Envirocon Uses Mud Cat™ Dredge for Innovative and Successful Steel Plant Waste Pond Remediation; Dredge used both for excavation and as spreader barge for cap material.
Mocks Pond is a 2-1/2 acre former limestone quarry that was used from the mid-1960s until 1988 as part of a wastewater treatment process by a midwestern steel and wire manufacturer. As part of the wastewater treatment, spent pickle liquor with associated sludge was piped from the plant to the pond for settlement. Over the years, sediments containing various heavy metals (including lead and zinc) accumulated in the bottom of the pond.
The task presented to Envirocon was to create a healthy, viable pond consistent with the overall plans for environmental conservation and protection. The approved remedial action plan involved dredging sediments to create a minimum of 10 feet of clear water throughout the pond, except where the existing limestone bottom was shallower than 10 feet. The removed sediments were to be dewatered through a belt filter press and the dried filter cake was to be shipped off site for disposal. Return water from the belt filter press operation was to be returned to Mocks Pond with total suspended solids (TSS) of less than 5 milligrams per liter. Upon completion of the dredging and dewatering activities, a stabilization fabric was to be placed over the bottom of the pond and covered with 2 feet of sand, thereby creating a "sub-aqueous cap" over any remaining pond sediments.
Envirocon began by establishing surveyed control points at various locations around Mocks Pond. To determine the existing basal contour of the pond and provide data for dredging operations, a pre-dredging survey was conducted.
An 8-inch Mud Cat MC915 hydraulic dredge equipped with Global Positioning System (GPS) capabilities was used to remove the sediment. GPS and a plotter were used to provide a continuous record of dredge position and sediment removal progress. Dredged sludge flows of approximately 1,200 gallons per minute were transported to holding tanks at the dewatering station through a floating dredge line attached to an 8-inch HDPE pipeline.
8" Mud Cat™ dredge equipped GPS capabilities
The dewatering station, built on a bermed and lined footprint, received the dredged sediment into a series of five 20,000 gallon closed top holding tanks. The sediment was decanted and conditioned with an NSF-approved cationic polymer prior to being pumped into a 2.2 meter, high capacity belt filter press. The pressed/dewatered sediment (~40% solids) was conveyed to a holding area for transportation to an off-site disposal facility. The filtrate from sediment decanting and dewatering was filtered through a high capacity sand filter, a dual bag filter, and finally a cartridge filtration system to achieve the required 5 milligrams per liter of TSS prior to being returned to the pond. To ensure water quality, samples were taken daily for TSS and weekly for chemical oxygen demand (COD).
Upon removal of 1,085 dry tons of sediment, pond dredging was complete. A post-dredging survey was conducted to ensure all sediment was removed to the designed depth, thereby creating the required minimum 10 feet of clear water. After engineering review and acceptance of the post-dredging survey, Envirocon began placement of a "sub-aqueous" sand cap. First, a geotextile liner (Trevira 1120) was deployed in Mocks Pond in three sections. The material was floated across the pond and anchored. Sand was used, starting from the anchored side, to sink the geotextile allowing it to conform to the new pond bottom. The two subsequent panels were similarly anchored and strategically sunk to provide a 10-foot overlap between panels.
By removing the dredge cutterhead, re-routing dredge piping and installing a velocity breaker box, Envirocon modified the 8-inch Mud Cat MC915 hydraulic dredge for use as a spreader barge. A sand/water slurry from a vortex-blending mixer was pumped to the spreader box where there was a velocity reduction which allowed the sand to settle uniformly onto the geotextile liner at the bottom of the pond. A 2-foot sand cap was placed in 3-inch lifts using a Z pattern to ensure proper overlap. To provide accurate lift placement and thickness, the spreader barge was equipped with a GPS (Trimble Pro XRS GPS unit) combined with a Horizontal Digital Depth Sounder. A subcontractor conducted weekly hydrographic surveys during sand cap placement to ensure the work was performed per the site-specific specifications.
After completion of the sand cap, a final hydrographic survey was conducted. The Client's engineer reviewed the results of the final independent survey and was satisfied that the placement was performed per design.Envirocon created a healthy, viable pond consistent with the Client's expectations. All work was performed in a safe, quality, cost effective, and timely manner.
Source: Environcon Web Site
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Paul Quinn, Director of Sales
Phone: 410-545-0240
Fax: 410-545-0293
Email: Paul Quinn
