Optimal Capacity Expansion with Off-Design Costs

The effect of off-design operating costs on operating and capacity expansion policy is considered for the planning of a water treatment facility. A mixed-integer programming model is presented for minimizing discounted costs of dynamically expanding a reverse-osmosis process that is required to remove increasing levels of salt over time from a brackish stream. Several possible product effluent qualities are considered as variables in the model. Results show the explicit consideration of off-design operating costs can influence capacity expansion planning and reduce actual system costs by a small percentage for desalting facilities. This (apparently small) percentage saving could amount to significant absolute savings in multimillion dollar projects. Analogous percentage savings in other process planning (e.g., municipal water and wastewater treatment, and power generation) could be more or less significant depending on relationship of design to off-design generating costs.