Optimizing hydraulic retention time for methane production from the hydrogenic effluent left over from the co-digestion of vinasse and spent brewer's yeast cell

• 10-day HRT optimized methane production from hydrogenic effluent. • MPR of 853.6 mL/L·d was achieved at the optimal HRT. • Methane production from hydrogenic effluent could reduced 1.02 M tCO 2 eq yearly. • Shorter HRT favors Bathyarchaeia taxa in methane production. • Dual methanogenic pathways boost efficiency at shorter HRTs. This study aims to optimize the hydraulic retention time (HRT) for the methane production from hydrogenic effluent derived from the co-digestion of vinasse and spent brewer's yeast cells. The experiments were conducted in a continuous stirred tank reactor (CSTR) at various HRTs ranging from 60 to 5 days. The results showed that optimal performance was achieved at HRT 10 days. Under this HRT, yielding a maximum methane production rate of 853.6 mL/L·d and a methane yield of 304.9 mL/g-VS, with a COD removal efficiency of 53.86 %. The microbial community analysis revealed distinct patterns across different HRTs, with shorter HRTs (5–15 days) dominated by Bathyarchaeia-related taxa and Thermoplasmatota, while longer HRTs (30–60 days) showed the predominance of traditional methanogenic archaea within the Euryarchaeota phylum. The methane production process involved both acetoclastic and hydrogenotrophic pathways, with enhanced efficiency observed under shorter HRTs where both pathways coexisted. The greenhouse gas reduction potential analysis revealed that implementing this process could potentially reduce emissions by 1,026,206 tCO 2 eq/year through the substitution of fossil fuel-based electricity with methane-derived power.