The Biological Safety of UV Disinfection Systems for Drinking Water

UV irradiation has become a common technique for drinking water disinfection since it was demonstrated to be very effective against (oo) cysts of Cryptosporidium and Giardia. One of the most important factors which limited UV applications in drinking water treatment was that it had no long term disinfecting effect in a distribution system. The results showed: (1) UV disinfection alone, did not impact the biofilm densities of steady-state significantly. The biofilm densities for the un-disinfected system were similar to the UV disinfected system, and it needed only 2 days to achieve steady-state, which was shorter than the UV disinfected system of 6∼8 days. (2) The bulk fluid concentrations and biofilm densities increased equally in the first few days of operation, and it increased with the biofilm densities decreased in steady-state. (3) It was considered to have biostability in a model distribution system when the chlorine dosage was above 0.7 mg/L in the experiment. The impacts of chloramine dosages on biofilm densities were not as significant as that of chlorine dosages. To add 0.4 mg/L chloramine had a similar effect as to when the chlorine dosage was 0.7 mg/L. Thus chloramine might be a more appropriate additional disinfectant after UV. (4) The biofilm densities and residual chlorine had a certain linear relationship. This also related to the bulk fluid concentration. The residual chlorine had more control effects when added chlorine than chloramine. The biofilm could keep growing with low residual chlorine.