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Due to high concentrations of organic matters and total solids, the effluent generated from recycled paper mill (RPME) requires appropriate treatment prior to discharge. The thermal condition of an anaerobic system has a major effect on the biodegradation processes of total solid substrates. In this study, the biodegradation of RPME was evaluated in a modified anaerobic inclining-baffled reactor (MAI-BR) under sub-mesophilic (29 °C), mesophilic (37 °C) and thermophilic (55 °C) conditions. The concentration of VSS, TDS, TSS lignin and VSS/TSS ratio in MAI-BR’s compartments were studied. The results showed that Compartment 1 had TSS of 15682, 13305 and 12145 mg/L; particles size of 621, 559 and 500 µm; VSS of 9662, 1776 and 10923 mg/L; and TDS of 594, 730 and 1873 mg/L at sub-mesophilic, mesophilic, and thermophilic conditions, respectively. An increase in the reactor temperature, an overall reduction in TSS and particles size, and an increase in VSS, VSS/TSS ratio, TDS, and lignin were observed. Under thermophilic conditions, the VSS/TSS ratio was higher than in mesophilic conditions, while lignin became harder to remove. These results conclude that operating the reactor at thermophilic has reduced the digested sludge within the system and enhanced the solid fraction biodegradation process of RPME. [ABSTRACT FROM AUTHOR]