EP1216207A1

EP1216207A1 - Composition useful for conditioning sludge derived from the treatment of an aqueous medium and uses thereof

Info

Publication number: EP1216207A1
Application number: EP00964332A
Authority: EP
Inventors: Yvette Pescher; Bruno Bavouzet; Michèle RAFFARD
Original assignee: Rhodia Chimie SAS
Current assignee: Rhodia Chimie SAS
Priority date: 1999-09-20
Filing date: 2000-09-20
Publication date: 2002-06-26
Also published as: FR2798652B1; AU7529200A; FR2798652A1; WO2001021532A1; US6855268B1

Abstract

The invention concerns a composition useful for conditioning sludge obtained by mixing an invert emulsion containing at least a cationic polyelectrolyte with an invert emulsion or an aqueous solution containing at least a mineral cation with a charge not less than two. The invention also concerns the corresponding applications.

Description

COMPOSITION USEFUL FOR THE CONDITIONING OF SLUDGE RESULTING FROM THE TREATMENT OF

AQUEOUS MEDIA AND ITS APPLICATIONS.

The present invention relates to a new composition which can be used in particular for the treatment of aqueous media such as waste or urban or industrial water and more particularly the conditioning of sludge prior to their dewatering operation.

The treatment of waste water, urban or industrial, in particular by biological means, in the purification stations, leads to the production of sludge. This sludge generally undergoes a mechanical dewatering operation (in particular filtration, centrifugation), before being transported to a landfill, agricultural spreading or incineration site.

The sludge to be treated consists mainly of water in which biomass is dispersed at a rate of 12 to 40 g / l. The treatments therefore aim to concentrate the dry matter as much as possible and to remove the water. The object of the present invention is precisely to propose a new composition which can be used effectively in the conditioning of this type of sludge, making it possible in particular to increase their dryness, that is to say to increase the dry extracts obtained during the subsequent operation. dehydration.

When the sludge is intended for a filter press, conventionally added to the sludge to be treated is an inorganic salt associated, where appropriate, with an electrolyte, commonly lime.

On the other hand, when the sludge is intended to be treated on a band filter or by centrifuge, the drainability necessary for the evacuation of the water is generally obtained by adding to them in sequence a inorganic salt, a cationic polymer and then optionally a polymer anionic.

The sludge treatment methods currently available are therefore different depending on the water / biomass separation technique used. On the other hand, they generally require the successive addition of several reagents. It is indeed difficult to formulate in a single composition and at significant concentrations, an organic salt of the aluminum polychloride type and a cationic polyelectrolyte. ^Aluminum is a complexing certain polyelectrolytes, their mixtures lead from a certain concentration in these compounds, to the formation of gels which are of course detrimental in terms of fluidity to the corresponding mixtures.

The object of the present invention is precisely to propose a universal composition, that is to say which can be used indifferently for the treatment of sludge according to one or the other of the techniques mentioned above.

Furthermore, the claimed composition has the advantage of combining, in the form of a mixture, an inorganic cation of charge greater than or equal to 2 and a cationic polyelectrolyte while not being subject to the gelling phenomenon discussed above.

More specifically, the main object of the present invention is a composition useful for conditioning sludge obtained by mixing at least one reverse emulsion containing at least one cationic polyelectrolyte with a reverse emulsion or an aqueous solution containing at least one inorganic filler cation. greater than or equal to two.

It thus extends to a composition useful for conditioning sludge, characterized in that it is in the form of an emulsion comprising in aqueous phase (s) and separately at least one mineral cation of charge greater than or equal to 2 and a cationic polyelectrolyte. According to a first embodiment, said composition is in the form of a reverse water-in-oil emulsion in which said mineral cation and said cationic polyelectrolyte are distributed in separate water droplets in the oily phase.

This type of composition is generally obtained by mixing, with stirring, a reverse emulsion containing said mineral cation with a reverse emulsion containing said cationic polyelectrolyte.

According to a second embodiment, said composition is in the form of a double water / oil / water emulsion in which the mineral cation is present at the level of the aqueous continuous phase and the cationic polyelectrolyte is distributed at least in part in water droplets constituting the second aqueous phase dispersed in the oily phase. This second type of emulsion can be obtained by mixing an inverse emulsion of the cationic polyelectrolyte with an aqueous solution of the mineral cation.

The claimed compositions are particularly advantageous insofar as they prove effective for all of the sludge treatment techniques, namely belt filtration and / or centrifugation or filtration on a press.

Furthermore, by allowing the mineral cation to be isolated from the cationic polyelectrolyte, they make it possible to formulate these reagents in higher concentrations. Thanks to this emulsion formulation, the risk of gelation is significantly reduced.

Particularly suitable for the formulation of a concentrated composition, the composition obtained by mixing the two inverse emulsions, namely that of the mineral cation and that of the cationic polyelectrolyte. Indeed, these two compounds are then distributed at the level of the emulsion in separate water droplets and are therefore effectively isolated from one another.

The mineral cation has a charge greater than or equal to 2. It is usually chosen from Mg ²⁺ , Al ³⁺ , Fe ³⁺ , La ³⁺ , Zr ^{4 *} and their polymerized forms when they exist. Very preferably, the cation is Al ³⁺ or one of its polymerized forms.

This mineral cation is present in the form of a water-soluble salt. As soluble salts, chlorides, nitrates, sulfates and acetates can be used.

In general, a soluble salt is used, free of the nitrogen element, which makes it possible to overcome any problems linked to its presence. Very preferably, a chloride is used. The cation is preferably an aluminum chloride or one of its polymerized forms and more preferably poly aluminum chloride.

The amount of mineral cation used is preferably between 0.05 and 2 moles, in particular between 0.49 and 1.8 moles per kg of the composition. The pH of the solution or emulsion is adjusted so as to prevent precipitation of the mineral salt in the continuous aqueous phase. This adjustment falls within the competence of man and art.

As regards the cationic polyelectrolyte, it is preferably of high molecular weight, that is to say of molecular weight greater than 1.10 ⁶ .

The polyelectrolytes preferably have a molecular weight from 1.10 ^e to approximately 20.10 ^e , more preferably from 1.10 ^e to approximately 10.10 ⁶ .

The cationic polyelectrolyte can be linear or branched and is preferably in a branched form. It was indeed noted that the ramifications had a beneficial effect on the kinetics of coagulation and flocculation.

Polyacrylamides, polyethylene oxides, polyvinylpyrrolidones, as well as polymers of natural origin such as starch and its derivatives or gum such as guar gum, are particularly suitable as cationic polyelectrolyte, insofar as they are cationic. Preferably, the polyelectrolyte is a polyacrylamide.

The polyacrylamide can be cationic up to 100% filler and is preferably cationic between about 0.1 and 15% filler.

Cationic polyacrylamides include copolymers of polyacrylamide with cationic monomers or polyacrylamides modified according to the Mannich reaction.

Examples of cationic polyacrylamide copolymers include acrylamide / halide copolymers, preferably diallyldialkylammonium chloride, diaminoalkylmethacrylate / acrylamide copolymers and dialkylaminoalkylmethacrylate / acrylamide copolymers, the C ₆ to C ₆ alkyl group.

Advantageously, the cationic polyelectrolyte is a polyacrylamide copolymer, and preferably the polyacrylamide / diallyldimethylammonium chloride copolymer with a molecular weight of the order of 3.10 ⁶ . It is more preferably combined with a polychloride of aluminum in the claimed composition.

The amount of cationic polyelectrolyte present in the claimed composition is preferably at most 10% by weight, in particular between 0.3% and 8% by weight of said composition. Generally, the reverse emulsion of the cationic polyelectrolyte, for example in the case of a high molecular weight polyacrylamide and the aqueous solution or the reverse emulsion of the mineral cation, for example in the case of polychloride of aluminum, are used. works in such a way that the molar ratio (mineral cation of charge greater than or equal to 2) / (cationic polyéectrolyte) is between 1.10 ² and 8.10 ^e in particular between 1.10 ³ and 8.10 ^e . In the particular case of a composition comprising polychloride of aluminum and a copolymer acrylamide / diallyldialkylammonium chloride of high molecular weight, one preferably uses a weight ratio polychloride of aluminum / polyacrylamide ranging between 0,1 and 15 and more particularly between 0.1 and 10.

As mentioned above, the claimed compositions containing the mineral cation and the cationic polyelectrolyte distributed in separate water droplets in the oily continuous phase are particularly advantageous for the formulation of a composition concentrated in these two compounds. This is how this mode of formulation will be favored for compositions containing a polyacrylamide concentration of the order of 4 to 10% by weight relative to the composition.

If necessary, the claimed composition can be stabilized in the form of an emulsion using surfactant (s). The surfactants are generally introduced at the level of the reverse emulsion (s) constituting said composition.

As regards the surfactant present in the oily phase of the emulsion, it is preferably a surfactant which naturally remains inert with respect to the mineral cation.

The liposoluble surfactants, which can be used in the emulsion according to the invention, can be chosen from liposoluble lecithins, esters of sorbitan and of fatty acids, of polyalkylenes dipolyhydroxystea spleens, fatty acids, monoglycerides, polygiycerol esters and lactic and tartaric acid esters.

Illustratively water-soluble surfactants, water-soluble lecithins may be cited, sucrose esters, fatty acid esters (including Tweens ^®), polyoxyethylene alkylamides, triglyceride sulphates, alkyl sulphates (including dodecyl sodium sulfate SDS), alkyl ether sulfates, alkyl sulfonates, alkylamine salts, fatty amines, lipoamino acids, modified polyesters and polymeric silica-born surfactants.

The amounts of surfactants are adjusted so as to stabilize the formulation (reverse or double emulsion) of the composition.

The claimed composition can be prepared either by mixing the two reverse emulsions containing the mineral cation and the polyelectrolyte respectively or by adding the reverse emulsion containing the polyelectrolyte in an aqueous solution of the polyelectrolyte. The mixtures are generally carried out at room temperature and with sufficient mechanical stirring to lead to an emulsion stabilized over time.

By way of illustration of the compounds capable of being used as oily phase according to the invention, mention may be made most particularly of hydrophobic materials such as in particular rosin esters, lanolin, petrolatum, waxes, polybutadienes of low molecular weights natural animal, vegetable or mineral oils and their mixtures.

The claimed compositions find a particularly advantageous application in the chemical conditioning of sludge, in particular of sludge from sewage treatment plants or waste or urban or industrial: its incorporation in sludge, which can be subjected before treatment anaerobic digestion, allows to structure them in such a way that the water contained in these sludges is better exuded during the dehydration operation which follows. The efficiency of the mechanical dehydration operation such as filtration or centrifugation can thus be improved, the volumes produced after this dehydration being reduced by obtaining a filtration cake of high dryness.

The amount of composition used during the conditioning of a sludge is such that it generally corresponds between 0.05 to 3 times, preferably between 0.1 to 2 times, the amount of theoretical cationic charge necessary to neutralize the amount of anionic charge of the sludge to be treated; in other words, the quantity of composition claimed is such that it has, in absolute value, a cationicity generally equal to 0.05 to 3 times, preferably between 0.1 to 2 times, the anionicity of the mud treat. Generally, the claimed compositions are diluted before use. This dilution is more particularly necessary for the composition in the form of a reverse emulsion obtained by mixing the reverse emulsion of the mineral cation and that of the polyelectrolyte to transform it into a direct emulsion. This phase inversion can also be carried out by adding an adequate surfactant. In the case where the compositions are in the form of double emulsions, this dilution operation is on the other hand optional.

The examples and figures which follow are presented by way of nonlimiting illustration of the invention.

FIGURES

Figure 1: Representation for composition A as well as for its constituents added separately, of the evolution of the volume drained after 2 minutes depending on the conditioning dose.

Figure 2: Representation for composition B as well as for its constituents added separately, of the evolution of the drained volume after 2 minutes depending on the conditioning dose.

Figure 3: Representation of the t / v monitoring graph as a function of v for composition A and its constituents added separately.

Figure 4: Representation of the t / v monitoring graph as a function of v for composition B and its constituents added separately.

Abbreviations: PAM = cationic polyacrylamide PAC = poly aluminum chloride

Equipment

Reverse emulsion of acrylamide / diallyldimalkylammonium chloride copolymer with a molecular weight of approximately 3.10 ^e , at 50% by weight, referred to below as "PAM emulsion". Acrylamide / diallyidimethylammonium chloride copolymer with a molecular weight of approximately 3.10 ⁶ in powder form.

Aquarhône 18 ^® (poly aluminum chloride solution marketed by Rhodia containing 4.56 moles of aluminum per kg - ES = 38.2%) Vaseline (Prolabo company)

Alkamuls S80 ^® : sorbitan oleate from HLB 4.3 (Rhodia)

Alkamuls S20 ^® : HLB 8.6 sorbitan monolaurate (Rhodia)

Span 85 ^® : sorbitan trioleate from HLB 1, 7 (Sigma-AIdrich)

Alkamuls T20 ^® : sorbitan monolaurate of 20OE HLB 16.6 (Rhodia) Alkamuls T85 ^® : sorbitan trioleate of 20OE HLB 11 (Rhodia)

PA35 ethoxylated phosphate esters (Rhodia)

EXAMPLE 1:

Obtaining a reverse PAM emulsion. Diallyidimethylammonium chloride of molecular weight approximately

3.10 ⁶ .

From 50 g of cationic polyacrylamide copolymer in the form of a powder, a reverse emulsion is produced in a water / vaseline mixture 1 g / 47.4 g using 1.6 g of a monolaurate / sorbitan oleate S20 mixture. / S80 88% / 12% (i.e. the required HLB of 8 for petroleum jelly).

EXAMPLE 2:

Obtaining a reverse PAC emulsion. a) dilute a solution, Aquarhône 18 ^® , with water (20g of water for 50g of Aqua 18). b) adding 5% surfactant S80 ^® vaseline oil (either 2g to 38g Vaseline S80).

After passing b) for 30s at the 8000 rpm utraturax, gradually pour a) into b) for approximately 5 minutes, always at the same speed. One minute is added at the speed 9500 rpm.

After a week of storage at 45 ° C., these emulsions, the size of which is less than 5 μm, sediment slightly (appearance of a small supernatant oil). However, everything is fairly easily restored by simple manual stirring.

EXAMPLE 3 Obtaining a PAC-PAM reverse emulsion.

92.35 g of the PAC emulsion prepared according to Example 2 and 7.6 g of the PAM emulsion prepared according to Example 1 are mixed with the uratratax.

A fluid reverse emulsion is then obtained comprising by weight: 16.1% in PAC, 3.8% in PAM, 42.8% in water, 35.7% in oil and at least 1.6% in surfactant.

The various tests carried out show that the mixing with the uretraturax of the two reverse emulsions obtained according to the preceding examples, makes it possible to obtain a PAC-PAM reverse emulsion whose behavior in stability is similar to the PAC emulsions alone. The inversion of a PAC-PAM emulsion kept for 13 days and then rehomogenized is obtained after the addition of a surfactant such as sorbitan monolaurate 20OE Alkamuls (T20 ^® ) in water. As soon as this emulsion is found direct, the viscosity of the system increases sharply and testifies to the formation of the polyacrylamide gel Al ³⁺ . This observation points towards a limited diffusion of PAM or PAC between the drops of water during the storage period.

The two compounds are therefore effectively separated within the reverse emulsion, thus preserving the latter a satisfactory fluidity.

EXAMPLE 4

Preparation of a double emulsion based on PAC and PAM.

This emulsion is prepared by mixing an aqueous solution of polychloride of aluminum (commercial solution Aquarhône 18 ^® ) and an inverse emulsion of PAM. The reverse PAM emulsion is prepared according to the protocol described above in example 1. It is then added with stirring to the Aquarhône 18 ^® . Two mixtures A and B are thus obtained at different PAC / PAM ratios.

- For a mixture called A: a 4% commercial PAM emulsion and an Aquarhône 18 ^® solution are mixed, so as to obtain a mixture of 18.2% in commercial Aquarhône 18 ^® and 3.27% in PAM emulsion, i.e. weight ratio of active ingredients PAC / MAP of 3.9.

- For a so-called B mixture: a 2% commercial PAM emulsion is mixed with an Aquarhône 18 ^{® solution} , to obtain a 20% mixture in commercial Aquarhône 18 ^® and 1.6% in PAM emulsion, i.e. weight ratio of active ingredients. PAC / PAM of 8.75.

The effectiveness of mixtures A and B is evaluated in drainability tests and piston filter and compared with that obtained with sludges in which have been incorporated in a separate and sequenced manner, Aquarhône 18 ^® and the PAM emulsion, in identical quantities to those present in the respective mixtures.

EXAMPLE 5.

Drainability test.

This is a test which makes it possible to identify the composition which is particularly suitable for dewatering the sludge on a band filter and / or centrifuge.

This test is carried out on 200 g of biological sludge from an urban wastewater treatment plant having a dry matter of 6.35% including 37.2% of mineral matter.

a) With mixture A and its control (separate addition and sequence of the two constituents).

During the tests, A is diluted to 1/6, so that the Aquarhône 18 ^{® content} goes from 18.2 to 3.03% and in PAM emulsion from 3.27 to 0.545%. Consequently, the respective amounts of the control constituents, that is to say added separately, are assessed as follows: to 10g of A at 1/6 (i.e. 1.67g of pure A) corresponds to 0.303g of commercial Aquarhône 18 ^® and 9.7g of PAM emulsion.

In the case of the drainability test with emulsion A, the latter is added to 200 g of mud with mechanical stirring with a pale lacerator at 700 rpm in a beaker, the mixture is then transferred to a filter and the quantity of d is evaluated. water that percolated over time.

In the case of the control drainability test, the sequence of the two products is added according to the following protocol:

200 g of mud are stirred at 700 rpm. The Aquarhône E 18 ^® is added thereto and the stirring is allowed to continue for 10 seconds. The pre-conditioned mud is then poured into a beaker where the PAM emulsion has been weighed beforehand.

The whole is transferred twice from one beaker to another and then stirred for 15 seconds at 700 rpm. It is then deposited on a filter and the quantity of percolated water is measured.

The results obtained with A and the control are presented in the form of graphs in FIG. 1.

b Mixture B and control mixture.

During the tests, B is diluted to 1/3, which means that the Aquarhône 18 ^{® content} goes from 20.0% to 6.67% and that in PAM emulsion from 1.6% to 0.53%.

Thus 10g of B at 1/6 (or 3.33g of pure B) corresponds to 0.667g of Aquarhône 18 ^® commercial and 9.33g of PAM emulsion at 0.57%. The addition of emulsion B and that of its two constituents, separately in the control test, is carried out according to the protocols described for the previous test.

The results obtained with B and its control are presented in the form of a graph in FIG. 2.

The set of graphs in Figures 1 and 2 gives for the mixtures A and B as well as for their constituents added separately, the volume drained after 2 minutes depending on the conditioning dose. We note that the packaging made by adding Aquarhône 18 ^® and then PAM emulsion gives lower drained volumes than in the case of packaging made from mixtures A and B. Consequently, the use of mixtures allows savings in product of 20 to 40% compared to the constituents added separately, while preserving an optimal efficiency.

Furthermore, mixtures A and B prove to be much less viscous than the 0.56% PAM solution.

EXAMPLE 6

Evaluation on piston filter.

This is a test to identify the compositions more particularly suitable for dewatering sludge on a filter press. This test is carried out with the following filtration conditions:

100 g of conditioned sludge are filtered on a piston filter at 2.10 ⁵ Pascal for 10 minutes in order to draw the time / volume graph (T / V) as a function of the volume which makes it possible to highlight the behavior on filtration. Filtration is then carried out at 1.10 ^e Pascal for 5 minutes in order to estimate the influence of the different conditioning on the limit dryness obtained.

The conditioning dose is determined by measuring the capillary suction time (CST) at different doses.

Two doses for packaging are used: the dose at the minimum CST time and the dose with a slight overdose.

a) Mixture A and its results control with A diluted 1/6.

The conditioning doses are determined by measuring the CST. In Table I below are presented the data obtained. BOARD

The doses used are: - 3g of A at 1/6 for 50g of mud, and - 3.5g of A at 1/6 for 50g of mud.

The filtration test is then carried out on a piston filter according to the protocol described above. The results obtained are presented in the form of a graph in FIG. 3.

In this figure 3, are also presented in the form of a graph, the results obtained for a sludge treated successively with the two constituents of A added separately.

The doses of these constituents are calculated in the same way as for the drainability test presented in example V.

b) Mixture B and its control

In the same way as for mixture B, the doses at A are determined by measurements of CST at different doses. They are listed in Table II below.

TABLE 11

The doses used are 2 g of 1/3 B for 50 g of mud and 3 g of 1/3 B for 50 g of mud.

Filtration tests are then carried out on the piston filter according to the protocol described above.

The results obtained with emulsion A and its control mixture as well as emulsion B and its control mixture are respectively presented in FIGS. 3 and 4. In these figures, the graphs for monitoring t / v as a function of v are represented. for the two mixtures and their constituents added separately. It appears that it is in terms of dryness or in terms of filtration behavior that the fact of adding the products constituting the mixtures separately reduces the effectiveness of the packaging. The specific resistances are indeed higher and the dryness lower.

Claims

1. Composition useful for the conditioning of sludge obtained by mixing at least one reverse emulsion containing at least one cationic polyelectrolyte with a reverse emulsion or an aqueous solution containing at least one mineral cation of charge greater than or equal to two.

2. Composition useful for conditioning sludge, characterized in that it is in the form of an emulsion comprising in aqueous phase (s) and separately at least one mineral cation with a charge greater than or equal to 2 and a cationic polyelectrolyte.

3. Composition according to claim 1 or 2 characterized in that it is in the form of a reverse water-in-oil emulsion in which said mineral cation and said cationic polyelectrolyte are distributed in separate water droplets, in the phase oily.

4. Composition according to claim 1 or 2, characterized in that it is in the form of a double water / oil / water emulsion in which the mineral cation is present in the aqueous continuous phase and the cationic polyelectrolyte is distributed at least in part in water droplets constituting the second aqueous phase dispersed in the oily phase.

5. Composition according to one of the preceding claims, characterized in that the mineral cation is chosen from Mg ² \ La ³⁺ , Fe ³⁺ , Al ³⁺ , Zr ⁴⁺ and their polymerized forms when they exist.

6. Composition according to claim 5, characterized in that said mineral cation is Al ³⁺ or one of its polymerized forms.

7. Composition according to one of the preceding claims, characterized in that the mineral cation is in the form of a water-soluble salt preferably chosen from chlorides, nitrates, sulfates and acetates.

8. Composition according to one of the preceding claims, characterized in that the mineral cation is an aluminum chloride or one of its polymerized forms.

9. Composition according to one of the preceding claims, characterized in that the mineral cation is used in an amount of 0.05 to 2 moles, in particular between 0.49 and 1.8 moles per kg of said composition.

10. Composition according to one of the preceding claims, characterized in that the cationic polyelectrolyte is of molecular weight greater than 1.10 ^e .

11. Composition according to one of the preceding claims, characterized in that -the cationic polyelectrolyte is chosen from polyacrylamides, polyethylene oxides, polyvinylpyrrolidones, as well as polymers of cationic natural origin.

12. Composition according to one of the preceding claims, characterized in that said cationic polyelectrolyte is a polyacrylamide having between 0.1% and 15% in cationic charge.

13. Composition according to one of the preceding claims, characterized in that the cationic polyelectrolyte is chosen from copolymers of polyacrylamide with cationic monomers or polyacrylamides modified according to the Mannich reaction.

14. Composition according to one of the preceding claims, characterized in that the said polyelectrolyte is a cationic polyacrylamide copolymer chosen from acrylamide / halide copolymers preferably diallyldialkylammonium chloride, diaminoalkylmethacrylate / acrylamide copolymers and diaikylaminoalkylmethacrylate / acrylamide copolymers.

15. Composition according to one of the preceding claims, characterized in that the cationic polyelectrolyte is used in an amount of at most 10% and preferably between 0.3% and 8% by weight of said composition.

16. Composition according to one of the preceding claims, characterized in that the mineral cation is an aluminum polychloride and the cationic polyelectrolyte is an acrylamide / diallyldimethyllammonium chloride copolymer of PM of the order of 3.10 ^e .

17. Composition according to one of the preceding claims, characterized in that the mineral cation and the polyelectrolyte are used in such a way that the mineral cation / cationic polyelectrolyte molar ratio is between 1.10 ² and 8.10 ^e , in particular between 1.10 ³ and 8.10 ^e .

18. Composition according to Claim 17, characterized in that when the inorganic cation is poly aluminum chloride and the polyelectrolyte, an acrylamide / diallyldimethyllammonium chloride copolymer, the mineral cation / cationic polyelectrolyte weight ratio is between 0.1 and 15 and more particularly between 0.1 and 10.

19. Application of a composition according to one of claims 1 to 18 for the treatment of aqueous media, in particular waste water, urban or industrial.

20. Application of a composition according to one of claims 1 to 18 for the treatment of biological sludges from the purification of waste or waste water, with a view to their dehydration.