Repression of photoreactivation and dark repair of coliform bacteria by TiO2-modified UV-C disinfection

Photoreactivation and dark repair of damaged DNA can occur after some bacteria are disinfected with UV-C irradiation and thus reduce the disinfection efficiency. We have discovered that the provision of 1 mg/L titanium dioxide (TiO2) in suspension during UV-C irradiation at normal disinfection doses repressed the photoreactivation and dark repair. The repressive effect was also observed when a TiO2-coated plate was used during UV-C exposure but removed thereafter, indicating the repressive effect does not require the presence of TiO2 in the post-UV event. The repressive effect was consistently observed with changes of bacteria species, temperature, salinities, UV sources, doses and intensities, and with/without nutrients. The repressive effect is likely associated with the additional radical attack during UV-TiO2 irradiation and/or formation of a small amount of stable residual oxidants (primarily hydrogen peroxide). The TiO2-modified UV-C disinfection represents an innovative means to disinfect water. It is particularly useful in situations where additions of residual chlorine or chloramines are not allowed or not possible. Nevertheless, the TiO2 shall be properly included, by either using the optimum dose in suspension or immobilizing it onto a surface, since too little TiO2 cannot provide the repression while too much TiO2 in suspension gives a detrimental effect on the UV-C disinfection.