Conditioning with zero-valent iron or Fe2+ activated peroxydisulfate at an acidic initial sludge pH removed intracellular antibiotic resistance genes but increased extracellular antibiotic resistance genes in sewage sludge

Sulfate radical ((SO4-)-S-center dot)-based conditioning methods, such as zero-valent iron (ZVI, i.e., Fe-0) or ferrous iron (Fe2+) activated peroxydisulfate (S2O82-), have recently developed to improve sludge dewaterability, but it remains unclear how they impact the intracellular and extracellular antibiotic resistance genes (ARGs) in sewage sludge. In this study, it was found that conditioning treatments that used ZVI/S2O82- or Fe2+/S2O82- system, at an acidic initial sludge pH, removed the intracellular ARGs and intI1 and the extracellular intI1 from sewage sludge, but led to the accumulation of extracellular ARGs of aadA-01, aadA-02, aadA1, aadA2-03, and strB in conditioned sludge. During sludge conditioning with ZVI/S2O82- or Fe2+/S2O82-, bacterial hosts of ARGs and intI1 were seriously lysed to release the intracellular ARGs and intI1 to the extracellular environment, thus removing intracellular ARGs and intI1 in sludge, while the released ARGs and intI1 were primarily degraded by the produced (SO4-)-S-center dot to attenuate most extracellular ARGs and intI1. However, the relatively lower degradation ability of (SO4-)-S-center dot for extracellular ARGs of aadA-01, aadA-02, aadA1, aadA2-03, and strB led to their accumulation in conditioned sludge. Therefore, (SO4-)-S-center dot-based conditioning methods can be employed to reduce ARGs in sludge, but the subsequent treatment of sludge dewatering filtrate requires more attention.