Anaerobic digestion of highly alkaline spent caustic from a distillery through cultivation of microbial consortia

Spent caustic, a byproduct of cleaning and neutralization in distilleries, is typically challenging to treat due to its high pH and elevated chemical oxygen demand (COD). To address this issue in a cost-effective manner, this study explored anaerobic digestion of spent caustic by blending it with naturally acidic distillery byproducts, centrate and condensate, without chemical neutralization. Two anaerobic sequencing batch reactors (one control and one inoculated with anaerobic granular sludge) were operated with feed mixtures containing centrate: spent caustic: condensate ratios of 4:1:10, 3:1:10, 2:1:10, 1:1:10, and 0.8:1:10. Results demonstrated that the inoculated reactor adapted well to increasing levels of spent caustic, maintaining stable effluent pH values around 7.8, achieving over 90% removal of total and soluble CODs, and efficiently consuming volatile fatty acids under optimal operating conditions (1:1:10 ratio and 5-d hydraulic retention time). Microbial community analysis revealed diverse syntrophic interactions among acetate-oxidizing bacteria and hydrogenotrophic methanogens, supporting effective organic matter degradation and biogas production under high alkaline conditions. This study demonstrates a sustainable and cost-effective waste-to-waste biotreatment strategy for spent caustic utilizing existing distillery waste streams, reducing chemical use and supporting circular waste management in distillery operations. • A waste-treating-waste concept was proposed for spent caustic treatment. • Blending centrate and condensate with spent caustic supported tolerant microbes. • This biological process required no chemical neutralization. • Centrate: spent caustic: condensate ratio of 1:1:10 and HRT of 3–5 d were optimal.