SK284697B6 - A method for conditioning sludge - Google Patents

Abstract

A method for conditioning sludge is described, whereby lime and at least one flocculent organic component are added and then the sludge is flocculated. The inventive method is characterised in that the added lime only increases the pH of the sludge to a value, which is above the pH that causes degradation of the applied flocculent organic component when the flocculation has finished.

Description

Technical field

The invention relates to a process for conditioning the sludge comprising adding lime and at least one organic flocculating agent to the sludge and flocculating it.

BACKGROUND OF THE INVENTION

For the purposes of the present invention, sludge is any kind of sludge whose pH is weakly basic, neutral or even acidic and preferably in the pH range of 6-8. Regardless of classification, for example, sludges from urban and agricultural wastewater treatment plants may be mentioned. It is also possible to take into account dredging sludge or other types of possibly acid sludge.

During waste water treatment, the sludge is generally first decanted and then thickened. Then the sludge is flocculated and dehydrated to facilitate transport and further handling.

Several methods of flocculation of preferably thickened sludge are known. They all use a flocculating agent. Among these, in particular four processes using mixtures of iron and lime salts, mixtures of aluminum and lime salts, organic polyelectrolyte and organic polyelectrolyte in conjunction with iron or aluminum salts are used as flocculating agents.

The disadvantages of using an iron or aluminum salt in conjunction with lime are a significant increase in the amount of dry matter of the sludge to be flocculated (by 50 to 60%) compared to a slight increase when using an organic flocculating agent (less than 1%). On the one hand, although iron and aluminum salts damage unoxidized steel and almost all metals, on the other hand, these salts in the solid state release irritating and even toxic dust. Their use therefore requires special precautions with respect to the material used and poses a danger to the people working with them.

Contrary to what is obtained from mixtures of FeCl ₃ and lime, the use of an organic flocculant does not increase the agricultural value of the treated sludge and the utility properties (cohesiveness, pelletability, spreadability) are unlikely to be good due to too low dry matter content in flocculated, dehydrated kale. Sludge that has not been treated with lime usually contains pathogenic germs and represents poor biological stability.

There have been efforts to avoid the disadvantages of these procedures. For example, it has been contemplated to treat lime-disinfected sludge with a particular copolymer capable of maintaining a flocculation effect at pH 12.

This treatment, however, has the disadvantage of using a completely special copolymer, which is therefore expensive. It further envisages separate addition of lime. The copolymer is added only when the pH reaches its maximum value of about 12, requiring two downstream processing operations (US-A-4 160 731).

On the other hand, the simultaneous use of the polymer as a flocculating agent and calcium milk has been contemplated some time ago for flocculation of sludge (see Wolf P., et al., Optimierung der Kläschlammentwässererung mit Kammerfilterpressen durch Polymer - Kalk-Conditionierung, AWT Abwassertechnik Abfalltechnik + Recycling +. 2, April 1993, pp. 46-48). This use aims to maintain all the benefits of lime, i.e., lime. j. increase of agricultural value of sludge, higher dry matter content after dehydration, good utility properties, hygienic treatment of sludge and its biological stability. When the filtration is carried out under pressure, the addition of lime facilitates the passage of the sludge and reduces the extrusion time.

This proposal does not seem to have been used so far. Indeed, when the organic flocculants that are commonly used operate correctly at neutral or only slightly basic pH, this is not the case at higher pH than when adding calcium milk to the sludge to be treated. At higher pH they are degraded very quickly and are therefore effective.

This degradation is an uncontrollable phenomenon which, as can be seen in the results obtained by P. Wolf et al., Causes:

i. The dry matter content obtained under these conditions is not reproducible.

ii. The dry matter content obtained is in many cases lower than if lime was added only after dehydration.

iii. The dry matter content obtained cannot be controlled by adjusting the amount of liming applied to the sludge.

In a similar manner to Wolf et al. JP-A-04-040286 has also been applied with the application of a water-soluble calcium compound to fish sludge and with the addition of a flocculating polymer for or after the application. Only the pH of the aqueous phase is considered, with the recommendation of neutralizing it with acid after addition of the basic calcium compound. In all of the examples, the treated sludge is initially strongly basic and neutralized upon application of the calcium compound prior to the addition of the flocculating polymer. The dry yields and the agricultural value of the sludge obtained were not investigated. While the sludge is neutralized, it becomes susceptible to contamination by bacteria and subsequent odor release.

SUMMARY OF THE INVENTION

It is an object of the present invention to remedy these drawbacks and to improve the sludge treatment process so that it is possible to obtain dehydrated and pure sludge with particularly good and uniform performance properties and a high dry material yield by means of commonly used organic flocculating agents.

To solve this problem, the process described above was considered, in which the addition of lime does not result in an increase in pH above the value causing the degradation of the organic flocculating component.

It has been found that it is possible in an unexpected way, by simple and reasonable control of pH development during flocculation, to obtain conditioned sludge having any properties of calcium sludge, using conventional organic flocculating agents with perfectly repeatable results.

According to one embodiment of the invention, the sludge conditioning method comprises adding lime to the sludge to increase the pH;

adding at least one organic flocculating agent to the sludge and flocculating the sludge, wherein the sludge to be conditioned has a pH of less than 8, wherein the pH increase comprises a first increase to a pH lower than the pH above which degradation of said at least one organic flocculating agent, and continuing to increase the pH above the level above which the organic flocculating agent degrades, and the added lime is selected from the group consisting of quicklime, slaked lime or slaked lime suspended in an aqueous phase with particles of sd _{5 (} at least 50 µm, quicklime, slaked lime containing a liquid additive capable of forming fine particle agglomerates, slaked lime containing a hydration inhibitor, slaked lime having agglomerated elemental particles, slaked lime to which a slaked lime activity inhibitor has been added; having a solids content greater than 20% by weight and a lime free of fine particles, the lime having a reactivity which causes a first increase in pH during flocculation and a continued increase in pH after flocculation has ended.

Preferably, the added lime causes an increase in pH to less than 9 during flocculation.

Thus, it has been shown in an interesting way that the less reactive the lime, the more reproducible the results of flocculation. Moreover, the dry matter yields were also fully reproducible in the same way, and in particular in the same way. Although this phenomenon has not yet been fully understood, the prevailing view is that, despite what was to be expected, lime is less reactive. Thus, the pH increase of the treated sludge is greatly retarded and flocculation initiated with a conventional organic flocculating agent can proceed correctly despite the presence of lime.

For the purposes of the invention, lime is a compound of formula (I):

[xCaO. (1x) MgO] _y H ₂ O (I), where x reaches a value of 0.5 to 1 and y reaches a value of 0 to 1. The compound may optionally be enriched with at least one additive. The lime may be present as burnt or slaked powder or as a slaked suspension in the aqueous phase. The quicklime is formed as a burning product of limestone and / or dolomite material at a temperature of about 100 ° C and contains calcium oxide (CaO) and / or calcium magnesium oxide (CaO.MgO). The slaked lime consists predominantly of calcium hydroxide [Ca (OH) ₂ ] and / or mixed Ca (OH) ₂ · Mg (OH) ₂ and is formed by hydrating (or quenching) quicklime and / or hydrating calcium magnesium oxide. Lime and / or calcium-magnesium milk is usually obtained either by hydrating quicklime and / or calcium-magnesium oxide in excess of water or by converting the slaked lime into an aqueous suspension.

The reactivity of lime is evaluated in different ways depending on whether it is quicklime or slaked lime. The reactivity of quicklime is determined by means of parameter T60, which corresponds to the time required to raise the temperature of the lime / water system to be extinguished to 60 ° C, the lime extinguishing being carried out according to the procedure given in DIN 1060-3. The parameter τ ₆₃ is used to determine the reactivity of slaked lime and corresponds to the time required to pass 63% of lime mixed with the volume of water into the solution under the following conditions: a lime volume corresponding to 0,1 g of slaked lime is added to 700 ml of water. The mixture was then stirred and maintained at 25 ° C. The conductivity of the solution is recorded until the lime is completely dissolved. It should be understood that dissolution of lime increases the conductivity of the solution, and that there is a linear relationship between the amount of dissolved lime and the observed increase in conductivity. The parameter τ ₆₃ is then determined on the basis of the conductivity obtained by determining the time required to reach 63% of the final conductivity. It should be emphasized that the faster the lime dissolves, the lower the ₆₃ .

According to another embodiment of the invention, the lime is added only after the flocculation has ended, for example at least 20 seconds after the addition of the flocculating agent. Thus, it is possible to add lime with moderate reactivity. However, the added lime should not be reactive enough to destroy the already formed precipitate and therefore extremely reactive lime is excluded in this process.

For lime with retarded reactivity, various products can be used. For example, it is contemplated to use powdered or slurry lime whose particles have a d _{50 of} at least about 100 µm, preferably at least 200 µm and most preferably at least 400 µm. The d ₅₀ value is a particle size from which half of the particle size distribution (% by volume) consists of larger particles. For example, defilerized lime may be used, which means lime in the form of a powder or suspension from which fines have been removed, especially those having dimensions less than 200 µm, and preferably dimensions less than 250 µm. Such lime can be obtained by sieving, cycloning or any other method designed to remove fine impurities.

Thus, it has been shown in an interesting way that the thicker the lime particles are, the more favorable the flocculation results are.

According to another embodiment of the invention, the added lime is burnt, for example lime having a T60 of 10 min, and available from Rheinkalk BmbH, Wulfrath, Germany.

According to yet another embodiment of the invention, quicklime comprises a liquid additive capable of agglomerating its smallest particles. Such a product is described, for example, in WO-98/23705. As a liquid additive of this kind, for example, but not limited to, compounds belonging to the group of mineral oils, polyolefins and mixtures thereof.

According to a preferred embodiment of the invention, the quicklime added contains an agent which retards hydration. Such lime can be prepared, for example, according to WO-98/02391. As a hydration retardant, for example, but not limited to, glycerol, glycols, lignosulfonates, amines, polyacrylates, alkali metal or alkaline earth sulphates, gypsum, sulfuric acid, phosphoric acid, sucrose, and theirs mixture. It is also contemplated, as described in WO-98/02391, to add a very small amount of water to quicklime without additional additives or in addition to the previous ingredients and in a controlled manner that allows the formation of a surface reaction retarding the later extinguishing reaction.

According to an embodiment of the invention, the lime in the slaked form may have a dry matter content of greater than 20% and preferably greater than 40%. For example, it is possible to use a Ca (OH) ₂ suspension as described, for example, in BE-A-1006655.

According to another embodiment of the invention, the lime may be in slaked form containing agglomerated particles. Thus, such a product is in the form of a powdered lump which can be easily transported and is very easy to handle. A product of this kind can be prepared, for example, according to the procedure outlined in BE-A-1006655 or also in the example below. Such an agglomeration result may take the form of an aggregate of calcium hydroxide micro-leaves.

According to yet another embodiment of the invention, the added lime is slaked with the addition of an agent reducing its activity. Examples of limiting activity include, but are not limited to, glycerol, glycols, lignosulfonates, amines, polyacrylates, alkali metal or alkaline earth metal sulphates, gypsum, sulfuric acid, phosphoric acid, sucrose. and mixtures thereof.

Preferred conditions are the addition of slaked lime in an amount where the liming rate is at least 10% by weight relative to the dry sludge mass and most preferably at least 20% by weight, without these limits being considered to be critical.

The lime may be added to the sludge before, during and / or after the organic fluffing agent. Thus, the embodiment of the invention gives users the possibility to choose the moment to add lime. Thus, most preferably, the simultaneous addition of lime and an organic flocculant is difficult to anticipate in view of the antagonistic properties of the lime and flocculant. It is therefore not necessary to make any changes to the existing sludge treatment technology. It is even possible to consider adding lime before using a completely conventional organic flocculating agent, unless there has been a significant increase in the pH of the sludge prior to its use. It is preferred that the use of an organic flocculating agent occurs within a maximum of 5 minutes. after adding the whole amount of lime.

It is preferred that the sludge to be treated be pre-decanted. After the flocculation according to the invention, the solid and liquid phases of the sludge can be separated by any suitable and conventional means, for example by means of filter belts, a pressure filter, centrifugation devices, and possibly later dehydration.

It is very advantageous if this separation is carried out immediately after the flocculation has ended, i.e. approximately before the pH reaches the values causing the degradation of the organic flocculating agent. The liquid phase is thus separated at neutral to slightly basic pH and can be discarded without further treatment, possibly after slight neutralization. In the solid phase, on the other hand, there is an increase in pH, preferably up to very high values, which allow recovery of the treated sludge.

As already mentioned, the choice of organic flocculants according to the invention is not critical and those which are most commonly used today for this purpose can be used. Depending on the properties of the sludge, nonionic, anionic or cationic organic flocculants are used alone or in admixture. Particularly suitable are acrylamide or acrylamide copolymer, for example with carboxylic or sulfonic acid, with dimethylaminoethyl acrylate or dimethylaminoethyldimethacrylate, with dialyldimethylammonium chloride or with acrylamidopropyltrimethylammonium chloride.

Further details and particularities of the embodiment of the invention are set forth in the claims.

It is important to note that the sludge obtained after flocculation and dehydration carried out according to the invention has a high lime content and is well disinfected. It can therefore be used well for the recovery of agricultural land.

The invention also relates to a sludge treated according to the method described in the invention, whereby a positive dry yield occurs with respect to the dry matter content introduced into the sludge during processing.

The invention will now be explained in more detail by way of examples, which are purely illustrative and not limiting.

DETAILED DESCRIPTION OF THE INVENTION

Example 1

Preparation of slaked lime with low reactivity from quicklime g Calcium sulphate dihydrate (CaSO ₄ .2H ₂ O) is dissolved in 5 liters of water. A suspension is then formed in this solution with stirring, by adding 1 kg of quick lime, the T60 of which, according to the method, lies between 1 and 5 minutes. During this operation, commonly known as extinguishing, the suspension temperature is maintained below 30 ° C. Once the lime slaking is complete, the calcium hydroxide slurry is filtered to give slaked lime with low reactivity. τ _{63 of} this slaked lime is determined as described above and the dry matter content is calculated according to Degrémont, Mémento Technique de I'eau, éd. Du Cinquantenaire, 9 'ed., Part I, p. 370, 1989. Particle size distribution was determined using a laser granulometer Coulter LS 230 d _o is determined based on this distribution.

r _{63 of} this slaked lime is 1010 s, the dry matter content is 80.5% and d ₅₀ is 748 pm. The particle size distribution is shown in black squares in the attached figure.

Example 2

Preparation of slaked lime with high reactivity from quicklime

200 g of micronized Ca (OH) ₂ having a d _{50 of} about 3 µm are poured into 1 liter of water while stirring. The suspension is stirred for a further 5 minutes. The same parameter determination methods as in Example 1 are applied.

r _{63 of} this slaked lime is 15 s, dry matter content is 25% and d ₅₀ is 3.3 µm. The particle size distribution is shown in white squares in the attached figure.

Example 3

Filtration of the sludge treated with a conventional cationic organic flocculating agent and low reactivity slaked lime added after the flocculating agent.

A quantity of thickened, cleaned sludge corresponding to 20 g of dry matter is taken into a liter vessel. This sludge sample is treated as follows.

Preparation of an aqueous solution containing a flocculating agent:

An aqueous solution of an organic cationic flocculating agent (acrylamide copolymer) at a concentration of 0.3% by weight is prepared by slowly adding 0.6 g of the flocculating agent as a powder to 200 ml of water in a 400 ml container. While pouring until a homogeneous solution is obtained, the solution is stirred with a magnetic stirrer.

Flocculation ml of this aqueous flocculant solution is added to the sludge by syringe. The mixture of sludge and flocculant is then stirred for 6 at 300 rpm using a conventional laboratory flocculator.

Liming

15 to 20 seconds after the addition of the flocculating agent, the lime prepared according to Example 1 is added to the flocculated sludge. The amount of lime added is adjusted so that the weight of the slaked lime corresponds to 40% of the original dry sludge mass. The mixture thus obtained is stirred for 10 at 200 rpm using a conventional laboratory flocculator.

At each step of the operation, the vessel is weighed to accurately determine the liming rate.

filtering

The sample thus obtained was transferred to a filter flask and allowed to drip for 4 minutes. Using a piston, a pressure of 2 to 4 bar is then applied to the dripping sludge in the time required to form a filter cake having a height of 3 to mm. This cake is then collected to determine the dry matter content.

In order to evaluate the effect of liming on the dry matter content of the filter cake, in the second experiment, the same, thickened sludge is flocculated and then filtered according to the procedure described above, but the liming-containing step is omitted. For the lime sludge filter cake, 16.5% dry matter was obtained, while the lime sludge filter cake contained 25.4% dry matter, representing a 17.2% relative dry yield. This dry yield was determined relative to the dry matter that would be obtained if an equal amount of lime was added to the non-calcined sludge after dehydration (S2). This value is calculated using the formula (II):

S1x (100 + t)

S2 = ----------- 100 + (S1 x (t / 100)) where SI is the dry matter content of the filter cake of non-calcined sludge, t is the ratio of added slaked lime expressed in% by mass on the dry matter contained in a thickened sludge.

The drip water fraction (λ ₀ u ap ap) of lime sludge is 0.51, while in the case of non-lime sludge, this fraction is 0.41. This fraction is obtained by dividing the drip water volume by the total volume of water contained in the treated sludge during the dehydration test. The drip water volume is understood as the volume of the filtrate obtained after 240 seconds of dripping.

Example 4

Filtration of sludge treated with a conventional cationic organic flocculating agent and several variations of slaked lime with varying reactivity, which were added after the flocculating agent.

A series of filtration of the purified flocculated sludge that was lime with several types of slaked lime with variable reactivity was performed according to the procedure described in Example 3. The different types of slaked lime were obtained by sieving the slaked lime obtained according to Example 1 into different granulometric fractions. The values of D ₅₀ , tpH = 9 and τ _{63 of the} different types of slaked lime used are given in Table 1. The t _pH = 9 indicates the time required for the lime type to increase the pH of the treated sludge to 9. Dry matter content of lime or lime sludge obtained after filtration, S1 and S2 are indicated in this table, respectively. Dry yields (SH = dry mass) using liming were determined using the method described. Finally, odkvapkané fractions are designated as X _gutter. It should be emphasized that a dripped fraction of 0.6 was obtained for flocculated, non-calcined sludge.

Table 1

sample 1 2 3 4 5 6 Granulometric limits (µm) 20-45 45-80 80-200 200-400 400-1000 > 400 D50 (um) 4 59 119 235 550 748 <5 20 36 53 80 551 t '3 (S) 12 80 180 350 800 1010 S1 (%) 15.5 15.5 14.9 14.9 14.9 14.9 S2 (%) 17.5 20.8 21.4 22 22 22.4 SH yield (%) rel.) - 17.3 2 8.9 12 12 14 0.50 0.49 0.63 0.63 0.61 0.63

The table clearly shows that in samples 4 to 6 a high and almost constant dry matter content is achieved and in the case of dry yield and dripping, the values already achieved in sample 3 are satisfactory.

Example 5

Filtration of the treated sludge using a conventional cationic organic flocculating agent and fine particle lime milk, which is added after the flocculating agent.

The concentrated clarified sludge was flocculated, calcined and then filtered according to the procedure described in Example 3, except that liming was performed in this case with the lime milk of Example 2.

In order to evaluate the effect of liming on the dry matter content of the filter cake, in a second experiment the same sludge was flocculated, then filtered according to the procedure described in Example 3, except that the liming step was omitted. Dry yields and X _gutters were determined as described in this example. For the non-calcined sludge filter cake, a dry matter content of 16.4% was obtained, while the lime sludge filter cake had a dry matter content of 19.1%. This represents a relative loss of approximately 11% dry. The drip water fraction from the lime sludge was 0.47, while the non-calcined sludge was 0.61.

Thus, it can be noted that the dripping has poor properties when very reactive lime is used, and thus deflocculation and, of course, dry loss, which is the opposite of the target, are likely to occur.

Example 6

Filtration of the treated sludge with a conventional cationic organic flocculating agent and low reactivity slaked lime, which is added before the flocculating agent.

A quantity of purified thickened sludge corresponding to 20 g dry matter is placed in a 1 liter vessel. This sludge sample is further processed according to the following procedure.

Preparation of an aqueous flocculating agent solution

An aqueous flocculant solution was prepared according to Example 3.

Liming

The slaked lime prepared according to Example 1 is added to the concentrated sludge. The amount of lime added is adjusted so that the weight of the slaked lime corresponds to 40% of its original dry mass. The mixture thus obtained is stirred for 10 at 200 rpm using a conventional laboratory flocculator. At each step of the liming operation, the vessel is weighed to accurately determine the liming rate.

Flocculation with the addition of lime, 20 ml of an aqueous solution of the flocculating agent is added to the lime sludge by syringe. The mixture of sludge and flocculant is then mixed with a standard laboratory flocculator 6 at a speed of 300 rpm.

filtering

The filtration is carried out according to Example 3.

In order to evaluate the effect of liming on the dry matter content of the filter cake, in a second experiment the same sludge was flocculated, then filtered according to the procedure described above, only the liming step was omitted. For the non-calcined sludge filter cake, 16.5% dry matter was obtained, while the lime sludge filter cake had a dry matter content of 25.8%, representing 19.3% relative dry yield. The drip water fraction from the lime sludge was 0.663, while the non-calcined sludge was 0.648.

Example 7

Filtration of the sludge treated with a conventional organic flocculating agent and burnt lime, which is added simultaneously with the flocculating agent.

A quantity of purified concentrated sludge corresponding to 20 g of its dry mass is placed in a 1 liter vessel. This sample is further processed according to the following procedure.

Preparation of an aqueous flocculating agent solution

An aqueous flocculant solution was prepared according to Example 3.

Flocculation and liming

20 ml of an aqueous flocculant solution are added to the sludge by syringe. At the same time, burnt lime (T60 = 28.8 min) is added with a weight corresponding to 30% of the original dry sludge mass. The mixture of thickened sludge, flocculant and lime is then mixed with a standard laboratory flocculator 6 at a speed of 300 rpm.

filtering

An aqueous flocculant solution was prepared according to Example 3.

In order to evaluate the effect of liming on the dry matter content of the filter cake, in a second experiment the same sludge was flocculated, then filtered according to the procedure described above, only the liming step was omitted. The dry yield was determined as described above, with a content of slaked lime used of 38.9%.

For the non-calcined sludge filter cake, 15.3% dry matter was obtained, while the lime sludge filter cake contained 20.9% dry matter, representing a 4.2% relative dry yield.

Example 8

Filtration of sludge treated with conventional organic flocculating agent and quicklime with one additive. Lime is added after the flocculating agent.

A quantity of purified concentrated sludge corresponding to 20 g of its dry mass is placed in a 1 liter vessel. This sludge sample is further flocculated and filtered according to the procedure described in Example 3.

During liming, however, not the slaked lime is added to the thickened sludge, but some of the quicklime obtained by the process described in WO-98/23705. The T60 of the lime thus obtained is 7.3 min. The amount of lime added is adjusted so that the weight of quicklime corresponds to 30% of the original dry sludge mass. The mixture thus obtained is stirred at a speed of 200 rpm using a conventional laboratory flocculator.

In order to evaluate the effect of liming on the dry matter content of the filter cake, in a second experiment the same sludge was flocculated, then filtered according to the procedure described above, only the liming step was omitted. The dry yield was determined as described with a content of slaked lime of 38.9%.

For the non-calcined sludge filter cake, 15.3% dry matter was obtained, while the lime sludge filter cake contained 20.4% dry matter, representing a 1.7% relative dry yield.

Example 9

Filtration of the sludge treated with a conventional cationic organic flocculating agent and defilerized quicklime, which is added simultaneously with the flocculating agent.

Filtration of the purified sludge, which was flocculated and calcined with quicklime, from which particles less than 250 µm in size were removed, was carried out as described in Example 7. In order to evaluate the effect of liming on the dry matter content of the filter cake, the second If the same sludge was flocculated, then filtered according to the procedure described, only the liming step was omitted. The dry yield was determined as described, the slaked lime content being 38.7%.

For the lime sludge filter cake, 18.1% dry matter was obtained, while the lime sludge filter cake contained 25.7% dry matter, representing a 8.3% relative dry yield.

During the experiment in which the sludge was calcined, the pH of the sludge before the addition of lime was 6.96. The pH of the lime slurry just prior to filtration was 8.8. The pH of the obtained filtrate was 9.8 and the pH of dehydrated lime sludge about 12.5 hours after dehydration.

Example 10

Filtration of sludge treated with a solution of cationic organic flocculating agent and slaked lime with low reactivity, which is added simultaneously with the flocculating agent.

Filtration of the purified sludge that was flocculated and limed with low reactivity slaked lime was carried out as described in Example 7. In order to evaluate the effect of liming on the dry matter content of the filter cake, in a second experiment the same sludge was flocculated, then filtered as described procedure, only the liming step was omitted. The dry yield was determined as described, the slaked lime content being 37.5%.

For the lime sludge filter cake, 18.1% dry matter was obtained, while the lime sludge filter cake contained 24.9% dry matter, representing a 5.8% relative dry yield.

During the lime sludge experiment, the slurry pH was 7.01 before the lime addition. The pH of the lime sludge just prior to filtration was 9.2. The pH of the obtained filtrate was 9.5 and the pH of the dehydrated lime sludge approximately 12.6 hours after dehydration.

It is to be understood that the present invention is not limited in any way by the embodiments described, and that many of the methods can be varied within the scope of the appended claims.

For example, it is contemplated to use a type of lime other than that described herein, the reactivity of which will be diminished in a manner other than that indicated in the preceding description.

Industrial usability

For example, the invention relating to the sludge treatment method can be used to treat sludge from urban and agricultural treatment stations, sludge obtained by dredging or waste water treatment.

Claims (12)

PATENT CLAIMS A method of sludge conditioning, comprising - adding lime to the sludge to increase the pH, - adding at least one organic flocculating agent to the sludge, and - sludge flocculation, characterized in that the sludge to be conditioned has a pH of less than 8, wherein the increase The pH comprises a first increase to a pH below which the at least one organic flocculating agent degrades, and continuing to increase the pH above that above the organic flocculating agent, and the added lime is selected from the group consisting of slaked lime, slaked lime or slaked lime suspended in an aqueous phase with particles having a d _{50 of} at least 50 µm, slaked slaked lime, slaked lime containing a fine particle agglomerate, slaked lime containing a hydration inhibitor, slaked lime having agglomerated slaked lime elementary particles to which a slaked lime activity inhibitor having a solids content of more than 20 wt. and fine-free lime, the lime having a reactivity that causes a first increase in pH during flocculation and a continued increase in pH after flocculation has ended. 2. The process of claim 1 wherein the lime is added prior to the addition of the at least one organic flocculating agent. A process according to claim 1, characterized in that the lime is added simultaneously with the addition of at least one organic flocculating agent. The method of claim 1, wherein the lime is added after the addition of at least one organic flocculating agent. The process according to any one of claims 1 to 4, characterized in that the added lime has a reactivity which causes the pH to rise to a value below 9 during flocculation. 6. The method of claim 5, wherein the time to reach pH 9 after the addition of lime is at least 20 seconds. A method according to any one of claims 1 to 6, wherein the added lime is a compound that corresponds to the formula [xCaO. (1x) MgO] _y H ₂ O, wherein x has a value of 0.5 to 1 and y has a value of from 0 to 1, optionally enriched with at least one additive. Method according to any one of claims 1 to 7, characterized in that the added lime is in the form of particles with a d _{50 of} at least 100 µm, preferably of at least 200 µm. Method according to any one of claims 1 to 8, characterized in that the sludge to be conditioned is selected from the group consisting of sewage sludge, dredged sludge and acid sludge. The method of any one of claims 1 to 9, comprising separating the solid phase from the liquid phase of the flocculated sludge and the lime treated sludge. Process according to claim 10, characterized in that the separation is carried out by filtration or centrifugation. Process according to either of claims 10 and 11, characterized in that the separation takes place approximately before reaching a pH value at which the organic flocculating agent is degraded, after which the aqueous phase has this pH and the solid phase causes the pH to continue to increase. to the above value.