ABSTRACT

The microbial profile during the anaerobic co-digestion of cassava peels and poultry droppings for biogas production was investigated. The aim was to evaluate the biogas production from mono and co-digestion of cassava peels and poultry droppings using cattle rumen fluid as inoculum, while characterising the microbiota extensively. To achieve this, four biodigesters of 20 liter capacity with gas collection systems were designed and constructed. Fresh cassava peels were collected from a cassava processing unit. Twenty-four-hour-old poultry droppings were collected from a poultry farm at Ahiaeke. These served as the feedstock. The rumen content was collected from freshly killed cattle at Ubakala abattoir. All were taken immediately after collection in sterile containers to the laboratory for analyses. The substrates were prepared and charged up to three-quarter of the biodigesters’ volume. An inoculum to substrate ratio (ISR) of 2:1 was employed. The experimental setup was operated under the batch-feed system at a hydraulic retention time of 45 days. The cattle rumen content was also used as the control. The proximate composition and physicochemical analyses were evaluated using standard methods, while the microbiota were characterised using the metagenomics approach. The quantity and components of biogas produced were determined using the water displacement method and gas chromatography respectively. The proximate composition analysis showed the substrates were good feedstock for biogas generation. The results of physicochemical analysis of the process parameters, gave a pH range of 4.5 to 7.5 and a temperature range of 30⁰C to 35⁰C throughout the digestion period. The Total Solids (TS) contents decreased from 87.64%, 86.00%, 89.35% and 80.98% to 61.00%, 60.25%, 66.75% and 59.77% while Volatile Solids (VS) contents decreased from 82.48%, 77%, 79.85% and 69.95% to 55%, 35.89%, 47.28% and 40% for (CP: RC), (PD: RC), (CP: PD: RC) and (RC) digesters respectively. The cumulative mean biogas yield from the four digesters where 1203.37 mL, 1929.34 mL, 1738.69 mL and 1978.26 mL (CP: RC), (PD: RC), (CP: PD: RC) and (RC) respectively. Gas chromatographic analysis of the biogas composition showed that the methane content was the highest amongst other gases present. Methane content ranged from 64.12% in the control (RC) digester to 52.35% substrate blend (CP: RC) digester. The metagenomic analysis identified 3 kingdoms, 18 phyla, 30 classes, 52 orders, 100 families, 165 genera and 222 species of the microbiota profile. The result revealed that the most abundant phylum was Firmicutes, most abundant class was Bacilli and Lactobacillales were the highest abundant order. The archaea community were less diverse compared to those of bacteria. The metagenomic result identified three classes of methanogens (archaeal kingdom) as methanobacteria, methanococci and methanomicrobia, three genera as Methanococcus, Methanosarcina and Methanobrevibacter and four species as Methaoncoccus maripaludis, Methanosarcina mazei, Methanosarcina acetivorans and Methanobrevibacter spp. One-way Analysis of variance (ANOVA) for the physicochemical parameters indicated a significant difference (P ≤ 0.05) in all the treatments. The results prove that the anaerobic co-digestion of cassava peels and poultry droppings can be used to produce biogas while giving a deep insight into the microbial community present.