Geoscience Reference
In-Depth Information
steps in the treatment process are shown in Case Fig. 1.3B. The first stage of the process is the
removal from the wet sediment of most of the coarse fraction by centrifugation. Any remain-
ing fine grains with the sand fraction are then removed in a fluidized bed under flowing water
conditions. Sand-sized material is dewatered and can be used in the construction industry. The
fine-grained material is dewatered through a series of flocculation and filtration procedures.
The resulting contaminated fine sediment is disposed of in a purpose-built disposal facility.
Annually, 1.2 to 1.4 million cubic metres of sediment are treated in this way, with 50% of this
volume being contaminated fine sediment placed in the disposal facility.
The removal of contaminated sediments via this dredging, sediment treatment and disposal
has removed about 30% of the heavy metal contaminants from the River Elbe and a similar
percentage of the organic contaminants. Therefore, this dredging acts as a pollutant filter to the
North Sea. This is not a sustainable solution, however, and it is now widely recognized that
minimization of contaminants at source (i.e. in the upper reaches of rivers) is the most effective
sediment pollution strategy.
Relevant reading
Adams, M.-S., Ballin, U., Gaumert, T., et al. (2001) Monitoring selected indicators of ecological change in the
River Elbe since the fall of the Iron Curtain.
Environmental Conservation
28
, 333 - 44.
Kroning, H. (1990) Separation and dewatering of silt from the Port of Hamburg.
Aufbereitungs-Technik
4
,
205 -14.
Netzband, A., Reincke, H. & Bergemann, M. (2002) The River Elbe: a case study for the ecological and eco-
nomical chain of sediments.
Journal of Soils and Sediments
2
, 112-16.
Sediment management approaches generally
take one of two forms.
1
Those that address issues resulting from the
presence of sediment (either a quality or quantity
issue) and where sediment requires removing or
remediating. This type of management is most
common in engineered or contaminated envir-
onments, e.g. urban environments (Chapter 6)
and estuaries (Chapter 7).
2
Those that use management strategies to trap
or retain sediments in a system to maximize
the ecological or environmental functioning of
that system. Such management practices are
particularly common in coastal environments
(see Chapters 7-9).
appropriate action. In order to determine if sedi-
ment is contaminated, baseline and threshold-
effect information is needed. In the case of
artificial compounds (e.g. pesticides, PCBs, some
radionuclides) the baseline value is zero, and
contamination assessment is relatively straight-
forward. Pre-industrial historical values need to
be established for elements with both natural and
anthropogenic sources (e.g. metals, phosphorus).
In rare cases this can be accomplished through
analysis of monitoring data or archived samples,
but in most cases values are determined through
the analysis of sediments that have accumulated
through time (Smol 2002). Threshold-effect values
are often determined through an assessment of
the physical, chemical or biological nature of the
sediment, or a combination of these. Increasingly,
the end result is a series of threshold-effect values
that collectively form Sediment Quality Guide-
lines, or other similar measures (e.g. critical level,
critical load, etc.).
1.7.1 Risk assessment and sediment guidelines
for contaminated sediments
A first step in addressing these issues is to assess
the environmental risk of the sediments, and take
