Low‐Temperature Thermal Pretreatment of Tobacco‐Stem Extraction Wastewater: Volatile Fatty Acid Redistribution and Short‐Term Methane Production Response

ABSTRACT Tobacco‐stem extraction wastewater from papermaking‐based reconstituted‐tobacco production is a small‐volume, acidic, high‐strength side‐stream. It can undermine downstream anaerobic wastewater treatment processes, such as upflow anaerobic sludge blanket (UASB) treatment, especially under short hydraulic retention time (HRT). The inhibition effects result from its high chemical oxygen demand (COD) and propionate‐skewed volatile fatty acid (VFA) profile. Given its small volume and the availability of low‐grade heat on‐site, a mild pretreatment compatible with existing infrastructure is desirable. In this study, low‐temperature thermal pretreatment (LTTP; 50°C–90°C, atmospheric operation) was evaluated as a low‐grade‐heat‐compatible conditioning step for improving the short‐term anaerobic response of tobacco‐stem extraction wastewater. The 48‐h short‐term methane production (STMP) peaked at 111.8 ± 6.1 mL CH 4 ·g COD −1 at 90°C for 90 min, corresponding to a 2.1‐fold increase relative to the untreated wastewater. LTTP also shifted the VFA profile from propionate‐dominant to acetate‐dominant: The acetate‐to‐propionate ratio increased from 1.0 at 50°C for 30 min to 11.5 at 90°C for 90 min. In parallel, COD, pH, and oxidation–reduction potential (ORP) changed only modestly, and no statistically detectable changes in bulk cellulose, hemicellulose, or lignin fractions were observed. The parallel factor analysis (PARAFAC) revealed broad increases in protein‐like and humic‐/fulvic‐like fluorescent DOM components. These results indicate that LTTP mainly induced liquid‐phase mobilization and VFA redistribution rather than measurable bulk lignocellulosic disintegration. Overall, LTTP may provide a gentle, low‐grade‐heat‐compatible side‐stream conditioning option for improving the short‐term anaerobic treatability of tobacco‐stem extraction wastewater.