WO1989011455A1 - Process and device for conditioning sludges which are difficult to de-water - Google Patents

Abstract

In a process and device for conditioning sludges which are difficult to de-water, in particular sludges with a high organic charge, the de-watering conditions can be improved without the use of any kind of chemical conditioning additives solely by conditioning the sludge in an electric constant-voltage field in a chemical cell in which at least one of the electrodes has an enlarged surface.

Description

 Method and device for conditioning sludge that is difficult to drain

description

The invention relates to a method and a device for conditioning sludge that is difficult to drain with the aim of influencing the drainage behavior in a positive sense.

Sludges from wastewater treatment plants occur with a high water content, which among other things depends on the processes used and the type and size of the solid particles. Sewage sludge from municipal wastewater treatment plants is thin and normally has a solids content of 1 to 5%. In this consistency, disposal is uneconomical and in most cases also not in accordance with the regulations. The sludge is therefore further dewatered into a solid waste product.

Next water sludges fall sharply in the aluminum industry in the processing of bauxite (red mud), in the processing of coal, in the electroplating industry and in many other industri ^¬ on economic processes.

The dewatering and subsequent disposal of sludge make a substantial portion of the investment and operating costs ^¬ from a plant.

According to the Technical Ordinance for Waste Removal - TVAB 26012 - natural processes such as sludge drying places and sludge ponds are used for sludge dewatering _. Processes of mechanical separation technology, such as Drying drum, selective dryer, floor dryer and spray dryer.

The highest degree of drainage, i.e. thermal drying provides the highest solids content. However, the operating and investment costs are uneconomically high. With the mechanical separation technology, solids contents of 15 to 25% are achieved; a salary that is not satisfactory because of the subsequent disposal.

Further dewatering can be achieved if the sludge is pre-treated. Thermal and chemical processes have become known for this.

In the case of thermal conditioning, the sludge is kept in reactors for a period of 30 minutes at temperatures up to approximately 220 ° C. and pressures up to approximately 20 bar and then cooled to 60 ° C. In a Entwässer¬ ung on filter presses, a filter cake is obtained with 45 to 55% ^"solids. The disadvantage of this method is the high energy requirement and the need for pressure vessels.

In chemical conditioning, lime milk and iron or aluminum salt solutions are mixed with the sludge, in some cases also organic flocculants. The drainability can hereby be improved to such an extent that filter cakes with 20 to 30% solids are obtained in vacuum cell filters and those with 30 to 50% solids are obtained in filter presses. A disadvantage of this process is the need for chemical additives, the resulting salting of the water and the need for measuring and regulating devices for monitoring the conditioning; an overdose leads to resuspension of the sludge.

In addition, processes for the separation of suspensions have become known which use electrophoretic and electroosmotic power; they are often referred to as electrofiltration. For this purpose, the suspensions are fed to an electrochemical cell which is divided into an anode and cathode space by a non-conductive diaphragm. If a direct voltage is now applied, the particles migrate under the influence of the electrical field to the diaphragm, where they are separated out as filter cakes with a high solids content. The water flows through the diaphragm and can be released as a filtrate. This method is used for dewatering mineral suspensions, preferably for clays, and polymer suspensions, for example PVC.

A disadvantage of this process is that only charged particles migrate here and the rate of migration depends on the type and size of the particles. This can lead to the fact that with a wide particle size distribution, as is to be expected in waste sludge, small particles lead to blockage of the diaphragm, while larger particles are hardly involved in the migration. This means a strong limitation of the area of application.

Furthermore, high demands are placed on the diaphragm. For one thing, the porosity must be so high that the Voltage losses remain low, on the other hand, the pores must not be so large that the particles can migrate through the diaphragm. Even during a long period of operation, the properties should not change due to contamination or other factors.

German patent application P 37 39 580.7-45 of November 23, 1987 describes a method and a device for splitting an aqueous emulsion which is subjected to a DC voltage in an electrochemical cell for this purpose. Sludge should not be conditioned with this procedure.

With regard to the prior art, reference is also made to G3-PS 1 527 692, which proposes to expose this to an electrical DC voltage for the treatment of water, which is superimposed on an AC voltage. Slurries cannot be conditioned using this procedure because the anode is isolated. The electrodes there act much more as a capacitor.

The present invention is therefore based on the object of overcoming the disadvantages of the known processes and of developing a process which allows waste sludges with different charge of the particles and a broad particle size distribution to be treated in such a way that Drainage behavior is influenced in a positive sense. The process is said to be largely insensitive to contamination.

It has now been shown that this problem is solved in a technically progressive manner if according to previous gender invention the difficult to drain sludge is exposed to a DC electrical field. It is known to influence sludges during the actual dewatering process by means of an electric field in such a way that the liquid phase flows to the cathode due to the electro-osmotic effect and flows out through perforations provided therein, and that the solids parts are separated by electrophoretic action collect at the anode. In connection with conditioning preceding the dewatering treatment, however, only an improvement in the filtration ability has been observed so far, especially for industrial, water-containing wastes previously treated with chemical conditioning agents.

The inventors, on the other hand, found that, in particular, highly organically contaminated sludges, such as those from municipal, sewage treatment plants, without any prior preparation with chemical agents, could be influenced in a very short time by the action of the electrical direct voltage field with sufficiently large electrode areas so that their sedimentation behavior and their filterability were considerably improved.

It was found that, in particular, a large surface area at the anode accelerated agglomeration of the suspended matter and colloids on larger particles, which improved drainage. This is probably due to the fact that the repelling effect of the mostly negatively charged sludge particles can be compensated to an increased extent by providing a large positive amount of charge. The device for carrying out the method according to the invention is technically simple and can be implemented with little effort. In addition to electrodes of the largest possible area, it preferably has only one separator for protecting the cathode. No special requirements are to be made of the DC voltage source, which must provide relatively low voltage values. The DC voltage can optionally remain constant or alternate during the treatment, or an AC voltage can be superimposed on it, for example at a frequency of approximately 50 kHz to 100 kHz.

On the one hand, the method according to the invention offers the possibility of extraordinarily fast conditioning, which, in addition to conditioning in batch operation, makes continuous flow through the chemical cell with direct transfer of the sludge for further processing only practical.

On the other hand, the economy of the process is greatly increased by the fact that the use of conditioning chemicals is completely eliminated. This saves both the costs for this and prevents the water from salting up. Furthermore, the amount of waste is not increased by the conditioning and is therefore more valuable

Landfill space saved. Furthermore, the expensive and fault-prone measuring and control devices are eliminated.

Another important advantage of not using any chemicals is the treatment of municipal sewage sludge, because here a disturbance of the subsequent digestion process by means of precipitants and flocculants cannot always be ruled out with certainty.

The invention is explained in more detail below on the basis of exemplary embodiments, from which further important features result. It shows:

Fig. 1 - schematically a first embodiment of a device for conditioning sludge;

Fig. 2 - a diagram with the results obtained with the arrangement according to Fig. 1 with regard to filtration properties;

3 shows a diagram with the results obtained with the arrangement according to FIG. 1 with regard to the sedimentation behavior;

4 - a laboratory device for the continuous treatment of the sludge. According to the embodiment shown in FIG. 1, water-containing sludge flows from a storage vessel 1 by gravity to an electrochemical cell 3 until it is filled to a predetermined height. The electrochemical cell 3 contains two electrodes 4 and 5, which are connected via terminals 6 and 7 to the poles of a DC voltage source.

The electrodes can be formed as a sheet or mesh; they are advantageously characterized by a high specific surface area. For this purpose, for example, several networks can be combined to form an electrode packet. It is also preferred if the electrodes are made of a corrosion-resistant material. This prevents the electrodes from corroding or dissolving. In addition to noble metals, nickel and graphite and platinized titanium, tantalum and niobium are also suitable as electrode material for the positive electrode (anode). Titanium, graphite, stainless steel, etc. are used for the negative electrode (cathode). suitable.

A voltage is applied to the electrodes, which is adjusted so that a predetermined limit value is not exceeded. The dewatering properties of the sludge are influenced in a positive way by the action of the electrical field in the cell; probably through the formation of larger sludge aggregates, releasing adhering water. The sludge conditioned in this way has a lower water binding capacity than sludges from conventional water treatment. So that will be Sedimentation, thickening and drainage behavior improved. The residence time of the sludge in the cell is chosen so that the sludge has achieved the desired properties.

The conditioned sludge then arrives in a buffer container 8 and is passed on from there for further treatment, such as sludge thickening and / or water separation.

At this point it should be mentioned that another preferred area of application of the method and the device according to the invention is the separation of metal ore suspensions from slurries or extraction processes in the context of ore processing and metal extraction.

The method can also be carried out in such a way that a separator 9 is provided in the cell 3 between the electrodes 4 and 5. It is advantageously arranged in such a way that the negative electrode is protected from direct flow of sludge particles. For example, large-mesh plastic nets are suitable as materials for this.

The drainage behavior is determined as a criterion for the efficiency of the process. For this purpose, the filtration and sedimentation behavior were measured.

Important parameters of the filtration behavior here are the filtration speed and the Fil ratmer.ee. To determine these parameters, a measured amount of sludge from 50 to 100 ml was placed on a filter, Filtered under gravity and the filtrate measured closely as a function of time.

The results of these measurements are shown in FIG. The amount of filtrate F (in ml) is plotted over time t (in hours), specifically for sludges which have been conditioned under the conditions according to the invention (curves II, III) and for comparison also for the unconditioned sludge (Curve I).

Important characteristics of the sedimentation behavior are the sedimentation speed and the volume of the sludge that is segmented. To determine these parameters, a measured amount of sludge was poured into a cylinder and the sedimentation under the influence of gravity was measured as a function of time.

3 shows the results of the measurements. The height H of the sludge volume (in mm) is plotted over time t (in hours); also here for conditioned sludge (curves II, III) and for comparison for unconditioned sludge (curve I).

Details of these figures are discussed in more detail below.

It is also possible to condition the sewage sludge in continuous operation. A preferred embodiment of a device is shown in FIG. 4. Basically, the electrochemical cell is the same structure as in Fig. 1 with respect thereto, and it uses pay v / ground the same reference ^¬. The electrodes are in this leadership form horizontally arranged, the positive electrode 4 is located in the lower region of the cell 3 below the negative electrode 3. In order to achieve a high specific surface, several metal nets stacked one above the other are used for the positive electrode.

According to the embodiment shown in FIG. 4, the water-containing sludge flows out of the storage vessel 1 by gravity or with the aid of a pump 2 to the electrochemical cell 3. The flow rate is predetermined by a valve or by a delivery rate of the pump, pumps which are to be used which Do not break pre-formed mud flakes. A DC voltage source is applied to current connections 6 and 7 and the voltage is regulated in such a way that a predetermined current, which depends on the type of sludge, is not exceeded.

The treatment time of the sludge corresponds essentially to the dwell time in the process shown in FIG. 1, but can advantageously be varied more easily here, for example by changing the flow rate. The conditioned sludge ^'then flows into the buffer vessel 8 and is further treated on closing.

In the examples below, tests are described which were carried out with laboratory systems of the two devices described. Sewage sludge from a municipal sewage treatment plant was used for the tests, specifically - Sludge after the digestion tower - in front of the belt press - with a sludge content of about 6%, referred to in the further description as sewage sludge 1 and

- Excess sludge - before the decanter - with a sludge content of about 1%, referred to as sewage sludge 2 in the further description.

Example 1:

200 ml of sewage sludge 1 were introduced into an electrochemical cell according to FIG. 1. Nickel expanded metal was used as electrodes for the positive side (anode) and titanium expanded metal for the negative side (cathode). The geometric surface of the electrodes

2 was about 50 cm, the distance between the electrodes 20 mm.

A DC voltage of 25 V was applied to the cell; the treatment time here was 3 hours.

100 ml of the treated sludge were placed on a pleated filter and the amount of filtrate was measured as a function of time. For comparison, the same amount of the untreated sludge was filtered.

The result is shown in FIG. 2. The filtration time in hours is plotted on the abscissa and the amount of filtrate in ml is plotted on the ordinate, namely curve I for the untreated sludge and curve II for the treated sludge.

The positive influence of the treatment can be clearly seen here; once at the higher Filtraticnsgeschwin igkei, on the other hand, the larger amount of filtrate. In the further sludge treatment, a larger sludge throughput is made possible and a sludge with a significantly increased solids content is obtained.

The filtrate is also clearly clearer, an indication that in particular organic suspended matter and colloids have accumulated on large particles or have combined to form larger agglomerates.

The sedimentation behavior is also positively influenced. While untreated sludge partially sediments and partially floats, the treatment causes all of the solids to sediment. This changed sedimentation behavior indicates that larger, heavier sludge particles have formed.

Example 2:

In this example, an electrode with a large geometric surface was installed for the positive side (anode). For this purpose, four chrome-nickel wire networks were combined to form an electrode package; the geometric top

2 area was about 200 cm. As negative Elektroαe

Another titanium expanded metal was used (cathode).

With this arrangement, similarly good results as described in Example 1 can be obtained with a lower voltage and a shorter treatment time. 200 ml of sewage sludge 1 were treated with a voltage of 4 V for a period of 30 minutes. The test result is shown as curve III in FIG. 2.

Example 3:

For this experiment, 100 ml of sewage sludge 2 with a sludge content of approximately 1% were treated in an arrangement as in experiment 1. A chromium-nickel steel mesh was installed as the positive electrode for the positive side and a platinized titanium expanded metal for the negative electrode. The applied DC voltage was 14 _V. the treatment time 10 min.

The sedimentation behavior was determined to characterize the sludge properties. For this purpose, v / ere 100 ml of treated sludge in a cylindrical glass vessel was added and the decrease ^'of the sludge level as a function of time measured. The initial height of the sludge was 40 mm. The same amount of untreated sludge was used for comparison.

The result is shown in FIG. 3. The sedimentation time in hours is plotted on the abscissa and the sludge height in mm is plotted on the ordinate, namely curve I for the untreated sludge and curve II for the treated sludge.

One can clearly see the positive influence of the treatment; the sedimentation rate is higher and the sediment volume is lower, in this example by about 33% after 1 hour. Example 4:

A significant improvement in sedimentation properties is also observed with a short treatment time. Under the same conditions as described in experiment 3, but with a treatment time of only 3 minutes, results are obtained which are plotted as curve III in FIG. 3. The sediment volume here is about 15% less after 1 h of sedimentation time.

Claims

iPatent claims 1. Process for conditioning sludge that is difficult to dewater, in particular with a high organic load, to positively influence dewatering behavior without conditioning additives, in which process a. the water-containing sludge is introduced into an electrochemical cell with at least one electrode with an enlarged surface and b. is exposed to an electric field for a predetermined period of time by applying a direct voltage to the electrodes of the cell. 2. The method according to claim 1, in which an alternating direct voltage is superimposed on the direct voltage. 3. The method according to any one of claims 1 and 2, in which the sludge of the electrochemical cell is fed from a storage container and, after it has flowed through the space between the electrodes, is introduced into a buffer container from which it is passed on for further treatment becomes. 4. Device for carrying out the method according to one of claims 1 to 3, comprising two electrodes (4, 5) connected with a direct voltage source and made of a network of nickel, stainless steel, graphite and / or platinized titanium. 5. The device of claim 4, in which the anode to increase its geometric surface. a pack of several such metal nets. 6. The device according to claim 4 or 5, in which the cathode through a separator (9) made of a plastic network ? with a mesh size of preferably 0.1 mm to 2 mm is protected against a direct flow through the mud. 7. Application of the method or the device. according to one of the preceding claims on the conditioning of sewage sludge, preferably from municipal sewage treatment plants. 8. Application of the method or the device according to one of claims 1 to 6 to the conditioning of metal ore suspensions, preferably from turbidity and / or extraction processes.