Dewatering is the process of removing as much water as practical from the dredged sediment. Contaminated sediment is processed through several stages before the remaining slurry is pumped to the filter presses. These stages include coarse debris separation, coarse and fine sand separation and pre-thickening.
Grain sizes greater than 43 microns are separated upstream from the presses because water drains readily from these grain sizes. PCBs do not adsorb well to the coarse grain sizes, therefore the PCB concentrations are very low in the coarse grain; generally low enough to allow beneficial re-use (not required to be landfilled).
The sediment slurry first passes over a single-deck vibrating screen that allows material less than ½" diameter to pass through. Following rinsing, coarse particles such as rock, gravel and debris in excess of ½" in diameter are deposited on a conveyor and stacked for transport to the landfill. All remaining material (i.e., sand, silt and finer grain sizes) will pass through the screen and enter a slurry holding tank. To ensure uninterrupted operation of this critical step, redundancy has been built into the design.
The slurry is then pumped through a 150 micron coarse sand separation unit to separate potentially re-usable, clean sand from the slurry. After polishing through an upstream clarifier, the separated sand is dewatered, using a dewatering screen, and stacked via a conveyor. The hydro-cyclone overflow, which contains grain sizes less than 150 microns, is collected in a second slurry holding tank.
Slurry from the second slurry holding tank is processed in two parallel Fine Sand Separation Units, in order to separate the fine sand from the slurry. This finer sand is dewatered using dewatering screens and also stacked outside the processing facility. Overflow, comprised of fine sands, collects in the Residue Tank.
A rotating rake pushes the settled sludge from the Residue Tank to the center; compacting and consolidating the material. Once there is a significant amount of sludge built up, underflow pumps move the thickened sludge toward the Sludge Holding Tanks.
From the Sludge Holding Tanks, the thickened sludge is pumped into the filter press. As the slurry is pumped into the presses, water passes through the filter fabric inside the plates, leaving the fine grained solids in the press and onto the water treatment equipment.
When the press has achieved capacity, the remaining free water is squeezed from the sludge by adding more pressure through the filter press membranes. After squeezing, the pressure is released and the filter press opens, allowing the filter cake to fall down into the filter cake conveyor system directly below the filter press.
The conveyor system transfers the filter cake to an area for stacking, transportation and subsequent disposal.
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