Quantification of seasonal photo-induced formation of reactive intermediates in a municipal sewage lagoon upon sunlight exposure.

ABSTRACT

Photochemically produced reactive oxygen species in wastewater lagoons upon sunlight exposure are important in the attenuation of emerging contaminants (ECs). The production of reactive radicals in wastewater lagoons depends on both environmental factors and the composition of effluent organic matter (EfOM) in the wastewater. Knowing the steady state concentrations of these reactive species produced in a particular lagoon wastewater is critical to the prediction of the persistence and attenuation of ECs in that sunlit wastewater treatment lagoon. This study quantified the formation of four photochemically produced reactive intermediates (PPRIs) hydroxyl radical, carbonate radical, singlet oxygen, and triplet excited state EfOM in 11 samples collected from a municipal wastewater lagoon over a full year. The temporal distribution of these key PPRIs in the lagoon under investigation was determined in relation to sunlight irradiance, wastewater composition and temperature. Greater sunlight intensity led to greater PPRI production over the year. Increasing wastewater temperature from 12 to 25 °C led to greater production of singlet oxygen, a moderate decrease in hydroxyl radical and increase in triplet excited state EfOM, and minimal impact on carbonate radical production. The optical properties of the lagoon wastewater of Napierian absorption coefficient (A300) and E2E3 ratio could be used as indicators of the formation of singlet oxygen (Pearson's r = 0.79) and triplet excited EfOM (Pearson's r = 0.76) produced upon solar irradiation. The concentration of carbonate radical formed was strongly correlated to the nitrate level in the wastewater (Pearson's r = 0.85). The findings could be used for modelling the seasonal sunlight-induced photolysis process of ECs during lagoon-based wastewater treatment, with a view to optimising the treatment process, predicting the efficacy of EC removal, and risk assessment of the treated water.