Abstract:
In West Africa, reliance on firewood for cooking contributes to deforestation and
household air pollution, while agro-industrial residues such as Okara and yam
peels are often discarded, creating environmental challenges. Valorizing these
wastes through anaerobic digestion (AD) offers a suitable alternative, though
mono-digestion of Okara is often unstable and can lead to low biogas yield.
Therefore, the objective of the present study is to evaluate the biogas potential of
co-digesting Okara with local lignocellulosic wastes such as yam peels.
Batch experiments were conducted under mesophilic conditions with three mixing
ratios (Okara:yam peels 1:3, 1:1, and 3:1) at substrate-to-inoculum (S/I) ratios of
0.5 and 0.6. The environmental implications of the best performing ratio were then
compared to landfilling using the ReCiPe 2016 Midpoint (H) method in OpenLCA
software, while biogas combustion impacts were contrasted with firewood use.
Credits from firewood substitution and digestate fertilizer displacement were also
considered in the net assessment.
Results confirmed the suitability of both substrates for AD due to their high
volatile solids and organic matter content. Mono-digestion yielded 610.58 mL/g VS
for Okara and 468.82 mL/g VS for yam peels. The co-digestion tests produced
between 609.10 to 661.00 mL/g VS, with the highest yield performed at the 3:1 mix
ratio. The findings highlight synergistic effects between nitrogen-rich Okara and
carbon-rich yam peels. The environmental assessment revealed that landfilling
1 kg of Okara and yam peels generated high burdens, particularly climate change
(0.55 kg/CO2-Eq) and terrestrial acidification, while AD significantly reduced
impacts. Replacing firewood with biogas further reduced emissions harmful to
human health and terrestrial ecosystems. Overall, AD reduces impacts and
became impact-avoiding when accounting for avoided burdens. Economically, the
system proved viable, with a benefit-cost ratio of 1.64 and a payback period of 3.20
years.
These findings support anaerobic co-digestion as a sustainable valorization route
for local bio-wastes in West Africa, offering simultaneously energy recovery,
environmental improvements, and economic benefits.
Description:
A Thesis submitted to the West African Science Service Centre on Climate Change and Adapted Land Use, the Université de Lomé, Togo, and the Universität Rostock in partial fulfillment of the requirements for the International Master Program in Renewable Energy and Green Hydrogen (Bioenergy/Biofuels & Green Hydrogen Technology)
