A wastewater purification device of the filtering mass type comprising two series of filters (12, 22, 13, 23; 102, 106, 103, 107) in parallel, intended to operate alternately, each series of filters comprising: - a first stage with fixed culture, freely drained with vertical flow consisting of at least one planted filter able to receive raw effluent and retain the sludge on the surface of said filter, the first stage of each series of filters which can be fed alternately by batch, so that on arrival of a batch of effluent, the water is distributed on the surface of the fed filter, then percolates vertically in a uniform manner towards the bottom, while sludge accumulates on the surface of said filter, - a second stage with fixed culture, saturated with water with vertical and horizontal flow, the first stage being disposed directly above said second stage, so that the water percolating through the first stage arrives in the second stage, said second stage comprising loading means to ensure that the water level is maintained, - a forced aeration system able to operate discontinuously, comprising a distribution network extending under the second stage, distributed over the entire surface of the bottom of the device, and able to supply oxygen to the first and second stages, and - a drain for recovering treated water, disposed at the bottom and at one end of the filter of the second stage perpendicular to a horizontal flow direction in said filter, said device comprising two collection systems of the treated effluent each disposed at the level of the second stage of a different series of filters, on either side of said device, and being intended to operate alternately, the filters of the two second stages being adjacent and the device being able to be operated so that the water flows vertically within one of the filters and then horizontally within the adjacent filter. - a first stage with fixed culture, freely drained with vertical flow consisting of at least one planted filter able to receive raw effluent and retain the sludge on the surface of said filter, the first stage of each series of filters which can be fed alternately by batch, so that on arrival of a batch of effluent, the water is distributed on the surface of the fed filter, then percolates vertically in a uniform manner towards the bottom, while sludge accumulates on the surface of said filter, - a second stage with fixed culture, saturated with water with vertical and horizontal flow, the first stage being disposed directly above said second stage, so that the water percolating through the first stage arrives in the second stage, said second stage comprising loading means to ensure that the water level is maintained, - a forced aeration system able to operate discontinuously, comprising a distribution network extending under the second stage, distributed over the entire surface of the bottom of the device, and able to supply oxygen to the first and second stages, and - a drain for recovering treated water, disposed at the bottom and at one end of the filter of the second stage perpendicular to a horizontal flow direction in said filter, said device comprising two collection systems of the treated effluent each disposed at the level of the second stage of a different series of filters, on either side of said device, and being intended to operate alternately, the filters of the two second stages being adjacent and the device being able to be operated so that the water flows vertically within one of the filters and then horizontally within the adjacent filter. <EOS>