Enrichment of antibiotic resistance genes in full-scale, two-phase co-digestion plants treating livestock and food waste

Antibiotic resistance genes (ARGs) have emerged as contaminants of growing concern in anaerobic co-digestion (AcoD) processes. The present work investigated the occurrence and transformation of 27 ARGs and the class 1 integron-integrase gene (intI1) across three full-scale, two-phase (acidogenic-methanogenic) AcoD plants treating livestock waste (LW) and food waste (FW). The majority of acidogenic reactor samples showed residual gene fractions (RGFs) of total ARGs exceeding 1.0, indicating an enrichment in absolute abundance. For example, certain samples exhibited markedly elevated RGFs of 84.49 and 88.57 for tetW and ermB, respectively. A notable rise in ARG levels was found in the methanogenic reactor, with tetW and ermB exhibiting RGFs of 416.59 and 268.51, respectively. Interestingly, the net shift in total ARGs from influent exceeded 1.0 across all plants. Based on the relative abundance of ARGs and intI1, a highly positive and significant association (r > 0.9, p < 0.01) was observed between intI1 and certain ARGs; however, several ARGs with increased relative abundance showed no correlation with intI1. Collectively, a full-scale, two-phase AcoD process may not sufficiently mitigate ARG proliferation and could serve as potential ARG reservoirs. To clarify this risk, long-term monitoring and more in-depth investigation of the behavior of ARGs within the two-phase AcoD are needed. Additionally, the development of a post-treatment process, such as the pyrolysis of anaerobically digested sludge, appears to be necessary for the effective removal of ARGs from the effluent of two-phase AcoD plants.