Studies on interactions between membranes (RO and NF) and pollutants(SiO2,NO3-, Mn++ and humic acid) in water

The comparison between two membrane separation processes, like reverse osmo sis (RO) and nanofiltration (NF), and the interaction between membrane and pollutant in water cleaning-up were studied. The separation efficiency of p ollutants like silica, nitrate, manganese and humic acids (HA) by means of RO and NF membranes (both of spiral wound type, in polyamide) was determine d. The interaction between the membrane and other ions (e.g. Cu++ casually present in water) also affected membrane cleaning. All pollutants were diss olved in tap water; in some tests they were singularly present, in others t hey were all together to simulate a polluted water. Particular attention to membrane washing was given testing various washing conditions for differen t fouling cases. In the separation of silica by RO, fouling problems were n ot observed in a three hours test at 600 mg/l concentration. In RO tests wi th water containing all four pollutant species at initial concentrations of 126 mg/l silica, 263.8 mg/l NO3-, 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 o bserved. These values of rejections were identical to that obtained when th e pollutants were present separately, showing absence of interactions among them. The permeate flow rate was 530 1/hxmodule with a recovery equal to 3 0% at an average transmembrane pressure of 30 bar. In the NF tests both wat er polluted with all four contaminants and torrent water (from torrent Emol i, Rende (CS)) were used. Obviously the mean rejections were lower than RO membrane, and equal to 35%, 6%, 80%, 35% respectively. Mn++ rejection was t he highest owing to the positive charge of the NF membrane. The permeate fl ow rate was equal to 530 1/hxmodule at a mean pressure equal to 11 bar and T=25 degreesC and comparable to that obtained using the RO module at 30 bar . The best washing for membrane cleaning was a NH3 aqueous solution 0.4% w/ v. A simple washing with water was not able to remove Mn++ and Cu++ ions (o wing to acid-base Lewis type interaction with polyamide membrane) and humic acids; the weak basic agent NH3 was able to avoid precipitation of insolub le hydroxides, like Mn(OH)(2), and consequent plugging of membrane pores, w ith respect to the strong base NaOH. The obtained results show the importan ce to know the type of interaction membrane-pollutants and the chemical beh aviour of pollutants to obtain the maximum benefit both in pollutant separa tion and in membrane cleaning.