The new strategies for high efficiency removal of antibiotics and antibiotic resistance genes by direct bio-drying of biogas slurry: Microbiological mechanisms

High levels of antibiotics and antibiotic resistance genes (ARGs) still exist in biogas slurry after anaerobic digestion of cow manure. In this study, direct bio-drying strategies of cow manure biogas slurry without solid-liquid separation for the removal of antibiotics and ARGs were explored. The results showed that, after direct bio-drying of biogas slurry, the moisture contents decreased to 25.2 %–31.5 %. The maximum temperatures of the piles reached 76.1–77.4 °C, which is close to ultra-high temperatures (>80 °C). Direct biogas slurry bio-drying (CK treatment) achieved efficient removal of antibiotics, ARGs, and mobile genetic elements (MGEs) (95.4 %, 98.6 % and 86.7 % removal, respectively). Compared to the CK treatment, molecular membrane covering (MMC) alone was the most effective in further significantly decreasing the antibiotic concentration and the abundance of ARGs and MGEs in the final bio-dried samples, followed by food waste hydrochar (FHC) addition alone. Methanogenic archaea were identified as potential hosts for ARGs based on Network analysis. FHC addition–MMC increased the abundance of potential hosts for ARGs and promoted the expression of microbial methane metabolism function relative to the CK treatment during the later stages of bio-drying, thereby decreasing the removal efficiency of ARGs. The results of structural equation model and redundancy analysis showed that MGEs had the most significant direct effect on ARGs and moisture content had the highest relative contribution to changes in ARGs. In summary, direct bio-drying strategies were able to efficiently remove antibiotics and ARGs from cow manure biogas slurry and also achieve biological dewatering of the biogas slurry.