EP2560744A1

EP2560744A1 - Method for treating water by weighted flocculation using a flocculation agent of natural origin

Info

Publication number: EP2560744A1
Application number: EP11714584A
Authority: EP
Inventors: Céline LEVECQ; Philippe Sauvignet
Original assignee: Veolia Water Solutions and Technologies Support SAS
Current assignee: Veolia Water Solutions and Technologies Support SAS
Priority date: 2010-04-20
Filing date: 2011-04-18
Publication date: 2013-02-27
Also published as: US20130168318A1; CN103108688A; JP5770830B2; FR2958927A1; AU2011244352B2; CA2796713A1; CN103108688B; FR2958927B1; AU2011244352A1; JP2013525096A; AR080935A1; NZ603683A; WO2011131632A1

Abstract

The invention relates to a method for treating water by weighted flocculation including a step of injection into said water of at least one flocculation agent, a step of injection into said water of at least one particulate material which is denser than water, and a step of recovery of a treated water, characterized in that said weighted flocculation is used under stirring using a mean speed gradient between 100 and 1400 s^-1 and in that said flocculation agent consists of at least one carbohydrate polymer of natural origin having an anionic charge density between -900 and -4000 μEq/g.

Description

Method of water treatment by weighted flocculation using a flocculating agent of natural origin

1. Field of the invention

The field of the invention is that of the treatment of any type of water with a view to its purification or its potabilization.

More specifically, the invention relates to a water treatment technique including a weighted flocculation step.

2. Prior Art

Processes of this type consist in adding to the waters to be treated one or more reagents for flocculating, that is to say, in the form of flocs, at least a large part of the pollutants present in the water, then to separate these flocs of pollutants from purified water.

Flocculation is usually preceded by coagulation.

Coagulation consists of injecting at least one coagulating reagent into the water to be treated in order to reduce or cancel the electrical charges carried by the pollutants present in the water in the form of colloidal particles in suspension, in order to favor their subsequent agglomeration in the form of flocs.

The flocculation consists in injecting at least one flocculating reagent into the water, which is preferably pre-coagulated, so as to form large easily separable particles or flocs, by agglomeration of the colloidal particles in suspension in water. Flocculation is facilitated by the prior implementation of coagulation.

Purified water is then obtained by separating the flocs suspended therein by settling.

According to one variant, a granular material denser than water, such as sand preferably having a particle size of between 60 and 300 microns, may be injected into the water to be treated upstream or during flocculation so as to ballast the flocks and thus promote and speed up their settling. Such a technique is described in particular in the French patent document published under the number FR2627704. A flocculation step during which, or upstream of which, a granular material denser than water, or ballast, is injected into the water is commonly called weighted flocculation.

In order to keep the ballast suspended in the water to be treated and thus to promote the formation of flocs around the ballast grains, the weighted flocculation is carried out with stirring. The flocculation step thus takes place inside a flocculation tank which usually houses a mechanical agitator of the paddle stirrer type.

Thus, for weighted flocculation to be effective, it is desired that the specific velocity be greater than 0.1 ms ^-1 in the vessel within which the weighted flocculation is carried out.

The specific speed is equal to the ratio between the pumping rate Q _p according to which the treated water is stirred in the flocculation tank and the ground surface of this tank.

Moreover, it is known that the power P of the stirrer can be calculated according to the following formula (N _P characterizes the coefficient of drag of the stirrer in the fluid):

p = pN _p N ³ D ⁵

and that the intensity of mixing in the tank can be evaluated by the average velocity gradient G:

The average speed gradient G can thus be calculated according to the following formula: where: Qp is the pumping rate (m .s ^" ) Np is the power number

N is the rotational speed of the agitator (tr.mn ¹ )

D is the diameter of the stirrer (m)

p is the fluid density (kg .m ³⁾

μ is the kinematic viscosity of the fluid (kg.s ^ .m ¹ )

V is the volume of the tank (m ³ )

P is the power of the stirrer (kg.m ² .s ³ )

G is the average speed gradient (s ¹ )

The implementation of such an agitator therefore allows to rule within the flocculation tank a gradient of average speed G generally between 100 and 1400 s ^"1 .

The average speed gradient prevailing inside the flocculation tank generates shear stresses on the flocs that are in suspension there.

The flocs must therefore have good mechanical strength so as not to disintegrate under the effect of these shear forces. For this purpose, the flocculating agents, also called flocculation adjuvants, used must give the flocs sufficient mechanical strength.

The flocculating agents currently used for the purpose of satisfying these constraints are organic in nature. These are most often synthetic oil derivatives.

The use of this type of flocculating agent is advantageous in that it contributes to creating flocs that are resistant to the hydraulic conditions inherent in ballasted flocculation and consequently makes it possible to produce an effectively weighted flocculation-treated water. This implementation, however, has some disadvantages.

3. Disadvantages of prior art

In particular, certain organic flocculating agents, such as for example polyacrylamide, are currently suspected of being carcinogenic. Therefore, their implementation might not be totally neutral about the state of health of the operators who handle them or consumers who use the treated water produced by processes using such organic flocculants.

In addition, these organic flocculating agents, which are found in the sludge obtained after separation of the flocs, are not biodegradable. This sludge is collected generally for incineration or as a fertilizer. The organic flocculants they contain can then be sources of pollution of the atmosphere or soils.

Under these circumstances, legal restrictions on the use of such organic flocculants should ultimately lead to the proscription of their use in water treatment.

In addition, in water treatment processes using a filtration of the treated water produced by weighted flocculation, the organic flocculants constitute a source of clogging of the filtration membranes used for this purpose.

Moreover, the current organic flocculants being petroleum by-products, the evolution of their cost follows that of the oil price, which presents a globally ascending slope which should be maintained considering its rarefaction. Their implementation has a significant impact on the overall cost of water treatment.

4. Objectives of the invention

The invention particularly aims to overcome these disadvantages of the prior art.

More specifically, an object of the invention is to provide a weighted flocculation water treatment technique whose implementation has a reduced or no ecological impact.

In particular, the invention aims to provide such a technique that has no effect on the state of health of the operators who implement them or consumers who use the treated water produced by such a technique. Another objective of the invention is to implement such a technique which has a level of efficiency equivalent if not close to that of current water treatment techniques by weighted flocculation.

The invention also aims to provide such a technique whose implementation is at least no more expensive than current water treatment techniques weighted flocculation.

In particular, the invention aims to provide such a technique which leads to limiting the clogging of membranes that can be implemented to filter treated water produced by ballasted flocculation.

5. Presentation of the invention

These objectives, as well as others which will appear later, are achieved by means of a weighted flocculation water treatment method comprising a step of injection into said water of at least one flocculating agent, a step of injecting into said water at least one particulate material denser than water, and a step of recovering a treated water, said weighted flocculation being carried out with stirring in a mean speed gradient of between 100 and 1,400 s ^-1 and said flocculant being at least one naturally occurring carbohydrate polymer having an anionic charge density of from -900 to -4,000 eq / g.

In the present description, the term "carbohydrate polymer of natural origin having an anionic charge density" is understood to mean:

any carbohydrate polymer extracted from plants, such as in particular starch, functionalized by grafting anionic reaction groups according to conventional techniques known to the chemist specialized in this field,

as well as :

any natural carbohydrate polymer extracted from plants and naturally having naturally anionic or anionic groups.

The general principle of the invention is therefore based on the implementation of carbohydrate polymers of natural origin functionalized as flocculants in a weighted flocculation water treatment under high average gradient conditions, said polymer having a selected filler in a particular range.

Such an implementation was by no means obvious from the prior art since the high average speed gradients reflect strong shear stresses on the flocs. In fact, it had already been proposed in the prior art to use carbohydrate polymers of natural origin, namely starches, in non-weighted flocculation processes, but these techniques had been abandoned because the flocs formed had bad behavior, in practice they disintegrated too easily and did not fulfill their role. However, non-ballasted flocculation techniques use much lower average speed gradients than those used in weighted flocculation techniques. Those skilled in the water treatment art therefore had no incentive to consider the use of such polymers in the context of these weighted flocculation techniques in view of this prior art.

In addition, the Applicant has, after much research, found that only certain carbohydrate polymers of natural origin could respond effectively to these constraints of high average speed gradients specific to weighted flocculation and that these polymers had to be have an anionic charge density selected in the range indicated above.

The invention makes it possible to produce a treated water having a quality equivalent to a water treated by weighted flocculation using a conventional synthetic organic flocculant polymer while being more respectful of the environment and having a limited impact on the health of individuals since the anionic flocculants based on natural carbohydrate polymer functionalized according to the invention are biodegradable.

Said flocculating agent is preferably composed of substituted starch. The substituted starches are preferred because less expensive and more easily available on the market. The range of anionicity ranging from -900 ε4 -4000 ε of a substituted starch corresponds to a degree of substitution of between about 0.1 and 0.5.

Also advantageously, the substituent or substituents of said substituted starch are preferably selected from the group consisting of carboxylate groups, sulfonates, phosphates, phosphonates.

According to a first preferred embodiment, a method according to the invention comprises a coagulation step upstream of said weighted flocculation step.

Also preferably said weighted flocculation step is followed by a decantation step.

According to a second preferred embodiment, a method according to the invention comprises a step of injecting activated carbon into said water upstream of said weighted flocculation step.

In this case, a process according to the invention preferably comprises a filtration step, in particular by membranes, in succession to said decantation step.

The implementation of a filtration downstream of the decantation makes it possible to separate from the treated water coal fines and an excess of flocculating agent.

Preferably, a method according to the invention comprises in this case an additional coagulation step implemented immediately before said filtration step.

The additional coagulation causes the formation of flocs with the excess of polymer contained in the water from the decantation. These flocs are larger in size than the particles initially present in the water coming from the settling. These flocs are deposited on the surface of the membranes of the filtration unit while the particles initially present in the water from the flocculation penetrate deeper. The implementation of the second coagulation is therefore advantageous because it limits the clogging of the filtration unit or at least makes it easier to unclogging. The inventors have also discovered that, surprisingly for polymers of natural origin, the ionic charge thereof should preferably be chosen as a function of the alkalinity of the water. In practice, the harder the water to be treated, the more the anionic charge density will be close to -4,000 eq / g. The less hard the water, the more the anionic charge density will be close to -900 μeq / g.

Thus, the process according to the invention comprises a preliminary step of choosing said flocculating agent as a function of the hardness of the water to be treated, the harder the water, the more flocculant being anionic.

Also according to a preferred variant of the invention, said step of injecting into said water at least one flocculating agent is carried out by adding said flocculating agent to water previously coagulated at a content of between 0.1 and 5 ppm, preferably between 0.1 and 2 ppm depending on the load of the water to be treated flocculating pollutants.

These contents are much higher than those used with synthetic polymers. Those skilled in the art could therefore have feared that the implementation of such high levels would have caused a strong increase in the biological COD (biodegradable dissolved organic carbon "DOCB"). The DOCB is estimated from the decay of dissolved organic carbon DOC after a long incubation period (28 days) in the presence of a suspension of bacteria (AFNOR T 90-318) or a fixed biomass (AFNOR T 90 -319). Surprisingly, it is not so.

Finally, according to a variant of the invention, said weighted flocculation is carried out with stirring in accordance with a mean velocity gradient of between 200 and 800 s ^-1 This range of speed gradient is that used in the majority of reactors flocculation.

6. List of figures

Other features and advantages of the invention will appear more clearly on reading the following description of preferred embodiments, given as simple illustrative and non-limiting examples, and appended drawings, among which:

FIG. 1 illustrates an installation for implementing a first embodiment of a method according to the invention;

FIG. 2 illustrates an installation for implementing a second embodiment of a method according to the invention;

FIGS. 3 and 4 are graphs showing comparisons of organic wastewater treatment performance in a plant of the type shown in FIG. 1, using on the one hand a synthetic organic flocculation agent, state of the art and secondly implementing a flocculating agent of natural origin according to the present invention;

Fig. 5 is a graph illustrating that the flocculant used according to the present invention is less clogging than conventional flocculation adjuvants.

7. Description of embodiments of the invention

7.1. Reminder of the general principle of invention

The general principle of the invention is based on the use of anionic flocculating agent consisting of carbohydrate polymer of natural origin in a weighted flocculation water treatment conducted at a high average rate gradient.

Such an implementation makes it possible to produce a treated water having a quality equivalent to a water treated by weighted flocculation using a synthetic organic flocculant polymer while being more respectful of the environment and individuals because the flocculants constituted of carbohydrate polymer of natural origin are biodegradable.

Moreover, this type of flocculating agent has the advantage, compared with organic flocculant agents of synthetic origin, to allow the production of treated water whose clogging power is lower. This can then be filtered while limiting the constraints associated with unclogging filtration units implemented for this purpose. 7.2. Example of a first embodiment

In relation to FIG. 1, a first embodiment of a weighted flocculation water treatment method according to the invention is presented.

Such a process consists in introducing a water to be treated 10, which is, for example, previously clarified or floated, in a coagulation tank 11 in which a coagulating agent 12 is injected, which in this embodiment consists of ferric chloride (FeCl ₃ ), commercial product.

The water thus coagulated 13 is introduced into a stirred ball of weighted flocculation 14 inside which are injected a flocculating agent 15 and a particulate material denser than water 16, or ballast, which in this embodiment is microsable. The flocculation tank 14 houses a paddle stirrer 20 which is implemented in such a way that there prevails inside this tank a mean velocity gradient of between 300 and 1400 s ^-1 .

The flocculating agent 15 consists of a carbohydrate polymer of natural origin, preferably of substituted starch, and has an anionic charge density preferably of between -900 and -4,000 eq / g. In the case of substituted starch, such a range of anionic charge density corresponds to a degree of substitution of between 0.1 and 0.5. The substituents are then advantageously chosen from the group comprising carboxylates, sulphonates, phosphates and phosphonates.

The coagulated and flocculated water 17 is introduced into a decanter 18 in the bottom of which are deposited sludge consisting of weighted flocs separated from a clarified water which is extracted overflow 19.

The sludge 21 is extracted from the decanter 18 by means of, for example, a recirculation pump 22 and is introduced, via a pipe 23, into a hydrocyclone 24 into which service water 25 is injected.

A mixture of sludge and microsand loaded with microsand 16 is poured under the hydrocyclone 24 into the weighted flocculation tank 14.

A mixture of sludge and microsand heavily loaded with sludge 27 is discharged overflow of Γ hydrocyclone 24 in a spillway channel 26.

A partly dehydrated mixture is extracted from the chute 26 by means of an extraction pump 28 and the effluent 29 from this dehydration is injected into the coagulation tank 11.

In a variant of this embodiment, it may be provided to implement no coagulation.

7.3. Example of a second embodiment

In relation to FIG. 2, a second embodiment of a weighted flocculation water treatment method according to the invention is presented.

This second embodiment differs from the previous embodiment in that:

the water to be treated 10 is introduced into a stirred pre-contact tank 31 into which powdered activated carbon 32 (CAP) is injected via a pump 33, and that

the effluent 27 from the spillway 26 is injected into this precontact tank 31.

The mixture of water to be treated and CAP 34 is then introduced into the coagulation tank 11.

This second embodiment is further distinguished from the first by the fact that the produced treated water 19 is introduced into a coagulation chamber 40 in which a coagulating agent is introduced, then into a filtration unit composed of a pre-filter 42 having a cut-off threshold at 150 microns and a membrane ultrafiltration module 41 having a cutoff threshold at 25 nm.

The implementation of a filtration downstream of the settling makes it possible to separate from the treated water coal fines and an excess of flocculating agent.

The coagulation implemented in the coagulation zone immediately preceding the filtration unit causes the formation of flocs with the excess of flocculating agent contained in the water leaving the decanter. These flocs present a larger size than the particles initially present in the water from the settling. These flocs are deposited on the surface of the membranes of the ultrafiltration unit while the particles initially present in the water coming from the settling penetrate the ultrafiltration unit further. The implementation of this coagulation is therefore advantageous because it makes unclogging of the ultrafiltration unit easier.

A method according to this second embodiment comprises cleaning phases of the ultrafiltration unit. These cleaning phases (preventive maintenance) are of two types: the hydraulic cleaning which consists of backwashing, and the chemical cleaning which implements chemical cleaning solutions.

7.4. Comparative tests

7.4.1. Tests in the context of the implementation of a method according to the first embodiment

Attempts have been made to compare the effectiveness of a ballasted flocculation using, according to the invention, a flocculant of natural origin based on starch and / or alginate modified, or, according to the prior art, an organic flocculant agent of synthetic origin.

A first test consisted of treating a raw water with a content of DOC (dissolved organic carbon) equal to 10.6 mg / l and an alkalinity of 5 ° f (ie 50 mg / l of CaCO ₃ ) by the implementation a ballasted coagulation-flocculation process according to the first embodiment described above with the following characteristics:

supplying the coagulation tank with water to be treated at a flow rate equal to

50 m ³ / h;

average velocity gradient in the flocculation tank equal to 800 s ^-1 ;

injection of a 150 ppm dose of ferric chloride FeCl ₃ (commercial product) as a coagulating agent;

injection of a dose of 0.2 ppm anionic polyacrylamide anionic charge density -1400 // eq / g sold under the name commercial FLOPAM AN905 by the company SNF FLOERGER, as an organic flocculant agent of synthetic origin.

As shown in FIG. 3, with reference to the first two columns thereof, the implementation of such a treatment has allowed:

- reduce by approximately 93% the turbidity of the raw water;

to reduce by approximately 80% the UV absorbance at 254 nm;

This treatment led to the production of treated water with a DOC content equal to 3.3 mg / 1. A second test consisted in treating a raw water having a DOC equal to 10.8 mg / l by the implementation of a weighted coagulation-flocculation process according to the first embodiment with the following characteristics: feeding of the tank coagulation water to be treated at a rate equal to

50 m ³ / h;

- average velocity gradient in the flocculation tank equal to 800 s ^-1 ;

a 150 ppm dose of ferric chloride FeCl ₃ (commercial product) as a coagulating agent;

a dose of 2 ppm of substituted starch marketed under the trade name C * plus 35704 by the company Cargill whose anionic charge density is equal to -900 eq / g) as a flocculating agent.

The anionic charge of this starch was measured using a device ("zametameter") MUTEK PCD 04 travel marketed in France by the company Noviprofibre under the reference X20128.

As shown in FIG. 3, with reference to the second two columns thereof, the implementation of such a treatment has allowed:

to reduce by approximately 87% the turbidity of the raw water;

to reduce by approximately 76% the UV absorbance at 254 nm;

This treatment led to the production of treated water with a DOC content equal to 3.6 mg / 1. Table 1 below indicates, for these two tests, the dose of polymer used, the dose of coagulant used, the biodegradable COD of the raw water and the treated water and the COD of the raw water and the water. treated water.

DCO *: the

accuracy of the

measure is

0.3mg / l

Table 1

The results of these two tests show that the use of a flocculant polymer of natural origin according to the invention makes it possible to produce treated water having a quality as satisfactory as that obtained by using a synthetic conventional organic flocculant polymer.

Obtaining such a level of quality, however, requires using quantities of polymer of natural origin much larger than conventional organic polymer. Nevertheless, polymers of natural origin are currently less expensive than conventional organic polymers. In addition, the cost of these, which are petroleum derivatives, should not stop growing. in the years to come. Also, the use of natural polymers in substitution of conventional organic polymers, certainly in greater proportions, should have no negative impact on the cost of production of water treated by weighted coagulation-flocculation.

In addition, the test results described above also show that the use of flocculent polymers of natural origin (in this case a substituted starch) in proportions of water in the form of weighted flocculation-coagulation-flocculation processes. Ten times higher than that of synthetic organic flocculant polymer makes it possible to produce treated water with levels of qualities that are fairly similar in terms of DOC content, turbidity and UV absorbance at 254 nm.

Surprisingly, the use of such large doses of flocculant polymer of natural origin does not lead to a significant increase in the biological COD in the treated water as it was normal for those skilled in the art to expect. .

For example, with reference to the table above, the water treated using a dose of 2 ppm of C * PLUS 35704 does not have a significantly greater biological COD than that treated using a dose of 0.2. ppm of FLOPAM AN905. However, adding 10 times more organic flocculation adjuvant in a water was obviously feared that the latter a risk of seeing the biological COD of the treated water increase significantly. Those skilled in the art were therefore in no way prompted to add to a water to be treated organic flocculation agent at dosages significantly higher than those used with adjuvants consisting of synthetic organic polymers.

Finally, it will also be added that polymers of natural origin are biodegradable. Their use therefore has no adverse effect on the environment or the state of health of individuals. Two other tests were reproduced under the same conditions as those of the two tests described above, but with a raw water less loaded with organic matter and having an alkalinity of 5 ° f (ie 50 mg / l CaCO ₃ ) with doses of coagulant and different flocculating agents as shown in Table 2 below.

The results of these tests on raw water turbidity reduction rates and UV absorbance abatement rates at 254 nm are shown in Figure 4.

Dose of

COD dose of

COD dot * coagulating polymer

Biodegradable polymer

sample (mg / l) used (FeC13)

(mg / 1) (ppm)

(Ppm)

Raw water 1, 5 4,5

Water released after C * PLUS

2 85 coagulation- 35704

0.4 1, 5

weighted flocculation and

décantaion

Raw water 1, 3 4,7

FLOPAM

Water output after 0.2 70 coagulation- AN905

0.3 1, 3

weighted flocculation and

décantaion

DCO *: precision

of the measure is of

0.3mg / l

Table 2

7.4.2. Tests in the context of the implementation of a method according to the second embodiment

Water was treated by the implementation of a method according to the second embodiment by varying the dose of iron chloride (FeCl ₃ ), a commercial product, injected into the water leaving the decanter, and measuring the loss of permeability recorded at the ultrafiltration unit.

The loss of permeability was 20.7 L / (h.bar.m ² ) for a dose of 0.05 ppm FeCl ₃ . It was equal to 4.5 L / (h.bar.m ² ) for a dose of 0.1 ppm FeCl ₃ . It was equal to 2.8 L / (h.bar.m ² ) for a dose of 0.15 ppm FeCl ₃ .

The results of these tests demonstrate that the fact of coagulating the water at the outlet of the clarifier makes it possible to reduce its clogging power and to consequently limit the clogging of the ultrafiltration unit placed downstream.

FIG. 5 is a graph illustrating the variation of permeability at the level of the ultrafiltration unit during the implementation of the process according to FIG. second embodiment on the one hand with the use of polymer of anionic synthetic origin of charge density equal to -1400 // eq / g and secondly with use of the polymer C * plus 35704 of natural origin (substituted starch) whose anionic charge density is -900 eq / g)). During these tests, washing phases of the ultrafiltration unit were implemented.

With the FLOPAM AN905 synthetic polymer, the filtration and washing sequences of the ultrafiltration unit were distributed as follows:

filtration for 40 minutes with pre-injection of 0.15 ppm ferric chloride, commercial product, in water from the decanter; every 40 minutes: backwashing for 40 seconds;

every 24 hours: maintenance cleaning including injection of soda for 25 seconds, soaking (maintaining sodium hydroxide in the ultrafiltration unit) for 10 minutes, rinsing for 80 seconds, filtration for 40 minutes, injection of acid during

25 seconds, soaking for 10 minutes, rinsing for 80 seconds.

With the substituted starch C * plus 35704, the filtration and washing sequences of the ultrafiltration unit were distributed as follows:

filtration for 40 minutes with prior injection of 0.15 ppm of ferric chloride, commercial product, in the water coming from the decanter; every 40 minutes: backwashing for 40 seconds;

every 26 hours: maintenance cleaning including injection of soda for 25 seconds, soaking (maintaining sodium hydroxide in the ultrafiltration unit) for 10 minutes, rinsing for 80 seconds.

The graph of FIG. 5 shows that the starch-based polymer which accumulates on the membrane surface of the ultrafiltration unit is more easily removed than the polymer of synthetic origin because:

the washes used during the use of the flocculant polymer of natural origin are less powerful and less frequent than those used when using polymer of synthetic origin, and that the difference in permeability of the ultrafiltration module between the end of a filtration cycle and the beginning The cycle following a maintenance cleaning is more important when using the flocculant polymer of natural origin (arrow A) than that of the polymer of synthetic origin (arrow B).

In addition to maintenance cleaning, in-place clean-ups must be performed as soon as the permeability reaches a predetermined low threshold which was set at 180 L / (hr.bar.m ² ) during testing. The linear regression line drawing on the graph illustrated in FIG. 7 from the permeability reached at the end of each filtration cycle (lines 71 and 72) makes it possible to evaluate the frequency with which cleaning operations will have to be carried out. in place. For an ultrafiltration module operating 24 hours a day with a starting water permeability of 550 L / (h.bar.m ² ), the cleanings in place were to be implemented every month as part of the use of the polymer of anionic synthetic origin with a charge density of -1400 // eq / g (straight line 71), and every 4 months in the context of the use of substituted starch-based polymer (straight line 72). ).

The starch-based polymer is therefore less viscous and less clogging than the organic polymer. Its implementation makes it possible to reduce the washing frequency of the ultrafiltration unit. This is particularly because it is biodegradable.

7.5. Main benefits

Flocculants consisting of natural carbohydrate polymers functionalized with anionic functional groups, such as substituted starches, having an anionic charge density of between -900 and -4000 eq / g have the following advantages in particular:

they allow to produce treated water having a quality comparable to that of a water produced by using organic flocculants synthetic origin;

they are biodegradable and have no impact on the environment or people;

they have a clogging power reduced;

they are easily released from the filtration membranes during unclogging operations;

they are cheap;

they are not derived from oil as organic flocculants of synthetic origin; their tariff is therefore not indexed to that of oil, which should not stop increasing in the years to come.

Claims

1. A method of water treatment by weighted flocculation comprising a step of injection into said water of at least one flocculating agent, a step of injection into said water of at least one particulate material denser than water, and a step of recovering a treated water,

characterized in that said weighted flocculation is carried out with stirring in a mean velocity gradient of between 100 and 1400 s ^"1 and in that said flocculating agent consists of at least one naturally occurring carbohydrate polymer having an anionic charge density of between -900 and -4000 eq / g.

2. Method according to claim 1, characterized in that said flocculating agent is a substituted starch.

3. Method according to claim 2, characterized in that said substituted starch has a degree of substitution of between 0.1 and 0.5.

4. Method according to any one of claims 1 to 3, characterized in that the substituent or substituents of said starch are selected from the group comprising the carboxylate substituents, sulfonates, phosphates, phosphonates.

5. Method according to any one of claims 1 to 4, characterized in that it comprises a coagulation step upstream of said weighted flocculation step.

6. Method according to any one of claims 1 to 5 characterized in that said weighted flocculation step is followed by a decantation step.

7. Method according to any one of claims 5 or 6, characterized in that it comprises a step of injecting activated carbon into said water upstream of said coagulation step.

8. Method according to any one of claims 6 or 7, characterized in that it comprises a filtration step in succession to said decantation step.

9. The method of claim 8, characterized in that it comprises an additional coagulation step implemented immediately before said filtration step.

10. Method according to any one of claims 1 to 9 characterized in that it comprises a preliminary step of choosing said flocculating agent according to the hardness of the water to be treated, the harder the water, the more difficult the flocculating agent being anionic.

11. Method according to any one of claims 5 to 10 characterized in that said step of injection into said water of at least one flocculating agent is carried out by adding said flocculating agent to the water previously coagulated at a rate of content between 0.1 and 5 ppm, preferably between 0 and 2 ppm.

12. Method according to any one of claims 1 to 11 characterized in that said weighted flocculation is carried out with stirring at a mean speed gradient of between 200 and 800 s ^"1 .