Alleviation of flux decline in cross flow nanofiltration of two-component dye and salt mixture by low frequency ultrasonic irradiation

• Flux decline can be effectively mitigated using low-frequency ultrasonic irradiation.
• Continuous mode of irradiation resulted in 3% to 4% increase in permeate flux.
• Concentration polarization resistance could be effectively decreased by ultrasonic irradiation.
• Substantial removal of color was achieved with a marked reduction in COD and TDS.
• Power consumption increased sharply with TMP and ultrasonic bath temperature.

Gradual reduction of steady state permeate flux (throughput) over a period of time often mars the performance of nanofiltration process. In the present study, nanofiltration of two-component dye and NaCl mixture containing Reactive Black 5 and C.I Reactive yellow 160 was carried out in a flat sheet membrane module with hydrophilized polyamide membrane (molecular weight cut off 400) over a range of operating conditions. Results indicate that the flux decline with time could be significantly mitigated under low frequency (34 ± 3 kHz) ultrasonic irradiation when compared with the conventional unassisted nanofiltration. Various filtration resistances were determined using a simple resistance-in-series model, which revealed that concentration polarization resistance could be effectively decreased by ultrasonic irradiation. Effect of several parameters such as mode of irradiation (viz. continuous and intermittent), trans-membrane pressure and temperature were studied and discussed with respect to existing theories. The power consumption of the high pressure plunger pump at different feed pressures and the ultrasonic power transferred to the water bath at three different temperatures were estimated. More than 96% of color, and 65% TDS were removed from the treated water accompanied by a marked reduction of COD. The process allowed the production of permeate stream with great reutilization possibilities.