The virological efficiency of individual treatment processes in a plant which produces drinking water from water of the river Oise was investigated. The plant had a capacity of 270,000 to 360,000 m3/d. The sequence of treatment processes was preozonation (0.5 - 1 mg/l)-storage - coagulation - flocculation - settling - sand filtration-second stage ozomtion (2.0 mg/l) - activated carbon filtration - post ozonation (0.4 mg/l free residual for 4 min)- chlorine dioxide (0.1 mg/l). Nine series of samples were taken after various treatment stages and analysed. Viruses were recovered by an absorption-elution method using electronegative (C 100-12 Balston) or electropositive (Virosorb 1 MDS Cuno) filters. The volume of water tested varied between 30 and 1 000 1, depending on filtration efficiency. BGM cells were used for virus detection, and all samples of raw water yielded positive results, with counts between 10 and 146 PFU/1 000 l.Viral counts decreased by 89%, 77% and 55% after intermittent preozonation and storage, settling and sand filtration, respectively. No viruses were recovered from the water after the second stage ozonation. The subsequent post-ozonation and final treatment with chlorine dioxide served as additional barriers to ensure a virologically safe drinking water supply.

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