The results of modelling of paper mill and textile wastewater biological treatment are presented. It is assumed that bioconversion of organic matter can be divided into two phases: intensive biodegradation and biodegradation limited by available substrate. The total substrate concentration is expressed as Chemical Oxygen Demand (CODtot), readily biodegradable substrate as Biochemical Oxygen Demand (BOD5) and biomass as volatile suspended solids (VSS). The real time courses of substrates and biomass concentrations are well described by simulated curves. Moreover, good agreement between calculated and experimental degree of biodegradation indicates that the applied mathematical model is of high predictability and can successfully be used for design and optimisation of industrial wastewater treatment processes.