Abstract: Industrial waste water, containing impurities which poison or otherwise inhibit bacterial forms employed in biological treatment of waste water, is purified by a process which comprises subjecting the waste water to a biological treatment process in the presence of activated carbon or fuller's earth. A gas containing molecular oxygen is distributed within the liquid mixture during treatment to provide oxidation means. Despite the presence of the poisonous or inhibiting impurities, the process efficiently reduces the biochemical oxygen demand, chemical oxygen demand and total organic content of the waste water in an unusually rapid manner, and removes material not normally removed by the biological treatment alone.