Microbial Diversity in the Active Slurries and Digestates of Selected Untreated and Alkali-pretreated Lignocellulose Substrates for Biogas Production

Abstract

This study aimed at assessing the microbial consortia during (active slurry) and after digestion (digestates) of selected lignocellulosic substrates used in biogas production. The substrates:   maize cob (MC), rice straw (RS) and water hyacinth (WH) were mechanically pretreated and the designated batches pretreated with alkali(10% NaOH) and loaded into 54 L capacity metallic batch anaerobic digesters. Codigestion was encouraged with the addition of cow rumen content (CR). The setups were allowed to run for 42 days under mesophilic conditions,  while stirring daily. Samples for microbiological analysis were collected on the 28th (for active slurry) and 42nd (for digestates) days respectively. Standard analytical methods were adopted. Higher total heterotrophic counts and diversity was recorded in the digestates than in the active slurries. The bacterial isolates include: Staphylococcus aureus, Enterococcus faecalis, Shigella sonnei, Bacillus cereus, Escherichia coli, Bacillus subtilis, Pseudomonas aeruginosa, Micrococcus luteus, Micrococcus roseus, Enterobacter aerogenes and Salmonella enteric while the fungal isolates include: Fusarium solani, Aspergillus niger, Saccharomyces cerevisiae and Penicillium notatum. The trend of biogas generation showed maximum displacement between the 28th – 32nd days for most of the setups, with the highest yield per day (858.36 ml) recorded in MC+WH+CR+AA (Alkali Augmentation) and the least yield (169.578 ml) in the untreated RS+CR. The slight increase in counts within the last week of digestion may be attributed to the nature of the substrates, extended hydrolysis rate and the fact that large populations of anaerobic and facultative anaerobic organisms are involved in the successive phases of anaerobic digestion.