Temperature is more important than solid retention time in biogas production and resistome dynamics in anaerobic digestion with recuperative thickening

Abstract

Temperature and solid retention time (SRT) are major parameters influencing performance and removal of antibiotic resistance genes (ARGs) during anaerobic digestion (AD). Recuperative thickening (RT) has been applied to isolate the effect of SRT from hydraulic retention time; however, RT-AD remains less explored under thermophilic conditions. The current study investigated the effects of SRT alone on AD performance using RT under both mesophilic and thermophilic conditions, microbial communities, functional profiles, and ARG abundances. Four conditions were tested: Meso-C (mesophilic, SRT 20 d), Thermo-C (thermophilic, SRT 20 d), Meso-R (mesophilic, SRT 40 d), and Thermo-R (thermophilic, SRT 40 d). Thermophilic conditions and extended SRT enhanced biogas production by extending microbial retention times and removing inhibitory materials. As a result, Meso-C had a biogas yield of 304.5 mL/g-VSadded, while Thermo-C yielded 320.3 mL/g-VSadded. Extended SRT improved biogas yield by 9.3 % (Meso-R) and 43.9 % (Thermo-R) compared to their controls. Temperature had a stronger influence than SRT in shaping microbial composition and functional profiles. Bacteroidetes dominated under mesophilic conditions, utilizing carbohydrate and carboxylate degradation, whereas Coprothermobacterota dominated thermophilic conditions with proteogenic amino acid degradation. Temperature also affected ARG-host microorganisms, influencing ARG composition. Moreover, low microbial diversity was observed with extended SRT under mesophilic conditions, and short SRT under thermophilic conditions improved ARG removal efficiency, limiting ARG transfer potential. Consequently, ARG removal efficiencies were higher in Meso-R (57.0 %) and Thermo-C (68.5 %) compared to Meso-C (55.2 %) and Thermo-R (59.0 %), respectively. These results underscore the dominant role of temperature over SRT in AD performance and ARG removal.