From Biodiesel Byproducts to Biogas: A Circular Economy Review of Oilseed Cakes, Glycerol, and Microalgal Biomass

Abstract

The global transition toward renewable energy has accelerated biodiesel production, generating substantial quantities of byproducts including oilseed cakes, crude glycerol, and microalgal biomass residues. Conventional disposal of these byproducts poses environmental challenges, yet they represent valuable feedstocks for biogas production through anaerobic digestion. This review synthesizes current knowledge on the valorization of biodiesel byproducts for biogas generation within a circular economy framework. Oilseed cakes—particularly palm kernel, rapeseed, jatropha, and sesame—exhibit significant biogas potential due to their nutrient-rich composition, with methane yields enhanced by 14–226% through co-digestion strategies. Crude glycerol, a major byproduct constituting approximately 10% of biodiesel output, demonstrates remarkable synergy as a co-substrate, improving biogas yields when maintained at concentrations below 8% v/v to avoid inhibition. Microalgal biomass residues, representing 60–70% of total biomass after oil extraction, offer additional energy recovery through anaerobic digestion, with co-digestion of rapeseed oil cake and microalgae achieving methane contents of 62.42% and energy yields of 3.76 kWh·kg⁻¹ DM. Nigeria's position as a major palm oil producer and emerging biodiesel market presents significant opportunities for integrating biodiesel byproduct valorization into the national energy mix. However, challenges including substrate C/N ratio imbalances, inhibitory compounds, limited technical capacity, and weak policy frameworks must be addressed. This review identifies critical research gaps and proposes a strategic framework for implementing biodiesel byproduct-to-biogas systems in Nigeria, contributing to waste reduction, energy security, and sustainable agricultural practices.