Annual dynamics of antimicrobials and resistance determinants in flocculent and aerobic granular sludge treatment systems

The occurrence and removal patterns of 24 antimicrobial agents and antimicrobial resistant determinants namely 6 antibiotic resistance genes (ARGs) and 2 mobile genetic elements (MGEs), and the fecal indicator E. coli were investigated in three full-scale wastewater treatment plants. Their waterlines and biosolids lines (including secondary treatment based on both granular and activated sludge) were sampled monthly throughout one year. Samples were analyzed by means of LC-MS/MS, qPCR and cell enumeration, respectively. The influence of rainfall, temperature, and turbidity on the occurrence and removal of the aforementioned agents was assessed through statistical linear mixed models. Ten of the antimicrobial agents (macrolides, fluoroquinolones, tetracyclines, and sulfonamides) were commonly found in influent in concentrations of 0.1-2 mu g L-1, and the predominant ARGs were ermB and sul1 (6.4 and 5.9 log 10 mL(-1) respectively). Warmer temperatures slightly reduced gene concentrations in influent whilst increasing that of E. coli and produced an uneven effect on the antimicrobial concentrations across plants. Rainfall diluted both E. coli (-0.25 logs, p < 0.001) and antimicrobials but not genes. The wastewater treatment reduced the absolute abundance of both genes (1.86 logs on average) and E. coli (2.31 logs on average). The antimicrobials agents were also partly removed, but 8 of them were still detectable after treatment, and 6 accumulated in the biosolids. ARGs were also found in biosolids with patterns resembling those of influent. No significant differences in the removal of antimicrobials, genes and E. coli were observed when comparing conventional activated sludge with aerobic granular sludge. Irrespective of the type of sludge treatment, the removal of genes was significantly reduced with increasing hydraulic loads caused by rainfall (-0.35 logs per Delta average daily flow p < 0.01), and slightly decreased with increasing turbidity (-0.02 logs per Delta 1 nephelometric turbidy unit p < 0.05). (c) 2020 The Authors. Published by Elsevier Ltd.

Automated summary

This study investigated the occurrence and removal patterns of antimicrobial agents, antibiotic resistance genes, and E. coli in wastewater treatment plants. The results showed that wastewater treatment reduced the abundance of genes and E. coli, while partially removing antimicrobial agents, with some still detectable after treatment and accumulating in biosolids. Irrespective of the type of sludge treatment, the removal of genes was significantly reduced with increasing hydraulic loads caused by rainfall, and slightly decreased with increasing turbidity.