Kinetic modeling of Escherichia coli and Enterococcus sp. inactivation in wastewater treatment by photo-Fenton and H2O2/UV–vis processes

• Enterococcus present a higher resistance that E. coli due to structural differences.
• Photo-Fenton at pH 5 did not improve the yield of H2O2/UV–vis treatments.
• In photo-Fenton, not dissolved iron at pH 5 reduce the bacterial inactivation rate.
• 25 mg/L H2O2/UV–vis treatments reach total inactivation of E. coli and Enterococcus sp.
• Bacteria population is divided in two subpopulation groups with different resistance.

This work aims to assess the effectiveness of the photo-Fenton system for the removal of fecal bacteria in treated urban wastewater effluents, using low reagent concentrations and near neutral pH. To achieve this objective, kinetic modeling was used as a tool in the study of the influence of the different factors such as the UV–vis irradiation source, kind of bacteria, presence of H2O2 and particularly the influence of precipitated iron at near neutral pH on the inactivation of Escherichia coli and Enterococcus sp. Three inactivation treatments at pH 5 were carried out, based on a combination of UV–vis irradiation (artificial and solar) with oxidizing agents such as hydrogen peroxide (25 mg H2O2/L) and iron salts (5 mg Fe3+/L) or with each agent individually. The fitting of the experimental inactivation values to the corresponding equations through nonlinear regression techniques was carried out using the Microsoft® Excel tool GInaFiT (Geeraerd and Van Impe Inactivation Fitting Tool; Geeraerd et al., 2005). Ten different mathematical models were studied (Linear, Hom, Linear with shoulder, Linear with tail, Linear with shoulder and tail, Weibull, Weibull with tail, Double Weibull, Biphasic and Biphasic with shoulder). The assessment of the inactivation rates is a useful tool to know the influence of the different variables of the treatments. Thus, this assessment shows clearly that the presence of undissolved iron at pH 5 significantly decreases the inactivation rate of Enterococcus sp. in photo-Fenton treatments compared to H2O2/UV–vis treatments, while in the case of E. coli the inactivation rates are similar in both treatments. Consequently, the amount of hydroxyl radicals and reactive oxygen species (ROS) generated because of the presence of precipitated iron at pH 5 in the photo-Fenton treatments is considered as insignificant compared to the H2O2/UV–vis irradiation treatments, worsening the inactivation of Enterococcus sp.