Microbial community dynamics in real-scale vertical flow treatment wetlands fed with raw municipal wastewaters

The aim of this study was to assess microbial community dynamics across space and time in vertical flow treatment systems. Four French vertical flow treatment wetlands (VFTW) treating domestic wastewater at a real scale were used in the molecular analysis of microbiota in the filters. The time-space approach was aimed at evaluating the biomass content horizontally and with respect to depth, rest and feed periods, season, and the age of the system. Thirty samples were characterized by DGGE analysis, and 15 samples were characterized by qPCR and 16S rRNA sequencing. Diversity and richness indices showed that there was no predominance of diversity in a specific system. The sludge layer greatly interferes with filtration in the bacterial community. This layer provides organic matter and nutrients for heterotrophic nitrification, thereby promoting nitrification and denitrification throughout the depth of the filter. Aeration drains induced an increase in ammonia-oxidizing bacteria (AOB) in the layers below the surface. The behaviour was also observed in winter. qPCR also indicated that a larger number of copies were found during the feeding period. DGGE analysis revealed that the 7th day of the rest period was highlighted by an apparent decline in the bacterial community. From 16S rRNA analysis, the most abundant phyla found were Proteobacteria, Bacteroidetes, Actinobacteria, Chloroflexi and Nitrospirae. Mycobacterium, Flavobacterium, Defluviicoccus and Nitrospira were found to be the predominant genera. These genera are linked to the degradation of organic matter and may promote heterotrophic nitrification and denitrification. Nitrospira overcame AOB and emerged in summer with a greater abundance at a depth of 30 cm with a slight increase in the rest period. The present study is the first to address this issue to improve our knowledge about microbial community dynamics in real-scale typical French systems, to examine pollutant removal mechanisms or to develop new ideas regarding these systems.