(541a) Mixed Mineral Scale Formation in Low-Pressure Reverse Osmosis Membrane

A major impediment to achieving high recovery operation in membrane desalination of inland water sources is membrane scaling, commonly by minerals of calcium sulfate, calcium carbonate, and silica, due to oversaturation of the RO retentate stream. As these minerals precipitate and scale the membrane surface, the resulting permeate flux decline ultimately leads to shortening of the membrane operational lifetime. Most studies on mineral scale formation have primarily focused on scale formation by a single mineral salt [1, 2]. However, mineral scaling encountered in field RO operations occurs in multi-ion systems in which co-precipitation of different mineral salts may take place. Although largely qualitative, several studies in recent years have suggested that the formation (kinetics and thermodynamics) and characteristics (e.g. tenacity) of mineral scale due to multi-salt precipitation can be sufficiently may be insufficiently represented by the simple additive scaling effect of the pure single mineral salts [3, 4].

In the present work, the impact of mixed minerals scale formation and kinetics on low pressure RO membranes performance was investigated in a laboratory RO system. The model scalants selected for the study were calcium sulfate dihydrate (gypsum) and calcium carbonate. Membrane scaling experiments studies were conducted in a high pressure laboratory plate-and-frame RO system with the capability for real-time visual monitoring of the membrane surface and thus the development of surface crystals. The system was operated in a total recycle mode with the retentate stream continuously microfiltered to trap bulk crystals to ensure surface crystallization as the dominant mode of scale formation [2]. The time evolution of crystal dimensions, crystal morphology and crystal number density were evaluated from digital image analysis. To allow comparison between mixed mineral formation on membrane surface and in bulk solution, gypsum crystal growth with calcium carbonate co-precipitation experiments were also conducted in a batch crystallizer. The kinetics of crystal growth was followed by monitoring calcium ion activity decline and elemental and image analysis of the formed precipitate.

The experimental results revealed that when gypsum was the dominant mineral scalant, the extent of gypsum scaling was reduced with increasing calcium carbonate supersaturation. SEM imaging also indicated alteration of the crystals morphology consistent with the above observation. The reduction in the rate of mineral formation with increasing concentration of carbonate ion indicated that co-precipitation of calcium carbonate competes and interferes with the growth of gypsum both in bulk solution and on the membrane surface. The present study suggests that the optimization of RO operating conditions and the development of scale mitigation strategies must consider co-precipitation kinetics for the specific source water to be desalted.

References:

[1] A. Rahardianto, W.-Y. Shih, R.-W. Lee, and Y. Cohen, J. Membr.Sci., In Press, Corrected Proof (2006).

[2] W.-Y. Shih, A. Rahardianto, R.-W. Lee, and Y. Cohen, J. Membr.Sci., 252 (2005) 253.

[3] P. Dydo, M. Turek, and J. Ciba, Desalination, 159 (2003) 245.

[4] R. Sheikholeslami, Desalination, 154 (2003) 117.