Ultrasonic irradiation control of silica fouling during membrane distillation process

• Ultrasonic irradiation was introduced into membrane distillation process.
• Influence of ultrasonic irradiation on silica fouling control was investigated.
• Calcium ion would destroy silica colloid stability and aggravate membrane fouling.
• Ultrasonic irradiation can mitigate silica fouling and maintain permeate flux.

Ultrasonic irradiation was introduced into membrane distillation process and the influence of ultrasonic irradiation on silica fouling control was investigated. A gradual decline of permeate flux during the concentration process of silica solution was observed due to the formation and deposition of colloidal polysilicic acid on the membrane surface. The PTFE hollow fiber membrane could maintain its mechanical properties and initial pore size distribution in the presence of ultrasonic irradiation. Ultrasonic wave brought significant mechanical and thermal effects, and generated powerful shock wave and microstreaming with high speed. The microstreaming, shock wave and acoustic vortex streaming stimulated the liquid–membrane interface continuously, therefore the membrane surface could be effectively kept clean and the permeate flux was hardly affected by the increasing of concentration factor. Due to the charge neutralization and compression of double charged layer induced by calcium ions, the silica colloid stability was eliminated and the colloids aggregated. A large amount of silica scaling was deposited on the membrane surface even at the beginning of concentration process. With ultrasonic irradiation, the permeate flux maintained stable and was enhanced by about 43%; the ultrasonic irradiation could effectively control silica fouling during membrane distillation.