2.3.1. Operating Cost

The operating cost takes into account the energy expense of the pumps and the cost of membrane replacement. The annual energy expense of the pumps is calculated as

$$C\_{\text{energy}} = \frac{E\_{\text{pump}}}{\eta} \times 8000 \times \frac{3600}{1000} \times \text{energy cost} \quad \text{[ $\$ /year]}\tag{23}$$

*<sup>E</sup>*pumps is the power supplied of the two pumps, feed pump and recirculation pump, and is evaluated by Equation (22). The assumption used in the calculation of annual energy expense is 8000 h/year operation (24 h/day and 333 days/year). η is the efficiency of the pumps, which is set to be 0.7. Energy cost is the electricity price as \$/kWh, and is supposed to be 0.08, which is the price for the industrial sector in the United States [18].

The membrane replacement cost is calculated as

$$\mathcal{L}\_{\text{membrane}} = A\_{\text{membrane}} \times c\_{\text{membrane}} \times \left(\frac{A}{P}\right) \text{ [\\$/year]} \tag{24}$$

where *C*membrane [\$/m<sup>2</sup>/year] is the membrane replacement cost calculated per year and *c*membrane [\$] is the membrane price per unit area. *AP* is the amortization factor, which presents the time value of money [19] and is calculated as a function of interest rate *i* and the membrane life.

The membrane price is usually about 200 \$/m<sup>2</sup> [9], and membrane life is 12–18 months. Therefore, the membrane replacement cost per year is roughly estimated as 200 \$/m<sup>2</sup>/year for the interest of *I* = 8%.
