Paper Title
Repeated-Batch Fermentation Ofmicroalgal Biomass for Long-Term Production of Bioethanol Using Immobilized Saccharomyces Cerevisiae

Abstract
The use of biomass in many technological processes is crucial for the development of biorefineries in terms of energy, economy and environmental sustainability. Among the biorefinery products, biofuel from non-food biomass could be used as a renewable fuel in the automotive sector. Bioethanol derived from microalgae have promise as carbon-neutral replacements for fossil fuel. Selective fermentation could improve microalgae-based biofuel by managing microbial processes. In this study, two different processes [(1) separate hydrolysis and fermentation (SHF) and (2) simultaneous saccharification and fermentation (SSF)]have been investigatedfor fermentative bioethanol production from a carbohydrate rich Chlamydomonas mexicanabiomass. SSF was selected as an efficient fermentation process to enhance the bioethanol yield from microalgal biomass through repeated-batches (7-cycles) using immobilized yeast cells. Combined sonication and enzymatic hydrolysis of C. mexicana released 445 mg/g of total reducing sugars (TRS), which led to a production of 10.5 and 8.48 g/L of bioethanol in SSF and SHF, respectively. Most of the TRS was consumed by the yeast reaching a relative consumption efficiency of 91-98%. A bioethanol yield of 0.5 g/g with volumetric productivity of 0.22 g/L/h was achieved after 48 h of SSF. Immobilized yeast cells enabled repetitive production of ethanol for 7 cycles. Alginate beads regenerated in yeast medium exhibited a fermentation efficiency up to 79% for five consecutive cycles. The maximum ethanol production was 9.7 g/L at 72 h for cycles 2 through 4. A total energy recovery of 85.81% was achieved from microalgal biomass in the form of bioethanol. In addition, repeated-batch SSF employing immobilized yeast cells demonstrated the possibility of cost-effective bioethanol production and increased bioethanol productivity.