The relationship between operating pressure and water flux and salt flux for Reversed osmosis water purification process using asymmetric cellulose acetate membrane. with dense layer thickness 1 micron, water concentration in feed was 0.2 g/cc, D value, =1x10-ocm-ls , D. =1x10*cm'/s, S. = 0.035 D, = 18cm' /mol water flux rate & %salt rejection that varies with the operating pressure drop Estimated osmotic pressure and operating pressure drop if 98 % salt rejection is specified.