Ciprofloxacin removal and antibiotic resistance genes response in the EGSB-AnMBR system treating swine wastewater: Performance, mechanism, and metagenomics.

Ciprofloxacin (CIP), a veterinary antibiotic in swine wastewater is an emerging contaminant with low concentration but significant environmental risk. Its inhibitory effects on biological treatment systems and the proliferation of antibiotic resistance genes have raised widespread public concern. An expanded granular sludge bed reactor was coupled with an anaerobic membrane bioreactor (EGSB-AnMBR) to treat swine wastewater containing CIP. During 320 days operation, the EGSB-AnMBR system achieved over 98.6 % chemical oxygen demand (COD) removal efficiency and 61.4 %-100 % CIP removal efficiency. During the initial operation phase, sludge adsorption served as the primary mechanism for CIP removal, whereas biodegradation became dominant in the last operational phase. 16S rRNA gene high-throughput sequencing analysis revealed that under CIP stress, the abundance of CIP-resistant Spirochaetota increased from 4.7 % to 9.5 %, whereas Patescibacteria abundance progressively decreased from 72.7 % to 15.7 %. Metagenomic analysis demonstrated microbes in anaerobic granular sludge (AnGS) achieved "defense-efflux" by activating macB/evgS efflux pumps while suppressing tetA, whereas membrane biofilm (MB) developed a "storage-retention" strategy through synergistic effects of tetA and evgS. This EGSB-AnMBR system exhibits promising application potential for swine wastewater treatment and antibiotic control, providing significant support for livestock pollution management.