FR2764818A1 - METHOD AND DEVICES FOR CLEANING FILTERS - Google Patents

Abstract

The invention concerns a method for cleaning filters characterised in that it consists of the following successive steps: a) storing the backwahsing or declogging water, consisitng of filtered water, in a storage chamber (3) located downstream of the filtering element (1) relative to the filtering direction; b) activating the stored backwashing water, by sudden expension of a predetermined gas volume V at a predetermined pressure P, said pressurised gas, stored in a chamber (7) being contacted with the backwashing water in said storage chamber (3); and c) selecting the value of pressure P and of volume V such that the instantaneous flow rate of the backwashing water obtained at the beginning of the expansion ensures that the particles accumulated on the filtering element (1) upstream wall are detached, said particles being re-suspended in the liquid to be filtered (2).

Description

 The present invention relates generally to the filtration on filter elements, of liquids containing suspended matter, in particular activated sludge from waste water.

 The filter elements of such filters can consist of cloths, membranes, filter materials, sintered, plastic or metal which clog quickly during the fittration by forming on the filter element a cake which consists of particles separated from the on said element. In some severe cases, activated sludge for example, such clogging occurs within a few minutes. It is therefore necessary to provide a device for periodically unclogging the filter elements in order to be able to ensure filtration in a practically continuous mode. The present invention proposes to provide improvements to this filter unclogging technique.

At present, the known unclogging solutions are the following:
- Backwashing with water in situ, that is to say by passing through the filter element a stream of clean water circulating against the current of the liquid to be filtered;
- backwashing with water on the drained filter element using a pressurized jet
- mechanical washing (for example using brushes) which can be combined with a pressure jet washing technique;
- backwashing with air, possibly with a pulsating effect.

These conventional techniques have a certain number of drawbacks and limits which can be summarized as follows:
- Backwashing with water in situ:
significant water consumption, need for high instantaneous flow rates, therefore large pumps and pipes, which are therefore expensive, limiting the volumes of water used sometimes involves installing ultra-fast isolation valves which are therefore expensive.

- Backwashing with water on a dewatered filter element (washing by jets):
need to dewater the filter element which requires the provision of pumps and / or valves or mechanical devices, or even to make this filter element mobile (for example filter drum, vacuum filter, etc.);
- Mechanical washing (brushes) combined or not with washing by jets:
large mechanical equipment, high overall dimensions (for example technical rotary drum filters.

- Backwashing in air (with pulsation effect or not):
Difficulties in distributing the air: - on large submerged filter elements (for example cloths) (the difference in height between the highest point and the lowest point of the filter element to be washed must be less than the pressure drop of the air through this element, taking into account the effects of the density difference between water and air); - between filter elements arranged in parallel, at different depths (effects of the density difference between water and air).

The present invention proposes to provide a filter washing technique which does not have the drawbacks of the previous solutions mentioned above, the technical problems to be solved by the invention being mainly the following:
- remove any mechanical device;
- wash with a low consumption of washing liquid;
- reduce the size of the washing fluid lines;
- reduce the power required for washing.

This problem is solved by the invention in the first place by a backwashing process of filters for liquid containing suspended matter, in particular for activated sludge from waste water, according to which the filters, comprising filtering elements such as fabrics, membranes, plastic or metallic sintered filter materials are periodically unclogged by circulation of a washing liquid against the current of the liquid to be filtered, characterized in that it comprises the successive stages consisting in:
a) store backwashing or unclogging water, consisting of filtered water, in a storage enclosure positioned downstream of the filtering element considering the direction of filtration;
b) setting in motion said stored backwash water, by sudden expansion of a determined volume V of a gas at a determined pressure P, said pressurized gas being brought into contact with backwashing water in said storage enclosure and ,
c) choose the value on the one hand of the pressure P and on the other hand, of the volume V so that the instantaneous flow of backwashing water being established at the start of the expansion ensures that the cake of particles accumulated on the upstream wall of the filter element, these particles being resuspended in the liquid to be filtered.

 According to the invention, the backwash gas is preferably air.

 According to one embodiment of this process, the pressure P and the volume V are chosen so that at the end of expansion, the gas has not driven all of the water contained in said storage enclosure , The gas / liquid backwash interface does not meet the wall of the filter element.

 According to another embodiment of the method according to the invention, the pressure P and the volume V are chosen so that at the end of expansion, the gas has driven all of the water contained in the storage enclosure, the gas under pressure being expelled through the filtering element, after passage of the cleaning water.

 According to the invention, the unclogging pressure P is adjusted as a function of the degree of clogging of the filter elements in order to adapt the backwashing energy to the washing energy requirements.

 According to the invention, the instantaneous backwashing power is adjusted as required, by adjusting the pressure (P) and the volume (V), as a function of the washing energy requirements, according to the degree of clogging of the filtering element. .

The invention also relates to a device for implementing the method as defined above, comprising at least one filter element immersed in the liquid to be filtered, through which said liquid passes under the effect of a pressure difference between the two sides of said filter element, the latter retaining the particles in suspension in the liquid to be treated, this device being characterized in that it comprises
- an unclogging water storage enclosure, in which filtered water is collected, the latter flowing towards recovery means through an outlet valve, this storage enclosure being able to be brought to exhaust through a purge valve,
a gas storage enclosure having a volume (V), in communication with said cleaning water storage enclosure through a valve and,
a means for pressurizing the gas contained in said gas storage enclosure, a valve being interposed between said means for pressurizing and said gas storage enclosure, this valve being closed when the pressure of the gas in said enclosure gas storage reaches a predetermined value (P), a storage / gas-storage-water communication valve being then opened to expel the unclogging water by sudden expansion of the volume (\ I) of gas at pressure (P).

 Other characteristics of this device are specified in the secondary claims appended to this description.

Various embodiments of the device which is the subject of the present invention will now be described by way of nonlimiting examples. In the course of this description, reference is made to the appended drawings in which:
Figure 1 is a schematic representation of a first embodiment of a device according to the present invention, wherein the filter element is arranged horizontally.

 Figures 2a and 2b illustrate two alternative embodiments of the device according to the invention in which the filter element is arranged vertically.

FIG. 3 is also a schematic representation of an exemplary embodiment of the device according to the invention using several horizontal filter elements;
FIG. 4 schematically represents another embodiment of the device according to the invention comprising several vertical filter elements and,
FIG. 5 illustrates an alternative embodiment comprising several vertical filter elements arranged in parallel.

 Reference is firstly made to FIG. 1 which schematically represents an embodiment of the device which is the subject of the present invention.

 In this figure, the reference element 1 designates the filter element which may for example be a filter cloth, this filter element being immersed in the liquid to be treated 2. In this embodiment, the filter element is horizontal and it is arranged so that the filtered water is collected in an enclosure or chamber 3.

Under the effect of the pressure difference between the two sides of the filter element 1, the liquid to be filtered 2 passes through the filter element 1 which retains the particles in suspension.

The filtered water collected in chamber 3 flows through line 15 to recovery means such as a pump 5 through an outlet valve 4. The pressure difference ensuring filtration can be obtained by setting in depression of enclosure 3 for example by pumping or siphoning.

 The device further comprises an enclosure 7 for storing pressurized gas, this enclosure having a volume V. The enclosure 7 is supplied with gas under pressure by means of pressurizing means 10 which can be produced for example in the form of a compressor or a pressurized gas tank. A valve 8 is interposed between the means 10 for pressurizing and the enclosure 7 for storing pressurized gas.

 This pressurized gas, which is preferably air, can be introduced into the enclosure 3 via a pipe 12. The latter communicates with the pressurized gas storage enclosure 7 through 'A valve 9 and there is further provided a purge valve 6 for the exhaust gas as will be described below.

 As specified above, the invention consists in backwashing the filter element 1 using filtered water, the latter coming from the chamber or enclosure 3 (here referred to as an enclosure or backwashing or unclogging water storage chamber), the backwashing water being set in motion by expansion of a defined quantity (Volume V) of a gas under pressure, coming from the gas storage enclosure 7, initially at a defined pressure (P), this gas being placed in communication with the enclosure 3 for storing the washing water.

The operation of this device is therefore as follows:
After a certain filtration time, a large quantity of particles is accumulated on the upstream wall (in the direction of fitration) of the filter element 1, thereby creating an increase in the pressure drop across the element assembly filter + deposits. It is then advisable to carry out a backwashing in order to take off the cake of particles.

 At the initial time of backwashing, the storage chamber 3 is filled with filtered water, the valve 4 for leaving the filtrate is open and the backwash gas purge valve 6 is closed.

 The gas storage enclosure 7 is filled with gas at atmospheric pressure, the gas outlet valve 9 being closed. This enclosure 7 has a volume V.

 The pressure gas inlet valve 8 is then opened and pressurized gas is introduced into this enclosure by means 10 such as a compressor or compressed gas tank. The valve 8 is closed when the pressure prevailing in the enclosure 7 has reached the predetermined value P. This pressure value can be controlled by using, for example, a pressure sensor Il which controls the closing of the valve 8.

The outlet valve 4 of the filtrate is then closed. The valve 9 is then opened and the gas contained in the storage enclosure 7 is released by expelling before it the filtered water contained in the pipe 12 and in the chamber 3. When the gas is expanded, it occupies part of this filtered water storage chamber 3. The values of pressure P and volume V can be chosen so that at the end of expansion, the gas has not driven all of the water contained in the storage enclosure 3,
The final gas-liquid interface 13 not meeting the wall of the filter element 1. In this case, the values on the one hand, of the volume V and on the other hand of the pressure P are chosen so that the volume expanded gas is less than the cumulative volumes of the enclosure 7 for storing pressurized gas, of the enclosure 3 for storing the unclogging water and of the connections between these enclosures.

 As a variant, it is possible to choose the pressure P and the volume V so as to force the gas through the filter element 1, after passage of the unclogging water, in order to ensure additional cleaning of the filter element. In this case, the values on the one hand of the volume V and on the other hand of the pressure P are chosen so that the volume of the expanded gas is greater than the cumulative volumes of the enclosure 7 for storing the gas under pressure, in enclosure 3 for storing cleaning water and connections between these two enclosures.

Of course, this variant implies a judicious choice of the filter element 1 in particular as regards its mechanical strength.

 In all cases, under the effect of the flush, the cake of particles accumulated on the upstream side of the filter element 1 is detached from the latter and resuspended in the water to be filtered 2.

According to the present invention, the pressure P is defined so as to supply by expansion of gas sufficient energy for unclogging the filter element 1. By way of example, P can be chosen between 1 and 10 × 10 4 Pa, preferably between 2 to 6.104
Pa.

 The pressure P and therefore the volume V are also chosen so that the instantaneous flow rate established at the start of the expansion is high enough to produce an optimal effect on the particle cake. By way of example, this instantaneous flow can be between 20 and 100 liters / s. m2.

 After unclogging, the valve 9 is closed, the purge valve 6 is opened so as to allow the gas contained in the chamber 3 to escape, this gas being replaced by filtered water coming from the liquid 2 after passing over the filter element 1. The valve 4 is then open, the means for taking up filtered water 5 are reactivated and thus, the filtration is put back into service.

 The backwashing operation must be repeated periodically. According to the invention, it is possible, for example, to provide a timer for periodically triggering the backwashing operation. It is also possible to provide a clogging detector for the filter element 1, this detector operating for example by measuring the pressure drop across said filter element and triggering the backwashing cycle when the clogging value of the filter element 1 that it detects exceeds a predetermined threshold.

Other variants of the invention can be envisaged, and in particular variants as regards its mode of implementation. So, for example:
- the purge valve 6 can be opened according to a progressive opening procedure in order to avoid jerks of the water flow during the flushing of the gas under pressure, and therefore the risks of rapid clogging of the filtering element 1 during such blows;
- the purge valve 6 can be opened before the end of the complete expansion of the gas which avoids a lag of depressurization
- the valve 9 can be closed before the complete expansion of the gas: in this case, unclogging is obtained by partial expansion of a volume V of gas at pressure
P initial, the expansion then being interrupted by the closing of the valve 9.

 It is also possible, according to the present invention, to envisage a certain number of variant embodiments and, by way of nonlimiting examples, a few will be described below with reference to FIGS. 2a to 5.

 In the embodiment described with reference to FIG. 1, the filter element 1, for example a filter cloth, is horizontal. We can consider other provisions including inclined or vertical. The embodiments illustrated in Figures 2a and 2b are characterized by filter elements 1 'arranged vertically. We find in these Figures 2a and 2b the other elements of the device according to the invention (designated by the same references as in Figure 1) to ensure unclogging of the vertical filter elements 1 '. In this case, means can be provided according to the invention for preventing the gas ensuring the flushing of the backwashing water from reaching the filter element 1 or 1 '. In the embodiment illustrated by FIG. 2a, these means are produced in the form of a lower extension 16 at the end of which is positioned the vertical filter element 1 ′, this extension 16 defining a submerged threshold as is clearly seen in this figure 2a. In the variant illustrated in FIG. 2b, these same means are produced in the form of a siphon partition 17.

 In the variants according to FIGS. 2a and 2b, the method of unclogging the vertical filter elements 1 ′ is identical to that described above with regard to FIG. 1.

 We now refer to Figure 3.

 In this exemplary embodiment, several filter elements are placed in parallel and in the embodiment shown, two horizontal filter elements 1a and 1b are placed in parallel. Each filter element has its own chamber or enclosure for storing cleaning water, respectively 3a and 3b, these chambers 3a and 3b being connected to the same device for producing backwash gas identical to the device described above in reference to FIG. 1 and which therefore comprises the elements designated by the references 7.8, 11.10. However, in this case, each unclogging water storage chamber or enclosure 3a, 3b must have its own isolation valve 9a and 9b respectively and its purge valve 6a and 6b respectively. Similarly, each enclosure 3a, 3b has its own means for taking up the filtered water: pumps 5a, 5b and outlet valves 4a, 4b. The operating mode for unclogging the filter elements 1a, 1b is identical to that described above with reference to FIG. 1, each filter element 1a, 1b being washed individually, successively.

 In the exemplary embodiment illustrated by FIG. 4, two filter elements are provided, such as for example filter cloths 1'a and 1lob, comprising a common chamber 3 for storing the filtered water. This common storage enclosure 3 is connected to a common means for producing backwash gas under pressure which is identical to that described above with reference in particular to FIG. 1 and in this FIG. 4 are found the constituent elements of this means which have been designated by the same references as in FIG. 1. In this embodiment, means are further provided for preventing the expanded backwash gas from reaching the filter elements a, 1'b. In the embodiment illustrated in FIG. 4, these means are produced in the form of internal partitions such as 18.

 The mode of operation of this variant of the invention is of course identical to that described above with reference to FIG. 1.

 In the variant illustrated in FIG. 5, the vertical filter elements 1 ′ a and 1'b also include a common storage chamber 3 but between the filter elements a and 1 ′ b, means of equal distribution are interposed of water respectively 19a and 19b which can be produced for example in the form of a diaphragm. This variant is also identical to the embodiment illustrated in Figure 4 and its mode of operation is the same.

 It is of course and without departing from the scope of the present invention, to envisage other alternative embodiments, for example a combination of the embodiments illustrated in FIGS. 4 and 5.

 According to the present invention, the backwashing energy can vary as required, by adjusting the value of the cleaning pressure P as a function of the washing energy requirements, according to the degree of clogging of the filter elements, within the limit of the volume. of the backwash water storage chamber 3 if the volume of the pressurized gas storage enclosure 7 is fixed.

Still according to the present invention, the simultaneous power and flow rate of backwashing can be made variable according to the needs by adjusting the pressure P and the cleaning volume V to the washing energy needs, according to the degree of clogging of the elements. filters: for example double pressure can be provided in a half enclosure 7. More generally, according to a variant of
The invention, the gas storage enclosure 7 can be divided into n isolable sub-enclosures, in which a pressure between P and nP can be applied depending on the number of sub-enclosures used.

Among the advantages provided by the present invention, the following may be mentioned in particular:
- low consumption of filtered water to perform unclogging operations
- mastery of even distribution on industrial multiple-chamber systems
- Adjustment of the pressure P and the volume V of unclogging back-up to the energy needs of backwashing;
- absence of mechanical device;
- backwash fluid lines reduced in size (the gas is transported, at given pressure drops, at speeds more than 10 times higher than what can be obtained in water: for the same backwash water flow rate filter element, the pipes will therefore be 3 to 5 times smaller in diameter)
- reduced electrical power (for the same energy supplied to the filter elements), the means 10 such as a compressor, or a pressurized gas tank, to obtain backwash gas under pressure P operating much longer than the time of the washes , its power is much lower than the instantaneous pressure of the washes. This results in a significant reduction in the costs of electrical equipment;
- compactness and ease of installation.

 It remains to be understood that the present invention is not limited to the embodiments described and / or shown but that it encompasses all variants thereof which come within the scope of the appended claims

Claims (25)

 1 - Process for backwashing filters for liquid containing suspended matter, in particular for activated sludge from waste water, according to which the filters, comprising filter elements such as fabrics, membranes, sintered plastic or metallic filter materials, are cleaned periodically by circulation of a washing liquid against the current of the liquid to be filtered, characterized in that it comprises the successive stages consisting in:  a) store backwashing or unclogging water, consisting of filtered water, in a storage enclosure positioned downstream of the filtering element considering the direction of filtration;  b) setting in motion said stored backwashing water, by sudden expansion of a determined volume V of a gas at a determined pressure P, said pressurized gas being brought into contact with backwashing water in said storage enclosure and,  c) choose the value on the one hand of the pressure P and on the other hand, of the volume V so that the instantaneous flow of backwashing water being established at the start of the expansion ensures that the cake of particles accumulated on the upstream wall of the filter element, these particles being resuspended in the liquid to be filtered.  2 - Process according to claim 1 characterized in that the pressure P and the volume V are chosen so that at the end of expansion, the gas has not driven all of the water contained in said storage enclosure , The gas / liquid backwash interface does not meet the wall of the filter element.  3 - A method according to claim 1 characterized in that the pressure P and the volume V are chosen so that at the end of expansion, the gas has driven all the water contained in the storage enclosure, the pressurized gas being expelled through the filter element, after passage of the cleaning water.  4 - Method according to any one of the preceding claims characterized in that the backwashing gas is air.  5 - Process according to any one of the preceding claims, characterized in that the unclogging pressure P is adjusted as a function of the degree of clogging of the filter elements in order to adapt the backwashing energy to the washing energy requirements.  6 - Method according to any one of claims 1 to 4 characterized in that one adjusts the instantaneous backwashing power as required, by adjusting the pressure (P) and volume (V), as required in washing energy, depending on the degree of clogging of the filter element.  7 - Device for implementing the method according to any one of claims 1 to 6 comprising at least one filter element (1) immersed in the liquid to be filtered (2), through which passes said liquid under the effect of a pressure difference between the two sides of said filter element, the latter retaining the particles in suspension in the liquid to be treated, this device being characterized in that it comprises:  - an enclosure (3) for cleaning unclogging water, in which filtered water is collected, the latter flowing towards recovery means (5) through an outlet valve (4), said storage enclosure (3) being exhausted through a purge valve (6);  an enclosure (7) for storing gas having a volume (yes), in communication with said enclosure (3) for storing cleaning water through a valve (9) and,  - A means (10) for pressurizing the gas contained in said enclosure (7) for storing gas, a valve (8) being interposed between said means (10) for pressurizing and said enclosure (7) for storing gas, this valve (8) being closed when the gas pressure in said gas storage enclosure reaches a predetermined value (P), a storage-gas - storage-water communication valve (9) then being open to expel the unclogging water by sudden expansion of the volume (V) of gas at pressure (P).  8 - Device according to claim 7 characterized in that it further comprises a pressure sensor (11) controlling the closing of said valve (8) when the pressure prevailing in the enclosure (7) of gas storage has reached the value P.  9 - Device according to one of claims 7 or 8 characterized in that the values, on the one hand of the volume (V), on the other hand of the pressure (P) are chosen so that the volume of the expanded gas is lower than the cumulative volumes of said enclosure (7) for storing gas, of said enclosure (3) for storing cleaning water and of the connections between said enclosures (7 and 3).  10 - Device according to one of claims 7 or 8 characterized in that the values, on the one hand of the volume (V), on the other hand of the pressure (P) are chosen so that the volume of the expanded gas is greater than the cumulative volumes of said enclosure (7) for storing gas, of said enclosure (3) for storing cleaning water and of the connections between said enclosures. (7 and 3).  11 - Device according to any one of claims 7 to 10 characterized in that the filter element or elements (1, 1 a, 1b) are horizontal.  12 - Device according to any one of claims 7 to 10 characterized in that the filter element or elements (1 ', a, I'b) are vertical.  13 - Device according to any one of claims 7 to 10 characterized in that the filter element or elements are inclined.  14 - Device according to one of claims 12 or 13 characterized in that the enclosure (3) for backwash water storage is designed so that the gas ensuring the flushing of backwash water does not reach to the filter element (1 s)  15 - Device according to claim 14 characterized in that the enclosure (3) for storing backwashing water is provided with a lower extension (16) at the end of which is positioned said filter element (s) (1 ') , this extension defining a submerged threshold.  16 - Device according to claim 14 characterized in that the enclosure (3) for backwashing water storage is provided with a siphon partition (17), positioned behind the filter element (s) (1 ').  17 - Device according to any one of claims 7 to 16 characterized in that it comprises several filter elements (la, lb) arranged in parallel, the backwashing device being common to said filter elements, each of said filter elements (la, lb ) being provided with its enclosure (3a, 3b respectively) for storing backwash water, said enclosures being connected to a common means for producing backwash gas under pressure (7, 8, 10) and provided with isolation valves own (9a, 9b respectively).  18 - Device according to any one of claims 7 to 17 characterized in that it comprises several filter elements (a, 1'b) arranged in parallel, said said filter elements comprising a common enclosure (3) for storing backwashing liquid connected to a common means (7, 8, 10) for producing backwash gas under pressure.  19 - Device according to claim 18 characterized in that The backwash water storage enclosure (3) includes means (18) for preventing the expanded backwash gas from reaching said filter elements (1'a, 1'b).  20 - Device according to claim 19 characterized in that said means preventing the expanded backwash gas from reaching said filter elements are constituted by internal partitions (18).  21 - Device according to any one of claims 7 to 20 characterized in that it comprises a timer for periodically triggering the backwashing operation.  22 - Device according to any one of claims 7 to 20 characterized in that it comprises a clogging detector of the filter element (s), operating in particular by measuring the pressure drop across the filter element (s), this trigger detector the backwashing cycle when the clogging value of the filter elements it detects exceeds a predetermined threshold.  23 - Device according to any one of claims 7 to 22 characterized in that said enclosure (7) for storing gases is supplied with the aid of a compressor.  24 - Device according to any one of claims 7 to 22 characterized in that said enclosure (7) for storing gases is supplied with gas under pressure via a reservoir of gas under pressure.  25 - Device according to any one of claims 7 to 24 characterized in that said enclosure (7) for unclogging gas storage is divided into n sub-insulated enclosures in which a pressure between P and nP is applied depending on the number of under speakers used.