Modelling of stimulus response experiments in the feed channel of spiralwound reverse osmosis membranes

Reverse osmosis is commonly used to produce ultrapure water which is necessary in hemodialysis to prepare the dialyzing liquid. We have managed to test some spiral-wound FT30-4040 membranes by analyzing their response to a stimulus injection of tracer. The shape of the distribution of the tracer in the rejection side is compared for different membranes, a new and two used ones, and different tracers in order to study the evolution of the interactions between the solutes and the membrane with wear. We used three tracing solutions: sodium chloride solution (1.5 M), hydrochloric acid (1.2 M) and sodium hydroxide (1.5 M) to reach acid or basic pH. The distributions of tracer detected in the rejection side are unimodal with a tail which is greater in the case of the hydrochloric tracing. These results enable us to establish an increasing affinity for NaOH, NaCl and HCl with the membrane. We represent the feed channel between the sheet of membranes by a series of perfect mixing cells with exchange. The modelling of the space between the membrane sheets by a series of perfect mixing cells with exchange, the dead zone representing the membrane, gives good results. It gives prominence to the existence of excursions of the solute in the membrane.