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Title: Anaerobic fermentation of organic wastes for chemical production by undefined mixed microbial cultures
Author: Bolaji, Efeoluwa Omotola
ISNI:       0000 0004 7228 7282
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2018
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Utilising organic wastes for energy and chemicals production as alternatives to petroleum-based resources is a strategy to tackle the global dependence on fossil resources. Conventional biological processes for producing biobased chemicals and fuels are challenged by high process costs hence alternative routes are being considered. This research investigates the anaerobic mixed microbial fermentation (MMF) of organic wastes without chemical or physico-chemical pre-treatment. The techno-economic assessment of the role of anaerobic digestion (AD) for energy and chemicals production showed that 17-20% of the global energy consumption could potentially be met by AD. AD can potentially produce carboxylic acids at rates which are hundreds or thousands of times their current production rates while ethanol and hydrogen can be produced at lower rates. Laboratory experiments were conducted in lab-scale batch and semi-continuous reactors (SCRs) using three distinct substrates (microcrystalline cellulose, vegetable and salad waste (VSW), and pot ale). The study investigates the impact of process conditions on the efficiency of MMF processes. The results showed that the MMF of cellulose required prolonged retention time due to the recalcitrant nature of the substrate. Cellulose degradation only occurred after an extended period of microbial acclimation, with the extent of degradation ranging between 43 and 77% in the batch tests while the maximum degradation was 60% at 80 days SRT in the SCR. Acetate was the only fermentation product produced in substantial amount whereas methanogenesis dominated in the semi-continuous experiments. With VSW as the substrate, the maximum total product concentration in the buffered and unbuffered batch reactor was 43.3 and 18.5 g COD/l; the resulting yield being 62 and 27% (COD/COD) respectively. Substrate degradation was higher in the buffered SCR compared to the unbuffered-acidic reactor, but the product yield was similar due to biogas production in the buffered reactor. Acetate and butyrate were the common fermentation products at all investigated conditions followed by caproate although this was only detected at 20 and 30 days SRT. Similarly, the MMF of pot ale at prolonged SRT (10 and 20 days) and pH 5.0-6.0 tended towards the production of butyrate and caproate as the dominant products. For the batch reactors, the maximum product yield ranged between 24-50% (COD/COD). The substrate conversion and product yield efficiency increased with SRT in the SCRs. Maximum yield of 51% (COD/COD) was observed at feed pH 10.0 and 10 days SRT. In conclusion, the study demonstrated that the efficiency of a MMF process is influenced by the nature of the substrate. Therefore, it is necessary that the process conditions in MMF processes be adapted to the type of the substrate used.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Waste products as fuel ; Organic wastes ; Sewage