Transportable waste water treatment unit for purifying waste water, comprising - a box-shaped, watertight transport container which is open at its upper side, - a supply device for supplying waste water into the transport container, - a discharge device for discharging treated waste water, - a waste water treatment region provided in the transport container, which is provided with a granular substrate, and - reed plants suitable for decomposing impurities in the waste water and planted on the substrate, wherein the substrate is built up in layers and at least one layer contains a porous, light-weight substrate, characterised in that the waste water treatment region comprises at least two partially separated waste water treatment sections (27a, 27b), in that the supply device for the waste water comprises at least one inlet which is arranged above the substrate, in that the supply device comprises a first inlet for the first waste water treatment section and a second inlet for the second waste water treatment section, in that the discharge device comprises at least one outlet element running horizontally in the substrate and connected to a discharge line which leads out of the transport container in order to receive and discharge the purified waste water, in that a truncated pyramid-shaped impact element is arranged on the substrate below the inlet, wherein the waste water passes through a region of ambient air before impacting on the truncated pyramid-shaped impact element, in that the first inlet and the second inlet each open out above the truncated pyramid-shaped impact element, and in that the outlet element is arranged in the lower substrate layer. - a box-shaped, watertight transport container which is open at its upper side, - a supply device for supplying waste water into the transport container, - a discharge device for discharging treated waste water, - a waste water treatment region provided in the transport container, which is provided with a granular substrate, and - reed plants suitable for decomposing impurities in the waste water and planted on the substrate, wherein the substrate is built up in layers and at least one layer contains a porous, light-weight substrate, characterised in that the waste water treatment region comprises at least two partially separated waste water treatment sections (27a, 27b), in that the supply device for the waste water comprises at least one inlet which is arranged above the substrate, in that the supply device comprises a first inlet for the first waste water treatment section and a second inlet for the second waste water treatment section, in that the discharge device comprises at least one outlet element running horizontally in the substrate and connected to a discharge line which leads out of the transport container in order to receive and discharge the purified waste water, in that a truncated pyramid-shaped impact element is arranged on the substrate below the inlet, wherein the waste water passes through a region of ambient air before impacting on the truncated pyramid-shaped impact element, in that the first inlet and the second inlet each open out above the truncated pyramid-shaped impact element, and in that the outlet element is arranged in the lower substrate layer.