Morphological variations of precipitated salts on NF and RO membranes

Morphological variations of precipitated salts on nanofiltration (NF) and reverse osmosis (RO) membranes were compared for conditions where naturally occurring differences in feedwater composition, sometimes coupled with differences in hydrodynamics, produced different ratios of possible mineral foulants near the membrane. As often observed in practice, calcium carbonate (CaCO₃) and calcium sulfate (CaSO₄) were the main salts observed to precipitate on the membranes. Differences in precipitate morphology were matched with conditions in which there were relatively higher concentrations of carbonate ions, organic matter, sulfate, or magnesium near the membrane. CaCO₃ was the main precipitate when conditions favored relatively higher carbonate conditions. The tendency to precipitate CaSO ₄ vs. CaCO₃ was influenced by hydrodynamic conditions that affect concentration polarization as well as by the composition of feed water. Organic matter had little effect on the morphology of mineral precipitates in nanofiltration. Similarly, differences in magnesium in the feed waters did not affect the structure of precipitates formed on NF membranes. In contrast, CaCO₃ precipitates on reverse osmosis membranes tended to be elongated in the presence of higher concentrations of organic matter while magnesium functioned as a transition between single crystals under carbonate-limited conditions for the RO membrane.