Alum Treatment of Stormwater: the First Ten Years
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, Al2(SO4)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/litre 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 + 6H2O 4 AI(OH)3(s) + 3H3O+ (9.1)
Removal of additional dissolved phosphorus occurs as a result of direct formation of AlPO4 by:
Al+3 + HnPO4n-3 ? AlPO4(s) + nH+ (9.2)
The aluminum hydroxide precipitate, AI(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 AI(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 (AlPO4) is favored.
In 1985, a lake restoration project was initiated at Lake Ella, a shallow, 13.3 acre (5.4 ha) hypereutrophic lake in Tallahassee, Florida, which receives untreated stormwater runoff from approximately 163 acres (66 ha) 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. Since there was no available land surrounding Lake Ella that could be used for construction of traditional stormwater management facilities, and the cost of purchasing homes and businesses to acquire land for construction of these facilities was cost-prohibitive, alternate stormwater treatment methods were considered.
Chemical treatment of stormwater runoff was evaluated using various chemical coagulants, including alum, ferric salts and polymers. Alum consistently provided the highest removal efficiencies and produced the most stable end product. In view of successfu1jar test results on runoff samples collected from the Lake Ella watershed, the design of a prototype alum injection stormwater system was completed. Construction of the Lake Ella alum stormwater treatment system was completed in January 1987, resulting in a rapid and significant improvement in water quality.
The alum precipitate formed during coagulation of stormwater can be allowed to settle in receiving waterbodies or collected in small settling basins. Floc settling rates vary considerably, depending on the chemical characteristics of the stormwater, typically ranging from 250-3000 mm/hour. Alum precipitates are exceptionally stable in sediments and will not redissolve due to changes in redox potential or pH under conditions normally found in surface waterbodies. Over time, the freshly precipitated floc ages into even more stable complexes, eventually forming gibsite. The solubility of dissolved aluminum in tile treated water is regulated entirely by chemical equilibrium. As long as tile pH of the treated water is maintained within the range of 5.5-7.5, dissolved aluminum concentrations will be minimal. In many instances, the concentration of dissolved aluminum in the treated water will be less than the concentration in the raw untreated water due to adjustment of pH into the range of minimum solubility.
There are currently 23 alum stormwater treatment systems either operational or under construction in Florida and one experimental system in Seattle, Washington. Alum treatment of stormwater runoff has now been used as a viable stormwater treatment alternative in urban areas for over ten years. Over that time, a large amount of information has been collected related to optimum system configuration, water chemistry, sediment accumulation and stability, benthic impacts, construction and operation costs, comparisons with other stormwater management techniques, and floc collection and disposal. A summary of current knowledge in these areas is given in the following sections.
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