Anaerobic microbial hydrolysis and fermentation of food waste for volatile fatty acid production
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Landfill, incineration, compositing and anaerobic digestion (AD) are the principal food-waste (FW) treatment methods used in the European Union. Because of the EU landfill directive and waste-management policies on organic wastes, however, the landfill approach is no longer a sustainable strategy. The incineration of FW is generally perceived to be energy demanding and inappropriate because of the high water content (>70%) of FW. Composting and AD both fit well in the “3R” waste management hierarchy and are therefore the most appropriate strategies for FW treatment. AD is more attractive than composting, however, due to its ability to stabilise FW and to generate valuable end-products such as organic acids, biogas and fertilisers. The use of FW as sustainable feedstock for the production of these valuable products through AD processes would contribute to reduce the green house gas emission and could enable to meet the EU 2020 renewable energy target; it could also enable an increase in chemical supply. However when dealing with mixed feedstock such as FW, AD process for methane production as the sole beneficiary product is usually less attractive. An alternative approach to the anaerobic digestion of this type of biomass is to aim for production of organic acids which have higher added value than methane. The sustainability of this approach depends on the extent of FW stabilisation, however, as well as on the yields, rates and profiles of the organic acids that are generated. Although FW is generally regarded as being readily biodegradable because of its high volatile solid fraction (90% of total solids), its hydrolysis is still perceived as a rate-limiting step. The enhancement of the hydrolysis step during anaerobic digestion could improve the rate and yield of organic acid accumulation and shorten the solid retention time required for biodegradation. Furthermore, there is a need to uncover the microbial groups involved in the hydrolytic-acidogenic stage as this could help in selecting the best operational parameters for their growth, which in turn could improve the rate of the processes. The objective of this thesis was to investigate and optimise the accumulation of organic acids from restaurant food waste (RFW) AD. The first phase of this study (Chapter 2) was conducted to evaluate and optimise the biodegradation efficiency of RFW using biomethane potential assay; the effect of the FW composition (fat, protein, hemicellulose and cellulose) on biodegradation rates was assessed. In addition, a bioaugmentation strategy was used to enhance the hydrolysis efficiency of the RFW components. The RFW biodegradation efficiency was enhanced by 10 to 15% using the bioaugmentation approach, which consisted of supplementing the primary inoculum with enriched culture developed on pure substrates. The hydrolysis rate constant for the different fractions of the RFW indicated that hemicellulose fraction was easily hydrolysable, while fat was the most recalcitrant. Hemicellulose and cellulose were the two fractions of the RFW enhanced as the result of enriched cultures supplementation. Bacteroides graminisolvens and species affiliated with Porphyromonadaceae were identified as potential cellulose and hemicellulose hydrolysers (respectively) using 16S rRNA profiling. The data obtained suggested a fourteen-day solid retention time for maximum biodegradation of RFW and the possibility of shortening this time through a bioaugmentation strategy. In the second phase of this study (Chapter 3), three leach-bed reactors fed with RFW and initially inoculated with granular sludge were operated at 37oC in a semi-continuous mode. Based on the results obtained in Chapter 2, the solid retention time of fourteen days was applied; a ratio of 1:4 (inoculum:RFW) was chosen to favour the rapid accumulation of organic acids inside the reactors. Stable bioprocess performance was demonstrated, with volatile solid (VS) efficiency above 60%. The hydrolysis of the components of the RFW was believed to be efficient over the initial two days of the incubation, as indicated by the maximum soluble chemical oxygen demand (sCOD) accumulation over the same period. Leachate analysis revealed the accumulation of up to 49 g l-1 of volatile fatty acids (VFAs), of which circa 35% was butyric acid and 25% acetic acid. Microbial communities identified from 16S rRNA-based Illumina sequencing analysis of leach-bed reactors identified Enterococcus as potential hydrolysers. The important fermentative groups (identified as Lactobacillus, Clostridium and Bifidobacterium) were likely responsible for the production of lactic acid, butyric acid and acetic acid, respectively. The results gathered in this second phase suggested that it is feasible to biodegrade the RFW over short periods (two days) while at the same time accumulating organic acids. In the final phase of this study (Chapter 4), process optimisation strategies were investigated in terms of promoting VFAs accumulation; the feasibility to selectively produce caproic and butyric acid from RFW was also assessed. Based on the data generated in Chapter 3, which showed that maximum hydrolysis efficiency was achieved in two days, the solid retention time (SRT) in the leach-bed reactors was reduced from fourteen- to seven days (Chapter 4). Increasing the recirculation regime (frequency) from once to three times per day and reducing the starting liquor VFAs’ concentration from 15 to 6 g COD l-1 resulted in a 55% improvement of VFAs production. With these parameters, VS removal efficiency of over 70% was achieved; caproic acid at the concentration of 21.86 g COD l-1 was the highest VFA produced in the leach-bed reactors. The composition of VFAs was influenced by hydrolysis rate, pH, loading rate and the depletion rate of short chain volatile fatty acids (SCVFAs). The selective production of caproic acid from RFW leachate at the rate of 3 g l-1 d-1 was demonstrated in this study by using hydrogen or the combination of hydrogen and ethanol supplementation. Butyric acid accumulation was observed in the presence of ethanol. Microbial community analysis based on the 16S rRNA sequencing suggested the implication of Clostridium and Peptoniphilus in the generation of butyric acid, while Lactobacillus reuteri could play a role in the accumulation of caproic acid. This study has set some basis for the selective production of caproic and butyric acids from FW. This thesis demonstrates the feasibility of biodegrading RFW while promoting the accumulation of valuable organic acids (mainly VFAs) using leach-bed reactors. The combination of bioprocess monitoring with molecular analyses provided several valuable insights into the complex hydrolytic-acidogenic microbial communities which underpin these processes.