The knock-on effects of different wastewater feeding modes: Change in microbial communities versus resistance genes in pilot-scale aerobic sludge granulation reactors

To explore the effects of wastewater feeding modes on the formation of aerobic granular sludge (AGS) and the complex relationships between resistance genes and bacteria, two pilot-scale sequencing batch reactors (SBRs) were established. The SBR with influent wastewater introduced uniformly through pipes at bottom was designated as BSBR, and the SBR with inlet wastewater flowing directly from top was TSBR. BSBR formed dense AGS due to uniform wastewater feeding at bottom, while TSBR failed to cultivate AGS. Metagenomic sequencing illustrated that rapid growth of AGS in BSBR was accompanied with increase of antibiotic resistance genes (ARGs) abundance, but ARGs diminished when the size of AGS was stable. The ARGs continued to elevate in TSBR, and abundance of metal resistance genes (MRGs) was always higher than that in BSBR. Two reactors had markedly different bacterial community, microbes in BSBR owned stronger activity, conferred greater potential to proliferate. AdeF in two systems had the most complex gene-bacteria relationships which would undergo HGT within bacterial genus. The different feeding modes of wastewater directly led to the changing size of sludge, which caused knock-on effects of variations in the abundance of microbial communities and resistance genes. This study provided promising suggestions for the rapid cultivation of AGS and control of resistance genes at pilot-scale.

Automated summary

Two pilot-scale sequencing batch reactors (SBRs) were established to investigate the effects of different wastewater feeding modes on the formation of aerobic granular sludge (AGS) and the complex relationships between resistance genes and bacteria. The results showed that AGS formed in the reactor with uniform wastewater feeding at the bottom, while the reactor with inlet wastewater flowing directly from the top failed to cultivate AGS. Metagenomic sequencing revealed that the abundance of antibiotic resistance genes (ARGs) increased with the rapid growth of AGS in the bottom-fed reactor, but decreased when the size of AGS was stable. In contrast, the abundance of ARGs continued to elevate in the top-fed reactor, and the abundance of metal resistance genes (MRGs) was always higher than that in the bottom-fed reactor. The two reactors had distinct bacterial communities, with stronger microbial activity and greater potential for proliferation in the bottom-fed reactor. The gene-bacteria relationship of AdeF was the most complex in both systems, potentially undergoing horizontal gene transfer within the bacterial genus. The different feeding modes of wastewater directly affected the size of sludge, leading to variations in the abundance of microbial communities and resistance genes. This study provides valuable insights for the rapid cultivation of AGS and control of resistance genes at pilot-scale.