NL1023474C2 - Method for removing a contaminant from an aqueous medium with the aid of a membrane. - Google Patents

Description

Title: Method for removing a contamination from an aqueous medium with the aid of a membrane

The invention relates to a method for removing a contaminant from an aqueous medium with the aid of a membrane.

Aqueous media, such as drinking water, groundwater and surface water, are often contaminated with inorganic and / or organic contaminants that are undesirable from a public health and / or environmental point of view. Due to, among other things, agricultural and industrial activities, such substances can end up on a large scale in groundwater and surface water and, consequently, also in the water that is involved, in particular in drinking water. Moreover, as a result of process steps such as oxidation in water purification, undesirable harmful by-products can be formed which must still be removed. Known techniques for the removal of at least a number of such contaminants are ion exchange and electrodialysis (for ionic contamination), reverse osmosis, nanofiltration (for relatively large molecules), oxidation / ozonization (for oxidisable compounds), adsorption on activated carbon, biological conversion and combinations thereof.

Biological treatment of aqueous media is relatively inexpensive and suitable for the removal of many types of contaminants. The biological conversion can be carried out, for example, in a biofilm of bacteria that are immobilized in a packed bed reactor, a tube reactor or a fluidized bed reactor.

Anaerobic or anoxic conditions are required for the degradation of many contaminants. Examples of such impurities are NO3 ', NO2', pesticides, chlorinated hydrocarbons, azo dyes, (per) chlorate and bromate.

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Moreover, an electron donor is required to reduce impurities. If this is insufficiently present, this I must be added to the stream to be treated. Examples of suitable electron donors are methanol, ethanol and acetic acid. It is proposed in US-B-6 387 262 to remove nitrate and / or nitrite from water by treatment in a hollow-fiber membrane which is provided with a biofilm. Hydrogen gas is supplied as a reducer for this purpose.

EP-A-1 178 017 describes a method for removing nitrate from water using catalyst particles which have a metal such as palladium on a porous particle I such as a porous oxide, an expanded polymer or ( preferably) carbon flakes with a size of a few mm. Hydrogen can be used as the reducer.

However, there is a need for alternative methods for the removal of organic and / or inorganic contaminants.

It has now been found that such an alternative is offered by a method for removing (in particular allowing the reaction to be eliminated by means of reduction) of a contamination from an aqueous medium with the aid of a membrane provided with a biofilm. and from an I specific catalyst.

The present invention therefore relates to a method for removing a contaminant from an aqueous medium with the aid of a membrane which is provided with a biofilm comprising an anaerobic bacteria zone and a catalyst which is capable of under the process conditions. catalyze conversion of oxygen and hydrogen to water, comprising - contacting hydrogen gas with the aqueous medium through the membrane; 3 - reducing at least one compound present selected from the group of oxygen, nitrate and nitrite with hydrogen on the catalyst to form water or nitrogen; and anaerobically converting the contamination by biological reduction with the hydrogen in the anaerobic bacteria zone.

Figure 1 shows schematically a membrane during the implementation of a method according to the invention (Figure 1A) and a membrane according to the state of the art, without catalyst (Figure 1B).

The biofilm usually comprises a zone with a very low oxygen concentration in which the anaerobic conversion takes place. This zone is called the anaerobic zone.

Anaerobic herein means that there is insufficient oxygen (in the form of O2) present on site to inhibit the biological reduction of contaminants. More in particular, anaerobic is understood to mean that the on-site oxygen concentration is less than 1 pg / l, more preferably 1 ng / l to 1 pg / l.

In addition, the biofilm often comprises a zone located between the bulk of the aqueous medium on the one hand and the anaerobic zone and the membrane on the other hand, in which the oxygen concentration lies between the oxygen concentration of the anaerobic zone and the bulk. This zone is called the aerobic zone.

A method according to the invention has been found to be very suitable for the treatment of groundwater, drinking water or surface water, and in particular for the treatment of an aqueous medium (such as already partially cleaned soil * or surface water) in the preparation of drinking water.

It has been found that in known methods for removing a contaminant from aqueous media, oxygen is often present in the aqueous medium - for example in the range of between 1 and 9 mg / 1 -30 which has a considerable adverse effect on the desired anaerobic organic

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4 conversion. It has been found that under such conditions in particular the removal of impurities to a concentration (far) below the oxygen and / or nitrate concentration becomes more difficult or impossible to achieve in a known method. It has now surprisingly been found that this problem can be overcome by using it. making a method according to the invention wherein the membrane is provided with a catalyst which catalyses the conversion of oxygen to water in the presence of the hydrogen. It has been found that as a result the oxygen concentration can be kept sufficiently low to remove one or more other impurities with good effectiveness in addition to or instead of nitrate and / or nitrite. For this reduction, microorganisms can use the H2 dosed through the membrane as an electron donor.

The invention is thus extremely suitable for the treatment of an aqueous medium, wherein the weight content of oxygen and / or nitrate in the aqueous medium in the bulk at the beginning of the conversion is higher than the weight content of contaminants to be converted, preferably at least 5 times as high, more preferably 10 to 1000 times as high.

The invention is very suitable for the removal of nitrogen oxides such as NO 3, NO 2 * and / or N 2 O.

The invention has been found to be extremely suitable for removing one or more reducible inorganic impurities, in particular at least one impurity selected from the group consisting of bromate, chlorate, perchlorate and sulfate.

In principle, the invention can be used to break down all types of aromatics contaminants, including those aromatics (i.e., hydrocarbons with at least one benzene ring) that appear on the so-called "black list" published by the Ministry of Health, Spatial Planning and the Environment. ("Target values and intervention values 5 soil remediation7", Dutch Government Gazette, No. 39, February 24, 2000, p.

8-16) which list is hereby deemed to be inserted.

The invention has been found to be very suitable for removing one or more reducible organic impurities, in particular carbon compounds with at least one substituent selected from the group consisting of halogen, sulfoxy, nitro and azo groups, more in particular chloroaliphates - such as per-, tri- and dichloroethylene, vinyl chloride, 1,1,1-trichloroethane, 1,1-dichloroethane, 1,2-dichloroethane, chloroform and tetrachloromethane and chlorobenzenes, chlorophenols and the like.

The invention is very suitable for removing one or more pesticides, in particular one or more pesticides selected from the group consisting of atrazine, ametrine, terbuthyrin, prometryne, cyanizine, propazine, simizine, simaton, cyromyzine, ammelidine, melamine, DCP (1,2-dichloropropane) bromacil, bentazone, mectroprop, amitrol, MCPA (4-chloro-2-methylphenoxy acetic acid), DDT (dichlorodiphenyltrichloroethane), methoxychloro, lindane and TBA (trichlorobenzoic acid).

By means of the invention, it has been found possible to reduce the weight content of at least one convertible contamination, for example bromate, from a content in the range of, for example, about 20-100 ppb, to a content of at most 10 ppb, preferably to a content in the range of about 0.1-1 ppb, for example about 0.5 ppb.

The particular advantage of a method according to the invention is furthermore the possibility of very effectively removing a contamination with a membrane on which a relatively thin biofilm is present in comparison with a known method, preferably with a thickness of approximately 0.01-100 µm at more preferably from about 1-100 µm, more preferably from about 10-100 µm. A thin biofilm means less biomass in the system and a lower diffusion resistance of the bulk to the anaerobic zone for the substances to be converted, which makes it possible to use a smaller membrane surface area, which requires lower investment costs. In addition, the release of I biomass to the stream to be treated can be lower (lower erosion).

I A hollow fiber membrane is very suitable as a membrane. Hydrogen gas is herein preferably passed through the lumen of the hollow fiber, whereby the gas can diffuse out through the porous wall and then comes into contact with the catalyst, the biofilm and the aqueous medium. An example of such a fiber in an aqueous medium that is being purified is shown schematically in Figure 1, in which a hollow fiber is shown whose wall 2 is hydrogen-permeable, has a wall thickness dm 10 and a central cavity (lumen) 1 where the hydrogen gas is passed through it. The curves indicated by "O2" and "contamination" I, respectively, schematically show the gradient of these components.

Figure 1A shows a membrane in a method according to the invention.

On the outside of fiber wall 2 there is a porous catalyst layer (shaded layer in zone 3) in which anaerobic bacteria are also present. This layer has a very low oxygen content relative to the bulk water stream 6 and here forms the so-called anaerobic zone 3. The oxygen content is kept sufficiently low during the process under the influence of the catalyst.

Subsequent to the anaerobic zone 3, which comprises the catalyst layer H, a so-called aerobic zone 4 with bacteria can be present in which the oxygen concentration is high relative to the anaerobic zone.

During a method according to the invention, this zone 4 may be eroded without H problems by the flowing bulk water. This makes high longitudinal flow rates possible.

The aerobic and anaerobic zone with bacteria together form the biofilm with db + thickness. In Figure 1B this is indicated by db. In a method with a membrane according to Figure 1B, the thickness of the biofilm, 7 and in particular the aerobic zone therein, is generally considerably greater. The anaerobic zone 3 "is free of a catalyst layer herein.

Adjacent to the aerobic zone is the hydro-dynamic boundary layer 5 with thickness δ, over which an oxygen gradient may still exist. Adjacent to the hydrodynamic boundary layer is the bulk water stream with a substantially gradient-free oxygen concentration.

The biofilm preferably extends over at least a portion of the membrane wall side that is closest to the aqueous medium 10 (usually the outside in the case of a hollow-fiber membrane), more preferably substantially the entire side. The thickness, specific activity and composition of the biofilm (thickness and specific activity / composition) will adjust itself if the correct nutrients (including the contamination) are present.

Suitable bacteria are generally naturally present in the liquid to be cleaned and contribute to a suitable biofilm without special measures being necessary.

Examples of bacteria of which at least one species is preferably present are Ralstonia eutropia (such as ATCC 17697), Pseudomana fluorescens, Xantthomonas maltophila, Flavobacterium indologues, Alcaligens eutrophus, Pseudomonas maltophila and Pseudomanas putrefaciens.

The anaerobic bacteria zone and the catalyst are preferably at least partially mixed with each other (or are intertwined) and are preferably located in or on the side of the wall of the membrane that is closest to the aqueous medium. It has thus been found that an extremely good fixation of the biofilm and / or the catalyst occurs, so that erosion of at least a substantially anaerobic zone of the biofilm and / or the catalyst is substantially suppressed or even (almost) completely prevented.

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The membrane is preferably based on a hydrophobic support material, in particular in the case of a porous I support material. Hydrophobic is defined herein as a material that has an edge angle with water of more than 90 degrees, preferably up to substantially 180 degrees. This generally means that force must be exerted (work done) for water to penetrate into the pores.

Very suitable is a membrane with a polymer selected from polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride I (PVDF) as carrier material for the catalyst and the biofilm.

The wall of the membrane is usually porous. The porosity can be selected within wide limits, in general such that the hydrogen gas flows through it and at least substantially does not flow through the aqueous medium under the process conditions. Preferably the number average pore diameter is less than 0.5 µm. Very good results have been obtained with a number average pore diameter in the range of about 0.05-0.2 µm.

By the conversion of oxygen, nitrate and / or nitrite, the catalyst supports the anaerobic / anoxicity to the membrane in an aerobic environment.

The catalyst preferably comprises at least one noble metal I from group VIII of the periodic table, more preferably at least palladium. It has been found that it has a good oxygen-converting activity under conditions where the biofilm is active.

The amount of catalyst relative to the weight of the membrane support material can be selected within wide limits and is preferably less than 10 wt. %, more preferably I 0.1-1 wt. %.

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Optionally, one or more other metals may be added to the catalyst such as, for example, copper, preferably in a total content of less than 10% by weight relative to the catalytically active component. The addition of copper and the like has been found to favorably influence the selectivity of the conversion reaction from nitrate and nitrite to nitrogen.

The catalyst can be applied to the support material in any way, and is preferably present as a (finely divided) porous layer. The porosity of the catalyst layer is in practice such that it is permeable to both hydrogen gas, oxygen and water.

Very suitable is a membrane on which the catalyst is applied by means of electroless plating. Very good results have been achieved with a membrane obtained by means of such a method as described in Dutch patent application No. 1 023 364.

Very good results have been achieved with a membrane in which a cover layer is applied to the catalyst, which is permeable to oxygen and also to hydrogen, nitrogen and water. Such a layer protects the catalyst against impurities in the aqueous medium and / or products secreted by the bacteria.

The cover layer is preferably a polymer layer, which can be applied, for example, by dip coating from a diluted solution of the polymer and then evaporating the solvent. In particular, a polymer layer has been found to contribute to keeping the catalyst clean. In this connection, very good results have been achieved with a polypropylene oxide (PPO) and the like.

In a method according to the invention, the aqueous medium is preferably kept at overpressure with respect to the hydrogen gas pressure. As a result, bubble formation is suppressed or completely prevented, which limits the gas transfer to diffusion and prevents hydrogen gas loss as a result of bubble formation) and / or the integrity of the biofilm is favorably influenced. More preferably, the overpressure is at least about 0.05 bar I, more preferably about 0.1 to 0.5 bar.

In principle, a method can be applied at any temperature in which the catalyst and the biofilm are active. The person skilled in the art will be able to choose these on the basis of the catalyst and bacteria present. A method is generally very suitable in which at least I anaerobically converting - and preferably also the conversion of oxygen - I takes place at a temperature in the range of 4-90 ° C. For practical reasons (for example with a view to energy consumption), a temperature in the range of 15-30 ° C is particularly preferred.

As already mentioned above, the invention can very suitably be carried out at a high longitudinal flow rate, for example longitudinal flow rates corresponding to a Reynolds number (He) of about 1 to 500, more preferably of about 10 to 500. Re is based here at the interstitial linear velocity, that is, the actual liquid velocity along the membrane surface. The characteristic length I in the calculation of Re is determined by the system and, in particular in the case of a system with hollow fiber membranes, is the outer diameter of the fibers.

An advantage of a high longitudinal flow velocity is that the aerobic I layer 4 (which generally does not substantially contribute to the anaerobic removal of contaminants) remains relatively thin and a very good substance transfer takes place. As a result, the surface of the biofilm I (formed by the anaerobic zone and the aerobic zone) and the membrane I can be relatively low, which is advantageous from an investment point of view.

For compounds that are removed by a sequential conversion by aerobic and anaerobic compounds, such as for the removal of halogenated aromatics, the longitudinal flow rate is preferably relatively low compared to a method for removing compounds in which the aerobic bacteria play no role or only play a minor role. For the removal of a compound by a sequential conversion by aerobic and anaerobic compounds, very good results have been achieved with a method in which the longitudinal flow rate corresponds to a Re in the range of 1 to 100.

The aqueous medium may contain one or more minerals, trace elements and / or vitamins. Examples of this are described in Dutch patent application No. 1 021 458.

Very suitable is a method in which the aqueous medium comprises at least one mineral, preferably selected from the group of phosphates, sulfates, nitrates. As counterions, one or more cations from the group formed by magnesium, potassium and sodium cations are very suitable.

Very suitable is a method in which the aqueous medium contains at least trace elements such as (i) the cations of iron, zinc, manganese, cobalt, nickel and / or copper; and / or (ii) acids and / or salts of molybdates (salts of M0O42 *), tungstates (salts of WO42 ·), selenates (salts of SeOa2 *) and / or borates. More preferably, substantially all of these trace elements are present.

Very good results have been obtained with a process in which the total content of minerals plus trace elements is 0.1-10 g / l.

Good results have been achieved, inter alia, with a method in which the aqueous medium contains one or more vitamins. The one or more vitamins are preferably selected from the group consisting of para-amino benzoic acid, folic acid, DT-lipoic acid, riboflavin, nicotinic acid amide, pyridoxine, panthotenate, vitamin B12 and biotin. More preferably, substantially all of these vitamins are present.

Very good results have been achieved with a total vitamin content of approximately 0.1-10 mg / 1.

The minerals, trace elements and vitamins contribute to a desired growth of the bacteria. A suitable amount can be routinely determined by those skilled in the art on the basis of general professional knowledge.

The pH can be suitably selected on the basis of the catalyst used, bacteria and contaminants to be converted). In general, a range of about 4.5 to 8.5 is very suitable, although a pH outside that range is also possible. If desired, the medium can be adjusted to pH in a conventional manner, for example by adding CO2 or a corresponding base.

Optionally, the invention can be used in combination with one or more purification processes for the removal of undesired components. Such processes are well known in the art.

The invention is, for example, very suitable for use on an aqueous medium that has first been subjected to an ozonization. With ozonization some bromate, chlorate and / or other contamination is often formed. A contamination formed during ozonization can then be removed by means of the invention.

The invention further relates to the use of a membrane as defined in the description and / or the claims in the preparation of drinking water, process water, waste water or ultrapure water (such as demineralized and / or distilled water).

The invention furthermore relates to the use of a membrane as defined in the description and / or the claims for maintaining (substantially) anaerobic conditions in a biofilm for the removal of a reducible contamination, and in particular to the use of said membrane when removing oxygen in the biofilm.

Claims (14)

A method for removing a contaminant from an aqueous medium with the aid of a membrane provided with a biofilm comprising an anaerobic zone with anaerobic bacteria and a catalyst capable of converting oxygen and hydrogen under the process conditions to catalyze to water, comprising - contacting hydrogen gas with the aqueous medium through the membrane - reducing oxygen, nitrite and / or nitrate present with hydrogen on the catalyst to form water and / or nitrogen 10. the anaerobically biological conversion of a contamination by reduction with the hydrogen in the anaerobic zone with anaerobic bacteria. Method according to claim 1, wherein the aqueous medium is kept at an overpressure with respect to the hydrogen gas pressure, preferably at an overpressure of 0.05 to 0.5 bar. A method according to any one of the preceding claims, wherein the temperature is in the range of about 4-90 ° C, preferably of about 15-30 ° C. The method of any one of the preceding claims, wherein the pH of the aqueous medium is in the range of about 4.5 to 8.5. The method according to any of the preceding claims, wherein the membrane is a hollow fiber membrane. The process according to any of the preceding claims, wherein the catalyst comprises at least one noble metal from group VIII of the periodic table, preferably at least palladium. A method according to any one of the preceding claims, wherein the anaerobic zone with anaerobic bacteria and the catalyst are at least partially mixed with each other. 8. A method according to any one of the preceding claims, wherein at least one impurity is converted, selected from the group consisting of bromate, chlorate, perchlorate, nitrate, nitrite, sulphate, N2O and carbon compounds with at least one substituent selected from the I 5 group consisting of halogen groups, preferably chlorine groups, sulfoxy, nitro and azo groups. 9. A method according to any one of the preceding claims, wherein the weight content of oxygen and / or nitrate in the aqueous medium at the start of the anaerobic conversion is higher than the weight content of impurities to be converted, preferably at least 5 times as high, more preferably 10 to 1000 times as high. A method according to any one of the preceding claims, wherein the aqueous medium is subjected to an ozonization prior to the anaerobic conversion of the contamination. I 15 A method according to any one of the preceding claims, wherein the catalyst is provided with a protective cover layer. 12. A method according to any one of the preceding claims, wherein the I longitudinal flow rate of the aqueous medium relative to the I membrane corresponds to a Reynolds number (Re) of approximately 1 to 500, preferably of approximately 10 to 500. Use of a membrane as defined in any one of claims 1, 5-7 or 11 in the preparation of drinking water, process water, waste water or ultrapure water. 14. Use of a membrane as defined in any one of claims 1, 5-7 or 11 for maintaining anaerobic conditions in an I biofilm to remove a reducible contamination.