Interactions between humic matter and bacteria when disinfecting water with UV light

Aims:  To investigate the impact of aquatic humic matter on the inactivation of Escherichia coli and Bacillus subtilis by ultraviolet (UV) light.
Methods and Results:  A bench-scale study investigated the potential for Aldrich^® humic acid (AHA) and Suwannee River natural organic matter (SR-NOM) to coat the surface of E. coli and B. subtilis and offer protection from low-pressure UV light. UV doses of 5 and 14 mJ cm⁻² were applied using a collimated beam at four concentrations of humic matter (0, 10, 50 and 120 mg l⁻¹) in reagent grade water. Both AHA and SR-NOM were found to offer statistically significant protection of both E. coli and B. subtilis at concentrations of 50 and 120 mg l⁻¹ for a UV dose of 14 mJ cm⁻².
Conclusions:  Both E. coli and B. subtilis are susceptible to coating by humic matter which can reduce the sensitivity of the cells to UV light.
Significance and impact of the study:  Micro-organisms in the environment may acquire characteristics through interaction with humic matter that render them more resistant to UV disinfection than would be predicted based on laboratory inactivation studies using clean cells.