SK284212B6 - Method for treating sludge - Google Patents

Abstract

The invention concerns a method for treating sludge consisting in feeding sludge to be treated into a mixer (1), adding to the sludge to be treated a quantity of at least an alkaline reagent, closely mixing the sludge with said at least alkaline reagent, and drawing from the mixer (1) treated sludge; measuring at least a cohesion value of the sludge to be treated before or during said feeding, and proportioning the added amount of said at least alkaline reagent to obtain a predetermined value of cohesion or of dry matter content for the sludge after treatment, based on said at least measured cohesion value of the sludge to be treated.

Description

Technical field

The invention relates to a process for treating sludge. The method comprises feeding the sludge to be treated into a mixing device, adding at least one alkaline agent to the treated sludge, thoroughly mixing the sludge with at least one alkaline agent, removing the treated sludge from the blending device, dosing at least one alkaline agent.

BACKGROUND OF THE INVENTION

According to applicable regulations, but also for practical reasons, sludges must be treated before being removed from the cleaning stations. One way of processing is liming.

The liming of sludge in purification stations has several positive effects: their stabilization by stopping fermentation, the elimination of most trace metals, the formation of a new structure after drying, their recovery by calcium enrichment and the destruction of pathogenic microorganisms (DEWANDRE et al. Le chaulage des boues. les nuisances, 1994, 174, 46-48).

In practicing this technique, however, the amount of lime added is predominantly estimated, and in most cases the lime dose is greater or less than optimal. Too small a dose will cause problems for the operator: it has no legal disposal for sludge (it is not sufficiently free of pathogenic organisms and cannot be used in agriculture, or the dry matter content is not sufficient to allow the sludge to be loaded and transported). conflict with the surrounding population and farmers. The use of excess lime leads to increased processing costs.

The gradual loss of calcium content after sludge treatment is another known problem caused by uncontrolled liming of sludge. If sludge is intended for recovery in agriculture, its uniform composition (A, P, K, Ca ....) is one of the main quality criteria, which is monitored not only by farmers but also by the relevant legislation.

It is also known that the determination of the dry matter content requires approximately 24 hours. In the current practice, the dry matter content of the purification stations is determined as follows: weigh the sludge sample, then dry at 105 ° C for 15 to 20 hours and reweigh. This lengthy and expensive method determines the dry matter content of the sludge to be treated and at the same time determines the amount of alkaline agent to be added to the sludge. A further disadvantage of this method is that the sludge properties may change over the course of 24 hours and, consequently, the amount of lime added may not be optimal.

The method mentioned in the introduction to this section is described in US-A-5401402. According to this patent, the sludge is mixed in a slug mixer with quicklime. The sludge leaves the slug of the mixer in the form of granules. The amount of lime added is controlled and controlled depending on the sludge height in the mixer or the friction moment on the slug. The disadvantage of this method is the high sensitivity to changes in the dry matter of the sludge prior to its treatment and to the variation in the reactivity of the lime used. It is well known that both of these parameters can be very variable and therefore this method can be considered inaccurate. At the start of mixing, the amount of lime added can only be determined approximately. Thus, the required amount of lime is added only as follows and still additionally, which shows that it is not possible to effectively control the optimum dosage, which would ensure the necessary properties of the treated sludge.

The prior art uses various methods of measuring the rheological properties of slurry and especially sludge. Among other things, penetrometry and tensile measurement can be named (see A. POITOU et al., Intemational Workshop, The Rheology of Sludges, Bari, March 17, 1997). Basic studies conducted with sludge have shown an exponential relationship between their cohesion and dry matter content (see, e.g., K. WICHMAN et al., Mechanical Properties of Waterwork Sludges-Shear Strength, Wat. Sci. Tech., Vol. 36, n ° 11, p. 43-50, 1997; S. GAZBAR, Evaluation and Amelioration of Performances for Precedence of Dehydration of Measurement of Bouis risiduaires, PhD thesis adopted on 13.12.1993 at the Institute of Nationale Polytecnique in Lorraine, J.-P. T1SOT and J.-P. M. ABADIE, Characterization of Sludge and Geometric Properties, Intemational Workshop, The Rheology of Sludges, Bari, March 17, 1997).

SUMMARY OF THE INVENTION

SUMMARY OF THE INVENTION The present invention is based on a simple sludge treatment method which allows the variation of the dry matter content of the untreated sludge to be taken into account and thus determines the exact amount of alkaline agent added from the very beginning of the sludge treatment. The process according to the invention makes it possible to process the sludge so that the final product has predetermined properties and, in particular, its cohesion corresponds to the minimum cohesion set for further handling of the treated sludge and / or dry matter content corresponding to the relevant sludge storage or utilization for agricultural land use. The method according to the invention is characterized in that it comprises inter alia:

- measuring at least one cohesive value of the sludge for processing before or during its filling into the mixing device,

- depending on said at least one measured sludge cohesive value for processing, the step of adding at least one alkaline agent to achieve a predetermined cohesion or dry matter content of the treated sludge.

By simply measuring the cohesion of the sludge, e.g. penetration test or tensile, the dry matter content can be determined quickly and easily. This measurement, which can be carried out before the sludge is treated and thus still mixed, makes it possible to determine the amount of reagent added so that the sludge after treatment has a predetermined cohesion value and thus a predetermined dry matter content. The dosing of the reagent assumes a predetermined final dry matter content based on the function C 1 = f (S 1), where C 1 represents the cohesive value of the sludge before processing and the Sj its dry matter content. After selecting the desired dry matter content of S ₂ , the corresponding amount of reagent added is determined.

It should be noted that the same amount of alkali added, e.g. of quicklime, causes a greater increase in dryness than the addition of an inert phase. The alkaline agent according to the invention is any substance which can increase the dryness of the sludge by adding dry matter and reduce the amount of water in the sludge due to:

1. water consumption by chemical reaction, e.g. formation of calcium hydroxide in the case of quicklime,

2. significant evaporation of water due to the heat released due to the exothermic reaction between the alkaline reagent and water;

3. by soaking water into the pores of the hydroxide formed.

An embodiment of the method of the invention preferably comprises:

- measuring at least one sludge cohesive value during or after treatment,

comparing each of these measured cohesion values with a predetermined value per treated sludge, and

depending on said comparison, optionally adjusting the amount of at least one alkaline agent added to achieve a predetermined cohesion value.

Such a process makes it possible to control the operation of the sludge treatment process very precisely, even if the initial properties of the sludge are varied. The method of the invention may advantageously comprise determining the dry matter content based on the cohesion measurement, either as a baseline measurement or added measurements, by means of said direct relationship between cohesion and dry matter content. Thus, the values are compared between the individual dry matter contents.

A particularly preferred embodiment of the method of the invention comprises:

- measurement of the sludge sample prior to processing, which includes the determination of the specific relationship between its cohesive value and dry matter content,

the choice of a predetermined value of cohesion and dry matter content, and

- depending on said relationship between cohesion and dry matter content, dosing a predetermined amount of at least one alkaline agent so as to achieve predetermined values of the treated sludge.

This process is particularly advantageous if the properties of the sludge are not known prior to its treatment, which is particularly characteristic of organic sludges, the properties of which are often unpredictable. A preliminary sample that is readily feasible e.g. by varying the sludge moisture, a gradual measurement can produce a sludge value curve that shows the characteristic relationship of its cohesion and dry matter. It is this functional relationship that will be utilized in the treatment of the sludge according to the invention and makes it possible to easily take into account changes in the dry matter values of the treated sludge.

An improved method of carrying out the invention comprises:

- before processing the sludge, samples thereof are obtained by adding varying amounts of at least one alkaline reagent. Sampling shall determine the specific relationship between the cohesive value and the dry matter content of the sludge to be treated, and

- where appropriate, placing the amount of alkaline reagent added with reference to this specific relationship, so that the resulting values correspond to the values that have been predetermined.

This method makes it possible to achieve with high precision and, if necessary, automatically the desired cohesion value of the treated sludge.

A preferred method of carrying out the method of the invention comprises:

- treating the sludge in a first mixing step with at least one alkaline agent and in a second mixing step of the sludge with at least one alkaline agent and / or an inert additive, and

at least in one of these steps, at least one measurement of the cohesive value of the treated sludge and, optionally, adjusting the amount of alkaline agent or inert additive added so that the monitored values of the treated sludge correspond to preselected values.

The process according to the invention makes it possible in a simple manner to treat sludge, the composition of which will be more balanced, in particular for their use in soil cultivation.

BRIEF DESCRIPTION OF THE DRAWINGS

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail by way of example with reference to the drawing, in which: FIG. 1 shows a first embodiment of the method according to the invention, FIG. 2 - the relation of cohesion - dry matter content in the case of urban sludge, fig. Fig. 3 - Cohesion - dry matter content prior to treatment of these sludges; 4 and 5 - two other embodiments of the method according to the invention, FIG. Fig. 6 shows the development of the cohesion value as a function of the dry matter content according to which quicklime (black squares) or an inert phase (white triangles) have been added; 7 is a diagram of a penetrometer used in connection with the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 shows a first embodiment of the method according to the invention. In this example, the sludge B for processing and the metering 3 (e.g. a snail feeder which is driven by a 3A motor) are fed continuously through line 2 to the mixing device 1 for processing and at least one reagent, e.g. quicklime, the particle size of which is preferably less than 5 mm (preferably 0.5 mm to 3 mm). The sludge treated with this reagent leaves the mixing device 1 continuously through line 4. The reagent is stored e.g. in Tray 5.

If the properties of the sludge are known, e.g. after carrying out preliminary tests, the sludge is characterized by determining the existing direct relationship between its cohesion and dry matter content. This relationship is established by measuring the cohesive forces using known devices, e.g. conical penetrometer. Water is added to determine sludge cohesion at the lowest dry matter content or after partial drying to determine sludge cohesion with higher dry matter content. In FIG. 2 shows an example of a cohesion-dry solids relationship that has been determined for water-free sludge coming from a sewage treatment plant. Such sludge contains 10 to 50% by weight, more usually 15 to 40% by weight of dry matter.

A measuring device 6 (e.g., a penetrometer, texturometer, elongation meter, and the like) determines the sludge cohesiveness for processing before it enters the mixing device 1. This meter measures e.g. every 20 minutes sludge cohesion, expressed in kPa. The sludge cohesiveness generally corresponds to the stress at which it is deformed. It is known that the sedimentation and accumulation of sludge depends on its cohesion. Therefore, it is in the interest of processors to control sludge cohesion.

The flow meter 7 continuously measures the sludge flow rate for processing. A flow meter is an instrument that not only measures but also roughly evaluates sludge flow, e.g. based on its weight.

The information from the meter 6 and the flow meter 7 arrives at the control unit 8, which is e.g. computer. This unit is equipped with memory or other software to create a cohesion curve - dry matter content of processed sludge. Depending on the desired dry matter content S2 and cohesion C1 measured by the measuring device 6, the control unit 8 calculates or determines the concentration of quicklime so that the treatment results in a sludge with the desired dry matter content. This calculation can be performed according to equation (1):

((S2 / S1) -1) te = ------------------------------- (I) + (0 (32) (Pc / 100) -S2 / 100 where: tc is the liming concentration expressed in% per mass unit of dry matter,

Pc is the amount (expressed in%) CaO reaction with H ₂ O is converted to Ca (OH) _2, the dry matter content of the sludge prior to the treatment, expressed as% per mass unit, which is determined by the relationship C = f (Sl), and is the dry matter content of the sludge after treatment, expressed in% per mass unit, which is the desired value.

In FIG. 2 shows an example relating to this calculation. After measuring the cohesion of C1, S1 is determined and it is desirable to increase the dry matter content by approximately 6% to achieve the desired dry matter content S2. Using formula (I) and determining Pc of 92%, 31.5% of lime is added to the original dry matter content.

The control unit 8 controls the drive of the motor 3A, which rotates the slug of the dispenser 3 depending on the amount of quicklime per kg dry matter of sludge to be processed and on the amount of dry matter so that the amount of quicklime fed to the mixer corresponds to for processing and the amount of its dry matter. The sludge discharged from the mixer then has a dry matter content close to the desired S2 content. It can be taken into account that the S2 value will be affected by the continued lime reaction after the treated sludge exits the mixer.

Every 20 minutes, the control unit can adjust the amount of lime fed per kg of sludge dry matter. The unit checks from the very beginning and continuously the amount of quicklime in relation to the amount of dry sludge treated. Said control and control is carried out by a known technique.

In the example described in FIG. 1, it is possible, but not necessary, to determine the cohesion of the treated sludge. Cohesion is measured by an instrument 9, which may be of the same type as the measuring instrument

6. The apparatus 9 sends a signal to the control unit 8, which controls the supply of quicklime to the mixing device. If the cohesion detected by the apparatus 9 exceeds the desired cohesion rate by more than one degree of a predetermined threshold, the control unit 8 automatically adjusts the amount of lime supplied per kg of dry sludge dry matter by the appropriate degree. The measuring device 9 may advantageously send a signal to the control unit 8 between the two signals sent by the measuring device 6.

Just as there is a direct exponential function for the sludge B1 prior to processing between its cohesion and dry matter content, there is another exponential function between the cohesion of this treated sludge B2 and its dry matter content. This functional relationship is shown in FIG. 3, in which the curve illustrates this function when adding different amounts of alkaline agent to the treated sludge. This relationship is of the type: C2 = f (S2).

The treatment sludge C is measured by the apparatus 6 (FIG. 1). Using curve B1 of FIG. 3, it can be concluded that the dry matter content is e.g. 23.5% (SI). The desired cohesion of C2 after sludge treatment is determined. Curve B2 determines the dry matter content S2 required after sludge treatment corresponding to C2. From the graph of FIG. 3 it is clear that this is a dry matter content of 32% by weight. Calculation concentration is calculated using Formula I.

This method can do without the control unit and all calculations and commands will be performed manually.

In FIG. 4 shows an embodiment of a method that differs from the example of FIG. 1 by comprising two steps of introducing an alkaline agent, e.g. quicklime.

In the first step, the sludge is treated with a minimum amount of reagent such that it has a certain degree of cohesion (e.g. at least 5 kPa) at the end of this step. This minimum amount of reagent depends on the crude amount of untreated sludge. The first step is carried out in a kneading device 1A, to which the quicklime comes from the dispenser 30. The dispenser drive motor 30A is controlled by the device 31, to which a signal from the sludge flow meter 7 entering the kneading device ΙΑ is received. In a second step, the meter 6 measures the sludge cohesion of the pretreated first step, and according to the cohesion-dry solids of the pretreated sludge, the reagent is metered into the mixer 1B such that the sludge cohesion after processing in the second step approaches predetermined cohesion. The control unit 8 controls the revolutions of the motor 3A, which drives the feed snail 3 conveying the necessary amount of lime to the second mixing device IB.

In the method shown in FIG. 4 also provides the dosing of the alkaline agent, which is carried out in a manner similar to that of FIG. First

The method of FIG. 5 includes, on the one hand, dosing of an alkaline agent, e.g. quicklime and, on the other hand, inert ingredients, e.g. calcium carbonate. An appropriate amount of quicklime is fed to the first mixing device 1A from the container 5 through the dispensing device 30. The dispenser motor 30A is controlled by a control unit 8 receiving a signal from the flow meter 7 and the meter 60 with a degree of cohesion. The sludge treated in the first step is then treated in the IB mixer with CaCO ₃ particles (supplied from the reservoir 50). Calcium carbonate is dosed by a metering 31 driven by a motor 32 whose speed is controlled by the control unit 81 according to the signals from the pre-treated sludge cohesion meter 61 with an alkaline reagent and a flow meter 7 and / or a secondary flow meter 70. an apparatus 9 which sends a signal to the control unit 81 controlling the supply of calcium carbonate to the kneading apparatus IB. If the cohesion detected by the device 9 is less than one degree lower than a predetermined threshold, the control unit 81 automatically increases the dosage by the appropriate degree of calcium carbonate per kg dry matter of the sludge to be treated. If the cohesion detected by the apparatus 9 is more than one degree of a predetermined threshold value, the control unit 81 automatically reduces the appropriate amount of lime per kg dry matter of the treated sludge.

Fig. 6 shows the direct effect of the addition of quicklime (curve D1) and calcium carbonate (curve D2) on sludge cohesion. It is clear from the graph that the addition of lime has a significant effect on cohesion from a lower dry matter content. Conversely, in the same sludge, the addition of calcium carbonate only begins to affect cohesion when the dry matter content of the sludge is greater than 35% by weight. The graph also shows that the addition of quicklime to sludge with a dry matter content greater than 35% by weight significantly increases its cohesion, which is less easy to control. The two-step process using alkaline reagents and inert additives allows rapid treatment of the initial cohesion with CaO (dotted line x-y) and final treatment with the additive (calcium carbonate) (dotted line y-z) and achieves the desired composition of the treated sludge.

SK 284212 Β6

As an alkaline agent, quicklime, burnt dolomite, cement, fly ash, water binders and mixtures thereof can be used. Among the inert binders which may be used simultaneously or after application of the agent, mention may be made of calcium, magnesium, potassium carbonates. These carbonates can be in the form of pulp, grains or powder, sand, cellulose fibers, paper fibers, crushed ore, silicates, aluminosilicates, crushed or ground phosphates, nitrates, iron or crushed sawdust, and mixtures thereof. The amount of additive used is preferably at least 25% by weight of the amount of agent used.

In FIG. 7 is a schematic representation of a penetrometer type cohesion measurement device used in the method of the invention.

The sludge is moved towards the kneading device through a line 100 which is open from above at one point. At this point, a scale 200 is placed above the pipe 100. The measuring cone 300 is lowered into the flow sludge and the depth of its penetration is measured on said scale.

The invention is not limited to the examples given, and other embodiments of the invention may be added within the scope of the claims.

Claims (11)

A process for treating sludge comprising filling sludge into a mixing device, adding at least one alkaline reagent to the treated sludge, thoroughly mixing the sludge with at least one alkaline reagent, and discharging the treated sludge from the blending device and dispensing at least one alkaline reagent, at least one alkaline reagent is added to achieve a predetermined cohesion value or dry matter content of the treated sludge, depending on said at least one measured cohesion value of said sludge. Method according to claim 1, characterized in that the dry matter content of the sludge to be treated is determined on the basis of each measured value of its cohesion and at least one reagent is metered in an amount determined on the basis of at least one determined dry matter content. Method according to claim 1, characterized in that at least one sludge cohesive value is measured during or after its treatment, each measured cohesive value is compared with a predetermined sludge cohesive value and adjusted according to said comparison. the amount of added at least one alkaline agent to achieve a predetermined value. Method according to claim 1, characterized in that at least one sludge cohesive value is measured during or after treatment, the dry matter content of the sludge during or after treatment is determined on the basis of each added measure of the cohesion rate; the value determined for the dry matter content of the sludge during or after completion of the treatment with a predetermined dry matter content for the treated sludge and, depending on said comparison, optionally adjusting the amount of at least one alkaline agent added to achieve a predetermined dry matter content. Method according to any one of claims 1 to 4, characterized in that samples of sludge are taken and measured before processing to determine a specific relationship between its cohesion rate and dry matter content, a predetermined value of cohesion and dry matter content is selected and depending on said relationship a predetermined amount of at least one alkaline agent is metered between the cohesion and the dry matter content to achieve predetermined values of the treated sludge. Method according to any one of claims 1 to 5, characterized in that the sludge samples are taken before processing and additional tests are carried out by adding a different amount of alkaline agent, the sampling comprising additionally determining the specific relationship between its cohesive rate and dry matter content and optionally adjusts the dose size of the at least one alkaline agent to achieve a predetermined value of cohesion and dry matter content. Method according to claims 5 and 6, characterized in that at least one cohesive value of the treated sludge is measured at successive intervals and optionally the measured sludge cohesive values are gradually adjusted on the basis of the cohesion-dry matter content relationship. Process according to any one of the preceding claims, characterized in that the sludge is treated by mixing with at least one alkaline agent and mixing the sludge with at least one alkaline agent and / or an inert additive and at least one of these steps measuring the sludge cohesiveness of the treated sludge. and optionally adjusting the amount of alkaline reagent or inert additive added so that the monitored sludge treated values correspond to preselected values. The method of any one of claims 1 to 8, wherein the alkaline agent is optionally quicklime, burnt dolomite, cement, fly ash, water binders and mixtures thereof. Method according to claim 8 and 9, characterized in that the inert additive is optionally calcium, magnesium, potassium, sand, cellulose fibers, paper fibers, crushed rocks, silicates, phosphates, nitrates, iron or wood sawdust, zeolites , silicas, aluminum oxides, sepiolites, aluminosilicates and their derivatives, activated carbon, gas black, clay, threshold of electric filters and mixtures of said substances. Method according to any one of claims 1 to 10, characterized in that the penetration of the object into the sludge is measured to detect at least one degree of cohesion.