Biological pretreatment methods of waste-activated sludge (WAS) have recently received attention because of their efficiency and relatively low investment. The application of hydrolysis/acidification step before methanization in anaerobic digestion or the two-phase anaerobic digestion process constitutes one of these methods. Two phase (thermophilic acidification and mesophilic methanogenesis) anaerobic digestion of WAS was investigated and reported to perform better in terms of chemical oxygen demand (COD) solubilization, volatile solids destruction, gas production, and pathogen indicator reduction. The solids loading rates (SLR) used in these studies were significantly high (6.5-28.9 g volatile solids (VS)/L . day). It is well-known that high SLRs along with low retention times lead to higher acidification in two-phase systems. However, WAS from different wastewater treatment plants is likely to have different concentrations due to different process configurations and operational practices that lead to lower solids loading rates. Therefore, this study investigated the performance of thermophilic anaerobic preacidification of WAS prior to conventional mesophilic (methanogenic) anaerobic digestion at low SLRs (0.98-3.93 g VS/L . day). To this purpose, continuous anaerobic acidogenic reactors with no recycle were operated at two different HRT (or SRT) values of 2 and 4 days, and three different SLR values of 0.98, 1.97, and 3.93 g VS/L . day. After the acidification reactors reached to steady-state conditions, the preacidified WAS samples (or the effluents of thermophilic acidogenic anaerobic reactors) and raw WAS samples (or the feeds to the thermophilic acidogenic anaerobic reactors) were subjected to biochemical methane potential (BMP) assay. Results indicated that thermophilic preacidification led to a 20.9-34.8% increase in dissolved (soluble) COD concentration and 18.2-33.3% CODt reduction in acidogenic reactors. When the CODt removals observed for preacidification and BMP were both considered, preacidification (or phase separation) has led to 26.2-49.4% additional CODt removal. It is apparent from these results that beneficial effect of preacidification on CODt removal is sustained for low SLRs (0.98-3.93 g VS/L . day).