TOrCs biotransformation, microbial composition and functional potential in biofiltration systems
Biofiltration processes can be applied for the removal of trace organic chemicals (TOrCs) both in wastewater and drinking water treatment systems. However, the TOrCs biotransformation mechanisms as well as the underlying drivers behind the transformation processes still remain elusive.In this study, we used laboratory batch incubations to investigate the biotransformation of 51 TOrCs by eight filter materials treating a range of water, from wastewater effluents to drinking water. Chemical analysis was complemented by 16S rRNA and metagenome sequencing for assessing associations between the biotransformation rate constants (kbiol), microbial composition and identified functional enzymes.We observed strong differences in the global transformation of TOrCs between the individual sand filters (0-58%) that were mirrored in microbial community composition, enzyme abundances and involved biotransformation pathways. The biotransformation potential, however, when factoring in biomass as ATP, which was strongly correlated with the kbio , the differences in the transformation rates became negligible, but still clustered into high biomass filters and low biomass filters. When comparing these two groups, Nitrospira was abundant in the high biomass filters (> 39.0%) but correlated with none of the TOrCs.  Hydrogenophaga showed the broadest correlation with TOrCs biotransformation despite of its minor proportion (0.04%), and it also had the most complete xenobiotics degradation pathways, rendering it as a potential biotransformation indicator genus. Moreover, the enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase gene may be a promising genetic marker for biotransformations. The drinking water filters showed a distinct microbial community with Pseudomonas being dominant, correlating negatively with kbiol, pointing to the slow biotransformation of TOrCs under oligotrophic condition.In general, this study provides new insights into so far rarely addressed associations between TOrCs biotransformation, microbial composition and functional potential which will be used to suggest novel biological indicators for assessing the removal performance of TOrCs in biofiltration systems. Moreover, it highlights the role of ATP in predicting the global transformation of TOrCs in sand filter systems.