 |
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The
system increased worker safety because the
dredge could be remote controlled from up to
2500 ft. (762 m) away
(Model RCLPEM)
|
Problem:
Cleanup crews wanted to dredge radioactive sludge from
hazardous waste pits without releasing contaminants to the
environment.
Solution:
A remote-control system encloses the radioactive material
during dredging and keeps workers at a safe distance.
The
Weldon Springs, Mo. Superfund site and remedial action project
has four pits containing low-level radioactive raffinate
debris and sludge. The sides and bottoms of the pits are lined
with native clay, and contaminated soils are mounded on top of
the lagoon. Contaminated sludge is kept in a slurry to prevent
radon emissions.
To
remediate radioactive materials, cleanup crews installed a
floating dredge typically used at wastewater treatment plants,
power plants, petroleum refineries, and other facilities,
according to Liquid Waste Technology, LLC (LWT) of Somerset,
Wis., which manufactures Pit Hog™ dredges.
Each
dredge is custom-built and equipped with remote-control
equipment. The dredge uses an underwater auger and agitator to
make a slurry that is pumped aboveground, where it is
transformed into grout to fill voids in disposal sites for
destroyed uranium buildings.
The
dredge at the Weldon Springs site uses a programmable logic
controller (PLC) to direct traversing movements, depth, and
rate of pumping. The PLC was delivered to the site
"custom tailored to suit our needs," according to
Bruce Fox, operations engineer at the Weldon Springs site.
The
system is automated to cover the entire pond by itself,
explains Don Mueller, LWT's Vice President. This is important
when dredging radioactive materials, he says, because it
allows workers to stand up to 762 m (2500 ft) away from the
toxic lagoon, increasing their safety.
The
Pit Hog™ system is designed to continuously deliver
consistent solids and flow volumes. The programmed PLC
provides control over the dredge's automated functions and
controls loops around the lagoon, minimizing labor and
maximizing production of solids at set flow rates.
Settled
solids are excavated from the lagoon by LWT's dredging unit.
The unit's movements are automated by a steel rail system
located at the ends of the lagoon. The PLC is programmed to
direct the dredge to make "sweeps" over the lagoon.
The dredge travels at a controlled speed, raking 2.4m-wide (8
ft-wide) passes through the solids bed. When the dredge
reaches the end of the run, it automatically accelerates into
high-reverse speed, slows the pumping rate, and travels until
it reaches the initial starting point for that pass. The
dredge then raises the auger head, shifts laterally to the
starting point of the next pass, lowers the auger head,
restores the slurry pump speed, and moves forward to continue
dredging.
The
variable-speed hydraulic pump used in the process features
heads up to 36.5m (120 ft) in diameter and can handle flows
ranging from 0 to 5677 L/min (0 to 1500 gal/min). the pump has
a cast iron volute and impeller, triple bearings, 76 mm (3
inch) solids handling capacity, and a stainless steel wear
sleeve. The efficiency of the pumps usually exceeds that of
typical fabricated pumps, according to LWT.
Engineers
at the site say they are pleased with the dredge's
performance. The dredge pumps between 16% and 22.6% solids,
exceeding the expected 15% solids slurry, Fox says. The
process is safe because the mixture remains in slurry and is
enclosed throughout the operation. "Waste-pit dredging is
the preferred method when waste is treated and disposed
onsite," says John Enger, project engineer at the site.
Source:
Wastewater Technology
