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The objective of this work is to simulate pretreatment steps on a laboratory scale with the purpose of producing a higher quality permeate for feed reverse osmosis process. Pretreatment steps involved in this work are a combination of physical and chemical processes, such as coagulation-flocculation, sand filtration, and microfiltration. Samples of seawater next to Itaqui thermoelectric power plant in Maranhão, State of Brazil, powered by coal, were collected and characterized. The characterization indicated high levels of turbidity, which is unusual for seawater, indicating the need of pretreatment to reverse osmosis. The combination of polyaluminum chloride dosages of 30 mg/L and 0.3 mg/L of Nalclear 8,173 anionic polymer allowed the reduction of turbidity values to below 1 Nephelometric Turbidity Unit (NTU). The use of coagulation and microfiltration membranes provided values of silt density index next to 3, while with sand filter, the silt density index values were higher than 4.

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