Based on the population growth analysis, a city’s wastewater is estimated to increase over time, as depicted in Figure 9.23 . Four phases for treatment plant expansion capacity are studied for the treatment of the future wastewater of this city in a 20-year planning time horizon. The capacity and initial investment of these projects are represented in Table 9.7 . The interest rate is considered to be 5%. Find the optimal sequence of implementing projects.

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Wastewater discharge (MCM/yr) 180 160 140 120 100 80 10 20 30 Year FIGURE 9.23 Four-phase treatment plant capacity expansion.