A composite membrane includes a hydrophobic porous membrane and a high water content hydrogel layer disposed on the surface of the hydrophobic porous membrane facing the hot end. The surface active agents contained in the wastewater at the hot end are blocked by the high water content hydrogel layer, thus the problem of pore wetting of the hydrophobic porous membrane can be prevented. Therefore, the membrane distillation technique can be utilized for processing wastewater containing surface active agents.