Novel dynamic and cyclic designs for ultra-high recovery waste and brackish water RO desalination

Abstract

There is a drive for ultra-high recoveries in waste and brackish water RO desalination due to high brine disposal costs. In this work, two novel dynamic and cyclic designs (i.e. feed reversal and retentate recycle with a time-varying ratio in stage 3, as well as stage rotation between 1 and 3) are proposed to achieve a 90% recovery while alleviating scaling potential. The designs are conducted following FilmTec's guidelines and are simulated using validated models published previously. It is shown that retentate recycle suppresses concentration polarization, despite a higher pressure drop. Feed reversal leads to a quick drop of salt concentration due to mixing of fresh feed and brine. However, if the feed reversal is applied to the same stage, the salt concentration on the other end may be higher than brine concentration for a short period of time. Such a behavior is not observed during rotations between stages 1 and 3, whether the feed is reversed or not. Interestingly, the designs at 90% recovery demand only a few percent more energy than traditional two-stage designs operated at 80% recovery. Moreover, cycle-to-cycle salt build-up in batch-type ROs is absent in both dynamic designs presented in this work.