Enhanced Anaerobic Digestion of Cotton Stalk Residues Using Rumen Microbiota as Hydrolytic and Fermentative Inoculum for Renewable Energy Production

Abstract The valorization of the organic content of cotton stalk via anaerobic digestion is a challenging issue, due to the high content of lignocellulose. By adding rumen fluid, biogas production increased by 6.6–16.1-fold compared to cotton stalk digestion without rumen fluid. Biogas production at the end of the incubation period was equal to 493 ± 85.20, 281 ± 43.90 and 203 ± 45.80 mL/g VS added at 4%, 6% and 12% w/v cotton stalk in rumen fluid, respectively, while in the control (without rumen fluid) was 30.60 ± 10.20 mL/g VS added . Operating Continuous Stirred-Tank Reactors (CSTRs) under an Organic Loading Rate (OLR) of 1 g VS/L.day, high biogas production of 0.70 L/g VS added (0.45 L CH 4 /g VS added ) was recorded at 5–7% w/v cotton stalk in rumen fluid. In the CSTR treating cotton stalk with rumen fluid (R cot ), acetate and propionate were detected at all OLR. In the R cot , high acetate and propionate concentrations were measured at OLR of 1 g VS/L.day, which gradually decreased by increasing OLR to 3 g VS/L.day. Candidatus Cloacimonas (24.58 ± 0.90%) dominated the R cot , whereas syntrophic bacteria reached up to 50% of the bacterial population. The acetoclastic Methanothrix predominated archaeal population, accounting for 85.60 ± 2.49% of the archaeal abundance.