Rejuvenation of discarded RO membrane for new applications

Abstract

Abstract The sensitivity of thin film composite RO membrane (TFCs) with chlorine has been widely reported. In the present paper, attempt has been made to modify the chemical structure of thin film composite RO membrane by exposing the membrane to free chlorine in order to evaluate potential of utilizing the discarded membrane after its useful life. The different variants of thin film composite RO membrane in terms of flux and rejection could be prepared by altering the exposure time and concentration of sodium hypochlorite solution. Tailoring the membrane for solute rejection offers advantages over the conventional process for instance: (1) It avoids re-mineralization and directly produces the water of required quality; (2) It enhances flux of membrane and thereby improves the productivity of the membrane; (3) Improved recovery leads to lower capacity pre-treatment and thereby improves the operational efficiency; (4) Number of membrane required for the given capacity will be less and the associated unit cost, e.g. pumping cost will also decrease. Successful attempt has been made to enhance permeability seven times that of the terminally fouled membrane by sequential hypochlorite treatment in controlled fashion, i.e. fouled seawater RO membrane with flow 3.36 l/min and solute rejection of 97.21% has been converted to the membrane with flow 24 l/min and solute rejection 82.37%.Moreover, the thin film composite RO membrane has been degraded totally till its solute rejection efficiency falls to approximately zero in order to convert it to microfiltration/ultra-filtration membrane, i.e. the membrane with pure water permeability 257 gfd @50 psig pressure has been made by exposing 3286 mg/l free chlorine for 96 h to TFC RO membrane. The characterization of this membrane was performed by measuring molecular weight cut off of the membrane and pure water permeability of the membrane to evaluate its performance as an ultra-filtration membrane. These membranes were also tested for micro-organism removal and found that their performance resembles the new ultra-filtration membrane. The hypochlorite treated membranes were also evaluated for their permeability after bio-film formation with different micro-organisms where it manifested higher permeability as compared to untreated membrane.