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11.2 PRETREATMENT SELECTION GUIDELINES 245
10 NTU; (2) the predominant size of algae in the source water; and (3) the content of THC in the water.
It should be pointed out that DAF clarifiers do not operate well when treating source wa- ters with turbidity lower than 10 NTU. If the year-around source water quality analysis for a specific project shows that periods of turbidity concentration over 10 NTU last less than 2 weeks per year, then gravity granular media filters will be a better choice for the first- stage pretreatment of the source water because they provide a much higher reduction of particulate and organic foulants at lower source water turbidities.
Over 10 years of experience with using DAF for seawater pretreatment in equatorial and tropical zones of the world shows that gravity granular media filters provide better removal of microalgae (e.g., algae with size < 20 mm) than DAF clarifiers. Therefore, the selection of one technology over the other should be based on the algal profiles completed in the intake area during the season of intensive algal blooms.
If the desalination plant intake is located in industrial or recreational ports, which are exposed to frequent oil spills, the use of DAF as a first pretreatment stage would be more ad- vantageous than the construction of gravity granular media filters because DAF can provide a significantly higher THC removal efficiency than filtration (95%e99% vs. 10%e15%). Usu- ally, granular media and UF/MF filters cannot provide effective protection of the RO mem- branes against colloidal fouling caused by oil spills when the THC concentration in the source water exceeds 0.1 mg/L.
11.2.5 Plants With Open Intake Exposed to Severe Algal Blooms
Prior experience shows that some enclosed saline water bodies, such as the Persian Gulf and Red Sea, could experience severe algal blooms during which the algal content in the seawater exceeds 60,000 algal cells/L. Such algal bloom for example occurred in the Persian Gulf in the winter of 2008/2009, when the algal content in the seawater exceeded 1 million cells/L and continued for over 4 months. During this algal bloom event, which has a repeat- ability of once every 10 years, the total organic carbon in the seawater exceeded 8 mg/L and turbidity varied between 20 and 30 NTU. The severe algal bloom was caused by microalgae Cochlodinium polykrikoides. Their cells have sizes between 5 and 20 mm and are poorly removed by DAF clarifiers. This algal bloom caused the shutdown of all SWRO desalination plants in the Persian Gulf, which used membrane pretreatment and resulted in derating of the fresh water production capacity by 20%e30% of most of SWRO desalination plants with single-stage granular media filtration pretreatment for a period of 2e3 months.
Severe algal blooms do not occur only in the Middle East. Such events are observed frequently in Australia, Southern California, Chile, South Africa, and other parts of the world. The saline source water quality reflecting typical severe algal bloom events is shown in Table 11.5.
As with RO desalination plants with open intakes exposed to high-intensity algal blooms, pretreatment in this case also includes two-stage system. However, instead of dual media (anthracite and sand) filters with normal filtration bed depth of 1.4e1.6 m (4.6e5.3 ft)d trimedia filters (anthracite, sand and garnet) with a total depth of 2.0 m (6.6 ft) or more are recommended to be used to provide enhanced biofiltration and thereby to reduce down to