Anaerobic digestion (AD) is a sustainable pathway towards recovering chemical energy from excess sludge, and humic substances (HSs) contained in sludge can inhibit energy (methane/CH ₄ ) conversion efficiency. This study aims to investigate the impact of humic acids (HA) on the various processes in a batch anaerobic digestion process. For this purpose, “clean” sludge was cultivated in a laboratory to avoid HSs presence. The cultivated sludge was used in a series of batch experiments, with humic acids added at different levels. A complete AD test, as well as three sub-phase tests (hydrolytic phase; acidogenic phase; methanogenic phase) was performed and analyzed with and without HA dosing. In the single-phase AD system, dosing with HA inhibited the methanogenic efficiency by 35.1% at HA:VSS = 15%. However, the effects of HA on the three sub-phases revealed something very different. HA inhibited hydrolytic efficiency by 38.2%, promoted acidogenic efficiency by 101.5%, and finally inhibited methanogenic efficiency by 52.2%. The combined efficiency of the three sub-phases without HA dosing is calculated at 15.7%; and with HA dosing (HA:VSS = 15%) at 10.2%. Overall, the combined inhibition efficiency of the three sub-phases is equal to 35.0%, which is almost identical (35.1%) to the result observed in the single-phase AD process. The possible mechanisms behind the phenomena were analyzed and summarized in the context.