Anaerobic mixed culture processes for bio-waste valorisation

Anaerobic mixed culture processes have great potential for bio-waste valorisation, as they convert organic matter to a range of value-added compounds. Of particular interest is acidogenic fermentation that accumulates the natural intermediates of anaerobic digestion, an already established technology. Delivering product selectivity is a challenge with complex feedstock, especially when aiming for cost-effective waste management by minimising chemical addition and operational complexity. This thesis explores operating strategies to direct fermentation of food waste towards medium chain carboxylic acids (MCCA) in single-stage stirred tank reactors. MCCA are generated via microbial chain elongation and have a higher value than other fermentation products. The substrates for chain elongation, i.e., volatile fatty acids and lactic acid or ethanol, are co-generated during food waste fermentation. A literature review on chain elongation by mixed cultures with complex organic feedstock identified a range of operating conditions and defined the research objectives of this thesis. Firstly, an operating strategy involving organic overload to drive an anaerobic digestion community to acidogenic fermentation in a semi-continuous single-stage stirred tank reactor was evaluated. Start-up at higher feed-to-microbial ratio (>5 gCODfed gVSinoculum-1) and organic loading rates (OLR) (8.5 gCOD L-1 d-1), compared to a parallel anaerobic digester (<1 gCOD gVS-1, 4.2 gCOD L-1 d-1), inhibited methanogenesis and produced volatile fatty acids. Chain elongation was stimulated by switching to a feedstock with higher organic content to give an OLR of up to 21 ± 2 gCOD L-1 d-1 with the same hydraulic retention time (HRT, 14 days). MCCA were produced at similar concentrations to more complicated reactor systems (22 ± 4 gCOD L-1 for n-caproic and 7 ± 2 gCOD L-1 for n-caprylic acid). A specialised community formed that showed in situ lactic acid production followed by chain elongation. The high-COD food waste deactivated methanogenesis and biogas production in anaerobic digestion, suggesting MCCA production is a better use of this feedstock. The OLR is determined by the organic strength of the food waste and the applied hydraulic retention time (HRT), hence the impacts of these parameters were evaluated. The main product from food waste fermentation at 12 gCOD L-1d-1 OLR and 8.5 day HRT was n-butyric acid (13 ± 2 gCOD L-1). Operating at the same 8.5 day HRT but at 20 gCOD L-1d-1 OLR resulted in lactic acid accumulation (34 ± 5 gCOD L-1). This is a similar OLR and lower HRT to the system that stimulated chain elongation in previous experiment. An OLR of 12 gCOD L-1d-1 and a higher HRT of 10.5 days, stimulated chain elongation (n-caproic acid up to 13.6 gCOD L-1). The microbial community was determined by the operating conditions and these together determined product profiles. Longer HRT resulted in greater abundance of lactic-acid producing genera such as Olsenella spp., known to aid chain elongation, and secondary fermenters such as chain elongating species. Operating at higher OLR led to greater abundance of the homolactic genus of Lactobacillus. Hence, the reactor operating strategy can direct product synthesis. This shows the potential for a biorefinery with a flexible product portfolio improving commercial viability and presents an opportunity to repurpose existing single-stage AD systems by adjusting operating strategy. The literature review also suggested the potential of a semi-continuous feeding pattern to stimulate the consecutive fermentation steps required for MCCA. Hence, bi-weekly and daily feeding patterns were compared. Daily feeding resulted in a less stable process due to lactic acid accumulation, which acidified the reactor and required more pH correction. Bi-weekly feeding resulted in higher ethanol and n-caprylic acid yields. Analysis of the microbial communities and their correlation with product formation, coupled with fermentation pathway analysis, revealed a competitive interaction between homolactic Lactobacillus and a consortium of primary fermentation bacteria producing ethanol, acetic and lactic acid with secondary fermenters performing chain elongation. With daily feeding the homolactic Lactobacillus had a competitive edge. Thus, the work reveals competitive and syntrophic interactions in the mixed culture fermentation of food waste. Understanding these provides a route to optimise process design and targeted products. An effective waste management system requires a stable outcome regardless of natural fluctuations in the feedstock. Variations of the food waste collected during the project and their impact on fermentation was assessed via batch reactor studies. Fermenting food waste as employed in anaerobic digestion recycling centres led predominantly to carboxylic acid formation. By contrast, fermentation of fresh cafeteria food waste mainly generated lactic acid. Feedstock storage and pretreatment was shown to affect fermentation and, therefore, the necessary optimal operating conditions. Batch studies also evaluated the impact of sucrose supplementation of the feedstock. Sucrose was found to destabilise fermentation and confirmed the competition between MCCA and lactic acid as the main product. Finally, immiscible, and low density oils present in some food wastes promoted partitioning and concentration of MCCA from the aqueous fermentation broth. This leads to interesting opportunities for utilising oily feedstocks and enhancing downstream processing. The thesis concludes by proposing the necessary operating conditions to direct acidogenic fermentation towards MCCA production in a simple one-stage reactor configuration, such as existing anaerobic digestion assets. Proposals are made for advancing this research to develop a bio-waste valorisation technology that allows sustainable resource recovery contributing to a circular bio-economy.