Antibiotic resistome during two-stage partial nitritation/anammox process for sludge anaerobic digestion reject water treatment

Anaerobic digestion (AD) reject water serves as a significant reservoir for antibiotic resistance genes (ARGs), underscoring the importance of understanding ARGs dynamics during treatment processes. Partial nitritation /anammox (PN/A) has become an increasingly adopted process, while comprehensive investigation on ARG behavior within this system, especially in full-scale, remains limited. This study explores the distribution of ARGs in a full-scale two-stage PN/A system, with an anaerobic/anoxic/oxic (AAO) system for comparison. The sludge in partial nitritation (PN) stage exhibited higher ARG abundance (0.82 copy/cell) compared to that in anammox stage (0.21-0.26 copy/cell). In anammox sludge samples, 95.8-99.1 % of the mobile genetic elements (MGEs) were identified as tnpA, while the PN stage showing greater diversity. Some ARGs co-occur in the same contigs, and some of these ARGs belong to different ARGs types, which is related to multi-antibiotic resistance, with the highest frequency observed in the PN stage. The abundance of contigs with ARG-MGE co-occurrence, suggesting the possibility of horizontal gene transfer (HGT), was most prevalent in the AAO system. Potential ARG hosts were identified by metagenomic binning, and some functional bacteria, like Nitrosomonas, were regarded as ARGs host. This study offers a comprehensive analysis of the complexities of ARGs distribution within full-scale two-stage PN/A systems treating AD reject water.