Abstract
The operation of household scale anaerobic digesters is typically based on diluted animal dung, requiring stabled livestock and adequate water availability. This excludes many rural households in low-income countries from the benefits of a domestic biogas digester. Solid state anaerobic digestion (SSAD) can be operated with low process water demands, but the technology involves operational challenges, as e.g., risk of process acidification or low degradation rates. This study aimed at developing simple methods to perform SSAD of yam peelings in low-tech applications by testing different inoculation strategies and evaluating the necessity of dung addition as a supportive biomass. In initial lab scale trials 143 ± 4 mL CH4/g VS (volatile solids) were obtained from a mixture of yam peelings and dung digested in a multi-layer-inoculated batch reactor. In a consecutive incubation cycle in which adapted inoculum was applied, bottom inoculated digesters loaded without dung reached a yield of 140 ± 16 mL CH4/g VS. This indicates that SSAD of yam peelings is possible with simple inoculation methods and dung addition is unnecessary after microbial adaptation. A comparison with a conventional fixed dome digester indicated that SSAD can reduce process water demand and the digester volume necessary to supply a given biogas demand.
Highlights
Anaerobic digestion of organic waste or agricultural residues can be performed in technically simple digesters
Both solid inoculum and yam peelings are above the favorable chemical oxygen demand (COD)/N range for anaerobic digestion (AD) processes of 20–30 [19], while cow dung had COD/N ratio below that range
The biomethane potential (BMP) of the solid inoculum indicates that a considerable amount of convertible solids in the straw was still present after the incubation with sewage sludge
Summary
Anaerobic digestion of organic waste or agricultural residues can be performed in technically simple digesters. Users of such digesters can benefit from the obtained biogas as fuel for cooking while the digestate can be applied as fertilizer [1,2,3]. A successful operation of a domestic anaerobic digester depends on the availability and ease of supply of substrates and water. The net benefits for the users/owners can depend strongly on the workload associated with the digester in particular with water supply [3]. In Africa, only about 26,000 digesters are in operation [5], while many households are facing problems with energy supply as the availability of traditional fuels such as firewood is declining [8]
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