TREATMENT AND DISPOSAL OF DREDGED MATERIAL: AN UPDATE
U.S. Environmental Protection Agency Endorses use of Ellicott Series 370 Dredge for PCB Remediation
Broadly speaking the prime consideration for choosing between alternative disposal strategies for dredges sediment is the chemical and physical nature of the sediment, and the presence of any contaminants which must not be released into the environment by the dredging process, or during transportation, treatment and disposal. Other prime considerations include the quantity of material to be disposed of and the practicability and cost of implementing the proposed option.
Sediment at New Bedford harbor in southeastern Massachusetts (USA) have been the subject of study programs since the late 1970's when they were first observed to be heavily contaminated with polychlorinated biphenyls (PCB's) and heavy materials derived from industrial activities along the waterfront.
A major dredging operation at the port started in 1994 and was completed in 1995. Prior to commencing dredging the EPA divided the Bay area into three study areas known as "the hot spot area," the Acushnet River Estuary, and the Lower Harbor. The "hot spot area" was adjacent to a factory which manufactured electrical capacitors and was identified as the source of the PCB's. Here the concentration of PCB's was greater than 4,000 parts per mission (ppm), and concentrations of up to 100,000 ppm have been identified. Much of this area, some 0.6m -1.2m deep, also contained heavy metals such as cadmium, chromium, copper and lead in high concentrations.
After investigating a range of options, the EPA opted to undertake a dredging campaign to remove 7,650m3 of sediment using a hydraulic pipeline (cutterhead) dredge, and to incinerate the sediments after they had been treated and dewatered. The ash from the process would be disposed of in a confined disposal facility (CDF) which would then be capped.
The dredging operation was complicated by the physical constraints in the harbor, shallow waters, and limited access to the site. Resuspension of sediment was minimized using a silt curtain and an oil boom to contain resuspended material. The CDF was constructed along the New Bedford shoreline and was then modified to break it down into three cells for the water treatment process. A polyethylene geomembrane liner was fitted as was a floating cover over the larger of the three cells into which the dredged material was pumped. The water treatment facility treated 26.5 liters per second through an equalization tank, flocculation tank, secondary clarifier, automatic backwash filter unit, polishing filters, and an ultraviolet hydrogen peroxide system.
A monitoring system based on the net transport of PCB's was developed at the EPA's Environmental Research laboratory and an upper limit for the net quantity of PCB's above the background level caused by the dredging operation was set. Adjustments to the dredging operation were made to reflect the rate of contaminant release with respect to this upper limit. In response to local opposition to the incineration of the sediment, an air-monitoring program was also instituted.
Dredging work started in April 1994, using an Ellicott Series 370 dredge, pumping sediments through a pipeline of some 1.6km from the hot spot to the CDF. It quickly became apparent that the dredging process would have to be modified after a layer of PCB oils was brought to the surface causing elevated levels of airborne PCB's. To mitigate against this the dredge's swing speed was reduced to the slowest the operator would maintain, and the dredge was stopped from time to time to allow the PCB oils to be removed. A shroud was fabricated at the site and installed over the cutterhead to catch oil as it was released from the sediment. Although these modifications have slowed the job down considerably, the dredging has successfully removed contaminated sediment in the area of the hot spot without significant resuspension.
Source: Excerpted with permission from Port Engineering Management
