The ultrasonic process is investigated to remove refractory pollutants. The particular case of sodium pentachlorophenate is studied as a model molecule and several raw paper mill wastewaters and a landfill leachate are chosen as real industrial wastewaters. As the sonication requires a lot of energy, it is investigated as a pre- or a post-treatment in combination with biodegradation. For that, global indicators of the environmental pollution (COD, TOC, toxicity and biodegradability) are examined. The results show that ultrasound can decrease toxicity and enhance biodegradability. The amount of energy supplied and the position of the chemical treatment (pre- or post-treatment) are discussed.