CA2200304A1

CA2200304A1 - Compound removal using membrane and activated charcoal filtration

Info

Publication number: CA2200304A1
Application number: CA002200304A
Authority: CA
Inventors: Pierre Le Cloirec; Bernard Delanghe
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-09-27
Filing date: 1995-09-26
Publication date: 1996-04-04
Also published as: EP0784500A1; AU695619B2; JPH10506323A; FR2724849A1; FR2724849B1; AU3569895A; WO1996009877A1

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

~ ~ G ~ 3 ~ 4 Method and device for compound removal using membrane and activated charcoal ffltration.

The field of the invention is that of the treatment and the purification of S waters. More specifically, the present invention relates to a process for removing organic or inorganic compounds from aqueous effluents, and a device for its implementation, combining membrane filtration and adsorption onto activated charcoal.
Membrane filtration, especially ultrafiltration, is a technique which is 10 presently being used more and more for retaining high molecular weight molecules, bacteria or fine particles contained in aqueous or even gaseous efflnents, these polluting m~teri~l~ not being retained by usual filters. This technique generally intervenes at the end of the effluent tre~tment or during refimng.
Furthermore, during the water treatment or purification, the removal of organic or inorganic pollutants is often completed by the introduction of activated charcoal, as grains or fine powder, into the circuit of the effluent to be purified.
The micropollutants are thus adsorbed onto this material. This adsorption alwaysintervenes upstream or at the same time as the conventional filtration step (on sand for example) so as to retain the activated charcoal particles on the filter.
The advantages of these two techniques have been highlighte~l recently by a water tre~tment process which uses a combination of activated charcoal particles and an ultrafiltration unit. The device used is e. g. described in the document of Adham S. S. et al., Predicting and verifying 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 d'eau potable par combinaison de traitements:
ultrafiltration sur membranes organiques et adsorption sur charbon actif en poudre in Récents Progrès en Génie des Procédés, Tech & Doc, Lavoisier, 1991, S, 15, ,135-138. In these devices, the granular or powdered activated charcoal is either introduced into the unpuliried water supply, or is placed in the ultrafiltration concentrate recirculation loop where it adsorbs micropollutants, it is then retained by the ultrafiltration membrane.
Although such a process enables both organic materials and micropollutants to be stopped, the method does have disadvantages in that it is not possible for it to permit a simple and efficient regeneration of the activated charcoal whose pores are rapidly saturated or blocked in the presence of a ~ 0 3 0 4 concentrate of pollutant materials. The adsorptive capacity of the activated charcoal is therefore rapidly met and the adsorbant must be renewed frequently.
On the other hand, the fine activated charcoal particles (derived from the powder or from abrasion of the grains) also block the pores of the ultrafiltration 5 membrane or they damage its surface by abrasion, rendering it rapidly and permanently useless.
A particular object of the present invention is to alleviate these disadvantages.
More specifically, a first object of the invention is to find a process and to 10 implement a device for removing organic or inorganic compounds from aqueous effl~lent~ which allow both an efficient retention of high molecular weight molecules and the adsorption of smaller molecules.
Another object of the present invention is to avoid the blocking and the abrasion of the filtration membranes by the particles of the adsorbant.
These objects, along with others which will appear in what follows, are met by virtue of a process which consists on the one hand in using not free particles of activated charcoal (grains or powder), but larger surfaces, i. e.
activated charcoal in the form of membranes (woven or pressed fibres, or grains fixed onto a porous material), and on the other hand in placing these adsorbant membranes not upstream of the ultrafiltration, but downstream of it in order to treat the permeate(s) therefrom.
The process of organic or inorganic compound removal from aqueous effluents according to the invention is therefore characterised in that the aqueous effluent is firstly submitted to a membrane filtration step consisting in the passage of the ~ffl~lent through at least one first membrane capable of retaining high molecular weight substances excee-ling a cut-off threshold, and the permeate origin~ting from the membrane filtration step is placed in contact with at least one second membrane, constituted at least in part of activated charcoal, so as to adsorb residual substances of low molecular weight contained in the permeate.
Preferably, the membrane filtration is ultrafiltration.
Conventional filtration is understood to mean the retention of particles of size greater than several micrometers in suspension. Membrane filtration allows three types of separation according to the size of the compounds. If the membrane retains particles whose size is of several micrometers (e. g. bacteria of about 1 to 2 ,um dimension), microfiltration is used. The term ultrafiltration, for which thediameters of the pores of the membrane are between 0.001 and 0.1 ,um, is reserved 3 ~ 3 n 4 for the separation of compounds whose molecular mass is from 10,000 to 100,000 (or even 2,000 to 300,000). Nanofiltration, still finer, relates to the separation of compounds whose molecular mass is greater than several tens of grams.
The use of activated charcoal in the form of a membrane saves post-5 filtering the effluent after adsorption. The activated charcoal particles are in factretained by their rigid structure (activated charcoal fibres or binder between the particles). These activated charcoal-based membranes may therefore be used without problem downstream of an Illtr~filtration step, in treating the permeate, and may consequently be used at the end of the treatment or purification chain.
Furthermore, this membrane filtration (e. g. ultrafiltration) p~rmc~te, made up of the effluent under treatment from which high molecular weight molecules, micro-org~ni~m.~, particles, etc.. have already been removed, is much less ladenwith organic or inorganic m~teri~ the adsorbant will therefore be able to play its role to the m~xi,,,~l,,, of its capacities without premature blockage either of its surface or of the pores of the activated charcoal-based membrane. In addition, the adsorption competition which may take place between the molecules or the ions atthe surface of the activated charcoal are from this fact appreciably reduced, even removed. The activated charcoal membranes also have the advantage of being easier to regenerate.
The device for implementing the process according to the invention is characterised in that it comprises at least one first filtration membrane receiving the aqueous effluent to be treated and having a cut-off threshold for retaining high molecular weight substances exceeding this threshold, and at least one second membrane, made at least in part from activated charcoal, receiving the membrane filtration permeate so as to adsorb low molecular weight substances which are contained therein.
The first membrane has in essence a filtralion function, particularly for retaining high molecular weight substances. The membrane receives the effluent to be treated, which passes through its pores, and provides a permeate which is in turn in contact with the second membrane. This second membrane, which is activated charcoal-based, has as function to adsorb the residual small moleculesand/or ions contained in the permeate.
The nature of the first micro-, ultra- or nanofiltration membrane is immaterial. It may be an inorganic membrane, or it may be an organic membrane such as of cellulose acetate or polyamide.

3 ~ 4 In an advantageous manner, the device according to the invention may comprise several membranes disposed in series or in parallel, of identical or different sizes and nature. It may advantageously comprise first filtration membranes which are placed in series and which are crossed by the effluent which5 comes into contact with pores whose diameters are increasingly reduced.
The microfiltration membranes retain particles in suspension, bacteria and colloids. Molecules of high molecular weight ranging from 2,000 to 300,000 are removed from the effluent by the ultra- and nanofiltration membranes.
In a first variant of the invention, the membrane(s) of filtration and that 10 (those) of adsorption based on activated charcoal are flat, it being possible for an activated charcoal-based membrane to be applied onto the downstream part of a first filtration membrane, or to be separated from it by an interstice (a gap). Such an interstice gives the permeate the time to undergo turbulence which thus favours the transfer speed of the compounds present in the fluid towards the adsorbant by 15 a reduction in the concentration gradient in the fluid. The thickness of the interstice may be between 1 mm and several centimetres.
In a second variant of the invention, the first filtration membranes form a cylindrical bundle of hollow fibres held together by a tie or a macroporous coating. The effluent penetrates each fibre at one end, migrates into the interior in 20 the direction of the length of the cylindrical fibre, and by pressure on the fluid, releases the permeate perpendicular to the circumference.
In this second variant, the adsorbant activated charcoal-based membrane(s) come to wrap the bundle of hollow fibres which form the first membranes or each fibre individually. Such a wrap may be for example cylin~lric~l or made up of flat 25 membrane(s) wound into a spiral.
According to a preferential embodiment of the device according to the invention, the second activated charcoal-based membrane is made up of activated charcoal fibres. These fibres are obtained for example by calcination and activation (in an oxidising medium at high temperature) of polyacrylonitrile 30 fibres. Such fibres may be woven or pressed in order to form membranes resembling a fabric or felt.
According to a second embodiment, the second activated charcoal-based membrane is a polyether or polyester foam filled with activated charcoal grains held together with a binder.

According to a third embodiment, the second activated charcoal-based membrane is a porous surface formed from an act;vated charcoal powder and a binder.
Other characteristics and advantages of the present invention shall appear 5 upon reading the following description of a preferential implementation means of the invention, given in an illustrative way and not in a limiting way, and annexed drawings, in which: -Figure 1 shows an exploded view of the device according to a first variant of the invention, in which both the filtration and adsorption membranes are flat, and Figure 2 shows a view of the device according to a second variant of the invention in which the adsorption membrane wraps a filtration membrane bundle.
Figure 1 represents a mixed ultrafiltration and adsorption unit on an activated charcoal-based membrane. The diagram of this figure shows a horizontalsuperimposition of the different constituent elements, but it is obvious that these elements may be disposed vertically.
This unit (1), which is parallelepipedic, is constituted here of an upper compartment (2) which receives the effluent (e) to be treated, separated at the lower part thereof from the ultrafiltration membrane (3) by a joint (4). The activated charcoal-based adsorption membrane (5), which is of the same dimensions as the ultrafiltration membrane (3), is either applied against the latter (case of figure 1), or is spaced apart from it by the presence of a second joint (4').
At the base of the stack a lower colllpalLlllent (6) is found which is intended for receiving the purified effluent(s): it supports the whole of a unit (1) and is separated from the adsorption membrane (5) by a seal (4").
The respective lower and upper faces of the compartments (2) and (6) are pierced with slits (7) which allow a better flow and a good distribution of the fluid, on the one hand to the surface of the ultrafiltration membrane (3) and on the other hand to the entrance in the lower compartment (6).
The effluent to be treated (e), laden for example with various organic and inorganic compounds, enters the upper part of the unit (1) via the nozzle (8) placed for example on the back face of the upper compartment (2). It is distributed in the whole of the compartment (2) and, under the action of a pressure applied by pump systems or compressors (not represented in Figure 1) (relative pressure of 1 to 15 bars for example), is forced to cross the ultrafiltration membrane (3). The fraction of the effluent (e) comprising the fluid itself and the compounds whose S~ 6 2 2 0 n 3 0 ~

size is smaller than the diameters of the pores of the ultrafiltration membrane (3) crosses this membrane and constitutes the permeate. The concentrate (c), which contains the compounds forced back (high molecular weight compounds, micro-org~ni~m~, particles, colloids, etc...) leaves the upper colllpalllllent (2) via a 5 second nozzle (9), situated preferably on the face opposite to that which supports the nozzle (8). This ultrafiltration concentrate (c) is either removed or recycled in order to pass one or several times again into the upper colllpalLlllent (2) withcontact with said ultrafiltration membrane (3).
As for the permeate, it then makes contact with the second activated 10 charcoal-based membrane (5) which plays the role of an adsorbant of low molecular weight molecules rem~ining in the fluid. The fluid which arrives purified in the lower compartment (6) crosses this porous membrane (S) and is then evacuated via the nozzle (10).
Figure 2 shows a device according to a second variant of the invention.
15 The filtration membrane - ultrafiltration here - is made up of a bundle of hollow fibres, i. e. of cylindrical llltr~filtration membranes (11), held together by amacroporous coating (12), which is also cylindrical. The second activated charcoal-based membrane (5) is a cylindrical Wldppil~g wound round the bundle ofhollow fibres (11), the whole being disposed within a colllpalllllent (13) which20 receives the purified fluid(s) and which evacuates it via the nozzle (10).
As in the device according to the first variant of the invention, the effluent (e) penetrates the interior of each hollow fibre (11), migrates in the sense of the arrows (from left to right in Figure 2), and releases the p~rme~te perpendicular to the circumference of each fibre. This permeate crosses the wall (12) and then the 25 second adsorbant membrane (S) in order to arrive in the final colllpalllllent (13).
The concentrate (c) comes out at the right of the unit (1) and may be recycled (following a recycling loop not represented).

l~xample 1:
The aqueous effluent contains humic substances at a concentration in the order of S0 mg C/l and a micropollutant (phenol) at a concentration of 100 mg/l.This corresponds to a mixture of high molecular weight compounds (humic substances 1000 < M < 100,000) and low molecular weight compounds (phenol M
35 = 94). This effluent is ultrafiltered on an organic membrane (cellulose acetate type) at a relative pressure of 1 bar, and is then passed onto a 15 mm thick microporous activated charcoal felt of specific surface 1,800 m2/g. The device used for this test is such as represented in Figure 1.
A series of ultrafiltration tests on membranes of different cut-off thresholds has given the results shown in Table 1.

Table 1: Results of the tests of ultrafiltration of the mixture of Example 1.

Cut-off threshold 1,000 5,000 8,000 10,000 50,000 % carbon removal from the humic substances 95 94 85 78 65 Phenolremoved (%) 8 0 0 0 0 Although the humic substances are very well stopped by the ultrafiltration membrane for cut-off thresholds in the order of 1,000 to 5,000, the same does not apply to the phenol, which is not removed by this process. The few percent removed during the crossing of the 1,000 Dalton membrane may arise from a certain adsorption onto the uL~pll,;ric~tion membrane. On the contrary, the passage of the ultrafiltrate onto the activated charcoal felt membrane at a speed of about 2 m/h gives a total adsorption of phenol (100% removal) up to the m~im~l adsorption capacity of the felt which is found to be in the order of 130 mg phenol/g of activated charcoal.

Example 2:
Under the same operating conditions as Example 1, an aqueous effluent cont~ining 500,ug/1 of akazine, colloidal materials and suspended m~teri~ which give a turbidity of about 20 NTU, has been analysed after treatment by the ultrafiltration-activated carbon fibres coupling. The cut-off threshold of the ultrafiltration membrane was 10,000 Daltons. The results reveal a total removal of the turbidity (<0.1 NTU) and a residual atrazine concentration estimated at 2,ug/1.
These results were constant up until the saturation of the activated charcoal fibres which intervened for an amount of about 150 mg atrazine /g of activated carbon.
This system, such as presented in the invention, enables the removal of the totality of the pollution present in the effluent (small and large molecules).
Furthermore, unlike the old processes, which used granular or powdered activatedcharcoal in the recirculation loop, no adsorption competition exists in the crude -2 ~ ~ ~ 3 0 ~

effluent. Each process herein (filtration and adsorption) is separately efficient.
Moreover, the whole does not require the addition of particulate (granular or powdered) activated charcoal and is therefore very compact.

Example 3:

The device used is such as represented in Figure 2. It comprises a tube of circular cross-section of 2.4 cm diameter and 0.9 m in length, perforated longitll-lin~lly by a bundle of 19 hollow fibres (diameter 3.5 mm and 0.90 length) woven intemally of the filtering membrane (10 to 15 ,um thick). Ultrafiltration membranes of several dirrel~nt qualities are usable as a function of their pore diameter in the range 0.05 and 3 ,um. In this example, an ultrafiltration membrane is used whose pore diameter is 0.1 ,um. The tube, which constitutes the macroporous coating of the ultrafiltration fibres is covered by a microporous activated charcoal felt sleeve of specific surface 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 includes humic substances and phenol in aqueous solution.
The operating conditions are the following: relative pressure 3.6 bars, pH =
6.5 and the recirculation in the ultrafiltration loop is 4.57 m/s.
Under these conditions, a total disappearance of organic carbon is obtained in the purified effluent(s) indicating clearly that the device according to the invention allows removing a wide range of soluble molecules present at various concentrations in the water to be treated.

Claims

1. Process for removing organic or inorganic compounds from an aqueous effluent, comprising a membrane filtration step and an adsorption step using activated charcoal, characterised in that the aqueous effluent is firstly submitted to a membrane filtration step consisting in passage thereof through at least one first membrane capable of retaining high molecular weight substances exceeding a cut-off threshold, and the permeate originating from the membrane filtration step is placed in contact with at least one second membrane, constituted at least in part of activated charcoal, so as to adsorb residual substances of low molecular weight contained in the permeate.

2. Process according to claim 1, characterised in that the membrane filtration is ultrafiltration.

3. Device for removing organic or inorganic compounds from an aqueous effluent, comprising means of feeding effluent water to be treated, membrane filtration means and means of adsorption by activated charcoal, characterised in that the means of membrane filtration comprise at least in part one first membrane connected to the feeding means so as to receive the effluent to be treated, and have a cut-off threshold for retaining high molecular weight substances exceeding this threshold, and the adsorption means comprise at least one second membrane made at least in part from activated charcoal, and are coupled to the membrane filtration means so as to receive therein a permeate, and to adsorb residual substances of low molecular weight contained in the permeate.

4. Device according to claim 3, characterised in that the or each filtration membrane is tubular and is surrounded by an adsorption membrane.

5. Device according to claim 3, characterised in that it comprises a collection of several tubular filtration membranes which collection is surrounded by at least one adsorption membrane.

6. Device according to any one of claims 4 to 5, characterised in that the or each adsorption membrane is cylindrical.

7. Device according to any one of claims 4 and 5, characterised in that the or each adsorption membrane is spirally wound up.

8. Device according to claim 3, characterised in that the filtration and adsorption membranes are flat.

9. Device according to any one of claims 3 to 8, characterised in that it comprises at least one adsorption membrane comprising a fibrous structure of activated charcoal fibres.

10. Device according to claim 9, characterised in that the fibrous structure is a fabric.

11. Device according to claim 9, characterised in that the fibrous structure is a felt.

12. Device according to any one of claims 3 to 8, characterised in that it comprises at least one adsorption membrane comprising a foam loaded with activated charcoal grains held together by a top.

13. Device according to any one of claims 3 to 8, characterised in that it comprises at least one adsorption membrane in the form of a porous structure comprising an activated charcoal powder and a binder.