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Boifouling Potential In Open Sea And Adjacent Beach Well Intake Systems
Boifouling Potential In Open Sea And Adjacent Beach Well Intake Systems Imprimir
Miércoles, 27 de Abril de 2011 18:31
Radiación solar
Intense solar radiation and elevated water temperature are characteristics of the Red Sea particularly in the southern part. These factors make coast water conducive to biological growth.
Consequently, seawater reverse osmosis (SWRO) desalination plants sourced from open sea intake systems are subject to membrane fouling. Chronic membrane fouling in one such plant on the southwestern Red Sea coast of Saudi Arabia has necessitated the search for an alternative beachwell intake system. The beachwell system consists of three wells located within a distance of 20–30m from coast line and spaced about 75m from each other. Measurements of bacterial growth rates, bacterial biofilm formation, organic and inorganic nutrients were carried out in the beachwell water and compared to those of adjacent coastal seawater.
Beachwells showed significantly higher bacterial growth rates and increased inorganic nutrient concentrations, but significantly lower biofilm formation and decreased organic nutrient concentrations than seawater. Therefore, presence of organic nutrients is a determinative factor in biofilm formation and subsequent membrane fouling. The present data indicate that feed water for the plant can be sourced directly from beachwells without significant fouling problems. On the other hand, well water also possesses characteristics which promote improved plant operation. These include reduced silt density index (SDI) values of less than one and a constant, favorable temperature of about 33 °C.
Keywords: Open sea intake, Beachwell intake, Biofouling, Nutrients
Maintaining acceptable water quality for membrane desalination is essential for successful operation of reverse osmosis plants. Questionable source water quality results in operational problems for many reverse osmosis (RO) plants. Such problems are magnified in plants with surface intake, particularly in tropical and sub-tropical regions. In such regions, the hot climate promotes biological growth and membrane fouling. The feed water withdrawal pipes are usually located at shallow depths. Such a location allows the water column to become easily disturbed. This results in elevated concentrations of total suspended solids (TSS). At times, elevated levels of TSS create filtration problems, and plants may need to be totally shut down until silt density index (SDI) values normalize. The combination of bacterial fouling and high SDI are particularly manifested at the Saline Water Conversion Corporation (SWCC) Seawater Reverse Osmosis (SWRO) plant at Al-Birk. The fouling and filtration problems at this plant arise from unfavorable feed water drawn from an open sea intake system. This is already well documented [1,2,3].
Efforts to control fouling and pretreatment problems at this plant included operation without chlorination/dechlorination [2] and operation with a chlorine-tolerant membrane [3]. None of these measures alleviated operational problems. They failed
because of fluctuating seawater quality and particularly seasonal encounters of high silt load. Therefore, the marine environment does affect the operation and maintenance of coastal desalination and power plants. Consequently, high quality source water is a prerequisite for the successful operation of such plants. The adverse prevailing marine conditions on both the Gulf and Red Sea coasts require complex pretreatment. Such pretreatment is not yet available let alone economically feasible.
None the less, an immediate and attractive alternative to an open sea intake exists in the form of subsurface intake system of beachwells. Preliminary hydrogeologic studies at Al-Birk SWRO plant using bore holes revealed SDI and water characteristics which make the development of beachwells feasible. Subsequently, three beachwells were excavated about 20-30 m from the coast line and spaced about 75 m from each other. Elemental and chemical analyses showed beachwell water to be comparable with or superior to nearby seawater. Trace metal concentrations were within WHO standards [4]. The beachwell water turbidity is quite low thus, no further filtration may be required prior to sending this water for RO. Although both chemical and physical compositions of beachwell water are optimal for successful RO operation, it is still not yet known whether bacterial growth rates would be elevated enough to pose a potential source of fouling and thus prohibit successful desalination.
This paper reports on bacteriological and nutrient analyses conducted as part of a feasibility study of beachwells as a potential source of feed water for the Al-Birk SWRO plant...