Bioprocess engineering of microalgae to bioethanol

The increasing demand of energy, coupled with the decline in fossil fuel reserves and their increasing effect on carbon emissions, has driven global population to pursue sustainable and renewable energy sources. One of such alternative fuels is bioethanol, which has received a lot of attention recently toward process development for full-scale production. It mixes easily with gasoline fuel without engine modification and provides higher octane rating. It also has lower emission of carbon monoxide, particulate, unburned hydrocarbon and carbon dioxide emission though it has a slightly higher NOx emission. However, the exact economic and sustainability benefits are unclear as existing feedstock materials for bioethanol production, such as corn, sugar cane and wheat, creates ‘food versus fuel’ issue in terms of land competition and other agricultural requirements thus hindering its commercialization. Microalgae have been identified as an alternative feedstock for bioethanol production as it requires a smaller area of land for cultivation, possesses high growth rates and accumulates satisfactory amounts of carbohydrates. Microalgae are microscopic photosynthetic organisms that are found in both marine and freshwater environments. Their photosynthetic mechanisms are similar to that of land-based plants and are generally more efficient in converting solar energy into biomass. Additionally, microalgae utilises large quantities of CO2 during photosynthesis, thus CO2 released from bioethanol production can be recycled for microalgae cultivation to make the process more sustainable. This study explores the bioprocess engineering viability of microalgal biomass feedstock for bioethanol production via yeast fermentation. Overall results indicated that microalgal biomass is a promising bioethanol feedstock, displaying comparable and even higher bioethanol yields than existing feedstock under specific process conditions for specific microalgal strains. The key process steps included disruption, pre-treatment, hydrolysis and fermentation of microalgal biomass. Cell disruption by sonication resulted in ~40% wt reduction in biomass carbon content and was found to be the most effective technique amongst the tested physio-chemical techniques. Microwave exposure and high pressure homogenisation displayed ~20% wt and ~30%wt carbon content reduction respectively. The effect of biomass pretreatment using acidic or alkaline chemicals was also investigated in this study. The acid pre-treatment method displayed a significantly higher bioethanol yield of ~52wt% (g ethanol/g microalgae) compared to the alkaline method, ~27wt% (g ethanol/g microalgae). In addition, the utilization of lipid-extracted microalgae debris displayed bioethanol yield of ~38 wt% (g ethanol/g microalgae) which shows promise for coproduction of biodiesel, via transesterification extracted lipids, and bioethanol. The addition of enzyme (cellulase) during hydrolysis improved saccharification of microalgal biomass and the rate of production of fermentable sugars. The highest glucose yield of 64.2% (w/w) was obtained at a temperature of 40 ºC, pH 4.8, and a substrate concentration of 10 g/L of microalgal biomass. Different fermentation techniques, such as Separate Hydrolysis and Fermentation (SHF), Separate Hydrolysis and Co-Fermentation (SHCF) and Simultaneous Saccharification and Fermentation (SSF), were adopted to investigate their effect on bioethanol production yield. It was observed that SHF gave the highest bioethanol production yield of ~34 wt% (g ethanol/g microalgae) compared to SHCF and SSF which displayed ~30 wt% (g ethanol/g microalgae) and ~21wt% (g ethanol/g microalgae) respectively. The findings from this dissertation contribute significantly to the process development and answer fundamental questions essential to enable full-scale production of bioethanol using microalgal biomass as the feedstock.