Towards green whiskey production: Anaerobic digestion of distillery by-products and the effects of pretreatment

Using renewable biogas from the anaerobic digestion of distillery by-products as a low-carbon heat source can decarbonize the distillery process and support the distillery industry to transition to a more sustainable production process. This study investigated the anaerobic digestion performance of different types of whiskey by-products and the effects of acid pretreatment on the digestion of solid by-products. Results of biomethane potential assays showed that the methane yield from the unprocessed by-products was 330 mL/g volatile solids (VS) from draff, 495 mL/g VS from thin stillage, and 503 mL/g VS from thick stillage. For the processed by-products the specific methane yield was 370 mL/g VS from cake maize, 382 mL/g VS from wet distillers’ grains with solubles (WDGS), and 545 mL/g VS from syrup. Acid pretreatment (1% H₂SO₄ at 135 °C for 15 min) did not significantly improve the methane yield from solid by-products (such as draff and WDGS) but reduced the digestion time by 54.5% for cake maize. The microbial community analysis revealed that methane production from the untreated and acid-pretreated solid by-products (draff and WDGS) was mainly through the hydrogenotrophic methanogenesis pathway. The gross thermal energy in the form of methane produced from 100 tonnes of mixed unprocessed by-products (draff, thin stillage, and thick stillage) was calculated as 24.4 MW_thh equivalent to 60.6% of the thermal energy consumed in whiskey production, which effected the same percentage of CO₂ emissions reduction.