Minimum wastewater target

The true minimum wastewater target is ensured using constraints (6.60), which ensures that at the end of the time horizon there is no water in the storage vessel. Since one does not have any information on events subsequent to the time horizon of interest, it serves no purpose to store any water at the end of the time horizon. Furthermore, water stored at the end of the time horizon gives a false impression of the amount of water saved. 

The formulation presented in this chapter provides a means to take different storage considerations into account. The methodology is capable of dealing with multiple contaminants and determines the minimum wastewater target in such a system as well as the corresponding schedule. 

As mentioned previously the objective was the minimisation of the required storage vessel while still achieving the minimum wastewater target. This was done using the solution algorithm discussed earlier. 

Using the methodology derived by Majozi (2006) a central storage vessel with a minimum capacity of 300 kg was identified. However, as can be seen above, with the use of inherent storage this is not required. In both the solution found by Majozi (2006) and that above the minimum wastewater target identified was 1285.5 kg. 

In concentration interval (200 100 ppm), only process 1 exists, Fig. 12.3. Targeting in this concentration interval is shown in Fig. 12.6. The total amount of water required is 100 t at a concentration of 200 ppm. Since 102.5 t of water at a concentration of 200 ppm is available for reuse from the previous interval, no fresh water is required in this interval. This eventually sets 200 ppm as the pinch concentration, i.e. the concentration beyond which the water target does not change. The total amount of wastewater generated from processes is 102.5 t, which is the minimum wastewater target. 

Targeting Minimum Wastewater Treatment Flowrate for Single Contaminants 607... 

Figure 26.38 Targeting minimum wastewater treatment flowrate. (From Wang YP and Smith R, 1994, Client Eng Sci, 49 3127, reproduced by permission of Elsevier Ltd.

Figure 26.39b shows the construction for targeting minimum wastewater treatment flowrate when removal ratio has been specified. If the initial effluent treatment line shown dotted in Figure 26.39b is considered, then there is a point of origin... 

Complete the network to achieve the minimum fresh water usage target and the minimum wastewater discharge target. 

As can be seen from Fig. 7.9, the objective of this problem is to move the terminal wastewater node (w = 6) to the targeted location designated by the X. The challenge, is how should the other nodes by moved to meet this target at minimum cost ... 

These results can be used to construct the solution as shown in Fig. 7.14. The target for minimum CE discharge through segregation, mixing and direct recycle is 0.488 X 10 kg/s (about 15 kg/yr). The solution indicates that the optimal policy is to segregate the effluents of the two scrubbers, pass the effluent of the first scrubber to the reactor, recycle the aqueous effluent of the reactor to the hrst scrubber and dispose of the second scrubber effluent as the terminal wastewater stream. 

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