Literature Review & Summary Report Design Basis Memorandum St. Johns River Water Management District Contract # SK940AA
BCI Project No. 19-15089 September 2007 Page 8
2.1.1 Equipment Type – Geotextile Tubes
Geotextile tubes (Figure 4) are comprised of high-strength polypropylene fabric and are fabricated to the project’s requirements. Each tube is filled with dredged material. The tube retains fine-grain fill material while allowing effluent water to exit through the tube wall. With the addition of a polymer flocculation agent, excess water drains from the container through the geotextile resulting in effluent that is suitable for reuse. Volume reduction within the container typically allows for repeated filling. Slurry to the tube may be as dilute as that discharging directly from the dredge, or pre-thickened to 12 – 15 % solids. Following the final cycle of filling and dewatering, the solids may be removed from the tube when dryness goals are met.
Geotubes are custom made and supplied in various sizes and lengths to suit design and installation requirements. They are suitable for installations on dry land or in water depths to 15 feet. The containment capacity ranges from about 1 to over 3 cubic yards per linear foot.
Figure 4 Geotube
2.1.2 Confined Disposal Area (CDA)
Dredged lake sediments are pumped in dilute slurry (as low as 4% by weight) to a confined disposal area of varying acreage. The disposal area is typically surrounded by earthen dams with wall height dependent on available acreage. The area may be sectioned to facilitate filling and decanting of the released water.
The dredged slurry is typically treated with a polymer to enhance and expedite solids settling in the disposal area, and the supernatant, decanted from the disposal area, is treated prior to return with aluminum sulfate to remove dissolved phosphates. Return water may flow by gravity through a ditch system, or be pumped through a pipeline back to the lake.