The gas-works sewage , for the sake of their complex composition, beside inorganic pollutants contain also organic xenobiotics, mainly phenols and polycyclic aromatic hydrocarbons (PAH's). It is well known, that those substances are very dangerous to live organisms. Therefore, the commercial scale purification process of the sewage was elaborated and carried out on the way of biological treatment. For that purpose the bacteria strains of Pseudomonas and Bacillus type already existing in a surface layer of the sewage were applied as biopreparat and whey as a bacteria medium. The purification was carried out for two similar kinds of sewage: typical gas water (in InowrocB.aw) and sewage generated by dilution of a gas pitch (in ToruD.). That way the serious possibility of soil and water pollution was avoided.