Biomethanation of Napier Grass Mono-digestion in Single-Stage Anaerobic Completely Stirred Tank Reactors Seeded with Cow Manure and Anaerobic Sludge

This study aimed to evaluate methane productivity of Napier grass mono-digestion in single-stage anaerobic completely stirred tank reactors (CSTR) inoculated differently by cow manure (COM), anaerobic sludge (ANS), and a mix of COM and ANS for CSTR1 to 3, respectively. The different inoculation promoted operational performance, stability, and organic loading rate (OLR) differently. The produced biogas and methane varied significantly with OLR (p < 0.05). The maximum gas productivities between 0.42–0.48 L-biogas/g VSadded and 0.27–0.34 L-CH4/g VSadded were obtained at OLR of 1.0–1.5 g VS/L day. Later, the increase in OLR to 2.0 and 2.4 g VS/L day decreased biogas and methane yields by 24.85–39.65% and 11.66–52.89%. Co-inoculation of COM and ANS was a promising strategy that encouraged more stability in CSTR3 than the other reactors. Consequently, the methanogenic performance was developed in all digesters, depicted clearly by high methane content between 52.94 and 59.30% in produced biogas. A similar microbial structure was found in each CSTR-dominated predominantly by Proteobacteria, Bacteroidetes, and Formicates. About 0.5–3.0% of the whole populations were archaea identified distinctly to Methanocorpusculum, Methanospirillum, and Methanosaeta. This finding confirmed the importance of seed inoculation to practical gas productivity and confined organic loading capacity of Napier grass biomethanation in single-stage wet and mono-substrate digestion processes.