IDA World Congress-Maspalomas, Gran Canaria –Spain October 21-26, 2007 REF: MP07-127 -1- Chlorite and chlorate effect on the reverse osmosis membranes performance. Authors: Enrique Ferrero, Anna Llansana, Vanesa Ayala and Jorge J. Malfeito Presenter: Jorge J. Malfeito R&D Director – Acciona Agua – Spain Abstract Biofouling is one of the main problems in the long term performance of the membrane processes. In practice, several ways are followed to deal with this problem. Usually, the measures are focused on cleaning of the membranes and dose of chemicals. Experiences are that cleaning can be difficult when the membranes are severely fouled and, on the other hand, the use of biocides to control biofouling could be restricted. In the last years ClO 2 has been considered as a potential disinfectant because it is a weaker oxidant than sodium hypochlorite or ozone. A weaker oxidant is less damaging to the membrane, and apparently the diffusion of the ClO 2 gas into the biofilm is more effective for its degradation. Additionally, ClO 2 produces lower disinfection by-products than others disinfectants, even when high organics concentrations are present. But despite there is reported [ 1 ] the chlorine dioxide seems not affect to the membrane polyamide active layer at low concentrations, there are no data about the effect of their sub- products (ClO 2 - and ClO 3 - ), on the membrane performance. Laboratory tests were conducted in the present work to determine the resistance of various polyamide RO membranes on water solutions containing 100 mg l -1 of ClO 2 - or ClO 3 - . The membranes were tested for salt rejection and the active layer has been characterized in order to determine the potential chemical changes in the polyamide active layer.