This manuscript investigates the impact of changes in the Total Acid Number (TAN) on bitumen characteristics resulting from aging and Re-refined Engine Oil Bottoms (REOB) modification. The study explores the potential of TAN as a diagnostic tool for detecting bitumen sources, identifying the presence of REOB, and predicting long-term binder performance. The research topic is both interesting and pertinent. The following specific comments and suggestions are provided to assist the authors in improving the manuscript: 

1. Figure 1 shows that REOB modification was only applied to hard grade bitumen (Bit-K and P) to achieve rheological properties comparable to 70/100 PEN. However, presenting also the TAN values of Bit-K and Bit-P before and after REOB modification would provide a clearer understanding of the modification's impact on TAN. 

2. While the study highlights the potential of TAN in indicating the effects of aging and modification on bitumen characteristics, it appears that TAN alone may not suffice as a standalone diagnostic tool for detecting bitumen sources, the presence of REOB, or predicting long-term binder performance. Integrating TAN analysis with methodologies, such as SARA fractionation, Colloidal Instability Index (CII), Gel Permeation Chromatography (GPC), and Dynamic Shear Rheometry (DSR), would provide a more robust and comprehensive assessment. This might be a more accurate conclusion.