An integrated model for the composting process was developed. The structure of the model is such that it can be implemented in any mixture of different substrates, even in the case of co-composting of a solid waste with industrial wastewater. This paper presents a mathematical formulation of the physicochemical and biological principles that govern the composting process. The model of the co-composting ecosystem included mass transfer, heat transfer and biological processes. The biological processes included in the model were hydrolysis of particulate substrates, microbial growth and death. Two microbial populations (bacteria and fungi) were selected using Monod kinetics. Growth limiting functions of inhibitory factors, moisture and dissolved oxygen were added in the Monod kinetics. The bacteria were considered to utilise the easy biodegradable carbon hydrolysis product, fungi the difficult one, while both could degrade the carbon of wastewater. The mass balances of the most important nutrients, nitrogen and phosphorous, were also included in this approach. Model computer Simulations provided results that fitted satisfactory the experimental data. Conclusively, the model could be a useful tool for the prediction of the co-composting process performance in the future and Could be used to assist in the operation of co-composting plants. (C) 2009 Elsevier Ltd, All rights reserved.