Microbial immigration in full-scale activated sludge systems is influenced by operational and design parameters

Microbial immigration from influent wastewater influences community assembly in activated sludge systems, but our ability to quantify this has been limited. Using recently developed inference methods, we apply the neutral community model (NCM) to quantify immigration rates (N_Tm) using 16S rRNA gene amplicon datasets from 74 full-scale wastewater treatment plants. Reliable immigration rates were determined for 31 datasets. Immigration rates ranged from 1.8 × 10² to 2.5 × 10⁴ across activated sludge systems and correlated positively with solids retention time (SRT) but negatively with treatment capacity. Attached growth systems such as membrane bioreactors and aerobic granular sludge exhibited lower immigration rates than conventional suspended-growth activated sludge systems. We quantified taxon-specific immigration probabilities for genera associated with key functional guilds. Denitrifiers, polyphosphate-accumulating, and glycogen-accumulating organism abundances correlated with immigration probabilities, suggesting a dependence on microbial immigration from the influent wastewater. Conversely, nitrifiers had the lowest immigration rates, and were generally more abundant in activated sludge reactors than would be predicted by the NCM. These findings suggest that immigration rates can be influenced by operational and design parameters, and demonstrates that microbial immigration can shape full-scale activated sludge communities in functionally relevant ways.