This study investigated the impact of biomass-based fly ash (FA) pretreatment on the biodegradability of a thermomechanical pulping spent liquor (TMPL) in an aerobic system. In this study, FA was mixed with TMPL under the conditions of 6 wt.% based on TMPL, 25 degrees C and 10 h, which removed a part of recalcitrant organic materials and resulted in 68.0, 40.0, 60.1, 81.2 and 48.3% reductions in chemical oxygen demand (COD), biochemical oxygen demand (BOD), total organic carbon (TOC), lignin and sugar, respectively. FA-pretreated TMPL pressate (FA-TMPL) was biologically treated in an aerobic system of sequencing batch reactor (SBR). The performance of the biological treatment with and without FA pretreatment was studied in two parallel SBRs over three months. The combination of FA and biological treatments removed 97.3% of COD, 98.3% of BOD, 96.3% of lignin, 99.5% of sugar, and 98.1% of TOC. Without FA pretreatment, the biological system removed 87.3% of COD, 89% of BOD, 81.6% of lignin, 98.6% of sugars, and 90.5% of TOC. The results also confirmed that the settling ability of sludge, which was indicated as a sludge volume index, was reduced from 109.3 mL/g to 53.5 mL/g. In addition, the advantages of using FA pretreatment in aerobic systems were discussed in detail. (C) 2017 American Institute of Chemical Engineers