Estimation of biomass feedstock availability for anaerobic digestion in smallholder farming systems in South Africa
2020
Tolessa, Amsalu | Zantsi, Siphe | Louw, Tobias M. | Greyling, Jan C. | Goosen, Neill J.
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.
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