NOVELTY - (Pre)treating liquid manure or biogas plant reject involves use of an apparatus which includes at least two functionally sequential process tanks (5) comprising microbes for conducting a biological treatment on a matter to be treated; and stripping tower (22A) placed down stream of the process tanks for removing nitrogen from the biological treated matter.    USE - For treating and/or pre-treating liquid manure or biogas plant reject for elimination of harmful substances, particularly nitrogen, phosphorus and odor molecules (claimed).    ADVANTAGE - The ammonium content which has been lowered, enables reducing the toxicity and product inhibition of ammonia and enhances the biological process. The ability to enhance the process results in a shorter process residence time, where the size of reactors is reduced. During biological aerobic process, the liquid manure becomes odorless and more fluid. Thus, the liquid manure is easy to supply in and expel from the tower, nor does the air blasted through the tower cause odor problems in the vicinity. The process is economical.    DETAILED DESCRIPTION - Treating and/or pre-treating liquid manure or biogas plant reject for elimination of harmful substances, particularly nitrogen, phosphorus, and odor molecules, involves providing an apparatus which includes at least two functionally sequential process tanks (5) for conducting a biological treatment on a matter that is treated; an inlet (4) for supplying the tank with a substance that is treated; an outlet for passing the post-treatment matter out of the tank; and air inlet (2) for supplying the tank with air required by a biological process; providing each process tank first with microbe population for processing the matter; supplying the matter gradually into the first tank and then further into subsequent process tanks such that the microbe population first provided in the tanks displaces step by step the original microbe population present in the matter which is free of original microbes and present in a final process tank is returned into the first process tank for diluting the matter and fed into the first process tank; and conducting, downstream of at least one process tank, a nitrogen removal treatment by supplying stripping tower (22A) with the matter from the tank in which pH of the matter is raised by microbiological treatment to a level sufficient from the standpoint of nitrogen removal. An INDEPENDENT CLAIM is included for an apparatus for treating and/or pre-treating liquid manure or biogas plant reject for the elimination of harmful substances, particularly nitrogen, phosphorus, and odor molecules, including the at least two functionally sequential process tanks for conducting biological treatment on the matter, where the tanks are interconnected in such a way that the matter is able to flow from one tank to the next; an unit (8) for returning a matter which is substantially free of original microbes and present in the final process tank, into the first process tank for diluting the matter and fed into the first process tank; and stripping tower which is functionally positioned downstream of at least one process tank and adapted to receive the matter directly from the tank.    DESCRIPTION OF DRAWING(S) - The figure shows diagrammatic view of an apparatus for carrying out the method.    Feed pump (1, 9, 14, 20)    Air inlet (2)    Collecting tank (3)    Inlet (4)    Process tank (5)    Returning unit (8)    Chemical pump (11)    Magnesium oxide tank (12)    Fan (21)    Stripping towers (22, 22A)    Reference numeral (25)    Heater (26, 27)    Pipe (28)