Microorganisms in wastewater treatment plants (WWTPs) are essential for water purification to protect public and environmental health. However, their diversity and the factors that control it are poorly understood. Using a systematic global-sampling effort, we analyzed the 16S rRNA gene sequences from ~1,200 activated sludge samples taken from 269 WWTPs in 23 countries on 6 continents. Our analyses revealed that the global activated sludge bacterial communities contain ~1 billion bacterial phylotypes with a Poisson lognormal diversity distribution. Despite this high diversity, activated sludge has a small global core bacterial community (n = 28 OTUs) that is strongly linked to activated sludge performance. Meta-analyses with global datasets associate the activated sludge microbiomes most closely to freshwater populations. In contrast to macroorganism diversity, activated sludge bacterial communities show no latitudinal gradient. Furthermore, their spatial turnover is scale-dependent and appears to be largely driven by stochastic processes (dispersal, drift), although deterministic factors (temperature, organic input) also are important. Our findings enhance mechanistic understanding of the global diversity and biogeography of activated sludge bacterial communities within a theoretical ecology framework and have important implications for microbial ecology and wastewater treatment processes.

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In the version of this Article originally published, the name of the author 'Mathew Robert Brown' was incorrectly written as 'Mathew Brown' in the main author list and as 'Matthew Brown' in the Global Water Microbiome Consortium list. In addition, in the Global Water Microbiome Consortium list, the names of the authors 'Kevin F. Boehnke' , 'Janeth Sanabria' and ' Adalberto Noyola' were incorrectly written as 'Kevin Boehnke' , 'Janeth Sanabria Gómez' and ' Adalberto Noyola Robles' , respectively. The names have now been corrected and the author initials in the author contributions section updated accordingly.