Browsing by Author "Mossissa, Amsalu Tolessa"

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    Development of a techno-economic analysis tool for anaerobic digestion in smallholder farming systems in the context of the water-energy-food nexus
    (Stellenbosch : Stellenbosch University, 2022-04) Mossissa, Amsalu Tolessa; Goosen, Neill Jurgens; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Anaerobic digestion (AD) technology holds numerous potential benefits for the agricultural sector; however, challenges persist in terms of implementation and sustainability in the majority of the low- and middle-income countries. The main barriers to AD implementation in Sub-Saharan African countries include financial and technical factors. Implementation costs appear to remain financially prohibitive as many rural African households and smallholder farmers cannot afford the high initial investment costs of installing and maintaining AD plants. The technology also suffers shortcomings such as incomplete bioconversion, low methane yields, process instability and economic non-viability. A systematic evaluation tool that integrates both technical and economic performance will help smallholders and supporting governments identify an appropriate solution to a specific context. The first objective of this research project was to quantify the availability and potential of agricultural residue biomass feedstock for biogas and biofertilizer production in South African and Madagascan smallholder farming systems. The biomass estimations were done using a combination of smallholder surveys, literature models, and publicly available data. Annually, South African emerging smallholder farming households generate 12 and 121 tonnes of crop residues and animal manure, respectively. Agricultural households in Madagascar generate 7.3 and 19.4 tonnes of crop residues and animal manure on a fresh weight basis annually, respectively. The second objective was to assess the potential small-scale AD technologies to be used in smallholder farming systems using multi-criteria decision analysis (MCDA) methods. The Simple Multi-Attribute Rating Technique and the Analytical Hierarchy Process approaches of MCDA were used as a decision support tool, and the preferred AD technology (DIY Biobag digester design model) was selected from a list of potential small-scale AD technologies. Following completion of the first two objectives, objective three was included to establish the best co-digestion strategy for the potential agricultural substrates under different operating conditions using a combination of the Anaerobic Digestion Model No. 1 (ADM1) and a Monte Carlo approach for probabilistic simulation with the inclusion of parametric uncertainty. Results indicate that co-digestion increases mean methane content by up to 8% compared to mono-digestion of individual substrates. The resulting methane production probability distributions were fitted using a Gaussian Mixture Model, which can be used to account for uncertainty in techno-economic feasibility assessments. Despite all its potential advantages for farmers, AD technology implementation involves capital and operational costs to purchase, install and operate. Therefore, the fourth objective was to develop and demonstrate an analysis tool for techno-economic feasibility assessment, explicitly centred in the water-energy-food nexus context to evaluate the feasibility of anaerobic digestion. To this end, the co-production of biogas and biofertilizer resulted in the most sustainable solution in both financial and economic analysis, with a mean financial and economic rate of return of 14% and 67% and benefit-cost ratio of 1.40 and 5.97 respectively under different scenarios. Further work would be required to focus on the implementation and application of this methodology in other AD projects where a detailed analysis on the site for a particular application is required in the nexus context as resource availabilities of a specific village can vary. This could be done to maximize economic and/or environmental sustainability and contribute to rural economic development.