Adsorption outweighs biodegradation: Organic micropollutants exhibit compound-specific temperature responses in biologically active granular activated carbon filters.

In this study, we investigated the influence of water temperature fluctuations on biologically active granular activated carbon filters (GAC filters) used for advanced treatment in municipal wastewater treatment plants. In addition to adsorption, biodegradation processes influence the removal of organic micropollutants (OMPs) in GAC filters as the filter operation time increases. Both processes, adsorption and biodegradation, are individually influenced by temperature changes. However, it is largely unknown how the temperature-dependent behaviour of individual OMPs affects the combination of adsorption and biodegradation processes in GAC filters so far. Results indicate that the influence of temperature on the OMP removal in GAC filters varies greatly depending on substance-specific properties. Regarding temperature dependency of adsorption, the number of hydrophilic groups, represented by the hydrophilic-lipophilic balance (HLB) and molecular weight, as well as the temperature-dependent shift of the dissociation constant (pKa) were identified as decisive substance properties. The effect of the pKa shift was evident primarily for 1H-benzotriazole, both in laboratory experiments and in two pilot-scale GAC filters operated over a period of four years. Furthermore, laboratory tests show that an adapted microbial community in GAC filters especially improves the biodegradation of diclofenac, clarithromycin and amisulpride, compared to an activated sludge system. Especially for 1H-benzotriazole, elevated temperatures lead to increased effluent concentrations in the pilot scale GAC filters which can be explained that exothermic adsorption behaviour outweighs potentially increased biodegradation. With the exception of 1H-benzotriazole and ∑4-/5-methylbenzotriazole, all OMPs examined exhibit the opposite behaviour, namely a decrease in wastewater concentrations with increasing temperature.