Seasonal assessment of the structural and functional potential of microbial communities in a full-scale treatment wetland for combined sewer overflows

Seasonal assessment of the structural and functional potential of microbial communities was conducted in a full-scale vertical-flow wetland for combined sewer overflow treatment (CSO-TW). The filter bed extending over 2000 m 2 was sampled in four different layers along its vertical and horizontal expansion. Biofilm extracted from the filter substrate was subject to community-level physiological profiling to evaluate metabolic activity in winter and early summer, and 16S ribosomal RNA (rRNA) amplicon sequencing to further examine the microbial community structure in winter. The first-time investigation at this scale revealed that while the hydraulic flow had a measurable impact on the spatial metabolic activity, seasonal effects remained limited, supporting prior findings that CSO-TWs offer stable water treatment performance throughout the year. Results from 16S rRNA sequencing showed a similar hydraulic impact, but contrasting conclusions from the comparison with a parallel study on a pilot-scale CSO-TW underline the importance of using mature pilot-scale systems for adequate representation of effects on full-scale sites. Currently, investigations on pilot-scale studies often rely on young systems due to short project durations and funding cycles, which allow only for limited conclusions to the situation in matured full-scale systems. Further comparison with investigations on vertical-flow treatment wetlands treating domestic wastewater revealed similarities in microbial communities across studies despite varying designs, size and influent water. This is encouraging for further and more widely comparison and insight to clearly determine the microbial functional responses. It also points to the adaptive nature of TWs to water treatment requirements.