The effect of the pollutant load on the efficiency of aerobic biodegradation of potato slops with a mixed population of thermo- and mesophilic bacteria of the genus Bacillus was examined. Batch biodegradation processes were carried out at 45 degrees C, using slops with the initial chemical oxygen demand (COD) totalling 11.3, 18.0, 42.6, 58.0 and 74.0 g O-2/l. The extent of COD removal ranged from 80.4% (with COD of 11.3 9 O-2/l) to 88.7% (with COD of 58.0 g O-2/l). With potato slops of higher initial COD levels (58.0 and 74.0 9 O-2/l), the first 24 h of growth were characterised by a deficiency of oxygen and a considerable rise in the content of acetic acid, which was then removed. In the first 48 h of the process, irrespective of the initial COD level, the biodegradation of the pollutants removed in the course of the entire process exceeded 91%. The rate of COD removal calculated for that period was a linearly increasing function of the initial pollution load. (c) 2005 Elsevier Ltd. All rights reserved.