The Bacteriological Index of Bioslurry and the Fate of Pathogenic Bacterial Organisms during Anaerobic Digestion of Domestic Waste in a Biogas Plant

Abstract

The bacteriological index of bioslurry and the fate of pathogenic bacteria during anaerobic digestion of domestic waste were determined. The wastes (food waste, vegetable waste, plantain peels, yam peels, and cow dung) were collected from households and markets within the Cross River University of Technology, Clalabar, Nigeria, and the analysis conducted at the Microbiology Department of the University. Bacteriological index was examined by enumerating the total heterotrophic bacterial (THB) count and bacterial diversity during the digestion period using viable count method on nutrient agar plate. The fate of pathogenic bacteria was analysed at 2 week interval for a period of 28 days using Salmonella and Shigella species as a case study. Results showed that the THB count decreased (1.8x1010CFU/ml – 6.3x108CFU/ml) over the 28 day period of anaerobic digestion. The isolation and identification of different bacterial species associated with anaerobic digestion of waste revealed the presence of aerobic, facultative anaerobic and anaerobic bacteria in Days 1, 14 and 28 respectively. Pseudomonas spp, Bacillus spp, Lactobacillus spp, Klebsiella spp, Proteus spp, Escherichia coli and Staphylococcus spp were among the organisms isolated on Day 1, which indicates that the initial microbial hydrolytic activities on the waste materials are mediated by aerobic and facultative anaerobic bacteria. The presence of Staphylococcus spp, Enterococcus spp, Peptostreptococcus spp, Micrococcus spp and Fusobacterium spp were present in the sample analysed on Day 14 showing that the digester was becoming anaerobic. Isolation of Propionibacterium spp, Listeria spp, Erysipelothrix spp and Clostridium spp on Day 28 showed that the digester has turned anaerobic, the stage at which biogas is produced. The result of the fate of pathogenic bacteria revealed that Salmonella and Shigella species decreased with time during the digestion process, with complete die off at Day 21. These indicate that anaerobic digestion enhances pathogen die off and could be applied as a waste treatment option in an integrated waste treatment management. A study on the metagenomics of the bioslurry will further reveal the uncultured and genomic diversity of associated microorganisms during anaerobic digestion.