The elimination of humic acids was studied by the following unit processes or their combination : ozonation (0), clariflocculation (C) and immersed biological filtration (F). (0) assures good elimination of color, but has little influence on tannins and TOD, (C) with FeCl3as the flocculant eliminates color well and the tannins, but the residual concentration of ferric ions is high. (F) requires an adaptation of attached cells, which was not the case during tests (O)-(F) in combination. Clariflocculation efficiency was not enhanced by preozonation.