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.