Studies on interactions between membranes (RO and NF) and pollutants (SiO 2, NO 3−, Mn ++ and humic acid) in water

Abstract

The comparison between two membrane separation processes, like reverse osmosis (RO) and nanofiltration (NF), and the interaction between membrane and pollutant in water cleaning-up were studied. The separation efficiency of pollutants like silica, nitrate, manganese and humic acids (HA) by means of RO and NF membranes (both of spiral wound type, in polyamide) was determined. The interaction between the membrane and other ions (e.g. Cu ++ casually present in water) also affected membrane cleaning. All pollutants were dissolved in tap water; in some tests they were singularly present, in others they were all together to simulate a polluted water. Particular attention to membrane washing was given testing various washing conditions for different fouling cases. In the separation of silica by RO, fouling problems were not observed in a three hours test at 600 mg/l concentration. In RO tests with water containing all four pollutant species at initial concentrations of 126 mg/l silica, 263.8 mg/l NO 3 −, 123.9 mg/l Mn ++, 129.4 mg/l humic acids at pH=8, rejections equal to 98%, 94%, 99% and 95.5%, respectively, were observed. These values of rejections were identical to that obtained when the pollutants were present separately, showing absence of interactions among them. The permeate flow rate was 530 l/h×module with a recovery equal to 30% at an average transmembrane pressure of 30 bar. In the NF tests both water polluted with all four contaminants and torrent water (from torrent Emoli, Rende (CS)) were used. Obviously the mean rejections were lower than RO membrane, and equal to 35%, 6%, 80%, 35% respectively. Mn ++ rejection was the highest owing to the positive charge of the NF membrane. The permeate flow rate was equal to 530 l/h×module at a mean pressure equal to 11 bar and T=25°C and comparable to that obtained using the RO module at 30 bar. The best washing for membrane cleaning was a NH 3 aqueous solution 0.4% w/v. A simple washing with water was not able to remove Mn ++ and Cu ++ ions (owing to acid-base Lewis type interaction with polyamide membrane) and humic acids; the weak basic agent NH 3 was able to avoid precipitation of insoluble hydroxides, like Mn(OH) 2, and consequent plugging of membrane pores, with respect to the strong base NaOH. The obtained results show the importance to know the type of interaction membrane-pollutants and the chemical behaviour of pollutants to obtain the maximum benefit both in pollutant separation and in membrane cleaning.