The flow of water and sludge coming from the previous phases of treatment (whether biological or physical-chemical) is introduced into the intermediate zone of the tank, just below the lamellar pack. A uniform distribution of the effluent over the lamellar pack is effected by means of a diffuser. The rise velocity inside the channels of the pack is constant; consequently, the flocculant sludge, which has a density greater than that of water, tends to settle under gravity on the bottom wall of each channel. From here, thanks to the slope of the walls and to the near-zero velocity of the water in the proximity of the wetted perimeter, the sludge slides into the conical collecting receiver, from where is extracted at intervals whose frequency varies with the quantity. At the same time the clarified water reaches the free surface at the top of the clarifier and overflows into a collecting trough, from which it is channelled to subsequent treatment or, eventually, to final discharge.