Characterization and Comparison of Bacterial Communities Selected in Conventional Activated Sludge and Membrane Bioreactor Pilot Plants: A Focus on Nitrospira and Planctomycetes Bacterial Phyla

A pilot-scale membrane bioreactor (MBR) and a conventional activated sludge system (CAS) were in parallel operated to investigate the impact of the separation technology on the structure and functionality of the selected microbial community. Microbial communities as well as nitrogen removal efficiency of the biomass were characterized. Kinetics and microbial community structure turned out to be duly correlated. The impact of the separation technology on selective conditions and, in particular, the higher variability of solid separation efficiency in CAS with respect to MBR pilot plant possibly represented the main factor influencing the selection of bacterial communities. Concerning nitrifiers, bacteria of the genus Nitrospira were predominant in the MBR. This was in accordance with kinetics of nitrite-oxidizing bacteria that suggested the presence of k-strategists, while r-strategists were selected in the CAS plant, possibly because of the presence of transient higher concentrations of nitrite (in the range of 0.05–0.18 and of 0.05–4.4 mg  $ {\text{NO}}_{2}^{ - } $ -N L^?1 in the MBR and CAS effluents, respectively). An unexpectedly high presence of bacteria belonging to two specific phylogenetic clades of Planctomycetes was found in both reactors.