*3.2. Objective Function*

The objective function consists of both capital cost and operating cost expressed per year:

$$\text{minimum } f(\mathbf{x}) = \text{annual total cost} = \text{annual capital cost} + \text{annual operating cost} \tag{34}$$

The capital cost includes the cost of membranes, pumps, pipes, and valves, instruments and controls, tanks and frames, miscellaneous (buildings, storage, etc ... ).

The operating cost includes membrane replacements and pumping energy cost.
