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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 20

3.0

COMPARISON OF TECHNOLOGIES

Rapid dewatering techniques can range from singular elements such as chemical addition to complex combinations of chemical and mechanical elements in series. The utilization of rapid dewatering technology may increase or decrease the unit cost of dredging a project depending on the site specific factors including dredge material characteristics, disposal area location and size, pumping rates, etc.. In certain instances, the use of these methods could prove to be less expensive than purchasing large disposal areas. Rapid dewatering technology can produce dewatered solids that are capable being trucked to remote disposal sites and have the potential for immediate beneficial reuse in a variety of applications.

For the purpose of this report, rapid dewatering techniques are grouped into four general categories – mechanical, physical, passive and chemical. In most of the recent lake dredging projects that utilize rapid dewatering, chemical flocculants have been used to enhance the

dewatering and settling characteristics of the solids. There are a number chemicals used for this purpose, and several are included in the vendor list

companies will receive and analyze sediments from

effective

chemical(s)

for

the

project.

The

classes

a candidate lake to of chemicals used

of manufacturers of in Section 5. These determine the most for dredge material

dewatering include a wide variety In recent years, polyacrylamide dewatering of lake sediments.

of polyacrylamides, polymers seem to

as be

well the

as ferric chloride, lime, and others. chemical of choice for the rapid

As noted above chemical flocculants may be used as a stand alone rapid dewatering technique. In this case, polymers are injected into a dredge slurry, and the slurry is then placed in shallow drying beds. Solids content following the polymer addition may range from 10 to 15% solids (dry weight). Care must be taken handling the slurry following polymer addition to prevent the destruction of the flocs during pumping to the drying beds. Water naturally drains from the solids and the material is left in the beds for additional drying. Examples of this approach include Lake Hollingsworth in Polk County, Sippo Lake in Ohio and the C-51 Canal in Palm Beach County.

Other rapid dewatering techniques utilize some combination of dewatering chemicals and

mechanical, physical or passive technologies.

tailored

to

the

project.

For

example,

J.F.

The selection Brennan Co.

of equipment and processes is often offers an Integrated Dredging &

Dewatering (ID&D) process that may include the use of drying depending on the material and drying area. Other contractors, such fixed process train for rapidly dewatering dredged sediments, and are the operation by adding or removing the number of processing units.

beds or geotextile tubes as Genesis, offer a more able to appropriately size

Table 2, presents a comparison of the different processes methods. Additionally, a standardized set of dredging conditions is identified to provide for a basis of comparison. Properties such as, drying effectiveness, processing area requirements, disposal area requirements, equipment needs and unit costs are included in the comparison of methods. Where complete data was not available, performance numbers were assumed based on data given for similar equipment.

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