Desalination of effluents with highly concentrated salt by nanofiltration: From laboratory to pilot-plant

Nanofiltration (NF) has been widely used for treatment of industrial effluents, but very few work concerns NF process in concentrated saline solution, especially for NF-desalination aiming at permeation of monovalent salts and retention of organic solutes. In this study, NF270 membrane was chosen to treat model solutions and three industrial effluents with highly concentrated salt (crude iron dextran solution, iminodiacetic acid mother liquor, and raw soy sauce), showing that with increase of salt concentration, the retention of all the solutes decreased while concentration polarization was increased.

In the presence of charged organic solutes, inorganic salt retention would decline, even negative retention of monovalent salt was found. Increasing pH would induce membrane swelling in saline solution, which might be caused by the higher local salt concentration around the membrane polymers at higher pH.

As NF-desalination of industrial effluents with highly concentrated salt was scaled up from laboratory to pilot-plant, the dead-end stirred filtration at constant flux could provide some important information for pilot-plant tests, such as membrane selection, optimum operating parameters and mechanism analysis, but it was necessary to re-optimize operating mode and method for crossflow filtration at constant pressure, in order to control the concentration polarization at high salt concentration.