Disinfection effectiveness of organic chloramines, investigating the effect of pH

The disinfection effectiveness of three organic N-chloramines (chlorinated amino acids and peptides) on the bacteria Escherichia coli (E. coli) was investigated, including a more detailed study into the pH dependency of the disinfection effectiveness of N-chloroglycine. The organic N-chloramines were prepared by combining sodium hypochlorite with each amino acid or peptide (glycine, Ala-Ala and Arg-Gly-Asp-Ser), at a N:Cl molar ratio of 1:0.4, and then used to treat E. coli suspensions for 180 min. No evidence of inactivation was observed at pH 8.1 for any of the tested organic N-chloramines. At pH 6.0 and 6.9, E. coli inactivation with N-chloroglycine was characterized by an initial lag phase, during which little or no measurable inactivation occurred, followed by a pseudo-first-order inactivation. This is in accordance with other results in the literature and supports the two step microbial inactivation mechanism proposed by some authors. Inactivation rate coefficients (Chick-Watson and lag coefficients) were calculated by fitting the experimental data with the Rennecker-Mariñas model. pH-dependent inactivation kinetics were observed, with faster inactivation rates occurring at lower pH values, when temperature and chlorine-to-nitrogen ratio where kept constant. N-chloroglycine was determined to be the only contributor to the inactivation process in these experiments. The free chlorine contribution was considered to be negligible in all experiments due to its very low concentration. As well, given that the anionic form of N-chloroglycine is expected to be the single predominant species over the tested pH range, changes in residual N-chloroglycine speciation could not be responsible for the observed pH-dependency of E. coli inactivation. However, while pH stress was considered as a possible synergistic factor, no significant effect of pH stress on E. coli viability was observed at the tested pH levels.