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Nyttiggørelse, rensning og fraktionering af havneslam 

Summary and conclusions

Beneficial use of sediments from dredging in harbours, channels and fairways in Denmark is regulated by a number of laws. The general intention is facilitation of beneficial use.

According to the Danish act on the Marine Environment and Raw Materials, the authorities have to encourage beneficial use of sediments from marine dredging. To fulfil this purpose the act on taxes on Waste and Raw Material states that materials from dredging are suspended from the general Danish tax on raw materials (5,00 DKK/m³).

If sediment from dredging is used for beneficial purposes, it is raw material reclamation according to the act on Raw Materials and requires permission from the Ministry of Environment and Energy by the National Forest and Nature Agency.

There is also a possibility for beneficial use of unpolluted sediment for coastal protection. This requires a permission form the Directorate of Coastal Protection.

When evaluating the level of pollution in harbour sediment, focus in Denmark has been on heavy metals. However, a number of recent investigations show that the heavy metal load on the harbours is decreasing, as newly sedimented material in harbours only shows signs of diffuse pollution, while the pollution with organic contaminants, not at least biocides from ship paintings, is a major problem.

Many attempts have been made on making a precise description of the sources of pollution, but it is often difficult to get a clear picture. Often the main source of a heavy metal pollution of harbour sediment is an activity in the harbour area, e.g. a specific industry, treatment of seeds with fungicides or shipyards.

According to the Danish Regulation on Marine Disposal of Dredged Material, the regional authorities (Counties) can give instruction on sampling and analysis of sampled material to the parties responsible for dredging and marine disposal, in order to get a picture of the level of pollution of the dredged material. The County can decide that sampling is not necessary, if the possibility of the dredged material being polluted is very low.

Danish Counties normally require analysis for heavy metal in dredged material for marine disposal. Seven counties have completed investigation on biocides from ship paintings. More extensive investigations have been completed by the counties around the Little Belt and Aarhus County.

The cost of making analysis for organic contaminants and biocides from ship paintings is high as is handling and disposal of dredged material polluted by these contaminants. This cost is expected to be a major economical strain on many small harbours, as the requirements for analysis for organic contaminants and biocides in dredged material increase.

It also has to be expected that new investigations of harbour sediments will show that organic contaminants and biocides contaminate more harbours. This problem has to be evaluated further, including topics such as which and how many harbours have the problem, what are the sources of pollution, and which mitigation measures can be used.

The possibilities of beneficial use of sediments from dredging are concentrated on sandy sediments with a low content of organic matter (< 3-4%), e.g. from harbours and fairways on the west coast of Jutland.

In general, sand from dredging of harbours, channels and fairways is more expensive than sand from gravel pits because the cost of handling, transportation and temporary storage. The beneficial use of dredged material is also limited by the limited experiences and the need for complicated logistics.

Cost of transportation is evaluated as the most limiting factor for beneficial use. An important potential use of sand from dredging is the building of highways, and as a general rule sand from dredging will not be competitive with sand from gravel pits if transport exceeds 30 km. Also at sea, transport cost is a major limiting factor for beneficial use of dredged material - the typical cost is ca. 1 DKK/m³/nautical mile.

Use of sediments from dredging in harbours, channels and fairways for the elaboration of new harbour areas and for coastal protection is less complicated and more widespread in Denmark than other types of beneficial use. An explanation is that the beneficial use is more driven of the need "to do something" with the dredged material, than a demand for it. At the same time the person who owns the harbour area and plans the extension of it, is the same who owns the need to do something with the sediment.

When harbour sediment is considered unfit for marine disposal, it has to be disposed at land in a placement site. To ensure that most of the pollutants stay in the sediment, the optimum place is behind sealed walls, under the water level and without infiltration of acid precipitation saturated with oxygen. It is also important to have a beneficial use of the area after its use as a disposal site. Dredged material from harbours normally has a high content of water (2-3 times its content of dry matter). A covering with thin layers of sand can consolidate it. When the final covering of the area with sand is supplemented with vertical drainage tubes, even areas used for disposal of soft dredged material can end as valuable land, after it has been covered with grass or asphalt.

Separation of contaminated dredged material in a sand fraction that meets criteria for beneficial use and a fraction for disposal at a placement site fulfils both an objective of increasing the possibilities of beneficial use of dredged material and an objective of reducing the need for new placement sites.

Some of the largest European harbours, Hamburg and Rotterdam, have practical experience with separation and cleaning of contaminated dredged material, while no installations for separation and cleaning of contaminated dredged material have been made in Denmark until now. It is assessed that an installation for separation and cleaning of contaminated dredged material in Denmark has to be based on mechanical methods as in Hamburg and Rotterdam.

It is evaluated that there is a need for a comprehensive adjustment and elaboration of methods to make them fit for Danish conditions, before an successful plant for separation and cleaning of contaminated dredged material can be reality in Denmark. An important question to answer is whether the sand fraction can meet the criteria for beneficial use, or if a subsequent chemical treatment is necessary before the sand meets the criteria for beneficial use.

Further the costs in connection with separation and cleaning of contaminated dredged material is substantial compared with disposal in a placement site. Also for that reason it is important that harbours reserve areas for the future disposal of contaminated dredged material.

A mobile installation, financed by several harbours could be one of the means to make separation competitive compared to a "simple" disposal in a placement site. There is also a need for areas for temporary storage of cleaned sand from the separation plant, to make it possible to work continuously, as this is a precondition for economical efficiency of such a plant.