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Treatment Before and. During Dredging
Spoil materials vary widely in both physical and
chemical characteristics. While many types of spoil may be
considered "polluted" major problems are generally limited
to highly organic, petrochemical-laden silts and clays, and
domestic sewage sludges found in v/aterways bordered by heavy
population or industrial concentrations.
AERATION
Aeration of bays, harbors, and other areas is sometimes
possible where organic sludges are responsible for noxious,
anaerobic conditions. In these instances aeration can
result in changeover from anaerobic to aerobic decomposition
with a long term elimination of aesthetically displeasing
odors. Such techniques can be especially effective if
combined with the selective removal of organic bottom
sediments by dredging.
CHEMICAL TREATMENT
Chemical treatments to improve spoil prior to dredging
are feasible. Chemical oxidizers, such as chlorine, offer
the potential for spoil improvement prior to or during
dredging, and wetting agents offer a good possibility for
increasing the slurry density.
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DISPOSAL TREATMENT
Spoil raay also be treated before being redeoosited.
Such treatment, of course, will not effect the dredged site
but can aid in improving the spoil deposition area. Methods
ur>ed in treating spoil before redeposition include:
FLOCCULATION
Flocculation within a diked disposal area has been
successful both in speeding the natural precipitating
process and in clarifying the resultant effluent; however
this technique depends on fairly quiescent water, maximum
settling prior to addition of chemicals, and efficient
mixing. An open water disposal practice of some promise
would utilize a silt barrier to enclose a "treatment area"
in which dredges could deposit spoil where, after an initial
settling period of 15 or 20 min, flocculents would be
applied. Such a technique holds promise of limiting
possible undesirable effects of spoil disposal.
VACUUM FILTERS
Since the spoil placed in a confined disposal area is
.so often similar to domestic sewage, the use of vacuum
filters for initial dewatering appears feasible. In this
manner, the sludge can be separated out for eventual inland
disposal (by other means such as rail or road haul). The
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dispersed, thereby allowing a maximum of spoil-water
contact, or disposed of in such a manner as to minimize this
interaction. The above technique would maximize spoil-water
contact while limiting the area over which this action
occurs. If this system proves practical, dredging- could be
used to both improve the area from which the spoil was
removed (by virtue of removing unwanted organic spoil) and
improve overall water quality (by satisfying the oxygen
demand associated with organic materials). Aeration offers
the potential to deal x-;ith highly polluted spoil in a very
progressive, environmentally compatible manner and may give
some flexibility with regard to location.
INCINERATION
Much as in a municipal sanitary, system, the treatment
of highly organic spoil will require a primary (solid) and
secondary (liquid) phase. Incineration is a proven
technique that can be expected to handle solids disposal.
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Sludge with a sufficiently high volatile solids content is
readily available in a number of harbor areas but would
require preliminary dewatering through natural settling,
vacuum filtration, or some other technique. Other
incineration techniques include wet oxidation and fluid bed
incineration. The danger of air pollution'must be.
investigated in all cases.
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FILTERS
The use of filters of various types is a possibility.
If the spoil is of sufficient size, a screening or
centrifuge process car perform a fairly efficient wa'ter-——-—
solids separation. In the case of finer materials, sand bed
filters, as used in municipal water treatment plants, can
find application in effluent treatment. Such filters could
be built as integral parts of a diked area to either replace
or supplement existing weirs.
SEWAGE TREATMENT PLANTS
Disposal of highly organic spoil from waterways into a
city interceptor-treatment plant nystera may be practical in
some cases, but expensive and requiring long periods of time
to dispose of large quantities of solids. The nature and
volume of this spoil material simply overwhelms the capacity
of a typical treatment facility to process and dispose of
additional sludge. One major limitation inherent in such a
system is the requirement to cease dredge operations during
high sewage flow, such as following a rainstorm. For large
projects such a scheme is unworkable, but smaller projects
may find it a viable alternative, particularly if temporary
storage facilities are available to hold the spoil effluent
for further processing.
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Dredge £poi_l P-i,sIL°^2i Techniques
Dredge spoil can bs disposed of on land, on marshes, in
estuaries or in open .;atcr. So'.ne of the adverse effects can"
be mitigated or ameliorated by the urco of prooor disposal
site selection to minimize ecological effects.
Additionally, modified dredging techniques or the use of
peripheral equipment designed to reduce spoil losses can
also reduce adverse effects.
OPEN WATER DISPOSAL
Short terra open water effects on the benthic biological
community from drodge spoil disposal bar. been found to
destroy the less mobile forms whereas some types ware able
to surface and survive. Apparently the high turbidity did
not increase organism mortality.
Methods to minimize the effects of spoil disposal
include: the accurate placement of spoil, the use of
disposal techniques which minimize resuspension of material
and which cause the loss of spoil compaction, and the
investigation of current dispersal patterns before site
selection.
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If the spoil contains pollutant?; or toxic materials,
knowledge, of oxygen donar.ds created or the release of such
toxic compounds should bo oocur.iulateu before planning
disposal. Site selection or the use of alternative "disposn-t
nethods may be dictated by tho presence of such materials.
LAKD DISPOSAL
Land disposal is defined as disposal not in open water
and not on marsh land.. It includes disposal on upland areas
or on bars or islands. Such areas may be confined by dikes
or natural barriers or be unconfined,, Contained areas are
usually equipped with spillwuys or overflow weirs and
occasionally with settling basins.
Dikes are generally constructed on noox* foundation
materials and require careful design to prevent failure * In
many cases dikes have breeched causing extensive losses of
spoil materials. In cases where dikes are constructed to
contain polluted materials, these dikes may be pervious
allowing seepage through and beneath the dikes.
Ground water contamination by the disposal of polluted
spoil raay also occur. Examination for this potentiality is
seldom made. If such contamination isf'likely the use of
liners or other impervious materials may be required.
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In order to minimi r'.e l;he effects of spoil disposal
methods of improvement and utilisation have been developed.
Spoil can be characterised as having a significant
percentage of fine grained organic materials and high, water
contents. Guch water j.als oosr>e,rjr, very r>oor qualities as a
foundation material. Inproveraoo r oc foundation properties
can ba aeconnj is hod by reaoving th<; water r»ivl enhancing
conso.'Li da tion. Spoil drainage ocx;ur,c; at a very slow rate
becaunc the effect of dijies if? to cause perched water tables
and water retention.
Several methods have been incorporated into projects to
improve dredge spoil en-jineerin'j proDertit^ :
£ii£l}-li;£ "" r^il(J <;Jround'>/ater table can br.-> lo\;ered by ditches
which also provide drainage of .surface runoff. Removal of
the excess wi*ter promotes consolidation.
S_a_n_d £r^iJis_ - Vertical sand drains are a cylindrical column
of sand or granular material pieced in a vortical hole and
connected at the original surface with a drainage blanket.
These drains provide an avenue of escape for pore water and
promote consolidation.
Gr oimd i Surf a. ce £rai_ns_ - The disposal site would initially
be covered by a layer of sand before aoolving dredge fill.
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The horizontal sand blanket provider a permanent drain of
overlying water and pro.-aotes oor.solidati on*
Experimental projects have suggested the possible use
of vacuum wells, electrco'^osis and desalcat.i on for
i'nproving clredgo r.poil. Acluitional ra'3e?rr.h a.nd test.xng
will be required to evaluate these techniques .
IILA' JD DISPOS7i,L
-Many of tho 3 and rlisvor.al tec'inicjues previously
described are applicable to disnosal on narVoland .
Marsh land dir;;r;o;jal has been Goranou in the pa it bocauKr; of
access j.bility and the inc2;:nensivcj values o.C the land.
Becau.oe of the increasing oxponso oil other Icm-.l caused by
development of coastal arc^s, these marshlands were
frequently the only land disposal sites available.
Such disposal is frequently attacked by
conservationists and others because of biological habitat
destruction. The use of marsh areas for spoil disposal! will
probably continue in some areas. Although habitat
destruction to some species will always occur, research
continues to mitigate these adverse effects as much as
possible.
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Comparison of diked ve.rsun undiked dinr>or;al indicates
that undiked disposal in more dcoj ruble. It appears that
r.iarnh recovery in undih._-'I areas occurs at a more rapid rate
Work is also underway to develop-.:; system:; to coracle or rate
marshes so that the no5; 1. productive nur.cJhe::> can bo left
undisturbed,
Productive lines of Snoil
Rather tiu:n beinq conr.idcrofi entire;"Ly detrinontal,
£-poil can be. ur-ccl advant;.Kjeous.ly in certain circu,'i:;l ances«
Efforts are being expended to determine beneficial u?:es.
Geve.val of tliose uses arc outlii'ieu in the folio'.."ino-
rnate rial .
/ARTIFICIAL H/vBITAT CREATION
At the present time the most promising of the
artificial habitat creation schor.ies usinq dredge spoil are
the spoil island and the creation of marshes. Artificially
created spoil inlands may be naturally colonized by
indigenous terxestria-l vegetation or specialized v/aterfowl
feed plants may be planted. Within the spoil disposal area
itself, small lakes may be created for fish and wildlife
habitats.
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The use of spoil to construe-! artificial marshes is
also feasible . Careful r.noll p. I a '::.:••:•' .int. and rocolor.i nation
of marsh vegetation is rv;i ircd, r>;.nco many of the nation's
marshes have been des trove. <..' by previous dredging operations
and man induced factors, the creation of new marshes in such
areas is desirable to provide the ecological nursery and
habitat for fifth and wildlife.
Other possible artificial habitats include the
development of. r-he.ll 1'i^h ]rads in open Welter areas. In
conjunction with »poi3 islando and r.inrsh creation whole
ecosysten hcibitatr; can por.rtibly be-- .recreated.
LAHD Dl^VK
Land created by dredge spoil cl.isnosal has hirstorically
been urjod in harbor development ana for other construction
whether delibera tely placed or for tu.itously located. Piers,
access roads and warehourjep. have been constructed on dredge
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spoil.
Areas specifically designed for construction use
typically select special materials such as sand which has
good drainage and structural properties. The use of these
materials makes for usable land and because such materials
are generally non-polluted, the drainage and runoff cause
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