Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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.
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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.
2200304
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
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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
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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
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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.
