| A capacity expansion model, exploring the trade-off between economies of scale and the time-cost of early construction of wastewater treatment plants, was developed for a 224-km. stretch of the Chao Phraya River in Thailand. The model is designed to find the cheapest wastewater treatment capacity expansion path, including treatment plant sites, capacity increments, and associated biochemical oxygen demand (BOD) removal efficiencies to meet ambient water quality standards in every period over the planning horizon. The water quality model known as QUAL2E was used, in combination with what little is known about the characteristics of the Chao Phraya to produce a linearized version of the BOD/dissolved oxygen relation for this section of what is arguably Thailand's most important river. Using the very sparse available cost data, cost functions were formulated based on forms from the literature, in which BOD removal efficiency and flow capacity are major arguments. Because economies of scale imply a nonconvex feasible space, no analytical mechanisms are available for finding the global optimum of such a problem. A heuristic search method was developed that produces solutions varying with assumptions about parameter values in intuitively pleasing ways.; The data deficiencies may be analyzed in terms of their importance in determining the net present value of the optimal expansion path. These results suggest the special importance of tying down the water quality model parameters and the raw waste loads. Further, a comparison of the net present value of the cost of the optimal plan with the estimated costs of two, more ad hoc, government-generated plans aimed at the same result suggest that significant savings can be achieved by optimizing the timing of investments. |
