Anaerobic digestion of the primary sludge with or without scum addition and the role of pH were evaluated in four completely stirred tank reactors (CSTR) operated as batch systems at 35 degrees C (mesophilic). For investigating the scum influence, two CSTRs were only fed with the primary sludge (PS) whereas the other CSTRs were composed of the primary sludge with 15% scum (PS+Scum). The pH in two reactors was fixed at 6.5, whereas the pH in the other two reactors was left to be operated at their original values for evaluating the retardation in biodegradation rates at low pH values. Since scum is generally produced at most primary settling tanks and given into the anaerobic digesters, the behavior of the CSTR was examined with or without scum addition. The results indicated that scum addition favoured low pH levels and led to retardation in methanogenesis. Besides, pH control enhanced the biodegradation rates and led to methanogenesis to perform at shorter digestion times in the reactors. The destructions of TSS and VSS were better when the pH was controlled. When only primary sludge was used as the substrate, the reactors with or without pH control removed VSS with a corresponding production of VFAs and soluble COD. However, their productions ceased earlier and the complete VFAs consumption occurred 4 days earlier in the reactors with pH control. In the reactors consisting of PS+Scum, soluble COD productions continued during 4 days of digestion. However, soluble COD remained constant and almost no VFAs consumption occurred during the whole operation period without pH control whereas VFAs were consumed completely after around 11 days in the reactor with pH control. Overall, scum addition caused methanogenesis to perform at longer operation periods when the pH was controlled and kept above 6.5. When the pH was not controlled, scum favoured acidic conditions and did not allow methanogenesis to start due to the fact that methanogens could not perform well at low pH levels.