Temporal Succession of Cultivable Bacteria, Fungi and Presumptive Methanogens During Mesophilic Anaerobic Digestion of Fruit Peels and Cow Dung in Calabar, Nigeria

Anaerobic digestion can recover energy and nutrients from organic wastes, but the temporal behaviour of cultivable microorganisms determines both process performance and digestate safety. This study characterised bacterial, fungal and presumptive methanogenic succession during 45-day mesophilic batch digestion of banana, paw-paw and watermelon peels, cow dung, and their combinations in Calabar, Nigeria. Twelve 60-L digesters were monitored, and slurry samples were cultured at five-day intervals. Isolate-occurrence richness and Shannon diversity were calculated from the archived culture records. Bacterial occurrences declined from 67 on day 5 to one on day 45, while richness fell from 11 to one and Shannon diversity from 2.33 to 0.00. A Clostridium perfringens-like morphotype represented 26.70% of bacterial occurrences and remained detectable through day 45; enteric and opportunistic morphotypes, including Escherichia coli, Enterobacter aerogenes and Pseudomonas aeruginosa, were not detected after day 25. Fungal occurrences declined from 48 to zero by day 35, although yeast persisted through day 30. Presumptive methanogen occurrence peaked on day 30. The findings show strong ecological succession but incomplete hygienisation under mesophilic conditions. Digestate intended for food-crop production should therefore receive microbiological verification and, where necessary, post-treatment before field application.