Water treatment process on membranes integrating adsorption on micro-grain activated carbon.

Abstract

A method of treating water with a view to reducing its content of organic matter, micropollutants and pathogens, comprising supplying water to be treated directly into a membrane reactor containing at least one filtration membrane and an adsorbent material, stirring the mixture of water and adsorbent material and extracting the treated water, characterized in that the adsorbent material consists of micro-grains of activated carbon having a real density of at least 0.45, a settling speed of 30 to 50 m / H, a specific surface area of ​​400 to 2500 m2 / g and an average particle size of between 600 and 1300 μm, less than 5% by volume of said grains having a size of less than 400 μm, in that the concentration of micro-grains of activated carbon in the reactor is maintained between 5 and 100 g / L, in that no other material granular or particulate other than micro-grains of activated carbon is not used within the reactor,the agitation of said mixture of water and micro-grains within the reactor being at least partly carried out by injecting air into the mixture at a rate of 30 to 60 Nm3 / m2 / H and being sufficiently significant to avoid the deposition of micro-grains of activated carbon on said at least one filtration membrane. Figure 1,

Description

Title of the invention: Water treatment process on membranes integrating adsorption on micro-grain activated carbon. Technical field

[0001] The present invention relates to the field of water treatment. More specifically, the invention relates to a method for treating water with a view to reducing its organic matter content, including, where appropriate, its micropollutant content (pesticides, endocrine disruptors, drug residues, industrial product residues, etc.) and with a view to eliminating pathogenic agents (viruses, bacteria and parasites). The method according to the invention falls within the framework of water treatment methods using a membrane reactor. The method according to the invention finds its application in particular in the field of water purification, in the field of tertiary treatment of wastewater and in the field of treatment of industrial water with a view to its discharge into the natural environment or its reuse.

[0002] Prior art Various processes are implemented on wastewater or natural water to ensure its purification or purification. Their objective is to rid the water, in whole or in part, of organic matter, micropollutants, microorganisms, suspended matter, etc. that it contains.

[0003] Such methods frequently implement a coagulation-flocculation step in a first reactor followed by an adsorption step on an adsorbent powdery material in a second reactor. Powdered activated carbon (PAC) is, according to a definition commonly accepted by those skilled in the art, made up of particles having an average size of between 5 pm and 50 pm, preferably between 15 pm and 25 pm. This material is commonly used as an adsorbent material because it has high adsorption capacities. The PAC and the coagulation-flocculation reagents are then separated from the treated water.

[0004] The coagulation-flocculation and adsorption stages make it possible to considerably reduce the pollution contained in the water. However, these processes require infrastructure, in particular civil engineering, which is significant and therefore expensive. In addition, such processes imply the need to finely control the conditions of their implementation, in particular with regard to the PAC concentrations within the separator and the management of losses of this material in the underflow of the latter in order to minimize the quantities of new PAC to be added to minimize these losses. All these elements negatively impact the overall performance of such water treatment facilities and the cost of producing treated water. Other methods involve bringing the water to be treated into contact with PAC and then separating the treated water from the PAC by membrane filtration. The membranes used in this context may include nanofiltration, ultrafiltration or microfiltration membranes.

[0005] Although effective, these methods have the disadvantage that the PAC they use clogs the pores of the membranes or forms a cake on their surface. These thus tend to clog progressively, inducing an increase in pressure drop and a reduction in filtration flows through the membranes, and consequently an increase in the necessary energy consumption. The clogging of the membranes by the PAC, which can be partly irreversible, also tends to reduce the lifespan of the membranes. To combat the clogging of submerged membranes, it is therefore necessary to implement regular washing of these membranes, mainly using chemical products such as chlorine or alkaline solutions. However, such solutions have the disadvantage of being potentially harmful to the environment and there is a need to reduce their use. In addition, they also reduce the adsorption capacity of PACs. Finally, these chemicals induce premature aging of the membranes.

[0006] It should also be noted that it is common to inject air near the membranes to unclog them. A disadvantage of this type of technique is that the CAP tends to cause mechanical wear of the surface of the membranes, particularly when air is injected near them. The first consequence of this wear phenomenon is to reduce the lifespan of the membranes and, consequently, the efficiency of the filtration. It is therefore necessary to replace these membranes often, which requires the shutdown of the installations.

[0007] In order to solve this technical problem consisting of the abrasion of the filtration membranes by the CAP, it was proposed in FR3015463Al to use, in the membrane reactor, polymer particles having an average diameter of between 1 mm and 5 mm and a density of between 1.05 and 1.5 at a concentration of between 1 g / L and 10 g / L and concomitantly to stir the mixture consisting of water, CAP and polymer particles within the membrane reactor.

[0008] According to this technique, the agitation of the medium containing them near the membranes allows the polymeric particles to create a protective screen on them preventing the CAP from sticking to their surface. This protective screen makes it possible to avoid, or at least to limit, the clogging of the pores of the membranes by the CAP and at the same time makes it possible to prevent the CAP from rubbing against the surface of the membranes. filtration. The phenomenon of membrane wear is thus largely contained while preventing their clogging.

[0009] Thus, according to this technique, a material distinct from activated carbon, namely polymeric particles, is used to prevent the formation of a filter cake and to protect the filtration membranes and avoid direct contact between the membranes and the CAP.

[0010] It should be noted that this technique can integrate the supply of ozone, either upstream of the membrane reactor or directly in it, for example in water purification systems. The interest of ozone is not only linked to the coupling of the strong oxidizing power of ozone with the large adsorption capacities of activated carbon, but also to the acceleration of the decomposition of ozone into hydroxyl radicals by activated carbon.

[0011] While this technique is highly effective, it also has some drawbacks.

[0012] Indeed, the CAP has the disadvantage of not being able to be effectively regenerated thermally, that is to say that in the current state of knowledge, it is not possible, under economically feasible conditions at the industrial stage, to treat the CAP saturated with adsorbed compounds in order to restore its initial adsorption capacities. Even if it can be envisaged to roughly clean the CAP, for example by passing the water containing it through a hydrocyclone, and to recycle the CAP thus cleaned, the adsorption capacities of the CAP are exhausted fairly quickly and it is necessary to replace it regularly.

[0013] The mixture of water and spent PAC extracted in the form of sludge from the installations must undergo treatment to at least fix the pollution it contains and preferably eliminate this pollution. Such treatment generally includes thickening and dehydration which lead to significant quantities of solid residues. These solid residues must then preferably be subjected to additional treatments to degrade the pollution they contain. These different treatment stages increase the operating costs of the sectors.

[0014] Furthermore, even if this technique makes it possible to limit their number, chemical washes of the membranes must nevertheless be carried out regularly in order to maintain their performance. As indicated above, for environmental reasons, it is desirable to reduce the quantities of chemicals used for such washes.

[0015] Finally, other disadvantages can be observed when this technique is implemented in the presence of ozone. Whether the ozone is injected into the water to be treated before it arrives in the membrane reactor or directly into it, the transfer of ozone into the water is often not complete and ozone can be found in the sky. gaseous from the membrane reactor. To protect the health of operators, it is then necessary to cover the membrane reactor and add an ozone destructor to the vent. In addition, to reduce the residual ozone molecules in the water leaving the ozonation reactor, a reducer, such as sodium bisulfite, must be added. In addition, the physical separation of the injection and adsorption stages implies a high contact time between the water to be treated and the ozone, which creates favorable conditions for the formation of ozonation by-products, such as bromates, which begin to form after 2 to 3 minutes of contact, but also by-products from the organic matter present in the water, such as N-nitrosodimethylamine (NDMA). These by-products are not always adsorbable on the activated carbon particles and can accumulate in the water leaving the adsorption stage. It is therefore important to prevent their formation. Objectives of the invention

[0016] An objective of the present invention is to provide a water treatment method for reducing the content of dissolved organic pollution and micropollutants in this water by adsorption on activated carbon which significantly reduces the quantities of new activated carbon which have to be used.

[0017] Another objective of the present invention is to provide such a method which makes it possible to significantly reduce the quantity of solid residues produced, such as sludge consisting of activated carbon and adsorbed materials requiring costly subsequent treatment steps, for example thickening and dehydration.

[0018] Yet another objective of the present invention is to describe such a method which, in at least some of its embodiments, makes it possible to reduce the frequency of washing the membranes with chemicals and therefore the consumption of such chemicals.

[0019] Yet another objective of the present invention is to disclose such a method which makes it possible, in at least some of its embodiments, to prevent the formation of harmful by-products from ozone and therefore to avoid such by-products ending up in the treated water. Detailed description of the invention

[0020] These various objectives, or at least some of them, are achieved thanks to the invention which relates to a method for treating water with a view to reducing its content of organic matter, micropollutants and pathogenic agents comprising:

[0021] a step of supplying water to be treated via a pipeline directly into a membrane reactor containing at least one submerged filtration membrane,

[0022] a step of bringing said water into contact with an adsorbent material in said membrane reactor,

[0023] a step of filtration by said at least one immersed membrane of said water containing said adsorbent material in said membrane reactor,

[0024] stirring said mixture of water and adsorbent material within said membrane reactor during said filtration step,

[0025] a step of extracting treated water,

[0026] characterized in that:

[0027] said adsorbent material consists of micro-grains of activated carbon having a real density of at least 0.45, a settling speed of 30 to 50 m / H, a specific surface area of ​​400 to 2500 m2 / g, preferably between 1500 and 2500 m2 / g, and an average particle size of between 600 and 1300 qm, less than 5% by volume of said grains having a size of less than 400 qm,

[0028] in that the concentration of said micro-grains of activated carbon in said membrane reactor is maintained between 5 and 100 g / L, preferably between 5 and 50 g / L,

[0029] in that no other granular or particulate material other than the micro-grains of activated carbon is used within said reactor,

[0030] the agitation of said mixture of water and said micro-grains of activated carbon within said membrane reactor during said filtration step being at least partly carried out by injecting air into said mixture at a rate of 30 to 60 Nm3 / m2.H and being sufficiently significant to avoid the deposition of micro-grains of activated carbon on said at least one filtration membrane.

[0031] The micro-grains of activated carbon used in the context of the present invention are available on the market. In terms of particle size, they do not correspond to the classic definition of powdered activated carbon (PAC) - which, as indicated above, according to a definition commonly accepted by those skilled in the art, consists of particles having an average size of between 5 μm and 50 μm, preferably between 15 μm and 25 μm -, nor to the classic definition of granular activated carbon (GAC) - which, according to a definition commonly accepted by those skilled in the art, consists of carbon particles having an average size of between 1 mm and 3 mm -. Thus, such micro-grains have an average particle size smaller than that of GAC and much larger than that of PAC.In this regard, "average particle size" means the particle size for which 50% (by volume) of the particles are larger and 50% (by volume) of the particles are smaller.

[0032] They also have specific surfaces of the same order as those of the CAP and greater than those of the CAG which allow excellent adsorption of organic matter and micropollutants. They also have the advantage of being self-draining and therefore of being able to be drained very easily and very quickly, for example using simple filter bags, after having been used in a mixture with a water to be treated. Unlike powdered activated carbon, they also have the advantage of being easily regenerated using processes, generally thermal, which allow the desorption and mineralization of organic matter and micropollutants adsorbed on their surface.

[0033] According to the invention, the micro-grains of activated carbon used as an adsorbent material to adsorb the organic matter surprisingly make it possible to avoid the concomitant use of polymeric particles to avoid clogging and abrasion of the organic membranes. As indicated previously, according to the prior art, powdered activated carbon is a material known to have the cumulative disadvantages of clogging the pores of membranes and damaging them. Those skilled in the art were therefore encouraged to ensure that the membranes were protected from direct contact with the activated carbon.

[0034] However, the solution proposed here completely contradicts such an incentive. Indeed, according to the invention the membranes are indeed exposed to direct contact with the activated carbon.

[0035] The inventors found that, surprisingly, the micro-grains of activated carbon selected for the implementation of the invention not only had an average size large enough not to block the pores of the membrane but above all that the medium consisting of the stirred mixture of water and these micro-grains did not cause them to be deposited on the surface of these membranes in the form of a cake which would clog them as a person skilled in the art might expect.

[0036] Despite the direct contact of the membranes with activated carbon, the disadvantages of the prior art inherent in such contact do not occur in the context of the method according to the invention. This result is due to the characteristics of the invention according to which the activated carbon is used in the form of micro-grains having specific characteristics of particle size, specific surface area, settling speed and density and according to which the mixture of water and micro-grains is sufficiently agitated to prevent the micro-grains of activated carbon from being deposited on the membranes.

[0037] It follows that, thanks to the method according to the invention, it is much less often necessary to carry out membrane washing operations than in the techniques of the prior art and in particular that according to FR3015463A1.

[0038] Furthermore, since the quantities of micro-grains of activated carbon deposited on the membranes are very small, the majority of these micro-grains continue to fulfil their adsorption function for a maximum period of time. Consequently, savings are made on the quantities of activated carbon that have to be used compared to techniques using powdered activated carbon, which has to be renewed less often than in these prior art processes.

[0039] Thus, thanks to the method according to the invention, the quantities of exhausted activated carbon and the suspended matter from the washing of the membranes to be purged from the reactor is less significant than in the prior art techniques, in particular that according to FR3015463A1. Moreover, unlike PAC, micro-grain activated carbon is regenerable, the quantities of dry residues from the treatment of this sludge are much less significant.

[0040] It will be noted that the injection of air into the reactor may be intermittent but that it will preferably be continuous so as to continuously contribute to the preservation of the integrity of the membranes.

[0041] In a particularly advantageous embodiment of the invention, the method according to the latter further comprises a step of injecting ozone into the water passing through said pipe supplying water to be treated to said membrane reactor, said injection step being carried out by a venturi effect injector. The use of ozone makes it possible to oxidize some of the molecules present in the water to be treated. In particular, it improves the elimination of endocrine disruptors and drug residues. Ozonation also makes it possible to fragment large organic molecules into smaller molecules, which facilitates their subsequent adsorption and elimination. Finally, ozonation makes it possible to eliminate certain algal toxins or malodorous molecules. The inventors also noted that, in the context of the process according to the present invention, the addition of ozone allowed a reactivation of the adsorption sites of the micro-grains of activated carbon and thus contributed to optimizing the effectiveness of this material.

[0042] According to this preferred characteristic of the invention, ozone injection is carried out by suction generated by a Venturi effect, which has the advantage of avoiding any leakage of ozone into the atmosphere and allows it to be mixed with the water. The Venturi effect is a suction effect generated by a moving fluid which undergoes a depression. Thus, by the Venturi effect, the water to be treated undergoes a depression which will allow the ozone to be sucked into the water. Thanks to this technique, all of the injected ozone is incorporated and mixed into the water to be treated. It is therefore possible to use reduced quantities of ozone compared to the techniques known from the prior art.

[0043] Preferably, the ozone is injected at a rate of 0.5 to 10 mg / L, preferably 1 to 3 mg / L.

[0044] According to a variant of the present invention, a step of recirculating the mixture of water and said micro-grains of activated carbon in said membrane reactor contributes, alongside the injection of air into the reactor, to the agitation of said mixture.

[0045] According to a preferred variant of the invention, the concentration of said micro-grains of activated carbon in said membrane reactor is maintained between 5 and 100 g / L, preferably between 5 and 50 g / L.

[0046] Preferably, the method also comprises a step of extracting said reactor of used micro-grain activated carbon, and a step of draining this used activated carbon and a step of regeneration of the drained activated carbon.

[0047] The micro-grain activated carbon used in the context of the present invention has the advantage, when it is used, that is to say saturated with adsorbed organic matter, of being able to be freed from more than 80% of the water it contains by simple draining, and therefore of having, after this simple draining, a humidity level of less than 20% by weight. Such simple draining can in particular be carried out using bags. The micro-grain activated carbon thus drained can then undergo a regeneration phase, preferably by thermal means in order to restore most of its original adsorption capacities, and to be able to be reused in the context of the process according to the invention. The supply of new material is thus limited, which contributes to reducing the costs of implementing the process according to the invention compared to the costs of the processes of the prior art. Description of an embodiment

[0048] The method will now be described in more detail using the following description of a non-limiting embodiment thereof given with reference to:

[0049] [Fig.l] which schematically represents an installation allowing the implementation of the method according to the invention. According to [Fig.l], the installation comprises a pipe 1 for supplying water to be treated opening into a reactor 4 housing a membrane filtration module.

[0050] Ozone injection means, more precisely a Venturi effect injector 2, make it possible to inject ozone O3 into the pipe 1 supplying the water to be treated, in order to subject this water to an ozonation step. This ozonation step makes it possible to oxidize the pollutants contained in the water to be treated. It also makes it possible to fragment the macromolecules, facilitating their adsorption on an adsorbent powder material.

[0051] The filtration module contained in the reactor 4 is composed of immersed membranes 5 made of organic material MYCRODYN BIO-CEL ®. It will be noted that, depending on the embodiments, the membranes may be microfiltration or ultrafiltration or nanofiltration membranes.

[0052] Means 3 for supplying an adsorbent material are provided in the upper part of the reactor 4 and make it possible to supply therein an adsorbent material intended to adsorb the organic matter present in the water to be treated.

[0053] According to the invention, this particulate adsorbent material consists of micro-grains 6 of activated carbon having:

[0054] - a real density of 0.45; - a settling speed of 30 to 40 m / H; - a specific surface area of ​​1500 to 2500 m2 / g, - an average particle size between 600 pm and 1300 pm, less than 5% of said grains having a size less than 400 pm,

[0055] This micro-grain activated carbon is present in the reactor 4 in a quantity determined such that the concentration of micro-grains of activated carbon in the membrane reactor 4 is between 5 g / L and 50 g / L.

[0056] The installation also comprises means 7 for injecting air into the reactor 40. These injection means here comprise an injection ramp 8 located in the lower part of the reactor 4, under the membrane filtration module, connected to an air supply network (not shown). In the context of the method according to the present invention, these means are used to bring air into the reactor 4 at a rate of 50 Nm3 / m2.H. The injected air makes it possible to suspend the granular activated carbon in the water to be treated so that its distribution is essentially uniform within the reactor 4 but also to agitate the mixture of water and micro-grains.

[0057] The combined use of micro-grains of activated carbon 6 and adequate agitation of the water mixture containing them makes it possible, on the one hand, to prevent the deposition of these micro-grains on the surface of the membranes but also to gently detach the organic matter which may have been deposited on the surface of the membranes, without damaging them.

[0058] Water mixed with activated carbon passes through the filtration module in order to separate the treated water from the activated carbon on which the organic matter present in the water is adsorbed. The treated water is then evacuated through a pipe 9 equipped with a pump 10. The installation also includes a recirculation loop 11 on which there is a recirculation pump 12 and a purge 13. The purge 13 makes it possible to evacuate from the reactor 4 the excess sludge consisting of micro-grained activated carbon weighed down by the organic matter which has adsorbed there.

[0059] The inlet of the recirculation pipe 11 is located in the upper part of the reactor 4 while its outlet opens into the lower part thereof, thus forming a recirculation loop. This loop makes it possible to recirculate, at least in part, the mixture of water and micro-grain activated carbon contained in the reactor inside the latter, this recirculation makes it possible to generate additional agitation in the reactor.

[0060] In this embodiment, the stirring inside the reactor is maintained continuously.

[0061] By using micro-grains of activated carbon meeting the characteristics indicated above and correctly stirred, it is thus possible to prevent damage to the membranes by the activated carbon and to prevent the activated carbon from being deposited in the form of a cake on the membranes. It is therefore possible to proceed less often to chemical washing of these and thus to use less chemicals for this purpose. The use of micro-grain activated carbon leads to the production of less sludge than processes using activated carbon and polymer beads. Indeed, this micro-grain activated carbon is easily regenerated. It can also be recycled more in the reactor, thanks to the injection of ozone into the water to be treated, this compound allowing, as indicated above, to reactivate the adsorption sites of this material.

Claims

Claims

1. A method of treating water in order to reduce its content of organic matter, micropollutants and pathogens comprising: a step of supplying water to be treated via a pipeline directly into a membrane reactor containing at least one immersed filtration membrane, a step of bringing said water into contact with an adsorbent material in said membrane reactor, a step of filtering by said at least one immersed membrane said water containing said adsorbent material in said membrane reactor, stirring said mixture of water and adsorbent material within said membrane reactor during said filtration step, a step of extracting treated water, characterized in that: said adsorbent material consists of micro-grains of activated carbon having a real density of at least 0.45, a settling speed of 30 to 50 m / H, a specific surface area of ​​400 to 2500 m2 / g,preferably between 1500 and 2500 m2 / g and an average particle size of between 600 and 1300 qm, less than 5% by volume of said grains having a size of less than 400 qm, in that the concentration of said micro-grains of activated carbon in said membrane reactor is maintained between 5 and 100 g / L, preferably between 5 and 50 g / L, in that no other granular or particulate material other than the micro-grains of activated carbon is used within said reactor, the agitation of said mixture of water and said micro-grains of activated carbon within said membrane reactor during said filtration step being at least partly carried out by injecting air into said mixture at a rate of 30 to 60 Nm3 / m2.H and being sufficiently high to avoid the deposition of micro-grains of activated carbon on said at least one filtration membrane.,

2. Method according to claim 1 characterized in that it comprises a step of injecting ozone into the water passing through said water supply pipe to be treated to said membrane reactor, said injection step being carried out by a Venturi effect injector.

3. Method according to claim 2 characterized in that the ozone is injected at a rate of 0.5 to 10 mg / L, preferably 1 to 3 mg / L.

4. Method according to one of claims 1 to 3 characterized in that said injection of air into the membrane reactor is continuous.

5. Method according to one of claims 1 to 3 characterized in that said injection of air into the membrane reactor is sequenced.

6. Method according to one of claims 1 to 5 characterized in that it comprises a step of recirculating the mixture of water and said micrograins of activated carbon in said membrane reactor contributing to the agitation of said mixture.

7. Method according to any one of claims 1 to 6, characterized in that it comprises a step of extracting from said reactor the used micro-grain activated carbon, a step of draining this used activated carbon and a step of regenerating the drained activated carbon.

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