IITB have developed a consortium and method of using algal and bacterial cultures for biological treatment of aqueous effluents containing non-aqueous phase liquid (NAPL).; like oil / hydrocarbons that are often present in wastes from petrochemical industries, petroleum industries or motor oil reforming industries. Oil degrading bacteria were immobilized on oil acclimatized algae, the algae being responsible for maintaining a near neutral pH, immobilizing the bacteria and facilitating the settlement of suspended biomass. A method using a rotating biological contactor was also developed it had an immobilized biofilm comprising a consortium of oil degrading bacteria and oil acclimatized algae formed on the discs of the reactor, the algae being responsible for maintaining a near neutral pH, immobilizing the bacteria and facilitating settlement of the suspended biomass. The rotating biological contactor (RBC) used is known as a typical experimental set-up for attached growth biological treatment. The RBC used as stated in the invention consisted of a semi-circular trough with three stages. The treatment of oil waste is a ever growing problem, especially with oil slicks. This method developed at IITB with help in numerous application top help deal with non-aqueous wastes.