The invention relates to a small-scale treatment unit for waste water, comprising one or more series of solid-bottom trays (1, 9) arranged relative to each other descendingly in the flowing direction of the water to be treated and containing a replaceable or flushable filling material, and that the trays are set at an inclination of 0-10% and that the trays (1, 9) have at least one end wall or skirt thereof provided with openings (4, 13) for passing water in each series of trays always from a higher tray (1, 9) to the next tray therebelow, the waste water to be treated being forced to flow through each tray and a layer of filling material contained therein in a substantially horizontal flow to at least one end wall or skirt of the tray (1, 9), the openings (4, 13) present therein providing a passage for the water to flow into the next tray in the series, such that the waste water to be treated is forced to flow through all trays in each series of trays in the treatment unit and to discharge by way of the openings (4, 13) provided at the end wall or skirt of the lowermost tray in a series of trays.