Optimization of RO desalting using external RO monitoring
Membrane desalination of brackish water can lead to super-saturated solution conditions with respect to sparingly soluble mineral salts in the bulk solution and at the membrane surface. Under such conditions, mineral salt precipitation, in the bulk and at the membrane surface, can result in scale formation and thus reduction in membrane permeate productivity and shortening of the membrane lifetime. In the present work, a multi-pronged approach toward scale mitigation was undertaken in order to: (a) develop and implement a method for early detection of mineral scaling and monitoring surface mineral salt crystallization, (b) develop a rapid field method to determine the optimal RO process conditions to maximize permeate recovery while avoiding mineral scaling, and (c) develop and demonstrate scale-free RO desalting in a feed flow reversal mode of operation. Experimental demonstration of the above approach, focused on gypsum as the model scalant, given that permeate recovery in inland water desalination is often limited by this scalant.
Early detection of mineral scaling and quantification of crystallization kinetics was accomplished via direct monitoring of membrane scaling using an optically transparent RO monitoring (EROM) cell interfaced with a digital imaging system. Gypsum crystal growth and surface nucleation were correlated in terms of a single-crystal mass transfer coefficient and the degree of gypsum supersaturation at the membrane surface, respectively. The EROM system was shown to be an effective tool for rapid field determination of the recovery limit at the mineral scaling threshold. The EROM was subsequently interfaced with a spiral-wound RO pilot plant demonstrating that permeate recovery can be maximized by safe operating just below the scaling threshold, with the RO operation adjusted given input (regarding the onset of mineral scaling) from the EROM system. Scale-free operation was also shown to be feasible, for RO operation at which the solution was supersaturated at the membrane surface and without the use of antiscalants, via periodic reversal of the feed flow direction that was triggered upon the detection of mineral scaling by the EROM detector.
0543: Civil engineering