WO1999001200A1 - Apparatus and method for continuous vacuum filtering, with endless belt - Google Patents

Abstract

The invention concerns a continuous vacuum filtering apparatus, with endless belt, for example for filtering, drying and desiccating wastewater sludge in sedimentation tanks or industrial treatment plants, comprising a filtering support or medium (4) arranged in the form of an endless belt wound on two rotating cylinders (2, 3) one of which is coupled with a motor (5) driving it in rotation, the filtering support or medium upper side (4') being arranged between a hopper (9) receiving products to be filtered provided outside the loop constituted by the endless belt and a vacuum vessel (7) provided inside said loop. The invention is characterised in that the filtering support or medium (4) consists of two superposed cloths (4A, 4B) made integral with each other, i.e. an external filtering cloth (4A) and an internal cloth (4B), the internal cloth (4B) meshwork being larger than the external filtering cloth (4A) meshwork.

Description

Apparatus and method for continuous vacuum filtration, endless belt.

The present invention relates to a filter or apparatus for continuous vacuum filtration, endless belt. It also relates to a process aimed at improving the performance of this filtration apparatus, in a specific interesting application of the latter.

Interestingly, although in no way limiting, the filter according to the invention is applicable to the filtration, spinning and drying of residual sludge from settling basins, or industrial treatment plants, or from other sources and nature, such as, for example, filtration of liquid manure, effluents from cheese factories, washing water, aquaculture pond water, etc.

There is known, for example from document SU-1,001,972 A, a continuous vacuum filter, with an endless band, comprising, on the one hand, a filtering support constituted by a perforated endless band of drive wound on two drums , and, on the other hand, an endless filter cloth resting, via its upper strand, on the upper strand of the drive belt which can thus ensure the friction movement of said filter cloth, a vacuum placed below the superimposed upper strands of the drive belt and the filter cloth makes it possible to create a permanent depression favoring the passage of the liquid phase of the slurry or effluent to be filtered poured over the filter cloth .

This type of equipment has the following disadvantages:

an irregular entrainment of the filter cloth by the drive band due to frequent slippage between the surfaces in contact with said band and said cloth, which reduces the efficiency of the filters;

- an irregular deposit of effluents on the filter cloth due to this irregular entrainment, resulting in an irregular drying of the sludge or slurry spilled on the latter; poor distribution of the suction effect applied to the lower surface of the filter cloth, resulting in poor performance in the degrees of drying of the effluents or other slurries to be filtered.

The object of the invention is therefore in particular to remedy the drawbacks of endless band filters of the aforementioned type.

This objective is achieved, according to the invention, by a continuous vacuum filtration apparatus, with an endless belt, which, according to a first characteristic arrangement, is remarkable in that its endless filter medium consists of two superimposed fabrics rendered integral with each other, either an external filter cloth and an internal support cloth, the mesh of the internal support cloth being greater than the mesh of the external filter cloth. Advantageously, the internal woven fabric serving as a carrying mat has a mesh of the order of one to a few millimeters (for example five millimeters), while the external filter cloth has a mesh of the order of one to a few micrometers ( for example of the order of 1 to 30 micrometers).

Thanks to the characteristic features above, there is a non-deformable support or filtering medium in the form of an endless band allowing it to be mounted on two drums, one of which is a motor, and its drive at constant speed and without blow, and, therefore, an excellent yield, a regular distribution of effluents or slurries on said filter medium and a very thorough drying of the latter.

The aims, characteristics and advantages above, and others still, will emerge more clearly from the description which follows and from the appended drawings in which:

Figure 1 is an axial sectional view of the essential part of a first embodiment of the endless belt vacuum filtration apparatus according to the invention.

Figure 2 is a plan view, partially broken away, of this device. Figure 3 is an axial sectional view of another embodiment of the filtration apparatus according to the invention.

Figure 4 is a vertical sectional view of a device for injecting additives into the effluents, before their filtration.

Figure 5 is a detail view on a larger scale and in vertical section illustrating the means contributing to sealing, along the upper upstream and longitudinal edges of the vacuum tank.

Figure 6 is a detail view, on a larger scale and in vertical section, illustrating a means contributing to sealing at the downstream end of the vacuum tank.

Figure 7 is a detail view, on a larger scale and in cross section illustrating the constitution of one of the longitudinal edges of the endless filter mat and a means for applying it on one of the longitudinal edges of a slide.

Reference is made to said drawings to describe advantageous, although in no way limiting, embodiments of the vacuum filtration apparatus, with an endless belt, according to the invention.

This device comprises a frame 1 supporting two spaced drums, a motor drum 2 and a smooth driven drum 3. The drum 2 is advantageously of the squirrel cage type comprising two cheeks 2a assembled, near their periphery, by bars 2b regularly spaced, for example of the order of 30 to 60 mm. A strainer 2c is arranged around the motor drum 2 thus produced. This construction has the advantage of avoiding slipping and wrinkling of the support or filtering media during its continuous scrolling.

The filter medium produced in the form of an endless band and designated, as a whole, by the reference 4, is wound on these drums 2 and 3. The continuous advancement of the endless band constituting the filter medium or medium 4 is ensured by a motor, for example by an electric gear motor 5, the output of which is connected to the axis 6 of the motor drum 2, by a transmission system, for example with pulleys and belt, the drums 2 and 3 being driven in rotation according to arrows F1. The movement of the filter medium 4 is preferably controlled by an automatic device controlling the speed of rotation of the motor drum, so as to adjust the speed of advance of said filter medium as a function of the physical and chemical characteristics of the effluents or other suspensions to be filtered. and their solids concentration. The speed of rotation of the motor drum 2 can, for example, be regulated by means of an electronic speed variator. The speed of advance of the strip is generally of the order of 8 to 50 cm / second.

According to a first characteristic arrangement of the invention, the filtering support 4 consists of two superimposed fabrics 4A, 4B pressed against one another and made integral with one another, preferably by means of their side edges, for example by sewing, or by self-gripping tapes of the "Velcro" type (registered trademark) or otherwise. A seal is added to each edge of this endless band. The mesh of the internal woven fabric 4B which serves as a support mat is clearly greater than the mesh of the external filtering woven fabric 4A.

The internal woven fabric 4B, that is to say the one which is disposed inside the loop, preferably has a mesh of the order of 1 to a few millimeters and, for example, a mesh of the of the order of 5 millimeters, while the external filtering woven fabric 4A preferably has a mesh of the order of 1 to 30 micrometers.

The external filtering woven fabric 4A may have a width greater than that of the internal carrying strip 4B, its protruding longitudinal edges 4A ′ being, in this case, folded below the longitudinal edges of said internal strip, a sealing tape 4C being fixed (by sewing, gluing or otherwise), under the longitudinal edges of the filtering medium 4 thus produced (Figure 7).

The external filter cloth 4A can be constituted by a micro cloth woven in a single strand of synthetic textile yarn such as polyamide, polyester or other. The mesh size of the external filter cloth 4A is determined beforehand as a function of the degree of clarification or drying and of the flow required.

The internal support fabric 4B can be woven with a synthetic fiber, for example a fluorinated polymer such as Poly (tetrafluoroethylene) or PTFE, which has the advantage of being insensitive to temperature changes and not very sensitive to the effects of stretching or withdrawal.

The support or filtering medium 4 circulates continuously (according to arrows F2) and a vacuum tank 7 is arranged below its upper strand 4 ′.

The upper part of this vacuum tank is surmounted by a perforated smooth plate or wall 8, for example constituted by a perforated sheet of stainless steel, resting on transverse beams 7a fixed on the side walls 7b of said vacuum tank. This fixed rigid plate 8 constitutes a slide on which the upper strand 4 ′ of the endless band 4 moves. The perforations of the plate 8 have, for example, a diameter of the order of 10 mm to 12 mm, the edges longitudinal of the latter being devoid of perforations.

The motor drum 2 is housed, in part, in the downstream end of the vacuum tank 7, so that the vacuum effect is also exerted through said motor drum and tends to press the filtering medium on the latter and to make it adherent to said motor drum.

Above the upper strand 4 ′ of the filtering support 4, is arranged an upper hopper 9 intended for the discharge of the effluents to be filtered, this hopper comprising an open bottom. The upper strand 4 ′ of the filter support 4 is placed at the interface between the upper hopper 9 and the perforated slide 8 surmounting the upper opening of the vacuum tank 7.

In the lower part of the bottom of the vacuum tank 7, an outlet orifice 10 is formed, connected, by means of a suction pipe 11a, to a pump 11, for example constituted by a vane pump placed below the level where said outlet is located. This pump 11 which always works under load, creates a vacuum inside the tank and collect the filtered liquid or filtrate.

The drums 2 and 3 and the perforated slide 8 can advantageously be installed so that the upper strand 4 ′ of the filtering medium 4 and said slide have an orientation more or less inclined relative to the horizontal, this inclination can, for example be of the order of 3 ° to 15 °, depending on the applications envisaged. In certain specific applications, filtration of slurry for example, this inclination can go up to 30 °.

By construction, filtration is of the tangential type.

According to this arrangement, the upper strand 4 ′ of the endless strip 4 and the perforated slide 8 rise in the direction of advancement of the endless strip.

In order to improve the vacuum effect in the tank 7, seals 12a, 12b, constituted by flexible and smooth tongues, are fixedly arranged above the upstream end of the vacuum tank 7 and the longitudinal edges of the perforated slide 8, respectively. The support or filtering medium 4 passes under the flexible tongue constituting the transverse joint 12a, while the longitudinal edges of said support or filtering medium circulate under the flexible tongues constituting the longitudinal joints 12b. These seals which are fixed by means of one of their longitudinal edges, on the upstream and longitudinal lower edges of the upper hopper 9, tend to press the upper strand 4 ′ of the movable filter support 4 against the upper upstream edge of the vacuum tank 7 and against the upper surface of the non-perforated longitudinal edges of the perforated support plate 8, respectively. The rear transverse edge 13 of the vacuum tank 7 extends approximately to the place where the separation of the lower strand 4 "from the endless belt 4 and the motor drum 2 occurs, so as to promote this separation. can be fitted with a flexible mask or seal 13a, for example made of fluorinated polymer CPTFE, FEP, PFA, EPE, better known under the name of "Teflon": (Registered trademark), opposing the free penetration of l air at this point in the vacuum tank. Separation of the lower strand 4 "and the drive roller 2 is, on the other hand, facilitated by the tension exerted by a roller tensioner 14 on said lower strand, this tensioner roller being subject to a tension adjustment device 15, for example constituted by a jack or small telescopic spring arm.

A ramp 16 for blowing hot compressed air is installed below the motor drum 2, parallel to the axis 6 of said motor drum, and inside the loop formed by the endless filter support 4. The orifices of outlet or nozzles of this ramp are oriented towards the internal surface of the movable filter support 4. The action of blowing the hot compressed air outwards, through the filter media 4, makes it possible to increase the degree of drying of the layer of mud or slurry applied against said filtering medium. This action also allows a partial detachment of the mud layer B from its support 4 and thus facilitates the cleaning of the filtering support which is then subjected to a scraping action exerted by a scraper 17 constituted by a scraper knife disposed transversely outside. of the loop formed by said filter support at a level lower than that where the blowing ramp 16 is located. This scraper 17 rubs, by means of its upper active edge, raised upwards and which can be provided with a strip of "Teflon" (registered trademark) hard, on the external surface of the endless strip 4, preferably over the entire width thereof.

The cake Ba detached from the filter support or the scraping residues are poured into a receptacle (not shown) from where they are then directed, depending on the applications, to a total drying tunnel or transported by a tank to a storage place.

The flow of arrival of the effluent B to be filtered in the hopper 9 can be adjusted in any known manner, for example by means of a valve 18 placed on the supply line 19 opening into said hopper, and controlled by a float 20 housed therein. In this way, the float 20 controls the external valve 18 which regulates the flow of the effluent to be filtered. The control of this flow is important, because it allows to fix the thickness of the blade or layer of liquid effluent entrained on and by the filtering medium 4. The thinner the blade or layer of liquid effluent, the faster the spin Ba sludge retained on the filter media. The desirable thickness of this liquid effluent blade (a few millimeters) in depending on the physical and chemical characteristics of the product to be filtered and the purposes of filtration (clarification or drying), can be determined by means of preliminary laboratory tests.

As indicated above, an interesting application of the invention is the filtration, spinning and drying of the residual sludge from the settling tanks.

In this case, to improve the concentration flow rates, a device allowing the implementation of a process for the chemical treatment of coagulation and flocculation is provided upstream of the entry of the effluent into the filtration apparatus and of concentration previously described.

According to this process, at very low doses, and continuously, by means of metering pumps P1, P2, P3, P4, are added to the liquid effluent coming from the settling tank for liquid sludge:

- a) an oxidizing bactericidal solution such as ozone or bleach;

- b) at least one coagulant;

- c) a flocculant;

- d) a "siccator" product.

Tests have shown that the addition of a small quantity of a bactericidal and oxidizing product (for example: ozone, hydrogen peroxide, bleach) slows down then stops the fermentations for a more or less long time. During this fermentation stop, the effectiveness of coagulants and flocculants increases. Tests have also established that this efficiency increases significantly if the temperature of the mixture is above 25 °.

Heating the mixture also makes it possible to significantly reduce the viscosity of the coagulants and flocculants. This heating can be obtained in various ways, for example by heating the air used for mixing the mixture.

This heating can be obtained by using the hot air produced by a continuous air compressor with paddles (heat gain of the order of 30 °).

The non-fermentation of the rejected sludge can be maintained beyond the time limit if the elimination of the filtrate (rejected clear water) makes it possible to reach and exceed a concentration greater than 80%.

To obtain this figure, the addition of a "drying coagulant", for example of the type sold by the company "Métaux Chimie" BP 7115 - 95054 CERGY PONTOISE Cedex, under the name of "Cos 50", is desirable.

The device for mixing the liquid effluent and the abovementioned additives is illustrated in FIG. 4.

This device comprises a closed mixing tank 21, for example having a capacity of approximately 800 liters, into which penetrate, in the upper part, a supply line 22 connected to a tank or tank of liquid effluent (not shown), and, in the lower part, a pipe 23 connected to an air compressor (not shown).

The dosing pumps P1, P2, P3, P4 are connected to an outer portion of the supply line 22, preferably near the tank 21, said supply line opening into the lower part of said tank.

At its upper part, the tank 21 has an outlet orifice 24 connected to a pipe 25, the free end of which opens above the hopper 9 of the filter itself described above.

The abovementioned additives are injected separately and simultaneously, by means of metering pumps P1, P2, P3 and P4, the flow of which is, for example, of the order of 20 liters / hour each. The additives flow simultaneously into the tank 21 with the effluent introduced by gravity through the supply line 22 which opens into the lower part of said tank. The hot air under low pressure allowing the mixing of the effluent and the additives, arrives via the pipe 23.

Brewing takes three to four minutes. The liquid mixture leaves through the orifice 24 and this liquid mixture flows gravitatively through the pipe 25, into the hopper 9 of the filter.

The mixing tank is constructed in such a way that mixing and feeding can take place continuously.

For this, said tank will have a capacity at least equal to 120/100 of the maximum effluent flow rate received during the brewing time. For example: for an effluent supply rate <10 m ³ / hour for a brewing time of 4 minutes, the tank capacity will be, in m ³ , 10m ³ x 4 x 120/100,

60 or 0.799 m ³ rounded to 0.800 m ³ , or about 800 liters.

Claims

1. - Continuous vacuum filtration device, with endless belt, for example for the filtration, dewatering and desiccation of residual sludge from settling basins or industrial sewage treatment plants, comprising a support or filter media (4 ) produced in the form of an endless belt wound on two rotating drums (2, 3), one of which is coupled to a motor (5) allowing it to rotate, the upper strand (4') of the support or filter media being arranged between a receiving hopper (9) for the products to be filtered arranged outside the loop constituted by the endless belt and a vacuum tank (7) arranged inside said loop, characterized in that the support or filter media (4) consists of two superimposed fabrics (4A, 4B) made integral with each other, i.e. an external filter fabric (4A) and an internal fabric (4B), the mesh of the internal fabric (4B) being greater than the mesh size of the external filter cloth (4A). 2. - Continuous vacuum filtration apparatus, with endless belt, according to claim 1, characterized in that the motor drum (2) is of the type "squirrel cage" comprising two side cheeks (2a) assembled by regularly spaced bars (2b), this drum being preferably surrounded by a strainer (2c). 3. - Continuous vacuum filtration apparatus, with endless belt, according to one of claims 1 to 2, characterized in that the internal fabric (4B) serving as a carrier belt has a mesh size of the order of one to a few millimeters, for example five millimeters, while the external filter cloth (4A) has a mesh size of the order of a few micrometers, for example of the order of one to thirty micrometers. . - Continuous vacuum filtration device, endless belt, according to Tune . any of claims 1 to 3, characterized in that the external filter cloth (4A) and the internal cloth (4B) are made integral with each other, by assembling their lateral edges, for example by sewing or by means of ribbons self-gripping. 5. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 4, characterized in that the upper strand (4') of the endless band constituting the support or filter media (4) is applied against a rigid perforated plate (8) surmounting the upper opening of the vacuum tank (7). 6. - Continuous vacuum filtration apparatus, with endless belt, according to claim 5, characterized in that the upper strand (4 ') of the endless belt constituting the support or filtering media (4) is arranged at the interface between the open bottom of the upper hopper (9) and the perforated slide (8) surmounting the upper opening of the vacuum tank (7). 7. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 6, characterized in that joints (12a, 12b) are fixedly arranged above the upstream end of the depression tank (7) and the longitudinal edges of the perforated slide (8), respectively, these joints tending to press the upper strand (4') of the mobile filter support (4) against the upper upstream edge of the vacuum tank and against the two longitudinal edges of the support plate or perforated slide (8), respectively. 8. - Continuous vacuum filtration apparatus, with endless belt according to any one of claims 1 to 7, characterized in that the filter cloth external (4A) has a width greater than that of the internal support fabric (4B) and its overhanging longitudinal edges (4A') are folded below the longitudinal edges of said internal supporting strip (4B), a sealing tape ( 4C) being fixed under the longitudinal edges of the endless filter media (4) thus produced. 9. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 2 to 8, characterized in that the motor drum (2) is housed, in part, in the downstream end of the storage tank depression (7). 10. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 9, characterized in that it comprises a vane pump (11) connected to the outlet orifice (10) of the tank depression (7) via a suction pipe (11 a), this pump being placed below the level where said outlet orifice (10) is located, this pump which thus always works under load , allows you to create a depression inside the tank and collect the filtrate. 11. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 10 characterized in that the rear transverse edge (13) of the vacuum tank (7) extends approximately to the place where the separation of the lower strand (4") of the endless belt (4) and the motor drum (2) occurs, this rear transverse edge being, preferably, provided with a flexible seal (13a) opposing to the free penetration of air, at this location, into the depression tank (7). 12. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 11, characterized in that it comprises a compressed air blowing ramp (16) installed below the drum. motor (2), parallel to the axis of the latter and inside the loop constituted by the movable filter support (4), the outlet orifices of this ramp being oriented towards the internal surface of said filter support (4) . 13. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 12, characterized in that it comprises a scraper (17) arranged transversely, outside the loop constituted by the support mobile filter (4), this scraper (17) rubbing, via its active edge, on the external surface of the endless belt (4). 14. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 13, characterized in that a flow adjustment valve (18) is placed on the supply pipe (19) bringing the effluent to be filtered (B) in the hopper (9), this valve being controlled by a float (20) housed in said hopper. 15. - Continuous vacuum filtration apparatus, with endless belt, according to any one of claims 1 to 14, characterized in that it comprises a closed mixing tank (21) into which a supply pipe (22) penetrates making it possible to bring the effluent to be filtered (B) into this tank into which a pipe (23) connected to an air compressor also opens, said tank (21) comprising, in the upper part, an outlet orifice (24) connected to a pipe (25) opening above the hopper (9), metering pumps (P1, P2, P3, P4) connected to an external portion of the supply pipe (22), preferably near the tank (21), allowing additives to be injected into said effluent. 6. - Process for filtering a liquid effluent coming from a • liquid sludge settling basin, using an endless belt vacuum filter, according to any one of claims 1 to 15, characterized in that You add, in continuously, to the effluent to be filtered, by means of metering pumps (P1, P2, P3, P4): - an oxidizing bactericidal solution, for example consisting of ozone or bleach; - at least one coagulant; - a flocculant; a “sicator” product. 17. - Method according to claim 16, characterized in that the mixing of the liquid effluent and the additives takes place in a closed tank by means of injecting compressed hot air, for a period of between three and four minutes .