Influence of biological pretreatment of coconut husk lignocellulosic biomass for biogas production on archaeal community and volatile fatty acids

Lignocellulosic agricultural residues such as coconut husk are abundant but recalcitrant substrates that limit efficient biogas production during anaerobic digestion. We evaluated the effect of biological pretreatment of coconut husk on archaea, volatile fatty acid (VFA) profiles, and biogas production during anaerobic digestion. Coconut husk was pretreated for 50 days using a consortium of cellulolytic bacteria isolated from poultry feces and subsequently digested alongside untreated husk (control) in batch reactors. Cellulose-degrading bacteria were presumptively characterized based on morphological, cultural, and biochemical characteristics and included Bacillus megaterium, Bacillus licheniformis, Lactobacillus plantarum, Bacillus cereus, and Bacillus subtilis based on morphological, cultural, and biochemical characteristics. Ten archaeal genera were detected across treatments, including Methanobrevibacter, Methanomassiliicoccus, Methanobacterium, Methanoculleus, Methanosarcina, Methanomicrobium, Desulfurococcus, Thermosphaera, Methanogenium, and Methanoregula. The control digestate exhibited higher archaeal diversity with a low methane production. The pretreated reactor (Ch + Cdb) produced a cumulative biogas yield of 88,355 mL gVS⁻1, compared with 67,105 mL gVS⁻1 in the control, representing a significant 1.32-fold increase (p ≤ 0.05). Methane production commenced on day 3 in Ch + Cdb relative to day 15 in the control indicating a shortened lag phase and enhanced breakdown of substrate. Mean methane concentration increased from 44.52 ± 25.4% in the control (Ch) to 56.38 ± 16.54% in the Ch + Cdb treatment representing a 1.27-fold increase in methane content relative to the control. Carbon dioxide concentration also increased from 11.41 ± 6.24% in the control to 15.01 ± 5.54% in the treated sample, corresponding to a 1.32-fold increase. Total VFA concentration was higher in Ch + Cdb at 596.43 mg L⁻1 than in the control (562.24 mg L⁻1), with acetic acid as the dominant component. Increased concentrations of acetic acid, propionic acid, and butyric acid in the treated system did not inhibit methanogenesis, suggesting improved intermediate conversion and process stability. Biological pretreatment enhanced hydrolysis, accelerated methanogenesis, and improved biogas yield with a stable reactor performance, and highlight the whole-cell biocatalyst potential of cellulose-degrading bacteria for optimum anaerobic digestion of lignocellulosic biomass.