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Problem
Sludge removal problems, at one time difficult to solve, are now being handled in stride by specialized cleanout contractors. For Lukens Steel, the challenge was to clean out a 20-million-gallon industrial cooling reservoir in Coatesville, Pennsylvania, which had a 30-year accumulation of sediment.

Solution
Lukens' contractor, Mobile Dredging& Pumping Company (Chester, PA) used a transportable dredge, a Mud Cat™ SP-915, and four transportable belt press systems to:

1. Dredge the basin to 21 feet, twice the typical cleanout depth.
   
2. Prevent the sediment, once disturbed, from dispersing and ultimately getting into the plant to possibly stain Lukens' manufactured products. In-water dispersal was minimized first by a hydraulically-operated mud shield that confined the agitated material to the area surrounding the dredge auger and its pump intake, and by contractor-placed solid-vinyl silt curtains deployed between the plant-water pump intakes and the dredging operation.
   
3. Reduce the volume that had to be off-hauled. This was achieved by the efficiency of the contractor's four mobile belt presses which, from an incoming 7 to 8 percent solids, produced a completely landfill-acceptable sludge cake of 50 to 60 percent (versus the usual 15 to 20 percent).
   
4. Protect the environment, particularly from spillage, via a four-way solution involving:
   • Staging sludge slurry feed tanks and the four trailer-mounted press systems on a bermed asphalt containment area.
   • Providing a solid loading surface for the dewatered sludge by temporarily paving the area below the press delivery chutes with 4 inches of asphalt and placing movable concrete barrier walls behind them to help contain the cake.
   • Equipping sludge haul trucks with tailgate locks to prevent accidental discharge of dewatered sediment.
   • Carefully coordinating feed of the dredged slurry through two-way radios between dredge operator and shore technicians, thus minimizing chances of overfilling.

The contractor's portable hydraulic dredge, a Mud Cat™ SP-915, removed bottom deposits, 3 to 4 feet thick, consisting of solids from equipment washdowns plus sand and dirt pumped in with river water used to replace water lost in evaporation-type cooling. By specification, removal operations were conducted while the reservoir and all manufacturing operations continued to function.

At work, the Mud Cat's™ auger-type cutting head mechanically moved a slurry of water and sediment through a self-contained trash pump and a floating pipeline. Its working path was controlled by a winch system that guided the rig along a cable successively moved to closely-spaced shore anchor points located at the opposite sides of the 180' x 1000' concrete-lined reservoir. A controlled flow of 800 gpm ended up in two ground-based portable tanks, each 21,000 gallons in capacity, which acted as surge storage facilities between the dredging and dewatering operations.

Within the tanks, mixers rotated non-stop to homogenize the mix and keep the particles in suspension, ensuring a consistent feed to the trailer-mounted twin-belt filter presses.

Four of these portable units did the dewatering, the number having been selected to match dredge capacity and the desired production schedule.

Sludge pumps on each press were set to draw the volume required to keep output to an optimum level. Each 2.5 meter press was capable of handling up to 200 gpm. Input was adjusted through a manifold to match the varying incoming sludge percentages.

A granular organic water treatment polymer was fed in from four 500-gallon tanks; the additive conditioned and flocculated the sediment. From here, the mixture was channeled between sets of porous filter belts under increasing pressures applied by a series of rollers. This pressure forced out almost all the free water, producing a sludge cake more than half solids that was conveyed to a chute for deposit onto a ground-level staging area. The material was then loaded by a small conventional tractor shovel into dump trucks and removal accomplished by hauling the material through plant property and over state highways to disposal at a plant-owned state-approved industrial landfill.

Mobilization was completed within five working days of the starting date, and reservoir cleaning was completed 70 working days later. The schedule was attained as planned by working one 10-hour shift per day, six days per week. Demobilization was completed within six working days after the reservoir was cleaned.

Results
Project pricing was based on capacity restored to the reservoir, not on the amount of dewatered material generated, which eliminated such variables as the amount of dewatered sludge generated, its solids content, and the volume of dewatering conditioners used. Thus, Lukens Steel did not pay for additional weight or volume caused by lower-than-anticipated solids content or higher-than-anticipated conditioning agents. The project was successful in providing Lukens Steel with an additional 5 million gallons of cooling water storage capacity.

Reprinted from The National Environmental Journal