Figure 5: Sequential source allocation with a developed methodology for multiple contaminants
However, the optimal source allocation strategy in a multi-contaminant water network is not straightforward. The prioritisation of the cleaner source is no longer applicable for the multi-contaminant case as the ranking of the sources is not obvious. This is demonstrated in Figure 6.
Figure 6a shows the demonstration plot of the sink and source CC for a single sink and two sources. Source 1 and Source 2 are conflicting. The sources are arranged based on the ascending order of contaminant ’A’ concentration. In this example, the Pinch point occurs at the contaminant ’B’ if Source 1 is prioritised over Source 2, while for contaminant ’A’ Pinch is not reached. However, there is room for freshwater reduction by reducing Source 1 flowrate allocated to sink 1. This is because Source 1 has a higher concentration in Contaminant B than Source 2. Reducing the use of Source 1 helps to reduce the load for Contaminant B. As presented in Section 2.3 for the case of conflicting sources, the optimal freshwater requirement occurs when all contaminant limits are reached. To achieve this, Source 1 can be reduced until the distances from the endpoint of both sink CCs to both of the source CCs are identical (see Figure 6b), i.e. Pinch Points occur for all the contaminants. The source CCs can be shifted to the left until both Pinch points are reached (Figure 6c). This is the optimal source allocation strategy for the specific sink. Similar steps can be repeated for the subsequent sinks.
The determination of the sources flowrates by manually adjusting the curves can be time-consuming, especially when there are plenty of sources available. This issue can be solved by using Eqs(4-5). To determine the reduced flowrates of the sources, one can set the following condition from Eq(4):