The efficacy of long-lasting residual drinking water disinfectants based on hydrogen peroxide and silver

The aim of the present work was to evaluate the disinfectant capacity and the possible fields of application of a combined silver and hydrogen peroxide (HP) water disinfectant. The findings demonstrated the high bactericidal action of silver (on E. coli) and its relatively ineffective virucidal effect (on MS-2 phage). HP was found to have a small bactericidal effect and a mild virucidal one. When combined, silver and HP usually exhibited a synergistic action on the viability of E. coli and on the luminescence of recombinant luminescent E. coli. In some instances, the combined bactericidal effects were 1000-fold higher than the sum of the separate ones. No increased virucidal action was observed. The biocidal action of the combination generally increased with increasing temperature and pH, and decreased in secondary and tertiary effluents. The physiological effects and mechanisms of toxicity of HP, silver and their combinations, were assessed by monitoring the induction of stress promoters upon exposure to the active agents, and by assessing the sensitivity of E. coli mutated in major stress responses to HP, silver and their combinations. The results showed that HP induced a wide array of stress responses, that both silver and HP induced promoters regulated by the heat shock response, and that the dnaK promoter (regulated by the heat shock response) was synergistically induced. The mutant sensitivity tests showed that bacteria deficient in the ability to activate central cellular stress responses (SOS, heat shock, stationary phase, oxidative) were hypersensitive to both HP and silver. These results imply that cellular proteins, and possibly the DNA, are the cellular moieties chiefly affected. The above findings suggest that the potentiated effect of HP and silver is a metabolically dependant/related process that stems from a combination and/or accumulation of physiological effects exerted by the active ingredients. The physico-chemical properties of the combined disinfectant, and its disinfection capacity, points to its potential application as a long-term secondary residual disinfectant for water of relatively high quality.