Primary settling changes the microbial community of influent wastewater to wastewater treatment plants

The continuous immigration of bacteria in influent wastewater strongly impacts the microbial community of activated sludge (AS) in wastewater treatment plants (WWTP), both in terms of species composition and their abundance. Therefore, it is of interest to elucidate the route of immigrating bacteria into the biological tanks, including the effect of primary settlers. These are commonly used pretreatment units that can possibly selectively increase or reduce the relative abundance of certain bacteria. Species-level identification of the microbial composition of influent wastewater before and after primary settling was carried out in four full-scale municipal WWTPs biweekly over one year by 16S rRNA gene amplicon sequencing. Overall, 37–49% of incoming COD was removed in the primary settlers. Most genera and species were present in the wastewater to all four plants and the trend of these were investigated across the primary settlers. Approximately 50% of the genera had the same trend across at least three WWTPs. Few genera significantly increased in relative read abundance (3.7%) after settling, while 22.3% showed a significant reduction in relative abundance. We investigated process-critical species in AS, such as known nitrifiers, polyphosphate-accumulating organisms, and filamentous bacteria. Most taxa were affected similarly in all WWTPs including multiple genera involved in bulking in AS. However, some genera, e.g., important polyphosphate-accumulating bacteria, had inconsistent trends across WWTPs, suggesting that the characteristics of the wastewater are important for the trend of some bacteria through primary settling. In all cases, primary settling changed the microbial community of the influent wastewater, posing an obvious candidate for upstream control to optimize the assembly of the microbial communities in activated sludge.</p

“Candidatus Dechloromonas phosphoritropha” and “Ca. D. phosphorivorans”, novel polyphosphate accumulating organisms abundant in wastewater treatment systems

Members of the genus Dechloromonas are often abundant in enhanced biological phosphorus removal (EBPR) systems and are recognized putative polyphosphate accumulating organisms (PAOs), but their role in phosphate removal is still unclear. Here, we used 16S rRNA gene sequencing and fluorescence in situ hybridization (FISH) to investigate the abundance and distribution of Dechloromonas spp. in Danish and global wastewater treatment plants. The two most abundant species worldwide revealed in situ dynamics of important intracellular storage polymers, measured by FISH-Raman in activated sludge from four full-scale EBPR plants and from a lab-scale reactor fed with different substrates. Moreover, seven distinct Dechloromonas species were determined from a set of ten high-quality metagenome-assembled genomes (MAGs) from Danish EBPR plants, each encoding the potential for polyphosphate (poly-P), glycogen, and polyhydroxyalkanoates (PHA) accumulation. The two species exhibited an in situ phenotype in complete accordance with the metabolic information retrieved by the MAGs, with dynamic levels of poly-P, glycogen, and PHA during feast-famine anaerobic–aerobic cycling, legitimately placing these microorganisms among the important PAOs. They are potentially involved in denitrification showing niche partitioning within the genus and with other important PAOs. As no isolates are available for the two species, we propose the names Candidatus Dechloromonas phosphoritropha and Candidatus Dechloromonas phosphorivorans