IMPACT of Intermittent Feed Water Flushing on Surface Mineral Scaling during SEMI-Batch Reverse Osmosis Operation

Reverse osmosis (RO) membrane desalination technology has become the major treatment method for the generation of clean water supplies around the globe. Although RO provides a superior operational flexibility to treat various water sources, surface mineral scaling remains a challenge due to limitation it imposes on the upper level of attainable water recovery. In recent studies, semi-batch RO was proposed as an alternative RO operational mode that could potentially reduce the level of surface mineral scaling while enabling reduction in antiscalant utilization. In semi-batch RO, the retentate stream is continuously recycled in the filtration period to increase water recovery. Although the saturation indices of sparingly soluble mineral salts rapidly increase as salts accumulate in the feed flow channel during the filtration process (in which retentate recycle is employed), intermittent flushing of the membrane elements with the raw-feed water is implemented with the goal of resetting the induction period (or “induction clock”) for the onset of mineral scaling on the membrane surface. In order to assess the above hypothesis, an investigation was undertaken to evaluate the impact of repeated filtration-flushing cycles in semi-batch RO operation on mineral scaling. In order to unambiguously assess the mineral scaling under semi-batch operation, a direct real-time membrane surface imaging system was developed to detect the onset of membrane scaling. The membrane surface monitoring system consisted of an ex-situ plate-and-frame membrane cell with provisions for high resolution imaging of the membrane surface in the cell. In the current work gypsum (calcium sulfate anhydride) was selected as a model scalant. Membrane surface image analysis enabled quantification of gypsum scaling kinetics in terms of both the number density of gypsum crystals and the extent of membrane surface scale coverage. The surface monitoring system was interfaced with a spiral-wound pilot scale RO system, capable of achieving up to 90% water recovery. RO desalination tests in both semi-batch and steady-state operational modes were then carried out for comparative analysis between the above two modes with respect to the scaling induction period and mineral scaling kinetics at various water recoveries to evaluate the impact of: (a) intermittent raw-feed water flushing for semi-batch RO operation in the formation of nuclei and rate of crystal growth, (b) convective residence time in the RO system on the scale induction period, and (c) mineral salt supersaturation level in the RO feed due to concentrate recycling. Given the ability for unprecedented early detection of mineral scaling via the current direct and real time membrane surface imaging, the current has provided unambiguous demonstration that mineral scaling under certain operational conditions can be significantly exacerbated in semi-batch relative to steady state RO operation. The fundamental results and analysis approach provided by the current study provide a basis for optimizing the cyclic operational strategy of semi-batch RO with the aim of mitigate membrane scaling.