Ultrasonic assisted direct contact membrane distillation hybrid process for membrane scaling mitigation

• Ultrasonic assisted membrane distillation hybrid process was designed.
• The effect of ultrasonic irradiation on membrane scaling was investigated.
• Ultrasonic irradiation can effectively mitigate CaSO4 membrane scaling.
• CaCO3 had little effect on permeability with or without ultrasonic irradiation.
• Ultrasonic irradiation can restrain silica colloid deposit and maintain flux.

A novel ultrasonic assisted direct contact membrane distillation hybrid process was designed and the effect of ultrasonic irradiation on membrane scaling mitigation during membrane distillation process was investigated. Under ultrasonic irradiation, ultrasonic wave refreshed the liquid-membrane interface continuously and reduced concentration polarization in the boundary layer adjacent to membrane surface simultaneously. Therefore, ultrasonic irradiation could mitigate membrane scaling caused by CaSO4 and prevented the permeate flux decline. Although the ultrasonic irradiation could restrain the crystal size of CaCO3 deposits and clean membrane surface, the experimental results demonstrated that it was not necessary to introduce ultrasonic irradiation into membrane distillation process for CaCO3 membrane scaling mitigation. Due to fast precipitation rate, the concentration of CaCO3 solution would not cause notable permeate flux decline in the absence of ultrasonic irradiation. There would be a gradual permeate flux decline during the concentration process of silica solution because of the formation and deposition of colloidal polysilicic acid on membrane surface. The ultrasonic irradiation caused the zeta potential of silica colloids to approach neutral and enhanced the tendency of colloid deposit, but the membrane surface could be effectively kept clean and the permeate flux was hardly affected by concentration factor increasing.

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