Removal rate constants are not necessarily constant: the case of organic micropollutant removal in wastewater treatment plants

The removal of organic micropollutants (OMPs) in wastewater treatment plants (WWTPs) is critical to avoid pollution of the aquatic environment. While it is commonly assumed that removal rates are constant at low concentrations, this assumption remains untested across multiple WWTPs. The aim of this study was to test if removal rate constants of OMPs in activated sludge are indeed constant at low OMP concentrations. To that end, we related removal rate constants of 58 OMPs obtained from 14 WWTPs to influent OMP concentrations by applying linear mixed effect modelling in an all subsets modelling approach, also accounting for WWTP characteristics as well as physicochemical OMP properties. Influent OMP concentration and hydraulic retention time (HRT) were retained as predictors of removal rate constants in all best-supported models (within 2 AIC-units from the best model). The relationship between removal rate constant and concentration varied between OMPs. For most OMPs, the relationship was positive, except for valsartan and 2-hydroxyibuprofen, which may reflect toxic effects at higher concentrations. Our results indicate that the assumption of a constant removal rate at low concentrations is too simplistic and highlight the relevance of considering influent concentration in OMP fate predictions. This in turn may help in optimising OMP removal strategies, for example by concentrating wastewater in WWTPs.