Disinfection effect of electrochemically generated chlorine on surface associated Escherichia coli in a drinking water system

For many years, electrochemical treatment has been proposed as a potential alternative to conventional drinking water chlorination due to its simplicity, ease of use and ability to generate active disinfectant from ions naturally found in the drinking water The aim of this study was to evaluate the survival of Escherichia coli on the surfaces of water distribution system after exposure to in situ electrochemically generated chlorine. To analyse the effect of chlorine and its reaction intermediates, completely mixed reactor with or without ingenuous biofilm was supplied with natural drinking water containing low amount of chloride ions (<10 mg/L) and treated with non-stoichiometric titanium oxide electrodes (TiO2-x) at low current density (4.1-8 mA/cm(2)) which generate predominantly chlorine species. Various cell viability markers (cultivability, ability to divide as such and respiratory activity) were assessed in this study. The results showed that electrochemical disinfection was very effective to neutralize the suspended E. coli (>5 log decrease in cultivability and 2 log decrease in respiratory activity was obtained after 1 h of treatment). However, surface and biofilm analyses showed significantly lower inactivation rates (1.49-1.79 log after 1 h of treatment). Moreover, after 24 h, biofilm still contained 16% ability to divide E. coli. The study clearly showed that surface- or biofilm-attached E. coli is more resistant towards electrochemically generated chlorine than the suspended ones, and this should be taken into account when choosing optimal doses for electrochemical disinfection.