Sodium chloride stimulus-response experiments in spiral wound reverse osmosis membranes: a new method to detect fouling

Fouling is generally described in terms of salt rejection and permeation fl ow, but these data seem to evolve very slowly, and when they reach a dramat ic level fouling is irreversible. We propose a method to determine the stat e of the wear of membranes by analyzing sodium chloride stimulus-response e xperiments. Indeed, it turns out that the shape of the distribution (RTD) o f sodium chloride in the permeate flow of the membrane reveals the solute p ermeation mechanisms for used membranes. For new membranes the distribution of sodium chloride collected in the permeate side as well in the rejection side is unimodal. For fouled membranes we note a singular distribution for m with the presence of several modes. The existence of a salt leakage peak, as well as an earlier detection of salt for all the fouled membranes, give evidence of the membrane structure modification. The intensive use of the membranes might have created an enlargement of the pore sizes; salt and sol vent permeabilities are improved as well. It turns out that each permeate s ide sodium chloride distribution can be fitted by a weighted average of Gau ssian distributions, the new membrane RTD and the shifted new membrane RTD. The coefficients of this average give us the fraction of the membrane whic h is enlarged, hampered by the deposited layer and not modified.