NOVELTY - In processing contaminated waste water using a clarification plant and a mechanical purification station (MPS) (4), waste water and/or clarified sludge (12, 13) withdrawn downstream of the MPS is/are supplied to fermenter(s) (16) of a biogas plant (17) downstream of the MPS, together with a solid biomass charge (29), and fermented anaerobically such that the harmful component contents in the fermented mixture (27) are below specific thresholds.    USE - The method is specifically used for processing contaminated water using a clarification plant (especially a communal clarification plant) while simultaneously utilizing the clarified sludge formed in a fermenter of a biogas plant together with a solid biomass charge (especially plant and/or animal waste and/or renewable raw materials, particularly one or more of cow, pig or chicken manure, brewing residues, molasses, apple, fruit or grape pressing residues, vines or whole plant, grass or maize silage) (all claimed). The biogas produced in the fermenter is useful as an energy source, e.g. for powering a current generating combustion engine. The fermentation residue, having a reduced content of harmful components, can be used as an agricultural fertilizer.    ADVANTAGE - A simple, environmentally friendly method is provided for simultaneously purifying waste water and utilizing the clarified sludge.    DETAILED DESCRIPTION - In a method for processing contaminated waste water using a clarification plant and a mechanical purification station (MPS) (4) (specifically a screening station of the clarification plant), a predetermined amount of waste water and/or clarified sludge (12, 13) withdrawn downstream of the MPS is supplied to fermenter(s) (16) of a biogas plant (17) located downstream of the MPS, together with a predetermined amount of a solid biomass charge (29), and is fermented anaerobically. The amounts and compositions of the components supplied to the fermenter(s) are such that the contents of harmful components in the obtained fermented mixture (27) are below predetermined thresholds.    An INDEPENDENT CLAIM is included for corresponding water treatment apparatus.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic flow diagram of a communal clarification plant with a connected biogas plant.    Clarification plant (1)    Waste water inlet (2)    Rain discharge (3)    Screening station (4)    Sand trap (5)    Preclarification tank (6)    Intermediate storage vessel (7)    Living sludge tank (8)    Post-clarification tank (9)    Purified water (10)    Recycled sludge (11)    Partial stream of recycled sludge (12)    Clarified sludge sedimented in preclarification tank (13)    Fermenter (16)    Biogas plant (17)    Biomass mixing vessel (20)    Fermented mixture of biomass and clarified sludge (27)    Partial stream of purified water (28a)    Partial streams of prepurified waste water (28b, 28c, 28d, 28e)    Accurately weighed biomass charge (29)