Estimation of biomass feedstock availability for anaerobic digestion in smallholder farming systems in South Africa

Abstract

This paper assesses agricultural residues available for biogas production in smallholder farming systems in the KwaZulu-Natal and Limpopo provinces of South Africa, during 2017–2018. Biomass estimations were done using a combination of smallholder surveys, publicly available data, and literature models, and 11 residue types from 5 crops and manure from 5 types of livestock were estimated. During the investigated period, gross biomass generated from crop residue was 11 and 13 t per smallholder household, of which 63% and 84% are estimated as surplus in KwaZulu-Natal and Limpopo, respectively. The gross biomass generated from animal manure was 158 and 84 t per smallholder household in KwaZulu-Natal and Limpopo, respectively, of which 48% are estimated as surplus. Estimated biomethane potential from the surplus crop residue is approximately 1370 Nm3CH4 per year per smallholder household, equivalent to 49.3 GJ of heat energy. For methane, estimated potential from animal manure is 2105 and 1135 Nm3CH4 per year per smallholder household, equivalent to 76 and 41 GJ of heat energy, for KwaZulu-Natal and Limpopo, respectively. The estimated energy potential is higher than low-income South African households’ cooking (19.8 GJ y−1) and total (49.3 GJ y−1) energy demand, theoretically enabling the complete substitution of current domestic fuels. The total estimated digestate from all residues contains approximately 0.32 t nitrogen, equivalent to 0.70 t urea fertilizer, per year per smallholder household. The analysis indicates that anaerobic digestion based on agricultural residues can make a significant contribution to meeting energy and fertilizer needs of smallholder farmers in KwaZulu-Natal and Limpopo.