Significant Project Features
- Hydraulic dredging and mechanical dewatering of PCB contaminated sediments with highest concentrations of 7,650 ppm in a phased multi-year approach. Over 740,650 CY of PCB contaminated sediments have been removed
- Design, installation, operation and maintenance of a liquids/solids separation system and wastewater treatment system to handle a 2,000 GPM flow rate
- Dredge spoils conveyance system required 20,000 lf of dual containment piping
- 300 million gallons of filtrate successfully treated to meet criteria limits establish for metals and PCB’s (parts per trillion)
- Mechanical dredging of 100,000 CY of PCB contaminated sediments for disposal in subaqueous CAD cell
- Water depths ranging from 3-30 feet
- Tidal ranges of 3-6 feet
- Bottom dump scows used to transfer and place dredged sediments
- Use of Sevenson owned equipment including PC450, PC800, Sennebogen 850, 100 CY scows, 2.5 CY and 5.5 CY environmental buckets and support equipment
- Use of single beam surface bathymetry
- Water Quality (WQ) Management
- Large, non-homogenous dredging footprint
History & Location Details
The 18,000 acre New Bedford site is an urban tidal estuary with sediments which are highly contaminated with polychlorinated biphenyls (PCBs) and heavy metals. Manufacturers in the area used PCBs while producing electric devices from 1940 to the late 1970s, when the use of PCBs was banned by the EPA. These facilities discharged industrial wastes containing PCBs directly into the harbor and indirectly via the city’s sewerage system. As a result, the harbor is contaminated in varying degrees for at least 6 miles, from the upper Acushnet River into Buzzards Bay. Bioaccumulation of PCBs within the marine food chain has resulted in closing the area to lobstering and fishing, and recreational activities and harbor development have been limited by the widespread PCB problem.
SCOPE OF WORK
Sevenson has been contracted to perform sediment remediation activities to Jacobs Engineering under its long-term U.S. Army Corps of Engineers, New England District TERC. This includes both hydraulic and mechanical dredging. Sevenson initiated work in the Spring of 2004 with mobilization and establishment of site facilities. An existing 40,000 SF building was modified to accommodate Sevenson’s liquids/solids separation and wastewater treatment systems. Sevenson erected a 15,000 sf temporary structure to house the desanding operations. Both buildings are designed to operate this fully integrated dredging, desanding, dewatering, and wastewater treatment system to process up to 900,000 CY of contaminated sediments in a controlled environment.
The wastewater treatment system includes clarifiers, secondary settling, pH neutralization, and sand and carbon filtration systems. The desanding facility was incorporated to minimize waste treatment and disposal. Six (6) plate and frame filter presses which total 1,320 CF process filtrate.
Sediment removal is ongoing. Sevenson has encountered several challenges during operations. These include tidal influences and the need for cathodic protection to offset the potential for corrosion to equipment and treatment systems due to salt water. Modification to equipment and applications of special coatings were required to ensure the longevity of process components.
High horsepower slurry transfer pumps are strategically placed to pump dredge spoils long distances (20,000 lf) from the multiple dredge system (4 dredges). Sevenson utilizes two (2) H+H 12 inch dredges, one (1) H+H 10 inch dredge, and one (1) 12 inch Mudcat dredge. Debris removal is an important aspect of the dredging operation. Sevenson modified equipment with a rake attachment and vertical controls and conducts mechanical debris removal activities ahead of dredging to prevent dredge line clogs, equipment damage, and associated schedule impacts. This prevents driving contaminants deeper into the sediments, mitigates costly downtime for repairs and eliminates rework of dredged areas.
To support the dredge system, Sevenson has installed a series of single steel sheets. The sheet piles have provided the necessary cabling system tie-offs to allow dredges to operate dredge designated piles areas.
Thus far, dredging activities has been isolated in eight (8) specific areas. They include Areas A, B, C, D, G, H, I and the upper harbor. A total of 300,000 CY of sediment has been removed with PCB levels as high as 7,650 PPM.
The liquids/solids separation system includes eight (8) 160,000-gallon sludge slurry storage tanks, +10,000 lf of 6-12-inch piping and six (6) plate and frame filter presses, each 220 CF capacity. The presses consistently achieve high solids and yield a clean filtrate stream (<50 TSS) that is pumped to the wastewater treatment system. Filter cake is removed from presses and staged for sampling and analysis for contaminants, paint filter and percent solids. Finished piles greater than 50 ppm PCBs are loaded and shipped offsite to a permitted TSCA facility. We continuously improve our process by optimizing polymers, cycle times, and chemical dosages in the non-homogenous sediments to achieve our objectives with minimal materials and bulking and to maximize production.
Mechanical dredging from multiple shoreline locations has been added to our scope of work. Thus far, 76,700 CY of PCB contaminated soil and sediment has been removed. Materials are loaded into scows and transferred to the CDF for disposal.