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SPECIAL PUBLICATION SJ2008-SP11 - page 10 / 48





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Literature Review & Summary Report Design Basis Memorandum St. Johns River Water Management District Contract # SK940AA

BCI Project No. 19-15089 September 2007 Page 2

Figure 1 Dredge Slurry

Another advantage of rapid dewatering is the enhancement of the quality of water returned to the waterbody (or watershed) following the separation of water and solids. For example, the use of flocculants and coagulants can bind metals, nutrients and other pollutants to the settled solids, resulting in improved water quality of the return water.

Advances in rapid dewatering technologies has evolved in recent years as more water resource managers have incorporated dredging and sediment removal into their comprehensive watershed management programs. In Florida, this approach was first tested in large scale at Lake Hollingsworth in Polk County in 1996. Lake Hollingsworth was a 354-acre lake with over 3 million cubic yards of organic sediments on the lake bottom. A new process had been developed for the phosphate mining industry by the Florida Institute for Phosphate Research. The process mixed waste paper with polyacrilamide polymers to form flocs that were passed across a static screen. The water passed through the screen openings while the flocs traveled down the screen to a collection area. This process was tested at a pilot scale prior to the final design for the Hollingsworth dredging project and a new process was developed that did not use paper pulp but used two stage polymer injection to achieve suitable flocculation. During the final design, modifications to the process were made which, when combined with the changes that accompanied full-scale operations, resulted in poor flocculation and sediment thickening performance. Many modifications to the dewatering process were made during the four-year term of the project. (Figures 2 and 3) including the construction of an in-ground clarifier for sediment thickening following polymer addition.

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