Effect of solution chemistry on water softening using charged nanofiltration membranes

The negatively charged polyamide thin-film-composite nanofiltration membranes have been used to possibly treat the sodium-enriched water, particularly cooling tower water, to separate hardness producing solute like MgSO4 from univalent salts using ESNA and EDA nanofiltration membranes. The flux declines with increasing ionic strength of the feed solution. Rejection of objectionable hardness producing magnesium ions decreases with increasing ionic strength, a case hopefully not reported earlier, with hardly any change in the rejection of sodium chloride used to maintain the ionic strength of the solutions. The effect of pH has also been studied, which shows that flux decreases whereas rejection increases when the solution pH is either reasonably acidic or alkaline in nature. The rate of change in flux and rejection drops with increasing pH when the ionic strength was kept constant. The method is applied to the treatment of some tap waters, altering the ionic strength and pH, to show the validity of the findings. A good agreement in results confirms the phenomenon of variable rejection of hardness producing ions under different solution chemistry.

The forgoing discussion concludes that negatively charged polyamide thin-film-composite nanofiltration membranes can be used to treat the sodium-enriched cooling tower water for separating multivalent ions from monovalent solutes in aqueous solution. The flux will be declined in highly concentrated solution due to reduced effective force resulted owing to the change in the osmotic pressure of feed solution having different concentrations of solutes. The rejection efficiency of the process would be higher with the lowest possible ionic strength of the feed solution. The solution pH would have a remarkable role on the flux and the rejection. The alkaline feed solution, which is normally the case, would be suitable for high rejection of hardness producing solutes with hardly any change in the rejection of sodium chloride. The performance would be poor for acidic feed solutions because of the reduced charge density and thus lower repulsive force offered by the membrane surface. The membrane separation technique can substitute the conventional ion exchange resin method for water softening, particularly treating sodiumenriched cooling tower water.