Designing the ideal polyamide membrane for water purification

Abstract

The polyamide membrane is a crucial element in reverse osmosis water purification. Although it has more advantages than cellulose acetate membranes, it is still susceptible to biofilm formation and chlorine degradation. In order for a polyamide membrane to be considered ideal it must have good performance, long lifespan and low cost. This report has found that it is impossible to optimize all three aspects at the same time to achieve a truly ideal membrane. However there are ways which a membrane can be modified to approach an ideal case for a given situation. Performance can be adjusted by varying the preparation conditions during interfacial polymerization. Preparation conditions include choice of organic solvent, reaction time, pH of the aqueous phase etc. They mainly control the thickness, density and extent of cross-links of membranes which then affect the water flux and salt rejection. However it was found that using this method to increase water flux will usually lead to a decrease in salt rejection and vice versa. Chlorine resistance can be increased by methylating the amide nitrogen and by varying the type of amine used to synthesize the polyamide membrane. However methylating the amide nitrogen will result in a lower initial performance. Furthermore the group contribution method can be used to do a prediction on the performance polyamide membranes that lack experimental data to correlate their properties. This is useful for preliminary selection of the type of polyamide membrane.