Zero-valent iron enhanced in-situ advanced anaerobic digestion for the removal of antibiotics and antibiotic resistance genes in sewage sludge.

The in-situ advanced anaerobic digestion (AAD) enhanced with zero-valent iron powder (ZVI) under mesophilic condition was investigated to remove 5 antibiotics (sulfamerazine (SMR), sulfamethoxazole (SMZ), ofloxacin (OFL), tetracycline (TC), and roxithromycin (ROX)) and 11 antibiotic resistance genes (ARGs) (AAC (6')-IB-CR, qnrS, ermF, ermT, ermX, sul1, sul2, sul3, tetA, tetB, and tetG) in sewage sludge. The effects of different ZVI dosages, antibiotic concentrations, and solid retention time (SRTs) on the removal were explored. Also, the correlation coefficient of antibiotics and ARGs, microbial community structure, biogas production and methane yield were analyzed. All conducted treatments operated stably, and the modified Gompertz model described the cumulative methane yield well. The antibiotics, with the exception of OFL, were effectively removed in the sewage sludge at a dosage of 1000 mg/L ZVI, SRT 20 d, and an antibiotic concentration of 20 μg/L during AAD. The removal rates of SMZ, SMR, TC, and ROX reached 97.39%, 74.54%, 78.61%, and 56.58%, respectively. AAC (6')-IB-CR and tetB could be effectively reduced during the in-situ AAD. Through the redundancy analysis, AAC (6')-IB-CR, ermT, ermX, sul2, tetB, and tetG had strong positive correlations with the antibiotics in the reactor. The principle component analysis revealed that the community structure was similar when the SRT was 10 d and 20 d at the same amount of ZVI and antibiotic concentrations in the sludge. Under the operating parameters of 1000 mg/L ZVI dosage, SRT 20 d, and an antibiotic concentration of 20 μg/L, Erysipelotrichia, Verrucomicrobia, Clostridia, Caldiserica, and Alphaproteobacteria of the class were dominated microorganisms in the anaerobic digestion.

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Declaration of competing interest The authors declare that they have no conflict of interest.