Removal of inorganic charged micropollutants in an ion-exchange membrane bioreactor

The transport of anionic drinking water micropollutants-perchlorate, nitrate and arsenate as well as that of a cationic micropollutant ionic mercury-through selected commercial ion-exchange membranes was studied under Donnan dialysis conditions in order to evaluate the possibility of using these membranes for pollutant removal in an ion-exchange membrane bioreactor (IEMB). The anion-exchange membrane, Neosepta ACS, proved to be a good choice for perchlorate and nitrate but was not suitable for arsenate, for which the Ionac MA-3475 allowed more than a three-order magnitude increase in flux. The cation-exchange membrane, Nafion-112, showed promising results for the transport of mercuric ions, while the Neosepta CMX was almost impermeable to this micropollutant. Tap water from the Lisbon public distribution network was supplemented with 100 ppb of perchlorate and 60 ppm of nitrate and was continuously treated in the IEMB incorporating a Neosepta ACS membrane for a period of about 2 months. Highly selective simultaneous removal of both pollutants was observed, in spite of their very different concentrations in the contaminated water. Secondary pollution of the treated water by microbial cells and residual organics was not found. Since the nitrate concentration in the treated water stream (<1 ppm) was always much lower than the EU recommended one of 25 ppm, the amount of water that can be treated in the system was dependent of the desired degree of removal of perchlorate, for which the US EPA suggests an acceptable concentration range from 4 to 18 ppb. The water treatment rate was about 3 L/(m²h) when the objective was to achieve 4 ppb in the treated effluent (96% removal) but increased to 18 L/(m²h) when the treated water contained the upper suggested limit of 18 ppb of perchlorate (82% removal).