Soldiers Creek Regional Stormwater Alum Treatment Facility at CR 427

The Seminole County Engineering Department teamed with the Florida Department of Transportation to design and construct a regional stormwater treatment facility.  FBT provided the structural design for the innovative alum treatment system consisting of a braced steel sheet pile trough for collection and removal of the solids (precipitate) resulting from the alum treatment.  The maximum retained height of the sheet pile is 28 feet and the overall length of the trough is approximately 420 feet. 

 

Alum is injected into the trough, and the precipitate which forms (floc) settles out of the water and is guided by a sloping surface to the pumping system that removes the floc from the trough. This process greatly improves the water quality of the water which ultimately makes its way into Lake Jesup.  The project also included a buried vinyl sheet pile cut-off wall to separate the trough from an adjacent wetland feature, and an aluminum sheet pile weir structure.  The construction cost for the project was over $6,000,000.  FBT worked with Environmental Research and Design, Inc. (ERD) and Pegasus Engineering, LLC to produce the construction documents for this project.

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The process of adding aluminum sulfate salt—otherwise known as alum—to stormwater is called alum injection. Alum combines with fine particles to merge into larger particles. Alum injection is particularly effective at reducing concentrations of soluble and particulate phosphorus, but the process is also applicable to other pollutants, particularly those associated with sediments. When stormwater professionals inject alum into stormwater, it forms precipitates of aluminum phosphate and aluminum hydroxide. These precipitates can combine with suspended solids, heavy metals and some microorganisms to settle into the sediment in a highly stable, inert state (Harper, 2007). A common term for the collected mass of alum precipitates, pollutants and sediments is floc.