The Effect of Feedwater Pretreatment Chemicals on the Performance and Durability of Reverse Osmosis Membranes

摘要

The sensitivity of commercial reverse osmosis membranes has been studied in a variety of chemical environments. Membranes were supplied by five manufacturers representing three general types. These are cellulose acetate, thin film composite polyamide and homogeneous polyamide. Pretreatment chemicals consisted of halogens, ozone, and chlorine dioxide. Membrane exposure was conducted by soak tests in buffer solutions over a pH range of 3.0 to 8.6 and concentrations between 1.0 and 125 ppm. Assessment of membrane damage was carried out by performance testing in a flat plate laboratory reverse osmosis unit using 5000 ppm sodium chloride feed solution. Chemical attack on membrane polymer was indicated by an increase in water flux and a decrease in salt rejection. Cellulose acetate membranes were the most resistant to chemical attack. They responded only to ozone and were also hydrolized at low pH. Polyamide membranes were sensitive to ozone, halogens, and chlorine dioxide. They were also hydrolized but at low pH. The activity of all chemical agents tested was responsive to solution pH. This factor controls the distribution of halogen molecular and ionic species and consequently the rate and possibly the mechanism of membrane attack. Under conditions which maximize damage, the order of activity of chemicals under study were O3 > ClO2 > Cl2 > Br2 > I2.