Renewable Fuels and Chemicals from the Organic Fraction of Municipal Solid Waste

Municipal solid waste (MSW) is any non-industrial waste produced in households and public or commercial institutions. 3.4 billion tonnes of MSW will be produced annually by 2050, but unsustainable practices like landfilling and incineration currently dominate MSW management. The organic fraction of MSW (OMSW) typically comprises ~50% lignocellulose-rich material but is underexplored as a biomanufacturing feedstock.
This thesis investigated OMSW as a feedstock for producing renewable biofuels and chemicals. Uniquely, the OMSW-derived fibre used in this project was produced via a commercial autoclave pre-treatment from a realistic and reproducible MSW mixture. The OMSW fibre was subjected to comprehensive compositional analysis and hydrolysis, and OMSW hydrolysate was analysed for sugars, metals and marker inhibitors to evaluate fermentability. Waste residues were investigated as a feedstock for biogas production. Next, the growth and productivity of eight diverse and biotechnologically useful microbial species was characterised on OMSW fibre hydrolysate supplemented with 1% yeast extract and the best candidate was further characterised and improved for industrial applications.
The OMSW fibre contained a large polysaccharide fraction, comprising 38% cellulose and 4% hemicellulose. Hydrolysate of OMSW fibre was high in D-glucose and D-xylose, low in inhibitors, deficient in nitrogen and phosphate and abundant in potentially toxic metals. Hydrolysis residues contained a six-fold greater metal concentration but generated 33.4% more biomethane in anaerobic digestion compared to unhydrolysed fibre. Microbial screening identified three species that robustly and efficiently fermented OMSW fibre hydrolysate: Saccharomyces cerevisiae, Zymomonas mobilis and Rhodococcus opacus. These species could theoretically produce 139 Kg and 136 Kg of ethanol and 91 Kg of triacylglycerol (TAG) per tonne of OMSW, respectively. R. opacus had the highest fermentation productivity, concurrently using D-glucose and D-xylose and producing TAG to 72% of maximum theoretical yield. Expression of a heterologous thioesterase in R. opacus to augment lauric acid production proved unsuccessful and requires further work.
Overall, this study showed that OMSW is a promising renewable feedstock for biomanufacturing. The microorganisms identified through this work grew robustly and efficiently on OMSW fibre hydrolysate and are promising candidates for developing an OMSW biorefining platform.