Integrated reverse osmosis/vacuum membrane distillation for enhanced arsenic removal and water recovery of brackish water desalination

Brackish water has attracted increasing attention as a promising source to augment fresh water supply in many areas around the world. However, the ubiquitously high content of arsenic in brackish water presents a vexing technical challenge to reverse osmosis (RO) desalination plants. In this study, an integrated RO/vacuum membrane distillation (VMD) process was investigated for enhanced As(III) removal and water recoveries of brackish water desalination without requirement of pre-oxidation step as in other conventional treatment processes. The experimental results demonstrate that As(III) removal and water flux of the single RO process were profoundly affected by the conditions of the brackish water feed and the process water recovery. At pH 10, the As(III) removal maximized at 90%, and sharply dropped to below 80% as the process water recovery exceeded 70%. Exceeding the process water recovery of 70% also significantly reduced the RO process water flux. On the other hand, the VMD process was able to further treat concentrated RO brines with a 100% As(III) removal and stable water flux. Thanks to advantage of VMD, the integrated RO/VMD process at the global water recovery of 95.5% achieved adequate global As(III) removal, bringing the As(III) concentration in the product water down to below the 10-ppb allowable maximum limit.