Abstract As a means to minimize or eliminate brine disposal problems, high recovery RO must be accompanied by pre- or post-treatment to avoid scale precipitation, almost entirely by CaSO 4. All such methods will add to water cost. However, rigorous comparison must be made with the costs and constraints of brine disposal at recoveries low enough not to require these treatments. Such examination is invariably and totally site-specific. Prior to detailed design of any large RO facility, it is always prudent to invest in a pilot plant testing program, whereby membrane performance under varying recovery and flux may be evaluated on the raw feed to be used at full scale. Because of the even tighter design limits imposed at high recovery, pilot tests of combined membrane and treatment processes are absolutely essential.