Abstract: The invention relates to a bioreactor unit for the aerobic biological treatment of hydrocarbon contaminated waste water and off-gases. The bioreactor comprises a biotreatment chamber comprising a vertically extending housing unit forming the walls, floor and ceiling of the biotreatment chamber. The housing unit also comprises a door for accessing the biotreatment chamber. The biotreatment chamber also comprises waste water inlet means for discharging waste water under pressure from the upper portion of the biotreatment chamber, onto a plurality of superimposed water treatment trays disposed substantially horizontally inside the chamber, the water treatment trays being adapted for retrieval from the biotreatment chamber through the door of the housing unit. Each water treatment tray has at least one bottom porous section adapted to receive contact media.