Industrial wastewater from a Polish cokery was treated with ultrasound (frequency 40 kHz, amplitude 61.5 mu m) for 8 min and then purified by addn. of FeSO4 and H2O2 to remove org. impurities by advanced oxidn. The biol. to chem. O-2 demand ratio increased from 0.001 up to 0.21 (or up to 0.11 without sonification) at the H2O2/Fe2+ ratio 2.5.