10.1.1 Residues Generated in the Process of Bioethanol Production

Fuel ethanol production generates solid wastes, atmospheric emissions, and liquid effluents. The atmospheric emissions correspond mostly to the gas outlet stream from fermenters, which are washed with water in the scrubbers in order to recover the volatilized ethanol. The gases exiting from the scrubbers contain mainly car­bon dioxide that is released into the atmosphere. The CO2 can be used for produc­tion of dry ice and beverages.

However, if these gases are not utilized, they should be considered in the calcu­lation of the environmental impact of ethanol-producing facilities. In this regard, it should be emphasized that the bioethanol presents net emissions of nearly zero CO2 because the plant biomass already fixed the CO2 during its growth and only this carbon dioxide is released during the combustion of fuel ethanol in the engines. In contrast, the burning of fossil fuels releases into the atmosphere additional amounts of carbon dioxide that was fixed by the plant biomass millions years ago.

The solid wastes formed during production of fuel ethanol are strongly linked to the raw material from which it is produced. When sugarcane is used, huge amounts of sugarcane bagasse are produced. Fortunately, this solid material has multiple uses and applications. Its most important utilization is as solid biofuel due to its high energy content. In fact, bagasse combustion allows the generation of the thermal energy (steam) required not only during the conversion of sugar­cane into ethanol, but also during cane sugar production. The bagasse also can cover the electricity needs if co-generation units are used (see Chapter 11). In the
corn-to-ethanol process by the dry-milling technology, most solid residues are concentrated in the so-called distiller’s dried grains with solubles (DDGS), so the generation of solid wastes is limited. If wet-milling technology is employed, the solids generated produce part of the different co-products in the framework of a corn biorefinery, as corn gluten meal and feed. When cassava roots are used, the solids produced correspond to the root peels that can be utilized as feedstock for production of mushrooms as well as the fibrous residue contained in the stillage stream. This fibrous residue is obtained after centrifugation of the whole stillage to obtain the thin stillage and this solid material. In addition, this residue can be used for animal feed or as a substrate in solid-state fermentations.

The production of ethanol from lignocellulosic biomass, in turn, generates lignin as the most important solid residue. This polymer can be isolated during the pretreatment step if some pretreatment methods like the pretreatment with sol­vents (organosolv process) or oxidative delignification are employed (see Chapter 4, Section 4.3.3). Nevertheless, most pretreatment methods allow the lignin to remain in the solid fraction resulting from this processing step along with the cel­lulose. After enzymatic hydrolysis using cellulases, the lignin remains in the liquid suspension until the end of the process where it can be recovered from the stillage stream. The lignin has a high energy value and, therefore, is used as a solid biofuel for feeding boilers or co-generation units, as in the case of sugarcane bagasse.

The stillage (or vinasses) is the major effluent of all flowsheets for ethanol production involving the submerged fermentation of streams containing sugars or carbohydrate polymers. This is valid for feedstocks such as sugarcane juice, molasses, starchy materials, and pretreated lignocellulosic biomass. The still­age represents the residual liquid material obtained after distillation of ethanol from the fermented wort (wine) and contains both solid and soluble matter. The elevated organic load of the stillage is most responsible for the high polluting properties of this burden, thus this stream should undergo treatment to reduce this load and minimize the environmental impact during its discharge into the water streams.