UCLA

Reverse osmosis (RO) membrane desalination is increasingly used for production of potable water from seawater and brackish water and in municipal and industrial water reuse applications. However, RO processes at high recovery are impacted by membrane mineral scaling and the upper pressure barrier of RO elements, pumps and the associated energy expenditure. Membrane surface scaling decreases membrane water permeability and may reduce membrane lifetime. Membrane mineral scaling can be partially mitigated via antiscalant dosing of the RO feed, but this is at an added cost. An alternate approach to scale mitigation can be achieved via RO Feed flow reversal (FFR) which is a chemical-free method. By periodically reversing the direction of the raw RO feed, mineral scale that develops at the membrane stage exit can be removed via dissolution, thereby restoring the membrane permeability. Accordingly, an evaluation of the FFR process in a spiral-wound RO pilot system was carried out, without antiscalant dosing, with gypsum as a model scalant whereby the efficacy of the process was monitored via real time membrane surface monitoring.

The recovery of RO process depends on both the ability to mitigate scaling and overcoming the pressure barrier, particularly for highly saline source water. However, the upper pressure limit constraint on membrane elements and the added energy cost often place a practical limit on the achievable recovery. In order to overcome the pressure limitation, the present work explored the deployment of a hybrid RO-NF configuration. In this arrangement the RO membrane enable balancing the rejection of the target mineral salts while the NF membranes allow for further concentration of the RO concentrate, which is particularly beneficial for source water high in concentration of divalent ions. The NF membrane permeate is then recycled and directed to the RO membranes. The attained recovery range and energy utilization for the above process were explored demonstrating that a higher recovery can be attained at a lower applied pressure relative to conventional RO system configuration.