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This study was performed to determine the elimination of the different chemical species of some toxic metals - zinc, lead, cadmium - during biological sand filtration on surface water containing humic material which is able to promote the complexation of metals. The effect of preozonation was examined. The metal speciation was realised by means of Atomic Absorption Spectrometry and Differential Pulse Anodic Stripping Voltammetry. The survey of organic matters was carried out by measuring TOC, UV absorption and fluorescence. The biological treatment of raw water is quite effective for the removal of particu-late metals. The data bring out an important decrease for dissolved metals - about 50 to 60 % with cadmium and lead - especially noteworthy for the labile complexed metals. The preozonation of raw water provides the redissolution of particulate cadmium and zinc which are converted to free metals and seem then to be more eliminated by biological filtration. Ozonation appears to improve the efficiency of biological treatment which usually gives higher yields of purification than floculation (aluminium sulfate) in the same conditions.

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