Optimal intermittent ultrasound-assisted ultrafiltration for membrane fouling remediation

Abstract

One of the most critical issues encountered in membrane-based separation processes is the permeate flux decline over a long period of operation. This study considered the potential advantages of applying intermittent ultrasound (US) to the ultrafiltration (UF) processes to control long term membrane fouling. The particles in the feed solution tend to attach to the membrane's internal pores and accumulate on the membrane surface, which impedes the filtration process and limits separation efficiency and significantly increases the power consumption. US-generated microbubbles create local turbulence and shear effect near the membrane surface, which disrupt the concentration polarization and fouling layer, resulting in flux improvement. In this regard, remediation of ceramic membrane fouling by an in-line intermittent US system was investigated. Bubble generation under different frequencies (20 kHz, 28 kHz, and 40 kHz), power intensities (50 W, 60 W, and 100 W) and time intervals were evaluated to determine the optimal US condition for enhanced filtration. A real time in-situ imaging of US-generated bubbles was employed, using a synchrotron propagation-based imaging (PBI) at the Canadian Light Source (CLS). A novel non-invasive method was employed to improve the contrast between the liquid and gas inside the bubble (interface of liquid and gas) facilitating the qualitative and quantitative analysis of US-generated bubbles. In addition, a piezoelectric ultrasonic transducer was integrated into a UF unit that was used to filter a 0.5 wt% latex paint solution. The microbubble dynamics at different US conditions were analyzed in order to correlate it with latex particles’ detachment, cake formation control and flux enhancement from data obtained with filtration experiments. It was found that the flux enhancement at 20 kHz and 28 kHz were significant, while the flux enhancement at 100 kHz was not noticeable. It was also observed that the amount of deposited particles on the membrane decreased when the frequency was increased. Oscillation of large bubbles leads to liquid movement around the bubbles and reduces the probability of the particle-membrane attachment. Our results showed that the bubble velocity increased as the frequency was increased from 20 kHz to 40 kHz. In addition, more bubbles were created when the US irradiation was turned on; however, the bubbles did not disappear when the US was off, which represent the long lifetime of the bubbles. The optimal flux was achieved when the ultrasound was set at a frequency of 28 kHz and power intensity of 50 W, while the interval of intermittent ultrasound was set at 60 s.