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80 4. SALINE WATER INTAKES AND PRETREATMENT
of variable frequency drives for efficient power use control, which would ultimately increase the construction cost of such system and complicate its operation.
One important issue to be taken under consideration when assessing the viability of using subsurface intakes for a given project is the fact that source water salinity could change unpredict- ably over time when influenced by freshwater inflow to the coastal aquifer. This uncertainty of intake water quality increases the risk of uncontrollable increase in unit cost of water production over time and has to be taken in consideration when comparing the overall life-cycle costs of the desalination plant operations. Therefore, subsurface intake water quality has to be thoroughly characterized by installing a set of test wells and collecting water quality samples under variety of operational conditions for a period of 12e18 months. A common practice of running such test for 72 h only and collecting one to three source-water quality samples have shown to be inade- quate for predicting the potential source-water quality variations, which may occur over time and for selecting the most suitable desalination plant pretreatment system.
Temperature is a key source water quality parameter that has a measurable effect on the RO-system design feed pressure and membrane performance. Typically, the SWRO-system feed pressure needed for production of target freshwater production flow is reduced by 5%e8% on a linear scale for every 10C source water temperature increment in a temperature range of 12e40C.
Based on tests completed at the Carlsbad seawater desalination pilot plant on cold Pacific Ocean water in the winter, when the source water temperature drops below 12C, the tem- perature effect is even more dramaticdthe SWRO feed pressure increases with 5%e10% for every 2C of temperature drop on an exponential scale until the source water temperature reaches 4C, below which the source water would begin to freeze and seawater desalination is dramatically hindered. The accelerated exponential increase in the operational SWRO feed pressure for source water temperatures below 12C is explained by similar curvilinear increase in source water density in the temperature range of 4e12C combined with changes in membrane material behavior.
Source seawater of temperature above 40C typically has two adverse effects on membrane performance that may negate the positive effect higher temperature on membrane pressure: (1) change in membrane material behavior (membrane compaction/internal fouling), which could result in shorter membrane useful life; and (2) accelerated membrane biofouling due to the ef- fect of temperature on bacterial growth. An additional negative impact of temperature on mem- brane performance is the reduction in membrane salt rejection with the increase of source water temperature. Therefore, operation at high source water temperatures (typically, 30C and higher) may compromise meeting product water quality goals in terms of TDS, chlorides, bo- ron, sodium, and other product water quality requirements and may require the installation of additional treatment stepdpartial or full second RO passdto address the negative effect of temperature on the product water quality.
The SWRO system construction-cost increase associated with the installation of partial or full second RO pass is typically in a range of 10%e25% of the cost of a single-pass SWRO system. The additional O&M costs associated with the operation of second pass-system vary between 3% and 10% of the costs for operation of the first pass.
Because of the higher depth of source water collection, subsurface intakes usually yield seawater of lower temperature and, therefore, their use may result in higher energy demand for production of the same volume of desalinated water. As discussed above, the difference in