The biodestruction of the water-oil run-off hydrocarbons at a metallurgical plant by an immobilized association of microorganisms in a flowing cultivation installation was investigated. The 90-99% destruction of the 0,1-10 g/l concentration of mineral oil is achievable. Only 3 of the 9 original groups of the hydrocarbons paraffins, monocyclic naphthenes, alkylbenzenes were identified in the installation output. Among them was tendency to molecular mass reduction due to the naphthene and benzene rings opening and aliphatic fragment removal.