Coastal wastewater treatment and ocean outfall facilities are usually planned as two separate subsystems in the conventional design strategy. However, the technical efforts in planning these subsystems can be combined by considering the minimization of system costs and the maximization of utilization rate of environmental assimilative capacity. But the system uncertainties should be further addressed in the modelling framework as the variations of environmental factors become significant. The grey systems theory is found to be an effective tool to modify the deterministic models. Therefore, a grey nonlinear programming model is developed in this paper to support such an uncertainty analysis. The case study of the Guishuic wastewater treatment and disposal project in Taiwan is used as a numerical demonstration.