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ALUM TREATMENT OF STORMWATER: THE FIRST TEN YEARS Harvey H. Harper, Ph.D., P.E. and Jeffrey L. Herr, P.E. Environmental Research & Design, Inc. 3419 Trentwood Blvd., Suite 102; Orlando, FL 32812-4863 Phone: (407) 855-9465; Fax: (407) 826-0419; email: [email protected] Eric H. Livingston Florida Department of Environmental Protection Bureau of Water Resource Protection/Nonpoint Source Management Twin Towers Office Building - MS 3570 2600 Blair Stone Road; Tallahassee, FL 32399-2400 Phone: (850) 921-9915; Fax: (850) 921-5655; email: [email protected] 1. Introduction Since at least Roman times, salts of aluminum have been added to drinking water and surface water to reduce turbidity and improve appearance. Aluminum sulfate, Al 2 (SO 4 ) 3 , commonly called alum, has been used extensively as a flocculating agent in the treatment of wastewater for over 100 years. In 1970, Jernelov was apparently the first to use alum to remove phosphorus from the water column of a lake in a whole-lake alum application conducted as part of a lake restoration project on Lake Langsjon in Sweden. The first U.S. lake to be treated with a whole-lake alum application was Horseshoe Lake in Wisconsin which received a surface application of 2.6 mg Al/liter in May 1970. Twelve years later, phosphorus concentrations were still below the pre-treatment level (Garrison and Knauer, 1984). The addition of alum to water results in the production of chemical precipitates which remove pollutants by two primary mechanisms. Removal of suspended solids, algae, phosphorus, heavy metals and bacteria occurs primarily by enmeshment and adsorption onto aluminum hydroxide precipitate according to the following net reaction: Al +3 + 6H 2 O 6 Al(OH) 3(s) + 3H 3 O + (1) Removal of additional dissolved phosphorus occurs as a result of direct formation of AlPO 4 by: Al +3 + HnPO 4 n-3 6 AlPO 4(s) + nH + (2) The aluminum hydroxide precipitate, Al(OH) 3 , is a gelatinous floc which attracts and adsorbs colloidal particles onto the growing floc, thus clarifying the water. Phosphorus removal or entrapment can occur by several mechanisms, depending on the solution pH. Inorganic phosphorus is also effectively removed by adsorption to the Al(OH) 3 floc. Removal of particulate phosphorus is most effective in the pH range of 6-8 where maximum floc occurs (Cooke and Kennedy, 1981). At higher pH values, OH - begins to compete with phosphate ions for aluminum ions, and aluminum hydroxide-phosphate complexes begin to form. At lower pH values and higher inorganic phosphorus concentrations, the formation of aluminum phosphate (AlPO 4 ) is favored. In 1985, a lake restoration project was initiated at Lake Ella, a shallow, 13.3 ac hypereutrophic lake in Tallahassee, Florida, which receives untreated stormwater runoff from approximately 163 ac of highly impervious urban watershed areas. Initially, conventional stormwater treatment technologies, such as retention basins, exfiltration trenches and filter systems, were considered for reducing available stormwater loadings to Lake Ella in an effort to improve water quality within the lake.
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