Liquid purification or separation – Structural installation – Geographic
Reexamination Certificate
2001-03-20
2002-08-13
Upton, Christopher (Department: 1724)
Liquid purification or separation
Structural installation
Geographic
C210S202000, C210S203000, C210S257200, C210S242100, C037S341000, C037S345000, C037S318000, C405S010000
Reexamination Certificate
active
06432303
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This application relates to a process for removing contaminated sediments from the bottom of lakes, reservoirs, rivers, streams, and other water bodies, while at the same time minimizing the release of particulate to the ambient environment during the excavation process, and cleaning the waters extracted during this process.
BACKGROUND OF THE INVENTION
Dredging to extract and remove bottom sediments involves mechanically raking, grabbing, penetrating, cutting, or hydraulically scouring the bottom of the waterway to dislodge sediment. This is a special problem when the dredge site contains highly contaminated materials that must be removed without dispersing the sediments and contaminating alternate locations. Bottom sediments disturbed by dredging operations, but not removed from the water body, pose several environmental problems. If the sediments are contaminated, the resuspension of particles provides the means for contaminants to migrate from their original source to new locations, impacting marine life in these areas and ultimately the ecosystem and food chain.
Of particular concern is the resuspension of fine clay and organic sediment particles (micron- and submicron-sized). Such particles tend to concentrate contaminants due to their high absorptive properties and the large surface areas that are cumulatively available in this very small size range. In addition to sediment toxicity problems, excessive particulate resuspension in environmentally sensitive areas results in visible turbidity, which may inhibit fish migration or reproductive patterns, impair fish gills, or cover larvae, eggs or bottom-feeding invertebrates. United States Environmental Protection Agency, Assessment and Remediation of Contaminated Sediments (ARCS) Program, Remediation Guidance Document, USEPA 905-B94-003, October 1994.
Current methods of dredging can be divided into two general categories. They include mechanical dredging and hydraulic dredging. The fundamental difference between these categories is in the form in which the sediments are removed. Mechanical dredges remove the sediments directly with clamshell-type buckets. The operation consists of lowering the bucket with a crane to the bottom of the waterway, scooping or extracting the sediment, and bringing the sediment to the surface for disposal (typically in a dredge barge). Hydraulic dredges, sometimes referred to as vacuum dredges, are designed to vacuum up bottom sediments. Unless the sediments are very loose, vacuum dredges require cutter heads or alternative means to dislodge the dredge material so that the sediment can be vacuumed into the dredge head.
Mechanical dredging operations typically yield much lower liquid to solid ratios (30 to 70 percent by weight) compared to hydraulic dredges (less than 1 to 10 percent by weight). Mechanical dredges, however, have the potential to resuspend sediment and contaminate the ambient water as the bucket initially contacts and penetrates the sediment. This action scoops and rakes the sediment, extruding bottom sediments perpendicular to the cut) away from the bucket. It introduces contaminated sediments into the inside of the bucket, which contacts and displaces water inside the bucket, forcing this (now contaminated) water back into the water column. When the bucket is lifted it produces an upswell that releases additional sediment particles into the water column. In addition, buckets that are not adequately sealed or unable to close completely, during the excavation process, will drain contaminated sediment particles back into the water column during the entire lifting cycle.
While hydraulic dredges provide a vacuum to draw in particulate matter during the dredging operation, cutter blade sediment agitation and raking is a major source of particulate resuspension, and vacuum recovery of these resuspended particles in conventional hydraulic dredge systems, particularly if there is any current in the waterway, is typically not very high. In addition, because hydraulic or vacuum type dredges remove and transport sediment in a slurry form, large quantities of water are collected with the sediment and must be contained and treated. This is typically accomplished by discharging the solids and water into an impoundment area to permit solids removal (typically by settling). The liquid portion of the slurry is normally discharged back into the ambient water environment after the settling process. The use of large impoundment areas for the dredging of contaminated sediments introduces the risk of on-shore or groundwater contamination and would, in general, be considered a questionable practice with highly contaminated sediments. Finally, a high degree of treatment of this impounded water is necessary to ensure that the excess water can be safely discharged back into the ambient water environment.
Some conventional mechanical dredges have been redesigned in an attempt to minimize sediment resuspension (Ouwerkerk, R. and H. Greve (1994). “Developments in Dredges During the Last Decade.” Pages 690-699 in Dredging '94, Proceedings of the Second International Conference on Dredging and Dredged Material Placement, Edited by: E. C. McNair, Jr., American Society of Civil Engineers. 1994. Zappi, P. A. and D. F. Hayes. “Innovative Technologies for Dredging Contaminated Sediments.” Improvement of Operations and Maintenance Techniques Research Program, U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss. Miscellaneous Paper EL-91-20. September 1991. Herbich, J. B. Handbook of Dredging Engineering, McGraw Hill, Inc., N.Y. 1992.). The primary objective of most mechanical dredge equipment modifications is to create as tight a seal as possible on the bucket through the installation of rubber backing compressible seals and sensors (proximity switches) to ensure the bucket is closed prior to lifting in the hope of minimizng spillage. While these types of systems offer improved designs, they do not eliminate the impact due to raking, upswell, and water displacement that will occur on excavation.
Numerous modifications and subcategories of hydraulic dredges have been developed to mitigate problems associated with hydraulic dredging particle resuspension. Pneumatic dredges, a subcategory of hydraulic dredges, use alternating cycles of negative and positive air pressure in a submerged chamber to draw sediment through a pipe into the chamber and to propel the sediment to the surface. Hydraulic or pneumatic dredges characterized as airlift dredges, amphibex dredges, bucket wheel dredges, clean up dredges, cutterhead dredges, delta dredges, dustpan dredges, eddy pump dredges, horizontal auger dredges, plain suction dredges, pneuma pumps, and oozer dredges are dredges that make use of hydraulic pumps and/or air compressors to draw in sediment (Cleland, J., Advances in Dredging Contaminated Sediment, Scenic Hudson, Inc., 1997). Many have specific features that attempt to reduce sediment dispersion that results from cutterblade, jetting, or raking mechanisms. None of these methods, however, significantly reduce or have suitable provisions for managing large volumes of contaminated water that are generated in the process. Large impoundment areas and suitable treatment methods are still needed to contain fine contaminant particulates that are drawn up with these sediments.
Several recent patents have proposed methods that make use of compressed air and/or hydraulic pumping operations to collect contaminated sediments in a manner that minimizes sediment dispersion.
Lynch in his U.S. patent, Contaminated Marine Sediments Dredging Apparatus, U.S. Pat. No. 5,540,005, Jul. 30, 1996, proposes the use of a dredging device that is comprised of scoop buckets, retractable rigid silt curtains, and the means to introduce pressurized air into the dredging device. Lynch's device is intended to collect dredge matter in a manner that minimizes the extrusion of soil out the sides of the apparatus during the excavation process (the purpose of the rigid silt curtains) and minimize
Chesner Warren Howard
Melrose James
Upton Christopher
Walker Alfred M.
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