A novel ultrasonic assisted direct contact membrane distillation hybrid process was designed and the effect of ultrasonic irradiation on humic acid (HA) fouling mitigation during membrane distillation process was investigated. Although permeate flux declines were negligible (less than 5%) for the ranges of HA concentration studied, ultrasonic irradiation could enhance the permeate flux more than 30% without destroying HA rejection. The higher the concentration factor was, the larger the ultrasonic enhancement of permeate flux could be obtained. Severe permeate flux decline can be found when CaCl2 was added into the HA solution. The presence of Ca2+ would aggravate HA organic fouling phenomenon and resulted in a thick and dense HA fouling layer on membrane surface. The HA fouling increased heat transfer resistance and reduced the pores available for vapor transfer, both the permeate flux and thermal efficiency of the FIFE membrane declined. Under ultrasonic irradiation, ultrasonic wave refreshed the liquid-membrane interface continuously and alleviated the deposition of HA aggregates. Therefore, although there were still some HA fouling scattered on membrane surface, most of the membrane pores could be kept open and clean, the relative permeate flux can maintain above 94% and was hardly affected by concentration factor increasing. (C) 2015 Elsevier B.V. All rights reserved.