WO2005087400A1

WO2005087400A1 - Storage and cleaning of contaminated dredging sludge

Info

Publication number: WO2005087400A1
Application number: PCT/NL2005/000186
Authority: WO
Inventors: Johannes Gerhard Van Der Zande
Original assignee: Hak Milieutechniek B.V.
Priority date: 2004-03-15
Filing date: 2005-03-14
Publication date: 2005-09-22
Also published as: NL1025730C2

Abstract

The invention comprises the use of an open 'cage' as a supporting structure on top of which an aboveground road surface (10) is founded. From, on and in this open 'cage', various pipes (3, 4) can be suspended and/or mounted which can be used to clean contaminated sludge. In a liquid form, the sludge is pumped or dumped into this 'cage', after which the contamination is removed from the sludge in a number of years with the aid of electricity and biological stimulation.

Description

Title: Storage and cleaning of contaminated dredging sludge

The invention relates to a method for storing and cleaning sludge and to a traffic infrastructure arranged for using such a method.

Virtually every country, including the Netherlands, has enormous areas of land which are, often permanently, taken up by infrastructure with a road surface. Here, "road surface" is understood to mean the upper layer of a motorway and/or road for other road-users, a parking lot, for instance an asphalt layer or a layer of paving bricks, or a layer under the railroad track, etc. The use of such road surfaces by motor traffic causes a considerable contamination of sludge in the vicinity of the road surface. Ditches, brooks, rivers, canals and virtually all other large watercourses need periodic maintenance in order to guarantee the required draft for shipping traffic and/or a sufficient flow of water. This maintenance takes place by various types of dredging operations. Thus, local authorities will keep their watercourses clean with mowing and digging machines, while the large (central) authorities often contract out these activities to large contractors which are able to carry out these dredging operations with all kinds of sailing equipment. During these dredging operations, sludge is released which is contaminated to a greater or lesser extent. Incidentally, herein, sludge is not only understood to mean sludge from beds (dredge spoil), but also sewage sludge or industrial waste sludge, etc. The processing and storage of particularly the heavily contaminated sludge causes an increasingly greater problem to the government. In the Netherlands alone, it is estimated that, in the coming years, approximately 60 milhon m³ of dredge spoil which cannot be spread right away is released under the Dutch Department of Waterways and Pub He Works and another 40 million m³ under the other water authorities. In order to turn this sludge into a building material, the sludge needs to undergo a number of processes, such as dewatering and cleaning. Currently, the conventional methodology is to have the sludge mature in a depot for a number of years and to process the concentrated sludge into, for instance, sound walls or to discharge it and have it thermally cleaned, depending on the contamination concentration. This method requires a large number of depots, which meets an increasing resistance. The available depot space needs to be used optimally so that everything is directed to quick dewatering and obtaining solidity as soon as possible. Often, reuse is not possible and the soil needs to be discharged to and processed at a soil- cleaning company, which is very costly. It is one of the objects of the invention to provide a method for cleaning contaminated sludge in which no large amount of depot surface needs to be reserved exclusively for storing sludge. It is one of the objects of the invention to make it possible to also use the surface of road surfaces for cleaning sludge during use for traffic purposes. Further, it is an object of the invention to clean sludge at low costs (investment and exploitation), with a good applicability at present and in the long term, in a realistic manner, and/or with social added value. The claims describe methods and infrastructure according to aspects of the invention. The invention comprises the use of an open "cage" as supporting structure on which the aboveground infrastructure is founded. From, on and in this open "cage", various pipes can be suspended and/or mounted, which can be used to clean the contaminated sludge. The sludge is, optionally in a liquid form, pumped or discharged or dumped into this "cage", after which the contamination is removed from the sludge with the aid of electricity and biological stimulation in a number of years. By this "cage", the mechanical loads of the road surface and traffic are transferred to the deeper soil layers with more bearing capacity. The cage is preferably from plastic, but may also be from a different, not electrically conductive material. Depending on the use, in the future, use could be made of a "cage" from biopolymer, a degradable type of plastic based on cellulose, which slowly degrades to an organic substance. The "cage" may contain, for instance, pipe elements, crates or other plastic shapes, such as spatial polyhedrons. Inside the cage, electrodes are provided to apply an electrical field (AC or DC). The sludge is cleaned with the aid of these electrical fields.

These and other objects and advantages of the invention will be described in more. detail with reference to the appended drawings, in which:

Fig. 1 shows the cross-sectional section of the construction; Fig. 2 shows the longitudinal section of the construction; and Fig. 3 shows introduced electrodes under a road surface.

The basis of the concept is creating a self-supporting and self-cleaning road body from contaminated dredge spoil. This body needs to have sufficient internal strength quickly to be able to build a road thereon. So, the spoil has a constructive function, which is cleaned in the construction with the aid of existing in situ techniques from soil remediation. The use of dredge spoil can be maximized by additionally using products from dredge spoil. Examples thereof are foundations from cold-immobilized spoil and asphalt with artificial gravel from dredge spoil. In our vision, maximal use is necessary to be able to offer solutions for the large supply of spoil. The road body needs to have sufficient bearing capacity within a short time. Therefore we use a system of forced dewatering using drains in our concept. In this manner, the spoil is quickly dewatered, so that the necessary bearing capacity is obtained sooner than with natural dewatering. An alternative to quickly obtain bearing capacity is to (additionally) provide a reinforcement. We have chosen a reinforcement from biopolymer. This is a reinforcement from degradable plastic based on cellulose, which slowly degrades to organic substance under the influence of oxygen. This reinforcement provides, for the greater part, the required bearing capacity. The degradation of the plastic also contributes to the durability: should the road body ever need to be dug off, then this will cause no problems for the disposal of the soil then obtained.

Introduction of the concept The core of the road consists of a bearing construction with dredge spoil and from a combination of in situ cleaning techniques. For the bearing construction, a construction is used in which a reinforcement from plastic or a biopolymer provides, for the greater part, the bearing capacity. The road further consists of as many dredging products as possible. Dredge material is used in the road body, in the asphalt, in the shoulder and in the sound wall. In the road body, use is made of a reinforcement which is made from a plastic or a biopolymer (degradable plastic based on cellulose). This reinforcement consists of large open structures such as honeycombs, open crates, or open cylinders or open spatial polyhedrons over which and in which the dredge spoil is pumped. The construction lies in a tray or cunette and is screened from the underlying soil by a sand layer and a film thereunder. On top of the film, there is a drainage system for the discharge of water to the retention basins. Besides under the road body, the reinforcement is also under the obstacle -free zone of the shoulder. The reinforcement degrades slowly if use is made of a polymer based on cellulose. The degradation rate can be adapted to the life span of the road. The shoulder After providing the road construction, the dredge material can be provided in the shoulder. In both variants, the plastic reinforcement provides a shoulder with good bearing capacity, so that traffic which goes off the road does not sink. In due course, the (consolidated) sludge in the shoulder can be replenished again. The shoulder is then planted with vegetation which takes up contaminations from the dredge material.

The stretch of road (including shoulder) is approximately 50 meters wide. The road takes up 25 meters thereof. In the median strip, cables and pipes can be accommodated, optionally with minor protection. In the median strip, the public lighting is provided as well.

The cleaning The cleaning takes place according to two different techniques, where electrodes have been introduced into the dredge material, which heat the dredge material and introduce a voltage difference. Due to this voltage difference, contaminations are released from the sludge particles so that they can be removed via the electrode circulation system or be degraded by the organisms in the dredge material. In addition, the cleaning system contributes to the dewatering and consolidation of the dredge material. On the shoulder, containers are present in which the adjustments and the monitoring of the cleaning take place. THE DREDGE MATERIAL IN THE ROAD BODY (THE BEARER)

PRINCIPLE The bearing construction uses the space under the road as 'hidden' storage and cleaning place for dredge spoil. The road body is borne by walls from biopolymer placed in a honeycomb structure. The walls transmit the vertical loads of the road surface to a sand layer with good bearing capacity. The walls are kept as open as possible, so that, during filling, the spoil can flow freely in the space between the walls (the cells) and the cells are filled as uniformly as possible. In this manner, a so-called 'reinforced soil construction' is created. The water from the dredge material is pumped away via a drainage system and is let into retention basins after having been cleaned. In the bearing construction (under the road and under the shoulder), the cleaning process can be started. Figs. 1 and la show the cross-sectional section of the construction, and Fig. 2 shows the longitudinal section. The depth with which the bearing construction is designed depends on the underground. From top to bottom are shown: the underlying soil, a film, a sand bed with drainage therein, the biopolymer cells with the dredge material and the electrokinetic cleaning system therein, a cover film, sand bed for dewatering, hydraulic mixed granulates and the road surface. Fig. 1 shows a part of a cross section transverse to the road in which a plastic film 1 has been applied to an underground with good bearing capacity, with supporting elements 2 from plastic or biopolymer thereon. In the supporting elements 2, a pipe/electrode 3 has been provided for injection for the purpose of cleaning and dewatering sludge, which is part of or mounted on elements. Optionally, a filling pipe 4 has also been provided for filling or replenishing the supporting elements 2 with sludge. Further, optionally, a liquid -tight cover film 12 has been applied on the sludge. On the supporting elements 2, an optional foundation 5 has been provided. On top is the road body 10, in the Figure on foundation 5, or, if this has been left out, optionally directly on the supporting elements 2. The road surface of road body 10 contains, for instance, an asphalt layer or paving bricks. Instead of film 1, use may also be made of a sludge-tight clay layer. If the underground itself is already sufficiently impermeable, such an impermeable layer may even be dispensed with completely. Fig. la shows a cross section over the whole width of the road. This shows the same elements, as well as an optional discharge pipe 6 for drainage of water and/or processed sludge. Discharge pipe 6 opens into a pit or ditch 8, from where the water can be pumped further to a storage basin with the aid of a pump pipe 9. Fig. 2 shows a longitudinal cross section with the same elements as in Fig. 1. In the case of irregularities in the bearing underground, the height of the supporting structure (the number of vertically stacked supporting elements) can be varied in this, so that more storage space is realized. Fig. 3 shows the whole in overview, with an anode circulation system 30, and a cathode circulation system 32. Further, an optional sludge pit 34 next to the road is shown; this is preferably provided at regular distances along the road and connected to the supply pipes 4. Sludge pit 34 is preferably provided with a cutting pump and a water supply construction to spread the sludge, free from large lumps, in the space under the road surface via supply pipes 4. In use, sludge is dumped into the pits, there mixed with water and pumped under the road. Of course, the sludge may also be mixed with water in advance, or mixing is not necessary if the sludge itself is already sufficiently liquid, but this makes the supply more laborious. Supporting elements 2 preferably contain wall parts which run on from top to bottom to transmit the pressure from the road surface to the underground. In addition, the supporting elements can also comprise cross reinforcements between the walls. Anyhow, the supporting elements, however, have an "open" design in the sense that flow over a large length or width through holes in or along the wall parts running on from top to bottom is not materially hindered.

Design First, a 'tray' needs to be created in which the plastic or biopolymer walls are provided. This may be in the form of a cunette or by providing two dike bodies. In the tray, a film is spread or a clay layer is applied which separates the original soil layers from the package to be provided. On the film, the walls are placed. See also Figs. 1, la and 2. The dredge spoil is hydraulically introduced into the reinforced soil construction from above. This may take place by means of unloading with a barge unloading suction dredge or by means of unloading with a crane and pumping with a soil press. For a pilot project, unloading with a barge unloading suction dredge is not realistic, but for large-scale application it is possibly a good option. The dredge material provided will settle, after which fresh dredge material can be provided multiple times. Since a construction is involved which 'bears' the road, so that uniform settlement occurs, the settlement during the consolidation process is no hindrance. Here, the dredge spoil does not have a bearing function. The plastic cells are also used in cleaning the dredge spoil. Thus, at the bottom of the construction, electrode units for cleaning the dredge material are attached and horizontally perforated injection lances are provided to inject nutrients or to air. On the bottom side and the top side of the reinforced soil construction, drains are provided.

Advantages The great advantage of this construction is the speed. Hardly any time is needed to consolidate the dredge material. The road construction can be built rapidly, after which the cleaning process can be started. In addition, the presence of the reinforced construction is convenient for providing the cleaning techniques. All electrodes and supply and discharge pipes can be mounted on the construction.

Conditions The dredge spoil needs to have a sufficient moisture to fill the plastic elements. It may therefore be necessary to admix the dredge spoil to a mixture to be pumped. The transport in the pipe and the filling of the plastic elements need to be accurately adjusted to each other to prevent flowing over or an incomplete filling. In the design stage, the manner of filling will be elaborated in more detail.

THE CLEANING OF THE DREDGE MATERIAL (THE CLEANER)

PRINCIPLE

The cleaner works with two complementary cleaning methods: electro-reclamation and electro-bio-reclamation. These are adjusted to each other such that cleaning can take place all year round. These techniques have in common that electrodes are introduced into the dredge material. As a result of the voltage difference applied, ions or polar contaminations will be moved to the oppositely charged electrodes. The Joule effect that occurs will also result in a temperature increase of the dredge material. Then, the contaminations are collected in a liquid circulating around the electrodes or they are degraded by the organisms in the dredge material. With electro-reclamation, the dredge material is cleaned of heavy metals and polar compounds, such as nitrates, phosphates, PAH and polychlorobiphenyls. With electro -bio-reclamation, the dredge material is cleaned of organic contaminations such as oil, aromates and light PAH. The higher temperatures brought about by the electrodes accelerate these forms of cleaning in that volatile substances evaporate, contaminations are released more rapidly from the solid substance and organisms degrade contaminations better due to biological activity. The biological activity is also stimulated by addition of nutrients and/or electron donors and/or electron acceptors which are transported from the electrode liquids through the sludge under the influence of the electrical field. In addition, by continuous or periodical injection of (compressed) air into the sludge by means of injection lances, the aerobic conditions for biological degradation of the organic contamination can be promoted. Besides via the electrode liquids, the nutrients may also be injected into the sludge as a liquid mixture with the aid of compressed air via the injection lances.

The system is controlled from and cleaned in containers standing on the shoulder. The technique of electro-reclamation and electro-bio- reclamation is a known in situ remediation technique. Depending on the time within which the remediation result is to be achieved, a larger or smaller amount of energy is required. If a quick result is desired, a larger power is necessary. As a result, this will involve a higher temperature in the dredge spoil. With a long elapsed time, alternative, durable energy sources can be used, such as wind and solar energy and even energy generated via the asphalt. If a quick remediation result is desired, the required electricity will substantially be supplied via the mains or be generated with the aid of a generator. Design From the cell construction needed for bearing the road, electrodes are suspended. Likewise according to a particular pattern, extraction filters and, if expedient, fixed injection points for compressed air and nutrients are installed between the electrodes. The mutual distance of the electrodes in the dredge spoil depends on the location-specific conditions. The electrodes are integrated in separate circulation systems, with which electrolytes (conductive liquids) are circulated around the electrodes. Via these electrolytes, the acidity is maintained at a predetermined level and the contaminations, which pass into dissolution and move under the influence of the electrical field, are collected. In an aboveground installation, the electrolytes are conditioned and periodically purified, whereby contaminations are removed. The conditioning and purification of the electrolytes as well as the energy supply are accommodated in containers. These containers are located outside the obstacle-free zone on the shoulder.

Advantages The cleaning techniques have proved their worth in the past. The cleaning techniques can be used under roads and buildings without causing anyone any trouble in use. The cleaning speed and the corresponding annual outputs can be set. The techniques are suitable for serious contaminations and the contaminations are actually removed. The maximum outputs of the different techniques are large. During the remediation, the monitoring can be done from the shoulder and the conditions in the package to be cleaned can be adjusted. The table below shows the experience figures.

Table 1: outputs

Conditions In order to obtain a good cleaning output, electrical energy and a minimum moisture of 15-20% are needed. By means of monitoring, the output is continuously kept track of and can be adjusted when necessary. Thus, extra moisture and auxiliary substances can be added during the cleaning. Cables and pipes which are inside the voltage field need to be wrapped to prevent disturbance and damage due to the high temperatures (> 40°C). Remediation time The remediation time substantially depends on the lithology (sand, clay, peat) and the nature, concentration and magnitude of the contamination. Remediations are generally carried out in phases, namely: 1- the intensive phase: heating the soil and groundwater by electrodes placed in the soil, combined with groundwater and soil air extraction, and periodical nutrient injection. 2. the extenuated phase: emphasis on the biode gradation in the source areas and around them. The supply of electrical energy is stopped. Only air and water extraction and intermittent recirculation of (warm) groundwater and nutrient injection still take place. 3. the monitoring phase: monitoring and control of the spread and risks based on sampling and analyses. If necessary, the residue contamination is actively controlled.

₅ The duration of the intensive phase varies from a few months to a few years; the extenuated phase varies from 1 to 2 or 3 years. By means of the energy supply, the remediation time can be influenced.

ADDITIONAL CLEANING MEASURES0 Purifying vegetation Another possibility for cleaning is the use of so-called infragreen systems. Here, use is made of plants which filter contaminations, particularly heavy metals, from the soil and plants which take up5 contaminations from the air (phytoremediation). These plants store the substances in their leaves, stem or roots. This causes them to contain elevated metal contents compared to plants growing on non-contaminated soils. The metals thus extracted are discharged by harvesting the plants. o The biomass obtained is used as fuel in a power plant (coal-fired or biomass-fired power plant). In the combustion process in the plant, the metals partly end up in the combustion gases and partly in the ashes. However, the intended power plants have an advanced flue gas cleaning, so that the metals in combustion5 gases are virtually completely removed. These ashes can be reused, for instance as auxiliary material in asphalt. So, in this manner, the shoulder soil is cleaned, energy is obtained and the metals are removed from the plants. This means this is a durable solution. 0 Wetlands In addition to the cleaning of dredge soil in the road body and under the shoulder, artificial wetlands in which contaminated dredge material can be cleaned may be laid out in an interchange. During the building of a clove leaf, artificial wetlands may be laid out in the four leaves of the cloverleaf by deepening them and partly filling them with contaminated dredge spoil. Then, reed plants and a water layer can be provided. In a shallow design of such artificial wetlands, the reed plants ensure an oxygen-rich root zone in which organic contaminations such as mineral oil and PAH can be degraded microbiologically. A deep design is also possible, in which deep pits (20-40 m) are dug in the cloverleaf 4. The walls are then provided with an impermeable clay layer and then the contaminated dredge spoil can be introduced. Heavy metals will remain immobilized and chlorinated compounds can be microbiologically degraded in the presence of sufficient electron donors.

Mineral oil and PAH will only be degraded in the top layer if reed plants are used to introduce oxygen into the top layer (rhizosphere). Such deep wetlands have much in common with so-called dredge depots, with which risks of spread of contaminations are minimized.

ENERGY SUPPLY

The in situ cleaning of dredge material requires quite some electrical energy. The energy consumption of electrical systems depends on the extent of contamination, the exact composition of the sludge to be cleaned and the desired speed of cleaning. In view of the durable vision behind this concept, it is only logical to generate the required energy in a durable manner. Durable energy such as wind energy or solar energy has the great advantage over conventional energy from fossil fuels that it is present everywhere. In contrast to conventional energy, it need not be extracted first and be converted into the usable form; it only needs to be converted into electric power. Durable energy sources are not always present: for instance, there is no sunlight at night and it is not always windy. By drawing on a combination of different sources and storing the yield in a buffer, the availability becomes reliable. The buffer can be formed by a battery or the electricity grid. Therefore we have chosen a combination of solar energy and wind energy. If these sources cannot supply sufficient energy, a generator or the mains can supply the difference. The solar energy is collected via the road surface and via solar panels on sound plates or the guide rail. The wind energy is generated with small windmills on the road furniture.

WATER MANAGEMENT

The amount of water in the dredging construction is crucial. It cannot be too much but by all means not too little. During the consolidation process, the water level is kept as low as possible to be able to drain the spoil as well as possible. The drainage can be stopped as soon as the dredge spoil has consolidated sufficiently. For the cleaning, a minimum moisture of 15-20% is necessary. With the drainage system and the associated injection pipes, the moisture can be regulated. It is expected that the moisture will not drop below 20% and that injections of extra moisture will not be necessary.

Cleaning and discharge of released water During the consolidation and the cleaning, water is released. This water is discharged to the cleaning installation via drain pipes located in and under the honeycomb construction. The road water running off is likewise collected in drain pipes and transported to the cleaning installation.

Water released during building The water released during the sedimentation and consohdation of the dredge material is expected to be chemically sufficiently clean to be allowed to be discharged into the sewerage. The water released during the sedimentation process is expected to be of such a quality that it cannot be discharged directly into the sewerage or the surface water. The water released during consohdation is expected to be cleaner, but probably not clean enough. Therefore released the water is filtered, first by a disc separator, where the coarser floating parts are captured, and then by a sand filter where the smaller parts are filtered out. An additional advantage of the sand filter is that it can also be used to carry out any deferrization required. Depending on the available space, the quality of the sludge and the water and the discharge requirements, it may be useful to first collect the released water in a "sedimentation" basin. In the basin in which the water remains during a longer period (a few weeks), all kinds of floating components can thus be settled. Then, it may still be needed to remove the remaining floating parts from the water before the discharge, via a disc separator and/or a sand filter. The advantage of a temporary sedimentation basin is that water can infiltrate into the soil and the capacity of the purification is unrelated to the amount of water released during sedimentation and consolidation. This has a favorable influence on the costs. Of course, space needs to be available to place a sedimentation basin. Water released during cleaning During purification of the dredge material, water is released as well. The purification installations of the groundwater and the soil air are accommodated in the containers in which the purification is controlled and monitored. After purification, this water is discharged to the surface water or the sewerage.

THE ROAD CONSTRUCTION Road construction In the framework of this concept, the road construction is seen as the superstructure from the elevation with the dredging sludge. Seen from this elevation, a number of (constructive) functions need to be filled. These are, in principle, allotted per construction part as follows: - Sand bed: water management, stability, transfer and spread of forces; Foundation: stability, transfer and spread of forces; Lower layers of road paving: transfer and spread of forces; Top layers of road paving: safety (surface properties, water discharge), sound reduction, transfer and spread of forces.

It is expected that no deviating requirements be imposed on the road construction compared to a normal situation. Assuming average traffic intensities, average percentages of truck traffic and excluding deviating local conditions, the construction hereinbelow is a possibility. Of course, as soon as a test location is known, the proposed construction will need to be tested for the conditions and intensities specific to the location. The separation between the elevation and the actual road construction can be formed by a geotextile in order to thus prevent mixing of dredging sludge and sand. Then, a layer of approximately 1 meter of sand is applied. The height of the sand package ensures a stable underground, partly in view of the water management. Then, approximately 250 mm of hydraulic mixed granulates is applied. The hydraulic binding is exp ected to provide a sufficiently stiff foundation. Then, the application of the actual paving can start. For this, three layers of road-metal asphalt concrete are applied (standard asphalt concrete 0/22), for instance in thicknesses of 80, 60 and 60 mm, respectively. On these lower layers, then a two-layer porous asphalt concrete can be applied. The first layer will be approximately 50 mm thick (porous asphalt concrete 11/16), followed by a finer and thinner top layer (porous asphalt concrete 2/6, 20 mm thick). The choice of a two-layer porous asphalt concrete has two reasons: • In our vision, it is desirable not to lose sight of the environment (for people and nature) in the context of the cleaning road surface; a two-layer porous asphalt concrete combines, on the one hand, security in the field of asphalt technology and, on the other hand, a low sound emission. • Due to the specific structure of the two-layer construction, splash & spray is minimized. As a result, blowing about of the contaminated discharge water on the road will only occur to a slight extent.

Finally, express attention is paid to the gutter construction along the roadway. By building the road construction at a gradient (2%), and having this construction drain into a gutter, the contaminated discharge water (run-off) is collected. This is important to prevent this contaminated water from sinking into the soil and adversely affecting the cleaning process in this manner. The water collected in the gutter can then be discharged to the water purification and then to the wetlands. A last option is to process, into the asphalt of the lower layers, gravel which has been extracted from a process of thermal immobilization of dredge spoil instead of the conventional gravel types. Although immobilization techniques are not considered a cleaning technique in the framework of this invention, this is still considered an interesting option in addition to the possibilities to clean dredge material under the road or in the shoulder. It is expressly noted, however, that further research on a laboratory scale is needed to judge whether the asphalt mixture made with the above-mentioned gravel type is satisfactory. For this, time is reserved in the design stage. Depending on this laboratory research, it can be decided by mutual agreement whether this gravel is processed and any price consequences can be made explicit. The shoulder

Arrangement Besides the normal CROW requirements (requirements of the Dutch Information and Technology Centre for Transport and Infrastructure) for a safe road view and a safe place of shelter, there are no additional requirements for the cleaning road with regard to the arrangement of the shoulder. In the shoulder, a large number of provisions for cleaning the dredge material are built in. These need to be prevented from accidentally becoming damaged by, for instance, excavation work or by loading of the shoulder by vehicles and/or objects. It is therefore necessary, on the one hand, to treat the shoulder such that it is recognized during excavation work that provisions have been provided here, and, on the other hand, to provide the shoulder with sufficient bearing capacity so that the provisions do not become damaged by loading by vehicles and the monitoring containers. So, the shoulder should provide a calm road view, with sufficient space to offer stranded vehicles or vehicles gone off the road a safe haven without the technology for the cleaning becoming damaged thereby. The shoulder is further planted with plants which take up contaminations from the (dredge) soil. In order to be able to fulfill the different functions, the shoulder needs to be sufficiently wide, flat and rough and have sufficient bearing capacity, without rigid objects such as masts which cause great damage to the vehicles in case of collision. When this is, for certain reasons, not feasible, the placement of a screening provision can be considered.

Design In view of the requirements imposed on the shoulder both by the road user and due to the building-in of provisions for cleaning the dredge material, a semi-paved shoulder will be chosen in the design. Below, two options are indicated; a definitive choice has not yet been made. Hollow concrete stones or grass tiles Various suppliers market approximately equal concrete stones with hollow spaces. Depending on the quality of the underground, a foundation of sand or, for instance, mixed granulates can be desired. The hollow spaces of the stones are filled with soil which is poor in humus and then a suitable grass mixture is sown thereon. Practical experiments have shown that the hollow concrete stone is a good semi -paving with regard to bearing capacity and friction. Plastic plates The plastic plates consist of a lattice structure with a height of 50 to 75 mm and a hollow space of more than 90%. Depending on the quality of the underground, a foundation of mixed granulates may be desired. In that case, it is preferred to lay the plates directly on the foundation. The hollow spaces are then filled with soil which is poor in humus and then a standard grass mixture is sown thereon. Practical experiments have shown that the plastic plates are a good semi-paving with regard to bearing capacity and friction.

Placement of containers The containers in which the cleaning and monitoring are controlled outside the obstacle-free zone on the shoulder.

MANAGEMENT AND MAINTENANCE

With regard to the aspect management and maintenance, three items are important: green maintenance (shoulders, wetlands), road maintenance and the maintenance of the active systems for the purpose of the electro- reclamation.

Green maintenance In view of the special plantation of the shoulder and the use of eco-fencing on the shoulder, the shoulder maintenance requires extra attention. Depending on the composition of the sludge, the plantation chosen will vary. The frequency of mowing (harvesting) varies per type of plant. For annuals, this will logically be once a year. The mown material needs to be collected and be offered to a combustion plant (biomass -fired plant) equipped for this purpose, after which the contamination (heavy metals) accumulated in the plants will finally end up in the soil ashes. These ashes can be reused, for instance as auxiliary material in asphalt. Use of infragreen systems which can contribute to the natural air cleaning is possible in maintenance -free variants of the shoulder. These have specifically been developed with a view to minimizing the inconvenience which maintenance can possibly cause to traffic. The green maintenance of the wetlands mainly involves the annual or biannual harvesting of the vegetation. The mown or pruned material can be offered to a biomass-fired plant.

Road maintenance The road maintenance is limited to the regular management and maintenance regime. There is no reason to assume that there should be a deviation from this. With careful design and realization, it can be assumed that matters such as residual settlement are minimized. By paying attention to the monitoring of the road construction, any possibly occurring damages can quickly be observed and dealt with.

Maintenance of cleaning system In practice, a remediation set as anticipated in the future of sludge remediation under the road consists of an energy module and a water treatment module in combination with a number of external pumps which pump the electrolyte in the direction of the water treatment module. Depending on the number of electrodes, the contamination concentration and anticipated energy requirements, it appears that this set can consist of two 20"-containers or similar as regards dimensions. The energy module contains inter alia the transformers which convert the supplied electrical power into the voltages needed for the remediation, the power supply for the water treatment unit and other measuring and regulating equipment. Except for periodical maintenance and keeping clean, this unit is relatively maintenance -free. In practice, various couplings are possible to, for instance, a central computer via a telephone fine so that disturbances can be observed from a distance. The water treatment module contains various liquid tanks in which the electrode water from the cathode and anode is collected, treated and purified. A surplus of water can be collected and pumped away to the wetlands. In dry periods, it may be the case that, conversely, water needs to be added to the system. The water module contains relatively few rotating parts and is therefore relatively maintenance -free. The various sensors, valves and pumps are maintenance-sensitive, however. In practice, the setting up of the system for a standard soil remediation is rather labor-intensive. Due to the fact that sludge needs to be cleaned now and the sludge remediation can be spread over a period of 20 years, a drastic simplification seems possible precisely in this aspect. Adjustment needs to take place only periodically on the site or can take place at a distance by means of automation. Most care needs to be directed to the pump system which is placed (underground) in the immediate vicinity of the road, which provides circulation of the electrode water and control of the water level/moisture content of the groundwater. The electrodes and other pipes under the road are maintenance-free.

Durability aspects Processing of plant residues The plant residues of the infragreen system are discharged to a coal-fired or biomass-fired plant. There, these residues are converted into energy, thereby providing a durable solution. The metals taken up by the plants are captured in the flue gas cleaning of these installations. Safety of cleaning technique The whole system is closed and largely inaccessible. The power supply is under ground level and the anode and cathode wires are in pipes in the road body. In addition, the voltage is relatively low and comparable with lo -voltage current in the anticipated apphcation. The energy module will provide such a closure. The liquid flows take place via a closed pipe system. Exposure to any contaminated liquid flows is therefore impossible. Only slightly contaminated or clean water is discharged to surface water and/or wetlands.

FITTING IN THE LANDSCAPE, ECOLOGICAL VALUE AND PERCEPTION VALUE

The use of cleaning vegetation in the road shoulder (phytoremediation) does not only provide cleaning of the road, but also makes a positive contribution to the ecological and landscape value of the road shoulders. The plants which fix the contaminations give the shoulder a natural character and offer, for instance, food and cover to butterflies. Road shoulders cut across our intensively used country as green ribbons and networks. Just like roads for people, they form connections along which plants and animals can settle and spread. These connections require shoulders with a wealth of species, but often also rougher shoulders in contrast to the earlier intensively managed lawn shoulders. Some examples are attraction of butterflies by the stinging nettle, which fixes heavy metals. Underwood herbal vegetation is important to many birds as a cover, nesting place and source of food, and small mammals also move via the rough shoulders. A good fit and colorful overgrowth of shoulders further increases the perception value of the shoulder, both for the road user and for the neighboring residents.

LARGE-SCALE APPLICATION POSSIBILITIES

If the approach of cleaning described herein is realized over a period of 15-20 years, only a small amount of energy is required, supplied over a prolonged period. The power to supply this energy can be generated via solar collectors and/or windmills. For large-scale treatment, with, for instance, many tens of thousands or hundreds of thousands cubic meters, of course a series of electrokinetic installations is necessary, which can each treat a certain volume in a time period of 15 to 20 years. These installations could have the same magnitude as those of the pilot, but instead of 6,000 m³ of spoil, with one installation, for instance 5 times as much spoil could be treated.

Starting from the assumptions as shown in the table below, per linear meter of road, between 33 and 185 m3 of dredge spoil can be processed. If 100 m³ is taken as an average, then this is 100,000 m³ (150 tons) per km.

Height of the soil body: 1 to 5m high (2.5m on average) Road width: 33 - 37m built up from 2 x 12.5m of asphalt for the roadways

It will be clear that it would also be possible to drain the sludge in this manner without using a supporting structure, after which one could until the sludge has settled sufficiently to build a road surface on it.

However, this may take months, or even years, which makes the building of the road undesirably expensive and time-consuming. In this case, for instance, first a cunette is made or dike bodies are built on both sides of the future road. Herein, a watertight layer such as a film or a clay layer is applied if necessary, after which the sludge is dumped. After dumping the sludge, or preferably before that, a drainage system is built. When the sludge has dried sufficiently to be able to bear a road, the road surface is built. Here, at any time, but preferably before the dumping of the sludge, the electrodes are installed (installation before dumping has the advantage that a minimum of interventions is necessary).

Claims

1. A method for storing and cleaning contaminated sludge, wherein a road surface is applied on a supporting structure which is so open that it allows a sludge flow through the supporting structure in a direction parallel to the road surface; - sludge is introduced into the supporting structure; during use of the road surface for traffic purposes, an electrical field is applied through the sludge in the supporting structure over a substantial length of the road surface, with which contamination in the sludge is removed.

2. A method according to claim 1, wherein, after settling of the sludge in the supporting structure under the road surface, additional sludge is introduced into the supporting structure and cleaned with the electrical field.

3. A method according to any one of the preceding claims, wherein the supporting structure is laid in the form of a mat before the building of the road surface.

4. A method according to any one of the preceding claims, wherein a layer which is non-permeable to sludge is laid and the supporting structure and the sludge are then applied on top of the layer which is non-permeable to sludge.

5. A method according to any one of the preceding claims, wherein contamination is transported through the sludge in the supporting structure to the electrodes by electromigration of ions; the contamination is collected in electrode liquids circulating around the electrodes; and the contamination is removed from the electrode liquids by means of a purification installation.

6. A method according to any one of the preceding claims, wherein contamination is degraded by microbiological activity stimulated by heating of the sludge by current through the sludge in the supporting structure generated with the electrical field.

7. A method according to any one of the preceding claims, comprising the supply of nutrients and/or electron donors and/or electron acceptors via the electrode liquids; the spread of the nutrients through the sludge in the supporting structure by the current through the sludge generated with the electrical field.

8. A method according to any one of the preceding claims, comprising the supply of air through the sludge via injection lances in the supporting structure.

9. A method according to any one of the preceding claims, comprising the moistening of the sludge under the road surface in the supporting structure during cleaning.

10. A traffic infrastructure provided with a road surface; a supporting structure under the road surface, which supporting structure is so open that it allows a sludge flow through the supporting structure in a direction parallel to the road surface; electrodes installed such that, by applying an electrical voltage between the electrodes, an electrical field can be generated under the road surface in the supporting structure over a substantial length of the road surface.

11. A traffic infrastructure according to claim 10, wherein sludge has been dumped into the supporting structure.

12. A traffic infrastructure according to any one of claims 10-11, wherein the supporting structure has the form of a mat under the road surface.

13. A traffic infrastructure according to any one of claims 10-12, provided with a layer which is non-permeable to sludge under the supporting structure.

14. A traffic infrastructure according to any one of claims 10-13, provided with a feed opening for feeding sludge into the supporting structure under the road surface over a substantial length of the road surface.

15. A traffic infrastructure according to any one of claims 10-14, provided with pipes around at least a part of the electrodes for a liquid separated from the sludge and a supply and discharge pipe for supply and discharge of the liquid, which pipes are liquid -containing and permeable to ions.

16. A traffic infrastructure according to any one of claims 10-15, provided with lances in the supporting structure under the road surface over a substantial length of the road surface, for injecting air through the sludge.

17. A traffic infrastructure according to any one of claims 10-16, provided with a discharge opening for discharging washed-out sludge from the supporting structure from under the road surface over a substantial length of the road surface.

18. A traffic infrastructure according to any one of claims 10-17, provided with a moistening installation for moistening the sludge while the sludge is in the supporting structure.

19. A traffic infrastructure according to any one of claims 10-18, wherein the supporting structure is manufactured from a degradable type of material which is degraded during use for several years under the road surface.