Microbial Communities and Nitrogen-Utilizing Bacteria of Rotating Biological Contactors and Activated Sludge Treating Public Sewage and Night Soil/Johkasou Sludge

The public sewage (PS) and night soil mixed with johkasou sludge (JO) have similar chemical compositions; however, the concentrations of organic matter and nitrogen compounds were different. We investigated the microbial community of the rotating biological contactor (RBC) units treating PS and JO, in which the RBC was submerged in the mixed liquor of activated sludge. Here, we observed that the microbial community compositions at the phylum and class levels were similar between the PS-RBC and JO-RBC, whereas the relative abundances of several phyla (Euryarchaeota, Acidobacteria, Chloroflexi, Firmicutes, Patescibacteria, and Betaproteobacteria) significantly differed between them. The microbial community composition of RBC (an attached growth process) was similar to that of the activated sludge (a suspended growth process). The microbial community of activated sludge likely affected that of RBC. The relative abundance of total denitrifying bacteria in the PS-RBC was twice as much as that in JO-RBC, while nitrifying bacterial phylotypes had a similar relative abundance. The predominant denitrifying genera were different between the PS-RBC and JO-RBC, as well as in the cross-sectional layers of the PS-RBC, suggesting the functional diversity of denitrifying bacterial genera inhabiting the RBC.