In this study, chitosan was periodically added to a CSTR treating raw POME in order to retain more working microorganisms at high OLRs. The data indicated that the CSTR with chitosan addition can be operated with reactor stability at an OLR of 26.5 kg m(-3) d(-1) which is approximately 7.5 kg m(-3) d(-1) higher than that for the control CSTR without chitosan addition. In the control CSTR, the biogas production did not increase with increased OLR, and the overall process was limited by slow methanogenic rates. For the CSTR with chitosan addition, the biogas production was 9.39 m(3) m(-3) d(-1) with a methane volume fraction of 68% at an OLR of 26.5 kg m(-3) d(-1). Corresponding to this increase in methane production, it was found that Methanosarcinales numbers were significantly higher (P < 0.05) in the CSTR with chitosan addition than in the control CSTR. (C) 2016 Elsevier Ltd. All rights reserved.