NOVELTY - A waste water treatment device comprises an anaerobic reactor for anaerobic treatment of waste water with an anaerobic biomass; aerobic reactor for aerobic treatment of waste water with aerobic biomass; and intermediate compartment between the anaerobic and aerobic reactors, where between the aerobic reactor and the intermediate compartment, a return system is provided for feeding part of aerobic effluent and aerobic biomass from the aerobic reactor back to the intermediate compartment.    USE - For treatment of waste water (claimed).    ADVANTAGE - The waste water treatment device is compact, removes nitrogen from the waste water, and is economically feasible.    DETAILED DESCRIPTION - A waste water treatment device comprises anaerobic reactor for anaerobic treatment of the waste water with an anaerobic biomass; aerobic reactor for aerobic treatment of the waste water with aerobic biomass; and intermediate compartment between the anaerobic reactor and aerobic reactor. The anaerobic reactor comprises a fermentation chamber. The aerobic reactor comprises aerators on the underside for the supply of gas containing oxygen. The anaerobic reactor, intermediate compartment and the aerobic reactor in a column above each other are provided with the intermediate compartment between the anaerobic reactor and the aerobic reactor to receive anaerobic effluent originating from the anaerobic reactor and also to provide intermediate effluent originating from the intermediate compartment as aerobic influent for the aerobic reactor. Between the aerobic reactor and the intermediate compartment, a return system is provided for feeding part of the aerobic effluent and aerobic biomass from the aerobic reactor back to the intermediate compartment.    An INDEPENDENT CLAIM is also included for a method for treatment of waste water, comprising subjecting the waste water, in a vertical arrangement, to an anaerobic treatment and an aerobic treatment, one after the other and from bottom to top; and removing nitrate by partially feeding aerobic effluent and aerobic biomass originating from the aerobic treatment, such as aerobic effluent containing aerobic biomass, back to the anaerobic effluent from the anaerobic treatment, subjecting aerobic effluent, aerobic biomass and anaerobic effluent to an anoxic treatment, and feeding the anoxic effluent from the anoxic treatment to the aerobic treatment.