Figure 3a shows the plot of the relationship (Example 1a) between the freshwater target for SK1 and the flowrate of SR1 supplied to SK1, for both contaminants. Note that the flowrate of SR2 (FSR2,SK1) is dependent on SR1 and Fw, so its flowrate can be computed by performing mass balance around SK1, which is FSR2,SK1 = FSK1 -FFW,SK1- FSR1,SK1.
The shaded region represents the feasible region of the freshwater requirement. By observing Figure 3a, it is apparent that the minimum freshwater always falls into the boundaries of the feasible region, i.e. one of the impurity constraints would always be active. The interesting point is where both the boundary lines intercept, which represents both impurity constraints are active. As SR1 and SR2 are conflicting sources in both contaminants ’A’ and ’B’, the boundary lines are linear with the respective negative and positive gradient. Assuming no limitation on SR1 and SR2 flowrates, the minimum freshwater point is always at the point where all impurity constraints are active. If one of the sources has limited flowrate, the minimum point cannot be achieved. In such a case, any points along the lowest boundary line are the minima. Figure 3b shows the plot for non-conflicting sources (Example 1b). In this case, the intersection point between the boundary lines is no longer the minimum point, because both lines are with a positive gradient. The usage of SR1 should be minimised, to reduce the freshwater target, which means SR2 should be maximised. SR2 should be fully used up first before SR1 is used. The source prioritisation becomes similar to the single contaminant case.