A quantity of clean aggregate contained within a wall. Typically circular on plan, and from 3-30m high depending upon capacity. Effluent is spread onto the ‘media’ by a (usually) 4 bar perforated slowly rotating ‘spreader’. It spills onto the stones and percolates slowly to an annular drainage channel at the outside bottom. Microbial slimes develop on the surfaces of the media and catch and degrade the organic wastes borne in the sprayed effluent, reducing the COD as the effluent proceeds under gravity to the collecting channels. These units are suited to removing complex pollutants such as vegetable dyes and are commonly found in STPs receiving industrial effluent.