with improved membranes at low pressures have been pointed out. Cellulose acetate (CA) semipermeable membranes developed with low air‐exposure periods by Sirkar et al. from novel Manjikian‐type casting solutions having high CA content, partial replacement of acetone by dioxane, and small amounts of ZnCl₂ (or none) have been tested for the following combinations of operating pressure (lb/in.²)/feed brine concentration (in ppm): 250/5000; 300/7500; 400/10,000; 600/30,000; 700/40,000; 750/50,000; 800/60,000. In the pressure range of 250‐600 lb/in.² (psig), the pure water permeability (PWP) constants of membranes for a given salt transport parameter are greater than those of similar membranes tested earlier by Sirkar et al. at 600 psig with a 5000‐ppm brine feed. The PWP constants decrease faster with pressure as the casting solution CA concentration is reduced from 32% to 30% and the dioxane content of the solvent is raised from 40% to 60%. The intrinsic performances of membranes from a 32% CA casting solution with 40% dioxane improve as the pressure is increased from 250 to 600 psig. The solution structure‐rate of evaporation concept along with the phenomenon of fingerlike cavities and protrusions in membranes from high dioxane casting solutions explain most of the observations of this work. The superior low‐pressure performance of these CA membranes provide a basis for systematically lowering the operating pressure in reverse osmosis desalination.