alorization of Okara (Soy pulp) and yam peels for bioenergy: effect of anaerobic co-digestion on biogas generation efficiency

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.