FR2724849A1 - METHOD AND DEVICE FOR REMOVING COMPOUNDS BY MEMBRANE FILTRATION AND ACTIVATED CARBON - Google Patents

Abstract

A method and a device for compound removal using membrane and activated charcoal filtration. The method is useful for removing organic or inorganic compounds from gaseous or aqueous effluents and comprises a membrane filtration step (microfiltration, ultrafiltration or nanofiltration) whereafter the permeate is passed over or through at least one activated charcoal membrane. The device for carrying out the method comprises at least one first filtration membrane (3) receiving the effluent (e) to be processed and providing a permeate that is virtually free of high molecular weight substances, said first membrane (3) being coupled to at least one second activated charcoal membrane (5) for adsorbing residual pollutants from the filtration permeate. Said method and device are particularly suitable for processing water containing humic substances and micropollutants.

Description

 The field of the invention is that of the treatment and purification of water and gases. More specifically, the present invention relates to a process for removing organic or inorganic compounds from gaseous or aqueous effluents and a device for its implementation, combining membrane filtration and adsorption on activated carbon.

 Membrane filtration, in particular ultrafiltration, is a technique that is increasingly used today to retain high molecular weight molecules, bacteria or fine particles contained in aqueous or even gaseous effluents, these pollutants not being retained by the usual filters. This technique generally occurs at the end of effluent treatment and for refining.

 Furthermore, during the treatment or purification of water, the elimination of organic or mineral pollutants is often supplemented by the introduction, into the circuit of the effluent to be purified, of activated carbon in the form of grains or powder. fine. The micropollutants are thus adsorbed on this material. This adsorption always takes place upstream or at the same time as a conventional filtration stage (for example on sand), in order to retain particles of activated carbon on the filter.

 The advantages of these two techniques have been highlighted recently by a water treatment process using combined activated carbon particles and an ultrafiltration module. The device used is for example described in the document by Adham S.S. et al. : "Predicting and venfying organics removal by PAC in an ultrafiltration system", J. Am. Water Works Ass., 1991 ,.

83, 12, 81-91 and that of Baudin I. et al. : "Production of drinking water by combination of treatments: ultrafiltration on organic membranes and adsorption on powdered activated carbon" from Recent Advances in Engineering
Processes. Tech. & Doc, Lavoisier, 1991, 5, 15, 135-138. In these devices, the activated carbon in the form of grains or powder is either introduced into the raw water supply, or placed in the recirculation loop of the ultrafiltration concentrate where it adsorbs the micropollutants, and is then retained by the ultrafiltration membrane.

 Although it makes it possible to stop both organic matter and micropollutants, such a method has the drawbacks of not being able to allow simple and effective regeneration of activated carbon, the pores of which are rapidly saturated or clogged in the presence of a concentration of polluting materials. The adsorption capacity of activated carbon is therefore quickly reached and the adsorbent must be replaced frequently.

 On the other hand fine particles of activated carbon (from the powder or grain abrasion) also clog the pores of the ultrafiltration membrane or by abrasion deteriorate its surface, making it quickly and permanently unusable.

 The present invention aims in particular to overcome these drawbacks.

 More precisely, a first objective of the invention is to find a method and to implement a device for removing organic or mineral compounds from gaseous or aqueous effluents allowing both efficient retention of molecules with high molecular weight and adsorption of smaller molecules.

Another objective of the present invention is to avoid clogging and abrasion of the filtration membranes by the particles of the adsorbent.
These objectives, as well as others which will appear subsequently, are achieved thanks to a process which consists on the one hand in using not free particles of activated carbon (grains or powder) but larger surfaces, it is ie activated carbon in the form of membranes (woven or pressed fibers or grains fixed on a porous material) and on the other hand to place these adsorbent membranes not upstream of the ultrafiltration but downstream of it to treat it the permeate (s).

 The process for removing organic or mineral compounds from gaseous or aqueous effluents according to the invention is therefore characterized in that it comprises a membrane filtration step (microfiltration, ultrafiltration or nanofiltration) followed by the passage of the permeate over or through at least one membrane based on activated carbon.

 Preferably, the membrane filtration is the ultrafiltration.

 By conventional filtration is meant the retention of suspended particles larger than several micrometers. Membrane filtration has three types of separation depending on the size of the compounds. If the membrane retains particles whose size is a few micrometers (for example bacteria with a size of about 1 to 2 clam.), This is microfiltration. The term ultrafiltration, for which the diameters of the pores of the membrane are between 0.001 and 0.1 µm, is reserved for the separation of compounds with a molecular weight of 10,000 to 100,000 (or even 2,000 to 300,000 ). Nanofiltration, even finer, concerns the separation of compounds having a molar mass greater than a few tens of grams.

 The use of activated carbon in the form of a membrane frees from the need to post-filter the effluent after adsorption. In fact, the activated carbon particles are retained by their rigid structure (activated carbon fibers or binder between the particles). These activated carbon membranes can therefore be used without problem downstream of an ultrafiltration step, by treating the permeate, and therefore be used at the end of the treatment or purification chain.

 In addition, this membrane filtration permeate (for example ultrafiltration) consisting of the effluent being treated from which high molecular weight molecules, microorganisms, particles, etc. are already eliminated. less loaded with organic or mineral matter: the adsorbent can therefore play its role at the maximum of these capacities without early clogging of its surface or of the pores of the membrane based on activated carbon. In addition, the adsorption competitions that can take place between molecules or ions on the surface of the activated carbon are thereby considerably reduced, or even eliminated. Activated carbon membranes also have the advantage of easier regeneration.

 The device for implementing the method according to the invention is characterized in that it comprises one or more first filtration membrane (s) receiving the effluent to be treated and delivering a permeate practically free of substances with high molecular weight, the said first membrane being coupled to at least one second membrane based on activated carbon adsorbing the residual pollutants of the filtration permeate.

 The first membrane has an essentially filtration function, particularly for the retention of substances with high molecular weights. It receives the effluent to be treated, which passes through its pores, and delivers a permeate which is in turn brought into contact with the second membrane. This second membrane, based on activated carbon, has an adsorption function for the small molecules and / or residual ions contained in the permeate.

 The nature of the first membrane, micro-, ultra- or nanofiltration, is indifferent. It can be a mineral membrane or an organic membrane. such as cellulose acetate or polyamide.

 Advantageously, the device according to the invention can comprise several membranes arranged in series or in parallel, of identical and different sizes and nature. I1 can advantageously include first filtration membranes placed in series and crossed by the effluent which meets increasingly reduced pore diameters.

 Microfiltration membranes retain suspended particles and bacteria, colloids if the medium is liquid. Ultra- and nanofiltration membranes remove molecules of high molecular weight from the effluent, in a range from 2,000 to 300,000.

 In a first variant of the invention, the filtration membrane (s) and the adsorption membrane (s) based on activated carbon are flat, a membrane based on activated carbon can be pressed against the part " downstream "from a first filtration membrane, or be separated from it by a gap. Such an interstice gives the permeate time to undergo some turbulence which thus promotes the speed of transfer of the compounds present in the fluid to the adsorbent by a decrease in the concentration gradient in the fluid.

The thickness of the gap may be between 1 mm and several centimeters.

 In a second variant of the invention, the first filtration membranes consist of a cylindrical bundle of hollow fibers retained by a link or a macroporous coating. The effluent enters each fiber at one end, migrates inside the lengthwise of the cylindrical fiber, and by pressure on the fluid, releases the permeate perpendicular to the periphery.

 In this second variant, the adsorbent membrane (s) based on activated carbon envelop the bundle of hollow fibers forming the first membranes or each fiber individually. Such an envelope may for example be of cylindrical shape or made up of flat membrane (s) wound in a spiral.

 According to a preferred embodiment of the device according to the invention, the second membrane based on activated carbon consists of activated carbon fibers. These fibers are obtained, for example, by calcination and activation (in an oxidizing medium at high temperature) of polyacrylonitrile fibers. Such fibers can be woven or pressed to form membranes resembling fabrics or felts.

 According to a second embodiment, the second membrane based on activated carbon is a polyether or polyester foam loaded with grains of activated carbon retained by a binder.

 According to a third embodiment, the second membrane based on activated carbon is a porous surface formed from an activated carbon powder and a binder.

Other characteristics and advantages of the present invention will appear on reading the following description of a preferred embodiment of the invention, given by way of illustration and not limitation, and the appended drawings in which
Figure 1 shows an exploded diagram of the device according to a first variant of the invention, in which the two membranes, filtration and adsorption. are flat, and
Figure 2 shows a view of the device according to a second variant of the invention. in which the adsorption membrane envelops a bundle of filtration membranes.

 FIG. 1 represents a mixed module of ultrafiltration and adsorption on a membrane based on activated carbon. The diagram of this figure shows a horizontal superposition of the various constituent elements, but it is obvious that these elements can be arranged vertically.

 This module (1), parallelepiped, here consists of an upper compartment (2) receiving the effluent (e) to be treated, separated at its lower part from the ultrafiltration membrane (3) by a seal (4). The adsorption membrane based on activated carbon (5), which is the same size as the ultrafiltration membrane (3), is either pressed against the latter (case of Figure 1), or be removed by the presence a second seal (4 '). At the base of the stack is the lower compartment (6) intended to receive the purified effluent (s) it supports the whole of a module (1) and is separated from the adsorpton memrane (5) by a joint (4 ").

 The respectively lower and upper faces of the compartments (2) and (6) are pierced with slots (7) allowing better flow and good distribution of the fluid on the one hand on the surface of the ultrafiltration membrane (3) and d on the other hand at the entrance to the lower compartment (6).

 The effluent to be treated, loaded for example with various organic compounds and minerals, enters the upper part of the module (1) by the end piece (8) placed, for example, on the rear face of the upper compartment (2 ). n is distributed throughout the entire compartment (2) and, under the action of a pressure applied by pump or compressor systems (not shown in FIG. 1) (Relative pressure from 1 to 15 bars for example), is forced to pass through the ultrafiltration membrane (3). The fraction of the effluent (e) comprising the fluid itself and the compounds of size smaller than the pore diameters of the ultrafiltration membrane (3) passes through it and constitutes the permeate. The concentrate (c), which contains the repressed compounds (molecules with high molecular weight, microorganisms. particles, colloids, etc.) leaves the upper compartment (2) by a second end piece (9), preferably located on the face opposite to that which supports the end piece (8). This ultrafiltration concentrate (c) is either eliminated or recycled to pass once or several times in the upper compartment (2) in contact with said ultrafiltration membrane (3).

 As for the permeate, it then comes into contact with the second membrane (5), based on activated carbon, which acts as an adsorbent for molecules with low molecular weight remaining in the fluid. This porous membrane (5) is traversed by the fluid which arrives purified in the lower compartment (6) and is then evacuated by the nozzle (10).

 Figure 2 shows a device according to a second variant of the invention.

The filtration membrane - here ultrafiltration - consists of a bundle of hollow fibers, that is to say cylindrical ultrafiltration membranes (11), retained by a macroporous coating (12), also of cylindrical shape. The second membrane (5) based on activated carbon, is a cylindrical envelope surrounding the bundle of hollow fibers (11), the assembly being disposed inside a compartment (13) receiving the purified fluid (s) and discharging it through the end piece (10).

 As in the device according to the first variant of the invention, the effluent (e) penetrates inside each hollow fiber (11), migrates in the direction of the arrows (from left to right in FIG. 2), and releases the permeate perpendicular to the periphery of each fiber. This permeate crosses the wall (12) then the second adsorbent membrane (5) to arrive in the final compartment (13). The concentrate (c) comes out to the right of the module (1). This can be recycled (following a recycling loop not shown).

Example 1:
The aqueous effluent contains humic substances at a concentration of the order of 50 mg C / 1 and a micropollutant (phenol) at a concentration of 100 mg / l. This corresponds to a mixture of molecules of high molecular weight (humic substances 1000 <M <100,000) and low molecular weight (phenol
M = 94). This effluent is ultrafiltered on an organic membrane (cellulose acetate type) at a relative pressure of 1 bar, then passed over a microporous activated carbon felt with a specific surface of 1800 m2 / g and 15 cm thick. The device used for this test is as shown in Figure 1.

 A series of ultrafiltration tests on membranes with different cutoff thresholds gave the results presented in Table 1.

Cut-off threshold 1000 5000 8000 10,000 50,000% carbon removal 95 94 85 78 65 humic substances Phenol eliminated (S s 0 0 OO
Table 1: Results of ultrafiltration tests of the mixture of Example 1
If the humic substances are very well stopped by the ultrafiltration membrane for cutoff thresholds of the order of 1000 to 5000, it is not the same for the phenol which is is not eliminated by this process. The few percent eliminated during the crossing of the membrane of 1000 daltons can come from a certain adsorption on the ultrafiltration membrane. On the other hand, the passage of the ultrafiltrate on the membrane of felt of activated carbon at a speed of approximately 2 m / h gives a total adsorption of the phenol (elimination of 100%) up to the maximum adsorption capacity of the felt which is found of the order of 130 mg phenol / g of activated carbon.

Example 2:
Under the same operating conditions as in Example 1, an aqueous effluent containing atrazine at 500 Rg / l, colloidal materials and suspended materials giving a turbidity of approximately 20 NTU was analyzed after treatment by coupling. ultrafiltration - activated carbon fibers. The cutoff threshold for the ultrafiltration membrane was 10,000 daltons. The results show a total elimination of turbidity (<0.1 NTU) and a residual concentration of atrazine estimated at 2, ug / l. These results were constant until the saturation of the activated carbon fibers which intervened for an amount of approximately 150 mg atrazine / g of activated carbon.

 This system as presented in the invention makes it possible to eliminate all of the pollution present in the effluent (small and large molecules). In addition, unlike the old methods which used granular or powdered activated carbon in the recirculation loop, there is no adsorption competition in the raw effluent. Each process (filtration and adsorption) is here effective separately. In addition, the assembly does not require the addition of particulate activated carbon (in grain or powder) and is therefore very compact.

Example 3.

The device used is as shown in FIG. 2. It comprises a tube with a circular section 2.4 cm in diameter and 0.9 m long, perforated longitudinally with a bundle of 19 hollow fibers (diameter 3, 5 mm and length 0.90) lined internally with the filter membrane (thickness 10 to
15, ut). Several qualities of ultrafiltration membranes can be used depending on their pore diameter between 0.05 and 3 Rm. In this example. an ultrafiltration membrane is used, the pore diameter of which is 0.1 μm. The tube. constituting the macroporous coating of the ultrafiltration fibers, is covered by a felt sleeve of microporous activated carbon with a specific surface of 1800 m2 / g. The thickness of the felt is 1.5 cm.

 The effluent to be treated is identical to that presented in Example 1. It comprises humic substances and phenol in aqueous solution.

 The operating conditions are as follows: relative pressure 3.6 bars, pH = 6.5 and the speed of recirculation in the ultrafiltration loop is 4.57 m / s.

 Under these conditions, a total disappearance of the organic carbon in the purified effluent (s) is obtained, which clearly indicates that the device according to the invention makes it possible to eliminate a wide range of soluble molecules present at different concentrations in the water to be treated. .

Claims (10)

1. Method for eliminating organic or mineral compounds from gaseous or aqueous effluents, characterized in that it comprises a membrane filtration step (microfiltration, ultrafiltration or nanofiltration) followed by the passage of the penneat on or through at least one membrane based on activated carbon. 2. Method according to claim 1 characterized in that the membrane filtration is ultrafiltration. 3. Device for implementing the process for removing organic or mineral compounds from gaseous or aqueous effluents according to any one of claims 1 or 2 characterized in that it comprises one or more first membrane (s) ( s) filtration (3, 11) receiving the effluent (e) to be treated and delivering a permeate practically free of substances of high molecular weight, said first filtration membrane (3, 11) being coupled to at least a second membrane based on activated carbon (5) adsorbing residual pollutants from the filtration permeate. 4. Device according to claim 3 characterized in that the first filtration membrane (s) (11) are tubular and the (or the second (s) membrane (s) of activated carbon (5) envelope (s) each or l of the first filtration membranes. (ll) 5. Device according to any one of claims 3 or 4 characterized in that the (or the) second (s) membrane (s) of activated carbon (5) is (or are) of cylindrical shape. 6. Device according to any one of claims 3 or 4 characterized in that the (or membranes) of activated carbon is (or are) arranged (s) in a spiral around the (or) first (s) membrane ( s) filtration (11). 7. Device according to claim 3 characterized in that the first (3) and second membranes (5) are planar. 8. Device according to any one of claims 3 to 7 characterized in that the (or) second (s) membrane (s) of activated carbon (5) is (or are) made up of woven activated carbon fibers or pressed. 9. Device according to any one of claims 3 to 7 characterized in that the activated carbon membrane is a polyether or polyester foam loaded with grains of activated carbon retained by a binder. 10. Device according to any one of claims 3 to 7 characterized in that the activated carbon membrane is a porous surface formed from an activated carbon powder and a binder.