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12 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
2.6.3.1 Concentration and Speciation of
Microorganisms 31
2.6.3.2 Content of Easily
2.6.4.2 TOC:TN:TP Ratio 35 2.6.4.3 RO Train Differential
Pressure Rate Increase 35 2.6.4.4 Chlorophyll a 36 2.6.4.5 Algal Count and
Profile 37 2.6.5 Threshold Levels of Microbial
Foulants 37
2.7 Combined Effect of Various
Foulants on Membrane
Performance 39
References 41
Biodegradable Compounds in the Source Water
2.6.3.3 Concentration and Balance of Nutrients
32
in Source Water 33 2.6.3.4 Source Water
Temperature 34
2.6.4 Biofouling Measurement
Parameters and Methods 34 2.6.4.1 Total Organic Carbon 35
2.1 INTRODUCTION
Saline water is collected from a source using either subsurface intakes (wells, intake galleries, etc.) or open surface intakes. Subsurface intakes naturally prescreen and prefilter the collected saline water and thereby they remove coarse debris and most of the sand and particulates from this water. Open ocean intakes that collect ambient water directly from saline surface water source have equipment (bar racks, fine traveling screens, microscreens, and/or strainers) to prescreen large debris, floating materials, large aquatic organisms, coarse sand, and stringy materials from the source water.
As a result, after preliminary screening by the intake facilities, saline water typically con- tains the following five key groups of compounds that could cause RO-membrane fouling and therefore, would need to be removed by the pretreatment system:
1. Particulate foulants (mainly suspended solids and silt);
2. Colloidal foulantsdcompounds of relatively small size (0.2e1.0 mm) that are not in fully
dissolved form, which when concentrated during the membrane separation process may
coalesce and precipitate on the membrane surface (mainly clay-like substances);
3. Mineral-scaling foulantsdinorganic compounds (i.e., Ca, Mg, Ba, and Sr salts), which
during the salt separation process may precipitate and form scale on the membrane surface (such as calcium carbonate and sulfate, and magnesium hydroxide) or may block the membrane separation layer (such as iron and manganese);
4. Natural organic foulantsdnatural organic matter (NOM) that can attach to and foul the membranes;
5. Microbial foulantsdmarine organisms and organic compounds released by them, which can serve as food to the microorganisms that inhabit in the source water and can form fouling biofilm, reducing freshwater transport through the membranes.