The behaviour of RO membranes is determined under realistic service conditions by a large number of parameters. The composition of the feed, for example, is of major importance. In practice, not only the common salt normally used in tests is dissolved in the feed, but other salts too. The rejection for these different salts or ions varies enormously. The pH and the temperature of the salt solution have, for each salt or ion, an individual influence on the success of separation. 
 
In order to gain a better understanding of all these relationships, experts from LANXESS launched a research project that uses modern Design of Experiments (DoE) methods. Only in this way can the number of necessary trials be restricted to a reasonable level even where issues are complex. The high crosslinkage level leads to a low effective pore size, bringing about the high rejection ability of these membranes. They are also noted for their high stability even in extreme pH and temperature ranges. 
 
The experts were particularly interested in the separation properties of the membranes for different dissolved salts. Use was therefore made of a test mixture containing not only sodium chloride (2,000 mg/l), but also nitrate (200 mg/l), ammonium (35 mg/l), boron (6 mg/l) and silicate (75 mg/l). Such mixtures are found, for example, in industrial and household effluent, with some components also being found in agriculture, groundwater and seawater.