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SPECIAL PUBLICATION SJ2008-SP11 - page 35 / 48

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Literature Review & Summary Report Design Basis Memorandum

BCI Project No. 19-15089 September 2007

St. Johns River Water Management District Contract # SK940AA

Page 27

Thickener discharge may further thickened by means of mechanical dewatering device, or pumped to a containment for passive dewatering and drying.

Thickener capacities are specific to the application, ranging from small to tens of thousands of gallons per minute. Clarified water flows over a peripheral weir to a discharge point.

As with most mechanical dewatering systems, the need for settling basins, return water systems, and containment area restoration, as well as the piping and pumps to the settling area is eliminated.

Disadvantages:

Thickeners are large processing devices easily exceeding 100 feet in diameter, and multiple units may be required to accommodate typical dredge rates.

Thickener sludge pumped to a confined disposal area can require vast acreages of storage. In addition, engineered dams may be required, with the associated construction costs. Pumping logistics are to be considered with the generally increased distance, and as with most dewatering technologies, chemical treatment is necessary to enhance settlement. Consideration must also be given to sludge reuse timing as drying cannot occur until compartment filling is complete and the supernatant is removed.

4.3.5

Thermal Dryer

Advantages:

Thermal sludge drying eliminates water and diminishes the volume of the sludge by a factor of 4-5. This also lowers the transportation cost and facilitates storage. Thermal treatment may also increase sludge calorific value, allowing incineration without the need for additional fuel. Stability is achieved by drying sludge to a dry mass above 90%, which improves sludge structure for spreading by agricultural equipment.

Disadvantages:

Thermal drying is noted for its high-energy demand. An appropriate energy source, the best method of drying (drier type) for a particular type of sludge, and the organization of the sludge drying process so that it is efficient and safe for the environment are all problems impacting the quality of product obtained after sludge drying.

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