This Article presents a mathematical programming model for the mass integration of eco-industrial parks. The model considers the reuse of wastewater among different industries and the constraints given by the process sinks and the environmental regulations for waste streams discharged to the environment. The model allows the optimal selection of treatment units to satisfy the process and environmental regulations. The objective function consists of the minimization of the total annual cost, including the treatment unit costs, the piping costs, and the cost of fresh water. A new discretization approach is proposed for the model reformulation to handle the bilinear terms of the model as part of a global optimization strategy. Results show that significant savings can be achieved for the design of an integrated eco-industrial park with respect to the integration of each individual industry.