One common assumption used in the design of water/wastewater systems for single components is to fix the process outlet concentrations of the pollutant to its maximum allowed value. This converts the problem from one with nonlinear constraints into one with linear constraints. For problems minimizing freshwater consumption in single-contaminant systems, this assumption has been proven to lead to global optimality (Savelski and Bagajewicz, 2000). In this article, we investigate the effect of using this assumption in cases where it may not lead to global optimal solutions, namely when the number of connections is minimized and when the cost is minimized. We therefore show that the use of nonlinear models helps in dealing with degenerate solutions featuring the same freshwater consumption.