Browsing by Author "Njokweni, Sesethu Gift"

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    Exploring a rumen-modelled carboxylate platform for the conversion of agricultural wastes to green chemicals and fuels in South Africa
    (Stellenbosch : Stellenbosch University, 2021-03) Njokweni, Sesethu Gift; Van Zyl, Willem Heber; Botes, Marelize; Stellenbosch University. Faculty of Science. Dept. of Microbiology.
    ENGLISH ABSTRACT: The notion of a “biobased economy” in the context of a developing country such as South Africa (SA) necessitates research and development of technologies that i) utilize sustainable feedstocks ii) have simple and robust operation iii) could work at small scale and iv) produces a variety of valuable bioproducts, fitting the biorefinery concept. One of the technologies that have been gaining increased global interest is the carboxylate platform based on ruminants. This platform uses mixed/undefined anaerobic co-cultures of rumen microorganisms to convert a variety of organic biomass to volatile fatty acids (VFA). VFA are short-chain fatty acids with two to four carbon atoms, which include acetic acid (C2), propionic acid (C3) and butyric acid (C4). These fatty acids have a wide variety of applications including the production of hydrocarbon fuels through electrolysis. Technological modelling has shown that each VFA corresponds to the production of a mixture of alkane hydrocarbons when using Kolbe electrolysis that may range from methane to octane depending on the average chain length and yield of alkyl groups in the produced VFA. Therefore, in order to produce higher alkanes, there is a need to produce VFA with higher yields of alkyl groups. In turn, these yields depend on a variety of operational aspects and input feedstocks used that need to be optimized to make the process economically feasible. Some of the operational challenges that need to be addressed include directing the microbiome into the desired products; Optimization of fermentation parameters and developing strategies for the preservation of the core microbiome involved in the production of the VFA. This study aimed to assess the feasibility of utilizing the ruminal carboxylate platform in SA to convert locally abundant sustainable organic feedstocks to VFA towards hydrocarbon fuel production. The study was separated into two research chapters: 1) Optimisation of the in-vitro anaerobic rumen fermentation conditions and selection of ideal SA organic waste products including fruit wastes (apple, grape and citrus pomace) and invasive plants (Prosopis juliflora) in terms of degradability, VFA yield and utility to serve as substrates for electrochemical conversion to hydrocarbons. 2) Examine the potential of preserving the core ruminal microbiome involved in VFA production using the two most common cryogenic agents (glycerol and dimethyl sulfoxide) and analyze population dynamics to determine any shifts using β-diversity distance metrics. It was shown that all organic waste products have the potential to serve as substrates for electrochemical conversion with citrus pomace showing the highest utility with 136 mM concentration of VFA (fractional yield of 0.52 out of a possible 0.75) and 12.16 mmol alkyl.g-1 present for conversion. Comparison of fresh rumen with glycerol preserved rumen showed Similarity index (β-diversity scale of 0 to 0.8, with 0.8 being the most diverse) with unweighted unifrac between the communities was 0.359 while the comparison of fresh rumen and DMSO cryo-preserved rumen was 0.250. This showed that both glycerol and DMSO cryo-preserved the rumen fluid without dramatic shifts in the community but the DMSO cryo-preserved community better resembled the core microbiome on the fresh rumen inoculum. This was further supported by analysis of VFA production and digestibility of pomace substrates with the preserved microbiome showing significantly similar yields to the fresh rumen inoculum. The results of this study further highlighted the potential of using a rumen-based carboxylate platform as a small-scale alternative towards biofuels production.