Diversity and importance of filamentous bacteria in biological nutrient removal wastewater treatment plants – a worldwide survey

Filamentous bacteria are present in wastewater treatment plants (WWTPs) worldwide where they play an important role by providing structural backbone for activated sludge (AS) flocs and thus ensuring good settling properties. However, their excessive growth may lead to inter-floc bridging, which interferes with floc settleability, causing ‘bulking’. This phenomenon is dependent on the type and abundance of filaments present thus it is important to know the community composition in AS systems. In this study we utilized state-of-the-art molecular techniques to make a detailed survey of filamentous bacteria in full-scale nutrient removal WWTPs. 16S rRNA gene amplicon sequencing was applied to survey 24 Danish and 30 worldwide full-scale biological nutrient removal WWTPs (total of >550 samples), where all known bacterial genera possessing filamentous morphology were investigated. Candidatus Microthrix and members of the phylum Chloroflexi were among the most abundant filamentous bacteria in all investigated WWTPs, often making up 20-30% of the total read abundance. Candidatus Microthrix showed limited microdiversity with two OTUs dominating alternately in all investigated plants. The 9-year long survey of Danish WWTPs showed that most of the dominant phylotypes belonging to Chloroflexi showed little fluctuation in their abundance in time, however, a recently characterized phylotype C10_SB1A showed marked shifts in its abundance and was statistically associated with bulking in few case studies. This study demonstrated limited diversity of abundant filamentous bacteria in AS community around the globe presenting a hope for solution of sludge settling problems if we can couple the knowledge of filaments identity and their physiology.