Anaerobic treatment of solid wastes from potato processing was studied in completely stirred tank reactors (CSTR) at 55 degrees C. Special attention was paid to the effect of increased organic loading rate (OLR) on the biogas yield in long-term experiments. Both biogas yield and CH4 in the biogas decreased with the increase in OLR. For OLR in the range of 0.8 gl(-1) d(-1)-3.4 gl(-1) d(-1), biogas yield and CH4 obtained were 0.85 l g(-1)-0.65 l g(-1) and 58%-50%, respectively. Biogas yield y as a function of maximum biogas yield y(m), reaction rate constant k and HRT are described on the basis of a mass balance in a CSTR and a first order kinetic. The value of y(m) can be obtained from curve fitting or a simple batch test and k results from plotting y/(y(m)-y) against 1/OLR from long-term experiments. In the present study values for y(m) and k were obtained as 0.88 l g(-1) and 0.089 d(-1), respectively. The simple model equations can apply for dimensioning completely stirred tank reactors (CSTR) digesting organic wastes from food processing industries, animal waste slurries or biogas crops. (c) 2006 Elsevier Ltd. All rights reserved.