Industrial wastewater discharged to a municipal sewage system may often be a source of valuable organic substrate for biological processes at a municipal wastewater treatment plant (WWTP). In such situations, the pre-treatment of wastewater at an industrial plant should be minimized, in order to maintain the favourable structure of organic material in influent to a WWTP. A frequent problem is how to determine the maximum industrial pollution load that can be discharged, without compromising the treatment capability of a WWTP. This article presents a method based on computer simulation that can be used for this purpose. As a result, the method produced the permissible industrial pollution load that could be discharged safely, and the relationship between chemical oxygen demand (COD) load and required biomass concentration as a function of process temperature. The method was tested at a fruit-processing plant and municipal WWTP, located in Poland. The results of this study have shown that the bioreactor should be operated at a biomass concentration varying between 3,600 and 5,400 g*m-3 in function of the process temperature. Under such operational conditions the WWTP would be able to accept the permissible industrial COD load of above 9,000 kg COD*d-1 for a wide range of temperatures, namely 14 - 19 °C. If the WWTP was operated at a constant biomass concentration its permissible industrial COD load would be reduced to 7,500 - 9,000 kg COD*d-1. The method can be applied to other municipal WWTPs with activated sludge that receive industrial effluents.