This paper presents an MINLP model to design an eco-industrial plant by retrofitting existing water networks from different industrial plants in the same industrial zone. The proposed model is based on a superstructure and takes into account both in-plant and inter-plant structural modifications, such as the placement/reassignment of existing treatment units, the required increase of capacity and/or efficiency of existing treatment units, the placement of new treatment units either within the participating plants or in a new shared wastewater treatment facility, and the stream re-piping associated with installing new treatment units and retrofitting existing treatment units. The model allows the tracking of changes in the process performance as a result of the stream re-routing and retrofitting activities. Two examples were solved, and the results show the economic and environmental benefits of the retrofitted networks within an eco-industrial park compared to the stand-alone retrofitted networks. (C) 2012 Elsevier Ltd. All rights reserved.