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122 6. CONDITIONING OF SALINE WATER
desalination plants do not condition the coagulated saline source water with polymer. If used, only nonionic or anionic polymers are usually applied because most RO membrane el- ements carry a negative surface charge. The use of cationic polymer is likely to form a polymer film on the membrane surface, which will foul the RO membrane elements.
6.3.1 Types of Flocculation Chemicals and Feed Systems
The type and dosage of the most suitable polymer (nonionic or anionic) for a given project is typically determined by jar and/or pilot testing. Such a testing is usually completed along with the coagulant jar testing (Jamaly et al., 2014). Once the optimum coagulant dosage is established, flocculant of different dosages is added to determine its optimum application rate. Usually several different flocculants are tested in series. The main criteria for selecting optimum flocculant type and dosage are the size of the formed flocs, and the percent of turbidity reduction in the supernatant which are determined by direct visual observation and the settling rate over 30 min to 1-h period and by measuring the turbidity of the super- natant at time zero and at the end of the settling period.
Similar to coagulants, the solids removal efficiency of flocculants depends on temperature. Usually, the flocculation process is hindered in cold waters, i.e., waters with temperatures lower than 12C (54F) and could be enhanced by applying higher intensity mixing in the flocculation chambers.
Many polymers are inactivated by temperatures higher than 35C (95F). Therefore, if the source water temperature is likely to exceed this threshold limit for prolonged periods of time, it is recommended to select flocculants that are specifically designed to perform in high-temperature waters.
One of the most common challenges with flocculant addition is overdosing. The application of flocculants at dosage higher than 1.0 mg/L often results in a high content of unused polymer in the pretreated water. Such polymers usually accumulate in the filtration media (if direct filtration is used for pretreatment), on the surface of the cartridge filters, and on the RO membrane elements causing their premature fouling. To confirm that the flocculant is not overdosed, it is recommended to inspect the surface of the cartridge filters periodically and look for the formation of transparent to yellow-colored film on the surface of the cartridge filters, which is slimy on touch and appearance. Normally, the surface of the cartridge filters should be naturally rough and should not have a surface accumulation of polymer. Similar observation holds true for the front-end cap of the RO membrane elements located downstream of the cartridge filters.
Another common problem with flocculants is the content of biodegradable organics and metal (i.e., copper, nickel) impurities that can cause fouling or damage on the RO membranes. To determine the relative contribution of flocculant to the source water total organic carbon (TOC) content, it is recommended to measure the TOC concentration of the saline source wa- ter before and after the addition of flocculant to it at a target dosage. Flocculants suitable for RO desalination systems should not cause more than 0.2 mg/L of TOC increase. Use of floc- culants contributing over 0.5 mg/L of TOC to the source water is likely to result in acceler- ated RO membrane biofouling and cartridge filter replacement.
6.3.2 Planning and Design Considerations
The formation of large flocks that can be removed easily by the downstream sedimentation, dissolved air flotation (DAF), or filtration processes is a slower process, and therefore, it requires