With increasing salinity levels of traditional water sources and global water shortages, desalination is emerging as a necessity to maintain sustainability, as well as a solution for exploiting underutilized brackish water sources. However, desalination at inland locations is often not feasible due to the high costs associated with the disposal of high volume of concentrate residuals. Consequently, inland brackish water desalination requires operation at high product water recovery (>90%) to minimize concentrate volume and thus to improve the options for concentrate disposal. The main operational bottleneck to high recovery is membrane mineral scaling, a direct consequence of mineral salts (e.g., CaSO4, BaSO4, CaCO3) being concentrated above their solubility limits, resulting in membrane surface blockage, reduced membrane productivity, and eventual membrane damage. Moreover, the wide variety of brackish water composition suggests that mineral scaling, the type of mineral scalants and the mineral scaling process, are highly site specific. A new approach is thus required to overcome this operational bottleneck to high recovery desalting, utilizing fundamental and systematic methodologies that can robustly transform site-specific challenges into site specific solutions.

This poster will describe the development of brackish water desalting at high recovery using a multi-prong approach, considering both membrane-desalting operations and the integration of accelerated chemical demineralization in accordance to site-specific conditions. The development of membrane-desalting operations from initial assessment of water recovery limits to systematic selection of operating conditions will be presented, underlining the importance of both theoretical analysis and experimental evaluations. Novel membrane scaling evaluation techniques were developed, allowing determination of scale-free operating conditions. These techniques also allowed, for the first time, real-time observation of membrane scale formation, and therefore direct quantification of surface crystal growth kinetics during membrane-desalting operations, as well as antagonistic/synergistic mineral scaling phenomena that are not predicted by theoretical solubility analysis, the current standard in conventional practice

The poster will also present a new approach of integrating chemical demineralization with membrane-desalting operations, which were developed and demonstrated through successive bench-, pilot-, and production-scale operations. Fundamental understanding of the impact of chemical additives on both scale suppression and on mineral precipitation is an integral aspect in the development of the present high recovery approach. An example will highlight how such fundamental understanding can lead to a novel, less chemical-intensive process that may be suitable for high recovery desalting of certain source waters. The present approach enabled water recovery enhancement up to very high levels (at or above 95% water recovery). The work clearly demonstrates that successive switching of strategies (from scale-suppresion during membrane desalting operation to accelerated chemical precipitation during chemical demineralization) is both technically and economically feasible, and can be readily implemented in large-scale desalting.

Relevant publications include:

1. Shih, W.-Y., A. Rahardianto, R.-W. Lee, and Y. Cohen, Morphometric characterization of calcium sulfate dihydrate (gypsum) scale on reverse osmosis membranes, J. Membrane Science 252 (2005) 253

2. Rahardianto, A., W.-Y. Shih, R.-W. Lee, and Y. Cohen, Diagnostic characterization of gypsum scale formation and control in RO desalination of brackish water, J. Membrane Science 279 (2006) 655

3. Shih, W.-Y., J. Gao, A. Rahardianto, J. Glater, Y. Cohen, and C. J. Gabelich, Ranking of antiscalant performance for gypsum scale suppression in the presence of residual aluminum, Desalination 196 (2006) 280

4. Rahardianto, A., J. Gao, C. J. Gabelich, M. D. Williams, and Y. Cohen, High recovery membrane desalting to low-salinity brackish water: integration of accelerated precipitation softening with membrane RO, J. Membrane Science 289 (2007) 123

5. Uchymiak, M., A. Rahardianto, E. Lyster, J. Glater and Y.Cohen, A novel RO ex situ scale observation detector (EXSOD) for mineral scale characterization and early detection, J. Membrane Science 291 (2007) 86

6. Gabelich, C.J., M.D. Williams, A. Rahardianto, J.C. Franklin, and Y. Cohen, High-recovery reverse osmosis using intermediate chemical demineralization, Journal of Membrane Science 301 (2007) 131

7. Rahardianto, A., B. McCool, Y. Cohen, RO desalting of inland brackish water of high gypsum scaling propensity: kinetics and mitigation of membrane mineral scaling, Environmental Science and Technology, 10.1021/es702463a (Web release on February 6, 2008)

8. Zach-Maor, A., R. Semiat, A. Rahardianto, Y. Cohen, S. Wilson, S.R. Gray, Diagnostic analysis of the technical feasibility of RO desalting of treated wastewater, Desalination 230 (2008) 239