Techniques

As a daughter company of Jan De Nul Group, global leader in the dredging industry, Envisan is involved since 1996 in permanent maintenance dredging works, land reclamation projects and harbour extension works.  As a result, Envisan acquired extensive experience in the processing of dredged materials and can profile itself as the expert in the treatment of both contaminated and non-contaminated sediments. The most commonly applied techniques for this activity are described below.

Pre-treatment of sediments

Worldwide, canals, rivers and harbours contain sediments, sludge and dredge spoil that must be removed to ensure the passage of ships and/or tackle the related pollution.

Sediments consist of a mixture of water and fine particles. Dewatering them is a first, important step in the overall treatment process. It results in a considerable reduction of both weight and volume, only leaving behind the particles that need further treatment.

Dewatering is mostly executed off-site and can be done in two ways.

Lagooning
In this dewatering technique, sludge is spread over lagooning fields.  By aerating and reversing the sludge at regular intervals, part of the water evaporates whereas another part flows away through the drainage system. The sediments' dry substance content is increased.

When there is plenty of time and sufficient space, this dewatering technique is the best option.
Envisan has several lagooning fields at its disposal, both in Belgium and abroad.

Mechanical dewatering

Envisan applies state-of-the-art, monitored dewatering techniques to remove the residual water from sludge. Possible techniques are the use of filter presses and centrifuges.
This technique is used when dewatering must be executed and time and/or space are limited. 

Additional treatment of sediments

Sometimes, it is absolutely necessary to subject dewatered sediments to further treatment because only dewatering them will not always make them suited for the intended destination, such as their reuse in civil construction. Therefore, a full-fledged treatment technique is sometimes required.

These treatment techniques are mostly but not solely applied on dredged materials from harbours and rivers and on excavated spoil from unnavigable waterways and can be executed as follows:

Biological treatment
Envisan has adjusted techniques available for the biological treatment of dredged materials. A precondition is of course that the dredged materials are contaminated with biodegradable pollutions. By optimising the growth conditions of the inherent micro-organisms, organic pollutions in the contaminated sediments are decomposed. 

Physicochemical treatment
For sediments that cannot be treated biologically, physicochemical treatment is applied. The sediments are decomposed into fractions through several consecutive physical and chemical processes, separating them in a strongly polluted residue on the one hand and in purified basic flows on the other. The smaller share of concentrated pollution is transferred to a properly licensed dumpsite whereas the major part of the original sediments can be reused. 

Desanding through hydrocycloning
The purpose of this hydrocycloning technique is separating the dredged materials in a fine fraction on the one hand and in a coarse fraction on the other hand. The fine fraction contains most of the pollution. This technique delivers particularly good results for sandy sediments.

Immobilisation/stabilisation

If the pollution cannot be (entirely) removed or decomposed, the impact of the residual pollution can be decreased by immobilising the sediments. To this end, the sediments are mixed with specific additives in mobile mixing plants. As a result, the leachable fraction of the pollution is reduced to below a critical level and the sediments can be reused for some types of civil construction works.

New destination of sediments

After having been treated, the sediments are retransferred to their original location or used for other applications such as the processing of sediments in civil construction or the use of sediments for covering dumpsites. Envisan has, for instance, experience in manufacturing concrete blocks based on contaminated sludge from the port of Dunkirk (France). Instead of the common formulation with sand, part of the concrete mixture is replaced by polluted sediments.

In Belgium, Envisan built an 800m long compartmenting dike in the temporary flood plain Vlassenbroek using dredged materials from the tide branch to the river Scheldt. Additives were added to the dredged materials to obtain the required strength and impermeability. Envisan is convinced that such innovative projects offer a valuable alternative and definitely have a future in our increasingly environment-minded society.