Enhanced energy recovery through serial fermentations of microalgal biomass for the production of bioalcohols

Abstract

Enhanced energy recovery through serial fermentations of microalgal biomass for the production of bioalcohols Marwa Mohamed Abdelkader Eldalatony Department of Earth Resources and Environmental Engineering, The Graduate School, Hanyang University, South Korea Growing concerns regarding the impact of fossil fuel use upon the environment and the cost of production have led to a growth in the interest of obtaining energy from biomass. However, 1st and 2nd generation biomass types, are often criticized for their high energy requirements and environmental impacts. Algal biomass is considered a 3rd generation biomass which does not require arable land for cultivation, typically has a high productivity and can be converted to a wide variety of energy carriers. The most significant barriers for economical biofuel production using microalgae are the technological and engineering aspects, such as energy efficient pretreatment and production technologies. The use of all microalgal constituents toward the sustainable production of bio-fuels through serial processes would warrant the maximum utilization of biomasses for biofuel production. Despite research on the concept of producing energy from algal biomass dating back to the 1960s there has been limited commercial development and the environmental advantages are still in doubt. This thesis investigated the potential of algal biomass as a source of bioenergy feedstock by considering pretreatment of microalgal biomass and conversion of native cellulose to fermentable sugar to maximize fermentation efficiency and biofuel production. The research did not stop at this point, however, the use of leftover biomass after sugar fermentation was considered. In this research work, Sonication pretreatment of Chlamydomonas mexicana under optimum conditions (at 40 kHz, 2.2 Kw, 50ºC for 45 min) released 186.2±0.7 mg g-1 of total reducing sugars (TRS) of dry cell weight, while the combined sonication and enzymatic hydrolysis treatment enhanced the TRS yield by to 445.1±4.9 mg g-1. The optimal ratio of enzyme [E]: substrate [S] for maximum TRS yield was [1]:[5] at 50ºC and pH 5. Conversion of released monosaccharides to bioethanol is accomplished metabolically via glycolysis and fermentation. Sequential enzymatic hydrolysis and fermentation is defined as separate hydrolysis and fermentation (SHF). On the other hand, if the two process steps are performed simultaneously, the process is referred to as simultaneous saccharification and fermentation (SSF). SSF allows the sugars released via hydrolysis to be rapidly consumed by the microorganism, thereby minimizing feedback-inhibition of cellulolytic enzymes. Combined sonication and enzymatic hydrolysis of Chlamydomonas mexicana generated 10.5 and 8.48 g/L of ethanol in SSF and SHF, respectively. Yeast utilized maximum portion of total reducing sugar (TRS) reaching a consumption efficiency of 91-98%. A bioethanol yield of 0.5 g/g (88.2% of theoretical yield) and volumetric productivity of 0.22 g/L/h was obtained after 48 h of SSF. Immobilization of yeast cells is a potential alternative for enhancing ethanol productivity, since immobilized cells are more tolerant to ethanol and exhibit a lower degree of substrate inhibition. Indeed, approaches employing repeated batch fermentation by immobilization of yeast cells enable the fermenter to retain high cell concentrations without the need for separation or recirculation devices. Immobilized yeast cells enabled repetitive production of ethanol for 7 cycles displaying a fermentation efficiency upto 79% for five consecutive cycles. The maximum ethanol production was 9.7 g/L in 2nd-4th cycles. A total energy recovery of 85.81% was achieved from microalgal biomass in the form of bioethanol. The conversion efficiency (22.26-27.56%) of C. mexicana biomass into biofuel revealed that one third of the biomass has been converted into energy in the form of bioethanol. The strategies established in this study will provide additional insight into fermentations and will be useful in scaling-up of fermentation processes for improving commercial bioethanol production. Repeated-batch SSF demonstrated the possibility of cost-effective bioethanol production. Fermentation of the carbohydrate fraction enhanced the soluble protein content up to 56% (228.4 mg g-1) of total protein content. Conversion of carbohydrate fraction into bioethanol leads to the accumulation of bioavailable protein by-products and lipids. These protein by-products can be used for higher alcohol (C3-C5) production through biological regulation of fermentative microbes. Phase I (carbohydrate fermentation) produced 10.5 g/L of bioethanol; whereas, phase II (fermentation of the leftover biomass in phase I) generated a total of 5.23 g/L of higher alcohols (including butanol, propanol, isobutanol and 3-methyl butanol). Transesterification of the remained lipids showed 93 g/L of biodiesel in Phase III. We believe that such thorough utilization of Chlamydomonas mexicana will encourage the use of microalgal cells in applications such as high scale biofuel production. Thorough utilization of Chlamydomonas mexicana toward the biofuel production is key strategy toward the production of sustainable biofuels. Conversion of biomass into valuable chemicals (biofuel) with a production of less waste is new approach for maximum energy recovery from microalgal biomass. This research work demonstrated that serial fermentations is required for the improvements of biofuel production for the possibility of cost-effective microalgal biofuel production. ----------------------------------------------------------------------------------------------------------- Keywords: Microalgae, fermentation, distillation, biofuel, pretreatment, transesterification.