NOVELTY - A clarification plant (1) for processing contaminated waste water, having a sand trap (5) in which heavy, non-floating materials settle as sand trap sediment, includes sand trap sediment processing vessel(s) (15), supplied with predetermined amounts of the trap sediment (23) and (preferably aerobic) sediment-specific bacteria (12b, 24) with a specific residence time suitable for biological processing of the sediment.    USE - The plant is specifically used for processing contaminated water using a 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 can be used as an agricultural fertilizer.    ADVANTAGE - A simple, inexpensive, environmentally friendly method is provided for simultaneously purifying waste water and utilizing the intermediate products formed in the clarification process.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a corresponding water treatment process.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of a clarification plant.    Clarification plant (1)    Waste water inlet (2)    Rain discharge (3)    Screening station (4)    Sand trap (5)    Preclarification tank (6)    Intermediate storage vessel (7)    Biological purification stage (8)    Post-clarification tank (9)    Purified water (10)    Recycled sludge (11)    Recycled partially purified water (12a)    Recycled sludge from post-clarification tank (12b)    Clarified sludge sedimented in preclarification tank (13)    Living sludge tank (14)    Sand trap sediment processing tank (15)    Fermenter (16)    Biogas plant (17)    Heat exchangers (18, 19)    Biomass mixing vessel (20)    Inflow line (21)    Backflow line (22)    Sand trap sediment (23)    Living sludge (24)    Processed sand trap sediment line (26)    Fermented mixture of biomass and clarified sludge (27)    Partition (28)    Partial stream of purified water (28a)    Partial streams of prepurified waste water (28b, 28c, 28d, 28e)    Accurately weighed biomass charge (29)    Blower (30)    Fresh air stream (31)    Stirrer (32)    Sediment washing and separating station (33)    Gas-fuelled engine (34)    Gas supply line (35)