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9.6 PLANNING AND DESIGN CONSIDERATIONS 205 9.6.1 Source Water Turbidity
Saline source water quality has a significant impact on the configuration and design of the membrane pretreatment system. Saline water of higher turbidity will result in the need for system design around lower membrane flux and will usually yield lower overall recovery due to higher backwashing frequency requirements. Because the decrease of design mem- brane flux results in a proportional increase in the total needed membrane surface area (i.e., requires additional membrane modules and equipment) and lowering system recovery means that additional saline water would need to be collected to produce the same volume of filtered water, depending on the solids content of the source saline water, it may be more economical to remove a portion of the solids before membrane pretreatment by employing upstream sedimentation, dissolved air flotation or granular media filtration.
Typically, saline source water of annual average turbidity lower than 20 NTU, maximum turbidity of 50 NTU or less and SDI5 lower than 16 could be treated economically without upstream solids removal. If the saline source water turbidity and/or silt content are above these thresholds or if the saline water intake experiences frequent and extensive algal blooms and/or turbidity spikes, then the saline water solids removal by DAF, sedimentation, or coarse media filtration may be warranted and economical.
The most prudent approach to determine the effect of high turbidity and silt on the design of the membrane pretreatment system is to complete pilot testing during times of the year when the source water has the highest level of solids (e.g., conditions of algal blooms, frequent rain events, heavy ship traffic, or winds and/or currents carrying high amount of sand, silt or seaweeds in the plant intake). If rain- and algal bloom-related saline source water turbidity spikes occur during different periods of the year, pilot testing should encompass both of these periods.
9.6.2 Organic Content of the Source Water
Similar to turbidity, high organic content of the saline source water would result in the need to design and operate the MF/UF system at lower design flux and sometimes lower re- covery. Depending on the nature of the organic compounds in the saline water, pretreatment membranes can experience biofouling similar to that of SWRO membranes. If the source wa- ter organics consist mainly of natural organic matter that could be coagulated easily, such as humic acids, then coagulation, and flocculation upstream of the membrane filtration may result in a significant improvement of membrane flux and performance.
It should be underlined that MF and UF membranes are not very effective in removing dissolved organics and marine bacteria associated with algal bloom events, even if the saline source water is conditioned with coagulant. During algal blooms, usually over 80% of the or- ganics contained in the source water are dissolved and cannot be removed by coagulant addi- tion. Therefore, if the saline source water is exposed to frequent and intensive algal bloom events when the source water TOC is higher than 2 mg/L for prolonged periods of time (i.e., week or more), membrane pretreatment system would need to be designed for conser- vatively low flux (<65 lmh for pressure-driven MF/UF systems and <40 lmh for vacuum- driven systems). Under such conditions, MF/UF system recovery should be reduced to 90% compared to typical recoveries of 93%e95%.