Anaerobic co-digestion of sweet potato and dairy cattle manure: A technical and economic evaluation for energy and biofertilizer production

Abstract

Energy from biomass is essential for sustainable agricultural development. The generation of biogas and bio-fertilizer from the anaerobic co-digestion is fundamental in mitigating environmental problems and in the local generation of electricity. In this context, this study examines the technical and economic feasibility of the effects of co-digestion of sweet potato with dairy cattle manure on energy and nutrient recovery using semi-continuous digesters. The use of sweet potato in co-digestion is characterized as an innovation in the technological process of anaerobic digestion, the application of which on small and medium-sized farms is still a challenge in emerging economies. The assay was conducted using different proportions of sweet potato mixed with dairy cattle manure, which produced a linear increase in methane yield with concentrations of 63–65.5% and biogas capable of generating 2376.44 kWh d−1. In addition to the generation of cleaner electrical energy, the co-digestion process produces a bio-fertilizer with a potential production of 26.53 tons/year of N, 20.45 ton/year of P and 23.6 tons/year of K. These results are superior to the mono-digestion process. Regarding the necessity of using sweet potato, this new system of production was evaluated for a model medium-size farm dedicated to dairy production in the countryside of the state of São Paulo in Brazil. The results demonstrate the economic feasibility of the investment in the five analysed scenarios, as the internal rate of return varied between 46.8% and 57% and the payback was achieved in 2–3 years. This research demonstrates that the innovation of using sweet potato as an additional input to the digestion process can increase the sustainability of small and medium-sized dairy farms in a manner consistent with a circular economy.