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240 11. GUIDELINES FOR PRETREATMENT SYSTEM SELECTION
source water to levels, which allow for reliable, efficient, and cost-effective desalination by reverse osmosis (RO). However, the selection of pretreatment system configuration is mainly based on the concentrations of the particulate, colloidal, natural organic matter, and microbial foulants because they govern the design of the most widely used pretreatment technologies at present. Mineral scaling is not considered a factor determining the selection of pretreatment system configuration because all saline source waters have scaling compounds at levels that could influence RO system performance and, therefore, all desalination plants are equipped with antiscalant addition systems.
In the pretreatment methodology presented herein, the amount of particulate foulants is quantified by the levels of turbidity and silt density index of the saline source water; colloidal fouling is reflected by the content of total hydrocarbons (THC) and iron and manganese in the water; and natural organic matter and microbial fouling are quantified by total organic car- bon concentration. In addition, microbial fouling caused by algal blooms is taken into account by the total concentration of algal cells in the saline source water, which as shown in Table 2.8, defines the severity of the algal blooms that may occur in the intake area.
11.2 PRETREATMENT SELECTION GUIDELINES
The most viable membrane pretreatment configurations and technologies recommended for the site-specific conditions of a given desalination project are classified based on the type of the desalination plant intake and source water quality.
11.2.1 Plants With Subsurface Intake
As indicated in Chapter 4, subsurface intakes (e.g., beach wells, horizontal directionally drilled wells, infiltration galleries) usually collect saline water of superior quality than open intakes because their water is naturally prefiltered though the bottom sediments. As a result, typical sa- line source water collected by subsurface intakes will have the water quality shown in Table 11.1.
TABLE 11.1 Typical Source Water Quality of Plants With Subsurface Intakes
Source Water Quality Parameter Value
Turbidity, NTU <0.5 SDI15 <3.0 Total organic carbon, mg/L <1.0 Total hydrocarbons, mg/L <0.01 Total iron in reduced form, mg/L <0.05 Total manganese in reduced form, mg/L <0.02 Algae, cells/L 0.0 SDI, silt density index.