Influence of inorganic and organic compounds on the decay of peracetic acid in wastewater disinfection

The aim of this study was to evaluate the influence of the physical-chemical characteristics of wastewater on PAA decay, in multi-component solutions of inorganic and organic compounds (11 compounds in total) representative of secondary effluents of wastewater treatment plants, disinfected at various PAA concentrations (2-5 mg/L). Batch experiments were defined using the statistical method of the Design of Experiments (DoE) in order to evaluate the effect of each compound and their interaction on PAA decay. Results showed that the organics consumed immediately a considerable amount of PAA, independently from the initial PAA concentration, and consumption dropped rapidly to almost nil after 5 min, whereas PAA consumption due to the inorganics was slow, dependent on the initial PAA concentration and persisted until the end of the experiments (60 min). In detail, inorganics (such as reduced iron and orthophosphate) have shown to be the main drivers of the exponential decay: iron, particularly, has proved to directly consume PAA due to its catalysing capacity, whereas orthophosphate has shown to mainly interact with iron, acting as a chelating compound towards iron and consequently reducing the iron effect in consuming PAA. As for organics, proteins such as, casein and peptone, have been highlighted as the main cause of the initial PAA demand, probably due to the homolytic fission of PAA to generate peroxyl and hydroxyl radicals, which are known to have a high reactivity towards proteins. Finally, a model for predicting the residual PAA concentration was obtained and validated; uncertainty analysis was also performed by a series of Monte Carlo simulations to propagate input uncertainties to the model output.