Performance enhancement of a submerged vacuum membrane distillation (S-VMD) system using low-power ultrasound

Submerged vacuum membrane distillation (S-VMD) is a thermally driven separation process capable of desalinating water with high salt concentration. Agitation techniques such as aeration and circulation have been used to enhance the permeate flux and to mitigate the effect of temperature and concentration polarizations, a major drawback in S-VMD. In this study, an S-VMD system that uses ultrasonic energy as an agitation technique is proposed. The effects of ultrasonic power and frequency under different feed temperatures and concentrations were investigated experimentally. Results show that applying low-power ultrasound can improve the permeate flux up to 24% compared to the same process without ultrasonic energy under the same operating conditions. The ultrasound-assisted enhancement increases with higher ultrasonic power and lower frequency, as well as lower feed temperature and higher concentration. Possible heat and mass transfer enhancement mechanisms associated with integration of ultrasound were investigated and based on the variation of the permeate flux with frequency, the enhancement was mainly attributed to acoustic cavitation. The ultrasonic-assisted S-VMD system maintained a stable permeate flux and excellent water quality over a relatively long-term operation, indicating that ultrasonic energy is a promising and safe method to enhance the permeate flux in S-VMD systems.