Page 251 - ro membanes

234 10. COMPARISON OF GRANULAR MEDIA AND MEMBRANE PRETREATMENT
Table 10.2 presents an example of the cost comparison of conventional gravity dual-media filtration system and UF vacuum-driven pretreatment system for seawater reverse osmosis (SWRO) desalination plant with a production capacity of 50,000 m3/day (13.2 MGD). This example assumes conventional pretreatment that consists of single-stage dual media filters and membrane pretreatment employing vacuum-driven UF membranes. The conventional pretreatment system is assumed to use 5 mg/L of ferric chloride for source water coagulation, whereas the membrane pretreatment system is designed to operate without coagulant addition. As a result, the desalination plant with conventional pretreatment incorporates a solid handling system for treatment of spent filter backwash water, whereas the membrane pretreatment system does not include solids handling facilities, and the spent membrane wash water is assumed to be disposed to the ocean with the RO system concentrate without further treatment. This assumption represents a best-case solids handling scenario for desali- nation plant systems with membrane pretreatment.
Other assumptions in this example that favor membrane pretreatment are (1) relatively high land costs of the desalination plant site; (2) 12.5% higher design flux (and therefore smaller size) of the RO system using membrane pretreatment; (3) avoidance of cartridge filter installation upstream of the RO system with membrane pretreatment; (4) relatively long use- ful life of the membrane pretreatment filters (5 years); and (5) reduction of RO membrane cleaning costs due to membrane pretreatment, which typically would not be the case if the main type of RO membranes experience is microbial biofouling.
Review of Table 10.2 indicates that both capital and O&M costs for conventional pretreat- ment are lower than these for membrane pretreatment. To determine the amortized value of the capital costs, these costs are divided by a capital recovery factor (CRF). The CRF is a func- tion of the interest rate of the capital and the number of years over which the investment is repaid. The CRF can be calculated using the following formula:
CRF 1⁄4 fð1 þ iÞn  1g=fi ð1 þ iÞng (10.1)
where n is the period of repayment of capital expenditures and i is the interest rate of the amortized investment.
In this example, the total construction costs for both plants are amortized using CRF estimated for the interest rate of 5.7% over a period of 20 years. Applying Formula (10.1) for these conditions results in CRF of 11.752. The capital cost component of the seawater desalination costs will be US$0.48/m3 (US$1.82/1,000 gal) for a plant with conventional pretreatment and US$0.50/m3 (US$1.89/1,000 gal) for plant with membrane pretreatment. These costs are determined by dividing the construction costs in Table 10.2 by the CRF and by the annual production capacity of the plant. For example, for the plant with conventional pretreatment system, these costs are calculated as follows: US$102,200,000/(11.752  50,000 m3/day  365 days) 1⁄4 US$0.48/m3.
The annual O&M costs for the conventional pretreatment system for this example are estimated at US$9,470,000/year (see Table 10.2). When converted to the O&M cost of water component of the desalination plant, these costs are US$9,470,000 per year/ (50,000 m3/day  365 days) 1⁄4 US$0.52/m3 (US$1.96/1,000 gal). Similarly, the O&M costs for the desalination plant with membrane pretreatment system are calculated as US$0.53/ m3 (US$1.82/1,000 gal). Based on the capital and O&M cost estimates presented earlier, the