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Tampa Bay, located adjacent to the City of Tampa on the west coast of Florida, is an estuary of national significance included in the National Estuary Program. One of the primary goals of the Southwest Florida Water Management District's (SWFWMD) SWIM Program is to work cooperatively with local governments to construct retrofit projects to reduce pollutant loads from nonpoint sources to Tampa Bay. Two such retrofit projects, the Largo Regional Stormwater Treatment Facility and the East Lake Outfall Treatment Facility, have been designed and permitted with construction to be completed during 2000.

In 1986, an innovative chemical stormwater treatment system was introduced in a lake restoration project on Lake Ella in Tallahassee, FL, based on the flow-weighted injection of liquid aluminum sulfate, Al2(SO4)3 .18 H2O, commonly called alum into the stormwater runoff flowing inside stormsewer lines before discharging to the lake. Upon addition of liquid alum to stormwater, non-toxic precipitates of AlPO4 and Al(OH)3 form quickly. Aluminum hydroxide, Al(OH)3, is a gelatinous floc that attracts and adsorbs phosphorus, heavy metals, suspended solids, and bacteria, causing them to rapidly settle from the water column leaving clear treated water. Alum treatment of stormwater runoff has been shown to consistently reduce concentrations of orthophosphorus and total phosphorus by 85%-95%, heavy metals by 80%-90%, suspended solids by 95%, total nitrogen by 40%-60%, and coliform bacteria by more than 99%.

Since the Lake Ella project, over 30 chemical stormwater treatment systems have been designed and constructed in Florida, Washington, and Indiana. Most of the initial systems were direct injection systems with the resulting floc settling in the receiving water. Over the past 15 years, the systems have evolved to include floc settling ponds, automated floc disposal systems, automated pH control systems, and in-line or in-lake floc traps. These innovative systems typically require little or no land acquisition or stormwater basin construction, thus greatly reducing the capital cost of stormwater retrofit projects.

In early 2000, design and permitting for the Largo Regional Stormwater Treatment Facility was completed for the City of Largo, FL. Stormwater runoff from a 469 ha urbanized area will be diverted off-line through a 1.2 m × 2.4 m (4 ft × 8 ft) concrete box culvert into an existing wet borrow pit. Aluminum sulfate will be added to stormwater runoff in the concrete box culvert, with the resulting floc settling in the existing borrow pit. An automatic floc disposal system will transfer floc from the bottom of the settling pond into an adjacent wastewater collection system. The Largo Regional Treatment Facility will remove approximately 6400 kg of total phosphorus per year, 1600 kg of total nitrogen per year, and 26,000 kg of total suspended solids per year from the Tampa Bay Estuary. The 20-year present worth cost for the project is approximately 1.8 million, resulting in a present worth cost per mass total nitrogen removed of approximately 1000/kg.

In 1999, design and permitting was completed for the East Lake Outfall Treatment Facility located in Hillsborough County, FL. This chemical injection project treats water which discharges from East Lake, a 40 ha hypereutrophic lake with an urbanized watershed area of approximately 456 ha, prior to discharging into the Tampa Bay Estuary. Water from East Lake is diverted through a 0.9 m stormsewer into a 0.5 ha constructed floc settling pond. Liquid aluminum sulfate is added to the water in the stormsewer prior to discharge to the floc settling pond. An automatic floc disposal system transmits alum floc from the floc settling pond into an adjacent wastewater transmission system. The East Lake Outfall Treatment Project will remove approximately 4500 kg of total nitrogen per year from discharges to Tampa Bay at a 20-year present worth cost of approximately 1.4 million. The 300 present worth cost per kg of total nitrogen removed is approximately 30% of the cost of an equivalent wet detention treatment system.

Alum stormwater treatment combines an extremely cost-effective method of retrofitting nonpoint source discharges in urban areas with high removal rates of nutrients, heavy metals, and bacteria. This method of stormwater treatment is not only applicable to Florida but any area needing stormwater treatment and surface water quality improvement.
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Document Type: Research Article

Publication date: 2000-01-01

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