New insights into the effects of dairy manure collection method on the reduction of antibiotic resistance genes in anaerobic digestion.

In large-scale dairy farms, flushing wastewater are directed either into municipal pipelines or manure collection pipeline. These distinct collection methods result in varying concentrations of manure in collection tanks. However, the impact of manure substrate concentration on the reduction of antibiotic resistance genes (ARGs) during anaerobic digestion (AD) remains unclear. To address this gap, a 2-factor experiment involving temperature (37°C and 55°C) and dairy manure concentrations (low and high) was conducted, where manure sampling from collection tanks in 2 same scale farms. The results indicate that the abundance of ARGs in low-concentration substrates (5.15 × 10-1 and 4.64 × 10-2 copies/16S copies) was significantly lower than in high-concentration substrates (1.75 × 10° and 2.66 × 10° copies/16S copies) after AD. Statistical analysis revealed that the effect of substrate concentration on ARG reduction was more significant than temperatures. Further analysis using partial least square models revealed that physicochemical parameters especially NH4+-N levels associated with substrate concentration, played a greater role in influencing the variations of ARGs and their bacterial hosts than temperature. In summary, the study highlights the importance of manure collection method in influencing ARG reduction during AD processes. Combining wastewater with manure collection appears to be more effective in reducing ARGs, while high-concentration manure may require pre-treatments such as dilution and ammonia recovery to mitigate the risks of ARG spread.