New insights into the anaerobic digestion of high carbon wastewater with ciprofloxacin: Methane production and ARGs inhibition.

Ciprofloxacin (CIP), as a quinolone antibiotic, has broad-spectrum antibacterial properties and can affect methanogenic performance in anaerobic digestion (AD). While previous studies focused on synthetic wastewater, the fate of CIP in real distillery wastewater (RDW) and its impact on microbial adaptation mechanisms remain unclear (such as biotransformation pathways, population dynamics, and the enzymes involved) in RDW is largely unclear. In this study, we investigated AD performance, metabolic pathways, and antibiotic resistance gene (ARG) dynamics using real wastewater spiked with CIP (0.3-2 mg/L). Results indicate that 0.5 mg/L CIP (631.83 mL CH4/g·VS) enhanced the methane yield by 6.67 % (592.34 mL CH4/g·VS in control), correlating with upregulated enzyme in glycosis, TCA cycle, and methanogenesis (F420 increased). With full use of short-chain acids, transient volatile fatty acid (VFA) inhibition (≤ 50 mg/L on Day 3) was overcome by Day 10. Metagenomics revealed CIP promoted the production of stress proteins (e.g., cysteine synthase activity doubled). Furthermore, CIP (0.3-1 mg/L) suppressed mobile genetic elements (MGEs) encoding horizontal gene transfer, including isfinder (15.15 %) and integrases (6.25 %), while ARG and virulence factor abundances remained unchanged versus control. This study firstly shows that low-dose CIP in RDW increases methanogenesis via metabolic adaptation without exacerbating ARG risks. MGE suppression implies that CIP may lessen the possibility of ARG diffusion in AD systems. These results offer vital information for improving AD performance in the treatment of wastewater contaminated by antibiotics and developing methods to strike a balance between antibiotic removal and ARG control.