Alum Treatment of Stormwater Runoff _An Innovative BMP for Urban Runoff Problems

Alum Treatment of Stormwater Runoff _An Innovative BMP for Urban Runoff Problems

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205 Alum treatment of stormwater runoff was begun in 1986 as part of a lake restoration project at Lake Ella in Talla- hassee, Florida in 1986. Our system provides treatment of stormwater runoff entering the lake by injecting liquid alum into major stormsewer lines on a flow-weighted basis dur- ing rain events. When added to runoff, alum forms non- toxic precipitates of AI(OH) 3 and AIPO 4 which combine with phosphorus, suspended solids and heavy metals, caus- ing them to be rapidly removed from the treated water. The alum stormwater treatment system resulted in imme- diate and substantial improvements to water quality in Lake Ella which led to implementation of similar systems on other urban lakes. There are currently 23 alum stormwater treat- ment systems either operational or under construction in Florida, and one experimental system in Seattle, Wash- ington. Alum treatment of stormwater runoff has consistently achieved a 90% reduction in total phosphorus, 50-70% reduction in total nitrogen, 50-90% reduction in heavy metals, and >99% reduction in fecal coliform. Ultimate water quality improvements in the receiving water body have been related to the percentage of total inputs treated by the system. Heavy metal and phosphorus associations with alum floc have been shown to be extremely stable over a wide range of pH and redox conditions. In general, alum treatment of runoff is substantially less expensive than traditional treatment methods and often requires no additional land purchase. Recent designs have incorporated automatic floc collection and removal systems with disposal to drying beds or sanitary sewer. Introduction The addition of alum to water results in the production of chemical precipitates which remove pollutants by two pri- mary 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 AI(OH) 3(s) +3H 3 O + Removal of additional dissolved phosphorus occurs as a result of direct formation of AIPO 4 by: Al +3 + HnPO 4 n-3 AIPO 4(s) + Hn + The aluminum hydroxide precipitate, AI(OH) 3 , is a ge- latinous floc which attracts and adsorbs colloidal particles onto the growing floc, thus clarifying the water. Phospho- rus removal or entrapment can occur by several mecha- nisms, depending on the solution pH. Inorganic phospho- rus is also effectively removed by adsorption to the AI(OH)3 fIoc. 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 alu- minum hydroxide-phosphate complexes begin to form. At lower pH values and higher inorganic phosphorus concen- trations, the formation of aluminum phosphate (AIPO 4 ) is favored. In 1985, a lake restoration project was initiated at Lake
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Alum Treatment of Stormwater Runoff _An Innovative BMP for Urban Runoff Problems

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