Ultrasonic detection of cavitation in a membrane under osmotic pressure

Cavitation in a membrane has been proposed as the principal cause of the low flux in osmotically driven membrane processes (ODMPs). In this study, an ultrasonic signal combined with a fast Fourier transform (FFT) analysis was adopted to detect cavitation in the membrane of ODMPs. The ultrasonic amplitude decreased rapidly under an osmotic pressure of 88.59 bar at the initial phase, indicating that the cavitation mainly occurred at the beginning of the filtration. In addition, cavitation was significantly affected by osmotic pressure, which indicated a higher cavitation probability with larger osmotic pressure values. Cavitation, which significantly reduces the membrane flux in ODMPs, can be delayed by applying hydraulic pressure on a feed solution. The flux rose to 16.50 L.m- 2 h-1 from 10.61 L.m- 2 h-1 with a hydraulic pressure increase to 2 bar from 0 bar under an osmotic pressure of 88.59 bar. However, the membrane flux could not be improved indefinitely owing to concentration polarization.