The organic loading rate affects organic micropollutants' cometabolic biotransformation kinetics under heterotrophic conditions in activated sludge.

Several studies have shown that organic micropollutants (OMPs) are biotransformed cometabolically in activated sludge systems. However, the individual role of heterotrophs in the microbial consortium is still not clear, i.e., there is still a gap regarding the influence of the heterotrophic activity on the cometabolic biotransformation kinetics and yield of the OMPs. Aiming to answer these questions, experiments with increasing primary substrate concentrations were performed under aerobic heterotrophic conditions in a continuous stirred tank reactor operated at several organic loading rates (OLR) with fixed hydraulic retention time. Moreover, the individual kinetic parameters were determined in batch assays with different initial substrate concentrations using the sludges from the continuous reactor. A set of 15 OMPs displaying a variety of physicochemical properties were spiked to the feeding in the ng L - µg L range. Results reveal that the biodegradation of the primary carbon source and the biotransformation of the OMPs occur simultaneously, in clear evidence of cometabolic behavior. Moreover, we conclude that the OMPs biotransformation kinetic constant (k) shows a linear dependence with the OLR of the primary substrate for most of the compounds studied, suggesting that the heterotrophic activity seriously affects the OMPs biotransformation kinetics. However, under typical activated sludge systems operating conditions (hydraulic retention times above 8 h), their biotransformation yield would not be significantly affected.