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.