ABSTRACTThe upgrading of the Ivry‐sur‐Seine water‐treatment plant, which produces potable water for consumption in Paris, is described. This upgrading is based on the slow biological filtration stage. The pretreatment comprises a primary filtration stage through biolite, and a secondary filtration stage through sand, with the possibility of coagulation. The polishing treatment includes ozonation and biological filtration through granular activated carbon. Ozone is produced from pure oxygen, with two ozone generators operating at medium frequency and producing a total of 52 kg/h. A description of pilot tests of the upgraded plant is given, together with the first re