Partial Desalination With Charged Nanofiltration Membranes

As the world’s population is increasing, the demand for clean water is increasing as well.  Water desalination is becoming more popular among countries that lack freshwater resources.  There are a number of techniques being researched or applied for desalination, including reverse osmosis membranes, forward osmosis membranes, and membrane distillation. These methods can remove almost all salt, but require an extensive amount of energy to perform the separation. However, these methods currently do not show any selectivity towards certain dissolved ions. There are some applications in industry where it is acceptable or even favorable to allow some ions to permeate through the membrane. The proposed method is to use charged nanofiltration (NF) membranes to selectively allow certain ions to permeate through the membrane while rejecting others from a water solution. In contrast to RO, these NF membranes have considerably higher water flux. Two types of NF membranes and modules (positively and negatively charged) were tested for partial desalination applications. Both membrane modules were tested for rejection of various salt solutions at transmembrane pressures varying from 3.4 to 12.1 bar. The positively charged membrane (water permeability of 5.5 LMH/bar) showed rejection of 92% Ca²⁺ and 51% SO₄^2- from single salt feed solutions at 6.9 bar. A higher flux (water permeability of 12 LMH/bar) negatively charged membrane showed rejection of 60% Ca²⁺ and 38% SO₄^2- from single salt feed solutions at 6.9 bar. Rejection of the divalent ions changes in the presence of sodium chloride. Generally, for the mixed salt feed, the Ca²⁺ rejection decreases while the SO₄^2- rejection increases for both membranes as compared to the single salt feed solutions. Further investigation needs to be done to look at the possibility of tuning the selective rejection of the membranes by manipulating pressure and cross-flow velocity. The authors acknowledge the joint development work with Sepro Membrane Corporation.