How do WWTPs operational parameters affect the removal rates of EU Watch list compounds?

Abstract

This work aims at achieving a better understanding of the mechanisms and the operative conditions regulating the removal of a set of relevant micropollutants in conventional activated sludge (CAS) systems to maximize their removal and, if possible, biodegradation. Eight compounds from the EU Watch list (clothianidin, thiacloprid, methiocarb, E1, E2, EE2, diclofenac and erythromycin) were spiked at 2 μg/L in CAS systems and their behaviour was studied in 6-h batch tests. The role of sorption was also investigated. Information on the removal of the pesticides clothianidin, thiacloprid and methiocarb is here presented for the first time to the best of the authors' knowledge. With the aim of enhancing the removal of the selected compounds in wastewater treatment, four parameters were explored: biomass concentration, temperature, pH and redox conditions. For each parameter, a low and a high value were chosen, based on the ranges usually applied in wastewater treatment plants (WWTPs). Results show that biomass concentration is the most relevant parameter among the ones investigated, followed by the redox conditions. The operational conditions that maximized removal rates were: 5 g/L of biomass, aerobic conditions, 25 °C and pH 7.5. High variability in removal rates was observed for compounds such as E1, erythromycin and methiocarb. The pesticides clothianidin and thiacloprid did not prove to be easily degradable. The highest removal rates were recorded for the hormones, particularly E2, with a transformation rate of at least 96% under all conditions. Sorption proved to be a relevant removal route for EE2, for which the highest sorption rates were recorded, and diclofenac, where the adsorption mechanisms was hypothesised for its prevalence at lower pH values.