Bactericidal action of illuminated TiO2 on pure culture of Escherichia coli K 12 and bacterial consortium was studied. Photocatalytic E. coli inactivation rate was dependent on the biological parameters such as: physiological state, generation and initial concentration of bacteria. The behavior of the bacterial suspension during the subsequent dark period was discussed in order to estimate the potential of using the photocatalytic treatment process in a real water disinfection Situation. The effective disinfection time (EDT) was defined as the time required for total inactivation of bacteria without re-growth in a subsequent dark period referenced at 48 It. An increase of E. coli concentration was observed after illumination of bacteria without TiO2 (named here as photolysis). In the presence of TiO2 (photocatalysis), the decrease of bacteria continues in the dark, and no regrowth was observed within the following 60 h. The extent) applied during the illumination period. Thus, of this "residual disinfecting effect" was dependent on the light intensity (400 or 1000 W/m(2)) EDT was reached in photocatalytic but not in photolytic treatment. Bacterial consortia present in two different wastewaters were phototreated with and without TiO2 and a significant difference in bacterial sensitivity to both phototreatments was observed. Enterococcus sp. appear to be less sensitive than coliforms and other Gram-negative bacteria. Considerable differences in photoreactivity were observed for both samples taken at the same place but at different date. The first sample named as wastewater 1, exhibited a bacteriostatic but no bactericidal effect after both phototreatments; the addition of TiO2 accelerated the solar disinfection. In the second sample, wastewater 2, a bactericidal effect was observed in photolytic and photocatalytic experiments and the EDT was attained in both cases, even if the bacterial inactivation rate was the lowest in the presence of the catalyst. The chemical oxygen demand (COD) decreases during photocatalytic treatment, but not during the photolytic one; whereas the Dissolved organic carbon (DOC) increases in both cases. COD/DOC ratio decreases during phototreatrnents. (C) 2003 Elsevier B.V. All rights reserved.