Wastewater generation variables

In a situation where scheduling is not given beforehand, time is treated as an optimisation variable to provide a truly optimal schedule for freshwater demand and wastewater generation. The problem, in this particular case, can be stated as follows. Given the aforementioned (i)-(v ) conditions for each water using operation as well as ... 

This simple example illustrates that the wastewater target for a plant is dependent on the schedule of the tasks within the time horizon. It also illustrates that the least amount of wastewater generation can only be achieved if the schedule is flexible. This implies that time is treated as a variable instead of a parameter. Important to note is that the production over the time horizon has not been negatively affected by the changing of the schedule, both wastewater generation targets and production targets are achieved. 

The solution was found in a total time of 522.07 CPU seconds and the resulting formulation had 162 binary variables. The objective function had an optimal value of 1.869x 106c.u. with 9 time points. The total effluent generated was 504.63 t. As with the previous solution, had recycle/reuse not been considered the total effluent would have been 562 t of water for the same production. This relates to a 10.2% decrease in the amount of effluent generated by recycling/reusing wastewater. 

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