In this study it is presented how anaerobic digestion of the stillage affects a spruce-to-ethanol process in techno-economic aspects. Each scenario consists of the following steps: 1) steam pretreatment of the impregnated spruce chips; 2) diluting the whole slurry obtained in steam pretreatment; 3) separating part of the liquid by filtration; 4) ordinary bakers’ yeast cultivation on part of the liquid fraction supplemented with molasses; 5) cellulase enzyme fermentation on the remaining liquid fraction; 6) simultaneous saccharification and fermentation of the filtered slurry by adding the yeast and cellulase enzymes produced beforehand; 7) distillation of the broth and dehydration of the concentrated ethanol; 8) combined heat and power (CHP) production; 9) wastewater treatment.

For stillage treatment three options are compared. At option A considered as reference case the stillage is separated into solid and liquid streams, and the latter is concentrated by evaporation. The solid stream and the syrup are used in the CHP plant. At B and C the liquid stream and the whole stillage are subjected to anaerobic digestion, respectively. Depending on the scenario the solid stream, the biogas and the pressed sludge are fed to the CHP plant.