Agro-Industrial Wastewaters as Feedstocks: New Research on Bioethanol Production

Abstract

Bioethanol is one of the most important renewable fuels and it is classified into four different generations (G) based on the feedstocks used for their production: edible food crops such as rice, corn, sugarcane, wheat, and potato for first generation (1G)-bioethanol and lignocellulosic biomass including forest residue, woody and herbaceous biomass, and nonfood crops for 2G-bioethanol. Although the last one represents an attractive feedstock for ethanol production, their processing is not economical feasible yet due to problems arising from the recalcitrant nature of the biomass. Algal biomass and the captured carbon dioxide by using advanced technologies such as electrochemical synthesis or oxide electrolysis represent promising alternatives to produce 3G- and 4G-bioethanol, respectively. However, the knowledge and the technology are still immature and some bottlenecks should be overcome to develop economically feasible processes. Currently, almost all bioethanol is produced from starch- and sugar-based feedstocks. The high worldwide bioethanol demand exerts enormous pressure on natural resources and competition for land that could be used for food production. Despite the need to identify alternative renewable sources for sustainable bioethanol production, the industrial wastewaters remain as undervalued candidates. Neither they are usually mentioned as potential feedstocks for ethanol production nor they are included in ethanol generations.This chapter is focused on advances in the development of yeast-mediated processes for bioethanol production from industrial wastewaters, including high-strength wastewaters from a) the non-alcoholic sugar-sweetened beverage industry (soft drinks, enhanced waters, fruit juices, sports and energy drinks); b) winery, brewery and cider wastewaters; and c) dairy wastewaters; as well as d) soybean molasses, despite not being a classic wastewater like the previous ones. These feedstocks exhibit considerable fermentable sugar content (50−200 g/L). So, their direct use as carbon source was evaluated, whereas the purges of fermentation tanks of the brewery industry and corn steep water from the corn starch processing industry were considered as inoculum and nitrogen sources, respectively. Fermentations were performed in batch mode under anaerobic conditions, and the concentrations of sugar, biomass and ethanol were monitored over time. Complete sugars removal was achieved in less than 15 h with ethanol yields of 0.40 – 0.45 gethanol/gsugar. Because the processes can be performed using conventional and available enzymes and equipment (fermenters, distillation columns, filters, and decanters), the industrial wastewaters are ready-to-use feedstocks for bioethanol production. Finally, a new bioethanol generation is proposed: “wG-bioethanol is the one obtained through yeast-mediated fermentation using industrial wastewaters as feedstocks”.