Treatment of dyeing solution by NF membrane for decolorization and salt reduction

A synthesized dyeing solution consisted of reactive dye, salts, and polyvinyl alcohol was prepared and treated by two different types of commercial nanofiltration (NF) membranes, i.e. NF90 and NF270 under various process conditions. These NF membranes were evaluated for permeate flux, salt, and color rejection as well as fouling propensity for a certain period of operation. Results revealed that NF90 suffered significant flux decline compared to NF270 after four operation cycles of treatment process, indicating its high sensitivity to foulant attachment. With respect to color removal, it is found that NF270 demonstrated greater stability in maintaining its separation efficiency (between 94 and 98% rejection) regardless of the feed properties and number of operation cycles. Though the salt rejection of NF270 was not as high as NF90, its consistent separation performance throughout the study period proved its reliability in long run. Recommended chemical cleaning process was also conducted in an effort to recover the membrane water flux due to the fouling. The results revealed that 90% of the initial water flux of NF270 was retrieved after a chemical cleaning. The water flux of NF90, however, was not able to be recovered. A water flux coefficient of 1.74 l/m(2) h bar was reported after a chemical cleaning process in comparison to its virgin state of 5.43 l/m(2) h bar. The irreversible fouling on NF90 was strongly linked to the pore blockage resulted from the presence of dye molecules in the feed solution. Based on these findings, it is showed that NF270 is more reliable and sustainable to be used in treating industrial dyeing wastewater. The high water productivity of NF270 coupled with high resistance against dye absorption may outweigh its relatively low rejection rate of salt in industrial applications.