Anaerobic digestion of pulp and paper sludge (PPS) and monosodium glutamate waste liquor (MGWL) was studied in completely stirred tank reactors (CSTR) at 37 +/- 2 degrees C. This work focused on the effect of increased organic loading rate (OLR) on the methane production in long-term experiments. For OLR in the range of (1.5-5.0) kg m(-3) d(-1) based on VS fed, VFA and SCOD concentrations decreased for the first 10 days and then kept stable at about 2.3 kg m(-3) and 4.0 kg m(-3) respectively until to the critical OLR of 5.0 kg m(-3) d(-1); and the methane generation rate enhanced to 0.838 m(3) m(-3) d(-1) during this period until to the reactor failure. Additionally, reaction rate constant k and sludge retention time (SRT) are described on the basis of a mass balance in a CSTR followed a first order kinetic equation. In the present study, values for y(m) and k were obtained as 0.733 m(3) kg(-1) of removed VS and 0.07 d(-1), respectively. The simple model can apply for dimensioning a CSTR digesting of organic wastes from pulp and paper industries, food processing industries, sewage treatment plants or biogas crops. (C) 2011 Elsevier Ltd. All rights reserved.