WO2008049911A2

WO2008049911A2 - Method for treating sludge and relative treatment plant

Info

Publication number: WO2008049911A2
Application number: PCT/EP2007/061511
Authority: WO
Inventors: Teimuraz Mechurchlishvili
Original assignee: Impresa Taverna Spa
Priority date: 2006-10-27
Filing date: 2007-10-26
Publication date: 2008-05-02
Also published as: WO2008049911A3; ITUD20060230A1

Abstract

A method and relative plant (10) for treating sludge contaminated by pollutant substances and in which the aqueous means consists in particular of sea water, brackish water, industrial water, oil extraction water, or other water with a high saline and/or ionic content, comprises a first step a) in which a pre-treatment of the sludge is performed by means of pre-treatment means (11) and a second step b) in which a treatment of the sludge pre-treated in said step a) is performed by means of treatment means (12). The treatment comprises a foam-free floatation treatment of the sludge pre-treated in the first step a), so as to obtain a head floatate, rich in pollutants, and a tail floatate, poor in pollutants. The foam-free floatation treatment comprises at least two floatation passes of a different type from each other, of which at least one floatation pass is of the type with a neumatic floatation.

Description

"METHOD FOR TREATING SLUDGE AND RELATIVE TREATMENT PLANT"

FIELD OF THE INVENTION The present invention concerns a method for treating sludge, in particular contaminated by pollutant substances and in which the aqueous means consists in particular of brackish water, sea water, industrial water, oil extraction water, or other water with a high saline and/or ionic content. By sludge we mean soil, earth, sediment, mud and/or mud matrix with varying water content. The invention also concerns the relative treatment plant. In particular, the method and plant according to the present invention allow to separate the heavy metals and/or hydrocarbons, allowing to decontaminate the sludge and the water itself, and possibly to recover the pollutant substances, by means of a sequence of operations of floating the sludge. BACKGROUND OF THE INVENTION

It is known that in sludge with different water content, in particular salt water, sea water, brackish water, water from lagoons and marshy areas, pollutant substances of various types are present, often with concentrations above the limits laid down by law. Among the most polluting substances there are heavy metals, in organic and inorganic form and/or contained in colloidal forms and hydrocarbons, oxidized or not.

It is known that this sludge must be treated and restored, to return the concentration of pollutant substances present back within the limit values.

In order to effect this restoration, and to allow the sludge, possibly, to be subsequently re-used, traditional treatments provide to send it to purification plants where the sludge is treated in many ways, most frequently in auto clave at high pressure and temperature and which provide pre-treatment, stabilization, drying and de water ing steps.

Known treatment methods and plants, however, do not allow to obtain high levels of extraction and/or separation of the pollutants from the sludge, and a high quality of the separated effluent, with limited environmental impact and limited cost of the plant and of management.

Moreover, known methods and plants do not guarantee a reduced risk of pollution of the site where the sludge is finally conferred, or of the reception body, nor do they respect the norms of hygiene and health and quality of life for the operators who work on the plant and the population resident in the vicinity.

One purpose of the present invention is to perfect a method for treating sludge, and to achieve a relative treatment plant, which allow high levels of extraction and/or separation of the pollutants or contaminants present in the sludge and a high quality of the separated effluent, high standards of environmental quality and reduced complexity of management and maintenance, with reduced cost of the plant and of management. Another purpose of the present invention is to perfect a method for treating sludge, and to achieve a relative treatment plant, which guarantee minimum risk of pollution of the site where the sludge is finally conferred, or of the reception body, and which respect the norms of hygiene and health of the operators who work on the plant. The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION The present invention is set forth and characterized in the independent claims, while the relative dependent claims describe other characteristics of the invention or variants to the main inventive idea.

In accordance with the above purposes, a method for treating sludge, in particular contaminated with pollutant substances and in which the aqueous means consists in particular of brackish water, sea water, industrial water, oil extraction water, or other water with a high saline and/or ionic content, comprises:

- a first step a) in which a pre-treatment of the sludge is performed;

- a second step b) in which a treatment is performed of the pre-treated sludge.

According to a characteristic feature of the invention, the treatment comprises a foam-free floatation treatment of the sludge pre-treated in the first step a), to obtain a head floatate, rich in pollutants, and a tail floatate poor in pollutants.

The floatation treatment comprises at least two floatation passes, of a different type from each other, of which at least one floatation pass is a pneumatic floatation.

According to an embodiment, the pneumatic floatation is of the floatation cells type.

According to another embodiment, the pneumatic floatation is of the floatation columns type.

Advantageously, a second floatation pass is of the type chosen from a group comprising an ejector floatation and a mechanical floatation and can be effected before of after said pneumatic flotation pass.

The relative treatment plant comprises: - means for pre-treating the sludge;

- means for treating the pre-treated sludge.

According to a characteristic feature of the invention, the treatment means comprises foam-free floatation means able to obtain a head floatate, rich in pollutants, and a tail floatate poor in pollutants. The floatation means comprises at least two floatation units, of a different type from each other, of which one unit is of the pneumatic floatation type.

According to an embodiment, the pneumatic floatation unit is of the floatation cells type.

According to another embodiment, the pneumatic floatation unit is of the floatation columns type.

Advantageously, a second unit is of the type chosen from a group comprising an ejector floatation unit and a mechanical floatation unit and can be disposed before or after the pneumatic floatation unit.

The present invention also embraces different solutions, wherein a first flotation unit or pass can be pneumatic floatation and at least second unit or pass can be ejector or mechanical floatation or vice versa or wherein a first flotation unit or pass and at least a second floatation unit or pass are both pneumatic flotation, but also of a different type, the first being floatation cells and the at least second being floatation columns or vice versa. One advantage of the present invention is that it perfects a method for treating sludge, and achieves a relative treatment plant, which allow high levels of extraction and/or separation of the pollutants or contaminants present in the sludge and the separated effluent, high standards of environmental quality and a reduced complexity of management and maintenance, with a reduced cost of the plant and management.

Another advantage of the present invention is that it perfects a method for treating sludge, and achieves a relative treatment plant, which guarantee a minimum risk of pollution of the site where the sludge is finally conferred, or of the reception body, and which respect the norms of hygiene and health of the operators who work on the plant.

According to another advantageous feature of the invention, the method and plant allow to effectively treat sludge having a high saline and/or ionic content, for example, but not only, with a high concentration of chlorine ions, such as sea water or other waters with analogous characteristics.

This overcomes a known problem in the field of treating contaminated sludge by means of floatation, where the presence of high quantities of dissolved ions, especially chlorine ions, prevents the effective use and correct lay-out of traditional floatation units.

Advantageously, the method according to the present invention and the relative plant are applicable both in the case of treating sludge contaminated by heavy metals, such as for example, but not exclusively, mercury, chrome, cadmium, in their organic and inorganic forms and/or contained in colloidal forms, and also in the case of treating sludge contaminated by hydrocarbons, oxidized and/or non-oxidized.

Advantageously, the plant for carrying out either of these treatments is the same, with suitable plant by-passes provided between the various units, according to the treatment desired. In an advantageous form of execution, common to both treatments, the pre- treatment provides one or more mixings or dilutions with water and stirring to disaggregate the sludge, and also an operation of mechanical separation, for example screening and/or sifting, of the coarse fraction and the fine fraction of the sludge. Moreover, during the pre-treatment, in particular during one or more of the mixings performed, one or more suitable reagents are advantageously added to the sludge, for example in order to promote the floatation. Advantageously a large part - about 80-90% - of the reagents in their ionic molecular form, are also separated and recovered with the floatate at the end of the floatation treatment, limiting the pollution of the waste waters.

According to an advantageous form of execution and embodiment of the present invention, the tail of the floatation treatment may be subjected, if necessary, to a third post-treatment step c), by means of suitable post-treatment means, in which at least a dewatering step is provided, in order to recover, on the one hand, the water to be re-circulated to the pre-treatment or to be disposed of, and on the other hand the de-contaminated solid matrix, with low residual humidity and to be disposed of. The foam-free floatation used in the present invention guarantees, with respect to floatation with foam, a plurality of advantages, including:

- a high extraction yield of the material concerned (target), associated with high selectivity extraction;

- reduced use of stabilizing foaming agents and depressor agents, since the floatation and separation is guaranteed by the fact that the desired flow regimen is determined inside the floatation chamber and the optimized sizing of the floatation chamber, in particular its height, the length of the vertical trajectory of the air bubbles generated and the diameter of said air bubbles;

- a smaller quantity of primary water required at inlet for mixing and dilution; - a smaller residue of the separated material concerned (target), at exit from the plant, with reduced environmental impact;

- reduced, if not zero, pollution of the waste waters of the plant, due to the minimal or zero use of chemical reagents and simultaneous oxygen enrichment of the waters, thanks to greater aeration; - reduction of costs due to use of chemical reagents;

- reduction of costs due to electric energy consumed, which is 4 or 5 times less than floatation with foam;

- increase in the final sizes of the particles floated, thanks to the laminar regimen, with a Reynolds number (Re) comprised between about 3000 and 4000, which is established in the floatation chamber, and in particular in the pneumatic floatation chamber, and which allows to float even coarser particles with the effect of having better treatment performance with lower cost per unit of material to be treated; - possibility of separating the conditioning area, that is, measuring out the reagents, from the floatation area, allowing optimum specific conditions for every treatment cycle;

- 3 - 4 times increase in treatment speed with respect to floatation with foam; - reduction of the operating spaces required, equal to 10 - 12 % of those required for floatation with foam;

- possibility of assembling small-size mobile treatment modules;

- lower cost of equipping, maintenance and spare parts;

- lower cost of management of the smaller quantity of foam that may possibly be generated, since it is not persistent and dissolves rapidly;

- high efficiency in treating sludge contaminated both by heavy metals, in their organic and inorganic forms and/or contained in colloidal forms, and also by oxidized hydrocarbons, also called "aged", or non-oxidized, heavy oils, resins, asphaltenes, bituminous residues. BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of a preferential form of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is a schematic view of a plant for achieving the method according to the present invention;

- fig. 2 is a schematic view of one form of embodiment of the plant according to the present invention; and

- fig. 3 is a schematic representation of another form of execution of the plant according to the present invention. DETAILED DESCRIPTION OF A PREFERENTIAL FORM OF

EMBODIMENT

With reference to fig. 1 , a plant 10 for treating sludge, to perform the treatment method according to the present invention, comprises:

- a pre-treatment unit 1 1 to which the sludge to be treated is fed, as indicated by the arrow IN, and in which the sludge is mixed and diluted with water, in order to prepare it for subsequent treatments;

- a treatment unit 12 for the pre-treated sludge, to separate the pollutants, as indicated by the arrow S; and - a post-treatment unit 13, advantageously able to recover the processing water used and from which possible water to be disposed of and/or the de-contaminated dry substance exit, as indicated by the arrow OUT in fig. 1.

The treatment unit 12 is able to effect a foam-free floatation treatment of the pre-treated sludge.

By foam-free floatation we mean a floatation in which the foam which has formed on the surface does not persist and dissolves rapidly, since the invention provides to use minimum or no foaming agents. Therefore the foam dissolves when the production of bubbles which normally feed the formation of surface foam is terminated, that is, once the means generating the bubbles is stopped, whether it is pneumatic, mechanical, thermo-dynamic or fluid-dynamic, like an ejector, or the foam is removed from the floatation unit. In fact, the foam is unstable and dissolves as soon as it is removed from the relative floatation unit and the solid pollutant content, about 5 - 10 % w/w of the inlet dry matter, is deposited in a suitable container, to be then sent to the site of final conferment.

Here and hereafter, for all the floatation treatments cited, it is understood that they are of the foam-free type.

The floatation unit 12 is provided with a pneumatic floatation unit 14 and two further floatation units 15 and 16, that are of a different type form the floatation unit 14.

The pneumatic floatation unit 14 can be made either by means of floatation cells or by means of floatation columns.

Floatation cells and floatation columns belong to the same category of pneumatic floatation apparatus but they are different from each other from the mechanical and constructive point of view and are, thus, selected in the present invention depending on the treatment needs.

Hence, here and hereafter we also intend that floatation cells and floatation columns perform a pneumatic floatation of a different type from each other.

In the solution disclosed in the drawings, unit 15 is an ejector floatation unit, upstream of the unit 14, and unit 16 is a mechanical floatation unit, downstream of the unit 14.

Moreover, an alternative solution, not disclosed in the drawings, is within the scope of the invention, wherein unit 15 and 16 perform a pneumatic flotation of a type different from the pneumatic floatation performed in unit 14.

According to the present invention, the pneumatic floatation unit 14 is always present in the treatment unit 12 and is associated with one or more of said units 15 and 16, upstream or downstream thereof, according to needs. Moreover, in the floatation unit 12 a concentration or secondary floatation unit 17 is advantageously provided, in order to concentrate the head floatate arriving from each of said units 14, 15 and 16, as indicated by the arrows in the drawings. According to a first form of embodiment of the invention as shown in fig. 2, the plant 10 is described hereafter in its application to the treatment of lagoon sludge, in brackish water, polluted by heavy metals such as mercury, chrome, cadmium, vanadium or others, in their organic and inorganic forms and/or contained in colloidal forms. The relative treatment method will be clear in its steps to a person of skill in the art based on the following description.

Usually, the sludge to be treated, fed according to the arrow IN, is subjected to a preliminary assessment of its type, weight and humidity, at an analysis unit 18 for the sludge, so as to be able to correctly calibrate the subsequent passes.

The sludge to be treated can be sent to pre-treatment by a screw 19, a conveyor belt, a vacuum pump or other suitable device able to ensure a flow rate equal to the design potential of the plant. Then, a primary mixing of the sludge is carried out with other water, mainly recirculated water, fresh or salt, using for example a reactor with a mixer with a low number of revs, having a volume of about 150 liters.

According to the known values of weight and humidity, the sludge as such is taken from the initial water content, usually equal to about 40% w/w, to a value of dilution required for the process comprised between about 75 and 85% w/w of water. For example, in the case of a design potential equal to about 0.30 mVhr of sludge as such, the water required is comprised between about 0.40 m³/hr and about 0.90 m³/hr. The duration of the primary mixing is about 10 minutes and in this operation no measuring out of any chemical reagent is effected. Moreover, depending on the characteristics of the incoming sludge, a step of screening is performed, for example by means of a grid 21, for example of the manual type, with a spacing of about 3 mm, or even less or more if needed, and able to treat the whole delivery upon entry, that is, with reference to the previous case, able to have a minimum potential comprised between 0.70 m³/hr and 1.20 mVhr, to hold back all the extraneous objects found, which are removed as indicated by the arrow F.

Advantageously, apart from the screening step, a sifting step may also be performed, for example by means of a granulomere classifier, in order to separate other granulometric fractions, on which in any case no adsorption of heavy metals is registered, and which are distanced from the treatment plant and, if necessary, are subjected to bland traditional treatments.

The finest granulometric fraction of the sludge, less than about 0.2 mm - 0.3 mm is conveyed instead to a series of compartments where the sludge is disaggregated or de-agglomerated and dispersed, or homogenized, by means of mixing and/or stirring operations.

In particular, a step of dispersion and disaggregation is performed in a reactor 22, with a mixer, having a minimum volume of about 100 liters, in which a disaggregating mixing is carried out, in conditions of great turbulence, that is, with a Reynolds number (Re) comprised between about 10000 and 15000.

To guarantee an adequate de-agglomeration of the possible masses of clay possibly present, and to ensure that conditions of homogeneous fluid are reached, the minimum retention time of the sludge in the reactor 22 is about 10 minutes. To intensify the effect of the disaggregating treatment, suitable dispersing agents of a known type may be measured out and added, as indicated by the arrow G, advantageously not more than 10 grams per tonne of dry substance.

Once it has been subjected to disaggregating treatment, the sludge mixture is sent to a reactor 23 with a three-bladed marine propeller mixer, with a volume of about 300 liters, in which a step of secondary mixing is carried out, by means of stirring, with a duration of up to 15 minutes. In this case too, if necessary, suitable dispersing agents may be measured out and added, as well as activating modifying agents, such as sodium sulphide Na₂S and/or collector agents in order to hydrophobize the heavy metals, as indicated by the arrow L. The mixture of sludge arriving from the secondary mixing is conveyed to another analogous reactor 24 with a mixer, in which it is subjected to a step of final mixing, after measuring out, if required, the foaming agents, as indicated by the arrow M. For example, it is possible to add an activator modifier like sodium sulphide Na₂S, in a water solution 10% w/w, measured out in a quantity comprised between about 600 and 800 grams per tonne of contaminated sludge, expressed as dry substance. Furthermore, a collector can be added, belonging to the class of xantates or esters of thiocarbamate acids, in a water solution of 1 % w/w, measured out in a quantity comprised between about 60 and 100 grams per tonne of contaminated sludge, expressed as dry substance.

Foaming agents may also possibly be used, such as olein, in order to promote for example the floatation of the apolar molecules linked to the heavy metals, and polypropylene-glycol in order to promote the floatation for example of sulphides. On the oxidized and hydroxided surfaces of the particles of mercury and other heavy metals layers of bivalent sulphur S^2" form, caused by the sulphurization due to the sodium sulphide. The xantate ions are chemically absorbed on the layers of bivalent sulphur S^2", making the particles of mercury and other heavy metals hydrophobic, and allowing them to float. After the pre-treatment, the sludge with its additives is conveyed, with the initial lifting step performed by means of a pump 25, to the floatation unit 12, in order to perform the floatation treatment.

In the unit 15, the intake of atmospheric air into the cell, ensured by the ejector, induces the formation of bubbles of air which, rising up in the mass of sludge, form a foamy surface layer in which the hydrophobic substances, or substances made so, which have adhered to the interface surface between the air and water of the air bubble, in this case the hydrophobic minerals or metals, are easily removed, manually or mechanically.

The head floatate exiting from the ejector floatation unit 15 is subjected to a further concentration, whereas the tail floatate, exiting as effluent from the unit 15, is sent to the pneumatic floatation unit 14.

In the unit 14, the air is blown into the matrix through a suitable blowing device made by adopting a series of porous membranes connected to a collector and fed by means of a compressor with an adjustable air delivery. Advantageously, a laminar flow is established in the pneumatic floatation unit 14, with a Reynolds number (Re) comprised between about 3000 and 4000. The laminar flow, consisting of fluid threads or channels, promotes good performance or separation selectivity of the heavy metal. In fact, due to the coalescence of the air bubbles in the foam on the top of the floatation chamber, a further concentration of the particles of heavy metals is achieved.

This occurs because a competition is started between the particles of heavy metals in order to associate with the air bubbles and consequently to float, and at the same time, at the external limit of the foam, the particles of heavy metals weakly associated with the air bubbles, which again fall into the suspension, through the fluid threads or channels. This causes a high concentration of substrate with an active surface, that is, particles of heavy metals covered by floatation collectors, facilitating the good performance and selectivity of the floatation.

The head of the pneumatic floatation is subjected to concentration, whereas the tail, at outlet from the bottom of the unit 14, is sent to the unit 16, where a rotor or stirrer, having a variable rpm (round per minutes) value, is used, through the shaft of which air is introduced or sucked into the floatation cell, generally with a modest flow rate and at low pressure. The rotation of the stirrer causes the dispersion of the air bubbles introduced or sucked in through the tank of the cell, which as they rise draw the hydrophobic material, or material made so, upwards.

All in all, the time taken for the floatation treatment of the sludge is less than 40 minutes, while for some pollutants it may even be less than 30 minutes.

The head floatate, in the form of foam or dissolved foam, arriving from each of the units 14, 15 and 16, is made to flow to the unit 17 which functions as a secondary floatation unit, in this case a pneumatic floatation cell, in which, as we said, a further concentration is carried out. Alternately, for this concentration treatment, it is possible to use a battery of two mechanical floatation cells in series.

The contaminated and concentrated product obtained on the surface, equal to about 3 - 5 % w/w of the dry substance at inlet can now be collected and disposed of, as indicated by the arrow S, while the decontaminated mixture, or tail, emerging from the bottom of the unit 17, is conveyed to the head of the pre- treatment unit 1 1 in order to be subjected to a new floatation cycle.

The tail arriving from the unit 16 is sent to the post-treatment unit 13 by means of a final lifting step, with a pump 26. In this case, in the post-treatment unit 13 a step of dewatering of the tail is performed, for example by means of a suitably sized hydro-cyclone, or a bank of hydro-cyclones 27, in order to separate the process water, which constitutes a flow of potentially re-usable water, from the decontaminated inert fraction, with a cut range comprised between about 12 - 13 micron, equal to about 95 - 97% w/w of the dry substance entering.

The separated process water is stored in a store 28 and, in a subsequent recircling step, is recircled to the head of the plant, for example by means of a centrifugal pump, or disposed of, as indicated, respectively, by the continuous arrow N and P.

The solid that is separated in the hydro-cyclone 27 is stored in a tank 29 and then disposed of, as indicated by the arrow Q.

In this way a closed circle is achieved, limiting the discharge and cost of possible treatment of the surplus waters or effluent, the quantity of which is connected to the dewatering efficiency required.

The possible surplus waters or effluent can be conveyed to the sewerage system, after treatment, should the quality of the waters not be in conformity with the environmental norms in force.

The plant 10 described above is particularly effective for abating the pollutant content of heavy metals, in particular mercury, organic and inorganic and/or contained in colloidal forms, present in sludge in sea water or brackish water, such as waters of the lagoon.

In fact, the heavy metals too, which are associated with the organic fraction naturally present in the sludge, are effectively separated by means of the floatation treatment according to the present invention

According to a second form of embodiment of the present invention, the plant 10, with plant by-passes and particular strategies, such as the variation in type and/or order of the pre-treatments and/or treatments with respect to the treatment of heavy metals as described above, is also suitable to treat and remove hydrocarbons, oxidized and not oxidized, from sludge contaminated in water with a high saline and/or ionic content, in particular sea water, brackish water, or drilling water (fig. 3).

Where not indicated or expressly mentioned, it is understood that the steps of the method for treating sludge contaminated by hydrocarbons and the units of the relative plant are identical and in common with the those for the treatment of sludge contaminated by heavy metals.

The primary mixing step is analogous, with the difference that it has a maximum duration of about 20 minutes and, while it is being carried out, agents for modifying the pH are added to the sludge, for example a suitable base to increase the value of the pH, as indicated by the arrow R.

Compared with the treatment of heavy metals, the position of the reactors 22 and 23, where we have the dispersing mixing and the secondary mixing, is inverted and in the pre-treatment there is not the reactor 24 for the final mixing, which is moved to the post-treatment.

Moreover, the addition of disaggregating agents to the disaggregating mixing can be advantageously avoided.

Subsequently, we have the steps of ejector floatation, pneumatic floatation, mechanical floatation and concentration or secondary floatation; these steps are also identical to the floatation steps already described in the treatment of heavy metals, in order to make the hydrophobic substances, with a hydrocarbon base and adhering to the air bubbles, rise to the surface, and to remove them as foam or dissolved foam. At this point, the tail arriving from the unit 16 is sent to a reactor 30 with a mixer, having a minimum volume equal to about 300 liters, in which a tertiary mixing step is performed. Here agents to modify the pH are added to the tail, as indicated by the arrow T, such as suitable acids to correct the pH and return it to the required value, that is, of the original sludge as such, and also flocculant agents if it is required by the composition of the original sludge. The tail with its additives is then subjected to stirring, for a maximum of 10 minutes, and then sent to post-treatment.

Moreover, if necessary, the process water recovered by the dewatering operation and separated from the excess sludge can be conveyed to the reactor 24 described above, with a minimum volume of about 300 liters, in which a further correction of the pH is carried out by means of a suitable modifying agent, for example a base to increase the volume thereof, as indicated by the arrow V, and then there is a final mixing step, with stirring by means of a mixer, so as then to continue with the recircling.

According to an advantageous feature, the method according to the present invention and the relative plant can also be effectively used for treating industrial waters or oil extraction water.

It is clear, however, that modifications and/or additions of parts may be made to the method and the plant 10 as described heretofore, without departing from the field and scope of the present invention.

Claims

1. Method for treating sludge, contaminated by pollutant substances and in which the aqueous means consists in particular of sea water, brackish water, industrial water, oil extraction water, or other water with a high saline and/or ionic content, comprising a first step a) in which a pre-treatment of said sludge is performed and a second step b) in which a treatment of said sludge pre-treated in said step a) is performed, characterized in that said treatment comprises a foam-free floatation treatment of said sludge pre-treated in said step a), so as to obtain a head floatate, rich in said pollutant substances, and a tail floatate, poor in said pollutant substances, wherein said foam- free floatation treatment comprises at least two floatation passes of a different type from each other, of which at least one floatation pass is of the type with a pneumatic floatation.

2. Method as in claim 1, characterized in that the pneumatic floatation is of the floatation cells type.

3. Method as in claim 1 or 2, characterized in that the pneumatic floatation is of the floatation columns type.

4. Method as in any claims hereinbefore, characterized in that a second floatation pass is of the type chosen from a group comprising floatation with an ejector and a mechanical floatation and can be carried out before or after said pneumatic floatation pass.

5. Method as in any claims hereinbefore, characterized in that said floatation treatment comprises a third pass of the type chosen from a group comprising pneumatic floatation, floatation with an ejector and a mechanical floatation and can be carried out before or after said pneumatic floatation pass.

6. Method as in any claim hereinbefore, characterized in that said floatation treatment comprises a concentration sub-step, in which the head floatate of each floatation pass is subjected to a subsequent floatation pass of concentration, so as to obtain a concentrated head current, able to be disposed of, and a tail current, able to be returned to said pre-treatment.

7. Method as in any claim hereinbefore, characterized in that said pre-treatment comprises at least a pre-treatment step of the type chosen from a group comprising mixing, disaggregation, dispersion, dilution, screening, sifting.

8. Method as in any claim hereinbefore, characterized in that it also comprises a third step c) in which said floatation tail of said floatation pass is subjected to a post-treatment, able at least to separate a flow of re-usable water in said pre- treatment from a solid, decontaminated, low-humidity matrix.

9. Method as in any claim hereinbefore, characterized in that said pollutant substances comprise heavy metals and in which said pre-treatment comprises:

- a sub-step of primary mixing;

- a sub-step of screening and/or sifting;

- a sub-step of disaggregating mixing;

- a sub-step of secondary mixing; - a sub-step of final mixing.

10. Method as in claims 8 and 9, characterized in that said post-treatment comprises:

- a sub-step of dewatering said floatation tail, in order to separate said flow of water from said solid, decontaminated matrix; and - a recirculation sub-step in said primary mixing and/or disposal of said flow of water separated by means of said dewatering.

1 1. Method as in any claim from 1 to 8, characterized in that said pollutant substances comprise hydrocarbons and in which said pre-treatment comprises:

- a sub-step of primary mixing; - a sub-step of screening and/or sifting;

- a sub-step of secondary mixing;

- a sub-step of disaggregating mixing.

12. Method as in claim 11, characterized in that said floatation treatment of said second step b) comprises a sub-step of tertiary mixing performed on said floatation tail.

13. Method as in claim 8 and 11, characterized in that said post-treatment comprises a sub-step of dewatering said floatation tail in order to separate said flow of water from said decontaminated solid matrix, a sub-step of final mixing of said flow of separated water in said dewatering, and a recircling sub-step in said primary mixing and/or disposal, of said flow of water subjected to said final mixing.

14. Plant for treating sludge, contaminated by pollutant substances and in which the aqueous means consists in particular of sea water, brackish water, industrial water, oil extraction water, or other water with a high saline and/or ionic content, comprising means (1 1) for the pre-treatment of said sludge and means for the treatment of said pre-treated sludge, characterized in that said treatment means comprises foamless floatation means (12) for said pre-treated sludge, able to obtain a head floatate, rich in said pollutant substances, and a tail floatate, poor in said pollutant substances, wherein said floatation means ( 12) comprises at least two floatation units (14, 15, 16) of a different type from each other, of which at least one unit (14) is a pneumatic floatation unit.

15. Plant as in claim 14, characterized in that the pneumatic floatation unit is of the floatation cells type.

16. Plant as in claim 14, characterized in that the pneumatic floatation unit is of the floatation columns type.

17. Plant as in claim 14, 15 or 16, characterized in that a second unit (15, 16) is of the type chosen from a group comprising a floatation unit (15) with an ejector and a mechanical floatation unit (16) and can be disposed before or after said pneumatic floatation unit (14).