Desalination of brackish water and reverse osmotic retentate using nanofiltration membranes: effects of TMP and feed concentration on the treatment

Nanofiltration (NF) membrane with low-pressure operating conditions and high flux permeability seems to be an attractive alternative for water softening and desalination. In order to study the performance of NF membrane on treating real brackish water, with mixture of mono and divalent ions, two commercial flat sheet nanofiltration membranes (NF90 and NP030) were used. The experiments were carried out with transmembrane pressure from 4 to 12 bar with three brackish waters having different ions concentrations. The results obtained showed that the high hydrophilicity of NF90 and its small pore size were the main advantages that allow this membrane to have the highest permeability and salt rejection for both mono and divalent ions in comparison with NP030. The results also showed that the permeate flux and rejection increased linearly with increasing in TMP. NF90 membrane performance was assessed by studying the effect of feed ions concentration. It was observed that the permeate flux decreased with increasing in salts concentration due to concentration polarization. Additionally, the study of scaling problem showed that its contribution in permeate flux decreasing was not as much significant and that the membrane permeability, recovered after the cleaning step, was 90%. In the other hand, the salts rejection of NF90 remained high (more than 80%) for all the studied concentrations due to its separation mechanism.