Abstract:
Co-digestion of water hyacinth (WH) (E. crassipes) with ruminal slaughterhouse waste (RSW) has the
potential for improving biogas production by complementing process parameters. This study evaluated
microbial communities in co-digestion of WH with RSW at 32˚C by isolation, phenotype analysis, DNA
extraction and PCR amplification of the 16S rRNA genes. Digestion of WH and RSW separately
exhibited lag times of 5 to 20 days that were associated with pH drop to acidic zone. The pH drop was
attributed to rapid production of volatile fatty acids by acidogenesis process without corresponding
consumption by methanogenesis. Co-digestion at 30% RSW maintained alkaline pH and increased
biogas yield for WH from 47 to 95% of the 42.1 L CH4/kg observed for RSW suggesting synergy in the
co-digestion. Morphologies of colonies isolated from the reactors were dominated by short and long
rods bacilli with some cocci, and streptococcus mainly in WH samples. About 77% of the isolates were
Gram positive, suggesting dominance of Firmicutes phyla that includes Bacillus genus. Molecular
analysis observed a shift in microbial community during the acidic lag phase from Bacillus genus to
acetogenic bacteria Lysinibacillus and Solibacillus genera that consume volatile acids increasing pH.
Recovery of alkaline conditions resulted in emergence of diverse species of Bacillus sp. associated
with fermentation and syntrophic processes that included Bacillus aerophilus, Bacillus pumilus,
Bacillus cereus, Bacillus thuringiensis, Bacillus licheniformis, Bacillus glycinifermentans, and Bacillus
aquimaris. Co-digestion of WH with RSW collated processes parameters and constrained drop in pH to
alkaline levels conducive for growth of Bacillus sp.
