Changes in the activity and abundance of canonical nitrifiers and Comammox bacteria during stream switching: Shifting from a mainstream hybrid reactor for C, N, and P removal to a sidestream biofilm nitrification reactor

This study investigated how mainstream-sidestream switching affected the activity and abundance of canonical nitrifiers and Comammox bacteria. The experiment started in a mainstream hybrid reactor for C, N, and P removal with biomass in the form of activated sludge and biofilm. A highly effective nitrification sidestream reactor with pure moving bed technology was then obtained through gradual reduction of the reactor's organic loading rate (OLR), increase in the temperature (20–30 °C), and increase in the reactor's nitrogen loading rate (NLR). qPCR and kinetic batch test results suggest that Comammox bacteria could play an important role in nitrification in both the mainstream hybrid reactor and sidestream biofilm reactor, dominating even the canonical AOB. Biofilm provided a better environment for Comammox bacteria development, except when the system was transformed into a hybrid nitrifying reactor. In that case, the occurrence of Comammox bacteria in activated sludge was greater. Simultaneously with reduction of OLR an increase in ammonia oxidation rate was reported in both activated sludge and biofilm. An increase in NLR had a greater effect on the activity of NOB.