The invention relates to a water treatment device comprising a mixing tank comprising an inlet path (12) for an effluent to be treated, an optional inlet path (13) for reagents, a stirring source (14) for generating a turbulent stir in a given volume of said tank, an extraction path (15) for discharging sludge, and an extraction path (16) for treated effluent, and further comprising above and adjacent to the given volume, but below the treated-effluent outlet path, a settling structure (17) comprising a plurality of ducts extending from the bottom to the top and arranged in the form of a baffle so that no particle can flow through said layer along a rectilinear path.