Filter medium for water filtration, method of its production, water filtration method and uses of the filter medium

Abstract

The present invention relates to a filter medium for filtering fluids, such as water, comprising, based on the total weight of the filter medium, (A) up to 60 wt.% of fibrillated fibers (B) up to 65% of positively charged nanofibers, and (C) at least 1.0 wt.% of a positively charged binder.

Description

FILTER MEDIUM FOR WATER FILTRATION, METHOD OF ITS PRODUCTION, WATER FILTRATION METHOD AND USES OF THE FILTER MEDIUMCROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is an international filing which claims priority to and the benefit of United States application number 63 / 719,809, which was filed on November 13, 2024. The entire contents of the foregoing application is incorporated herein by reference.

[0002] The present invention relates to a filter medium comprising fibrillated fibers, positively charged fiber and a positively charged binder with excellent lifetime and filtration efficiency.INTRODUCTION

[0003] Purification of water for human consumption, industrial use, and waste treatment is a worldwide problem. Most water purification technologies involve some form of mechanical filtration or size exclusion. These techniques generally involve the use of submicron filters to remove pathogens (such as bacteria and viruses), metals, and particulate matter from the water.

[0004] A variety of water filter media are known. These typically include particles comprising one or more of activated carbon, zeolites, metal oxides, clays, diatomaceous earth, and other materials, which are usually dispersed within a polymeric binder that holds the particles in position during filtration and reduces entrainment of the particles into the filtered water.

[0005] Filter media of the prior art include substrates having a nanoalumina (aluminum oxide / hydroxide) coating. For example, U.S. Pat. No. 9,309,131 describes powdered siliceous components (including diatomaceous earth, perlite, talc, vermiculite, sand, and calcine composites) on which nanoalumina has been precipitated as being suitable sorbents for purifying water. Siliceous components have been defined as materials which have silica as a primary component, typically in an amount of at least 40 wt. %. On the other hand, sintered ceramic filters comprising fibers of metal oxide obtained by electrospinning and powdery nanoalumina incorporated into the fibers or coated thereon are disclosed in US 2016 / 0244373 AL Although these typesof filter media are known to be capable of removing contaminants from water to a moderate extent, there is scope for improvement in their performance.

[0006] US 2024 / 0050881 Al relates to a filter medium, more particularly, to a filter medium comprising AI2O3 containing particles or fibers that have been coated with nanoalumina, which may be used for filtering contaminants such as positively charged species from fluids such as water. The drawback of the filter media described herein is that the lifetime and its removal

[0007] It is hence the object of the present invention to provide a filter medium with enhanced filtration efficiency and a longer lifetime.DEFINITIONS:

[0008] For the purpose of the present invention the following definitions of parameters and measurement methods thereof are used:

[0009] The pore size is measured according to the American Society of Testing and Materials (ASTM) Standard 316-03 (2011). The pore size is measured using a technique known as capillary flow porosimetry. The sample is first wetted with a wetting fluid such that all the pores in the sample are filled. A nonreacting gas of increasing pressure is applied to one side of the wet sample to displace the liquid from the pores. The gas pressure and gas flowrate downstream of the sample are measured and plotted for the wet sample. After the sample is dry, the test is repeated to plot a gas flow vs. the applied pressure curve for the dry sample. Using such capillary porosimetry technique, the “maximum pore size,” “minimum pore size” and “mean flow pore size” can be determined.o Maximum Pore Size: The gas pressure using the capillary flow porosimetry technique described hereinabove at which air flow through the medium is first detected (i.e., the pressure at which the bubbles first begin to flow) is used to calculate the maximum pore size.o Minimum Pore Size is determined from the pressure at which the wet flow rate curve merges with dry curve using the capillary flow porosimetry technique described hereinabove.o Mean Flow Pore Size is the pore diameter at which the flow through a wetted medium is 50% of the flow through the dry medium at the same pressuredrop using the capillary flow porosimetry technique described hereinabove.

[0010] In the context of filtration, the terms “6-1 og bacterial removal” and “4-log or 5-log virus removal” refer to the effectiveness of the filter medium in reducing the concentration of bacteria and viruses, respectively. These terms use logarithmic scales to quantify the reduction in microbial load. A 1-log reduction corresponds to a 10-fold (or 90%) reduction in the number of microorganisms. A 2-1 og reduction corresponds to a 100-fold (or 99%) reduction. A 3-log reduction corresponds to a 1,000-fold (or 99.9%) reduction. A 6-log reduction corresponds to a 1,000,000-fold (or 99.9999%) reduction.“Fibers” refers to material with aspect ratio above 3:1 and average lengths above 0.2mm.“Fibrillation” refers to a process where the fibers are mechanically or chemically treated to create fine, hair-like strands or fibrils on their surface.“Fibrillated fibers” means fibers that have been further acted upon to create numerous fibrils. Fibrillated Fibers, are preferably characterized by a Canadian Standard Freeness, measured according to TAPPI T227 om-94, of about 300 mL or less, preferably about 200 mL or less, typically between about 1 to about 200 mL. “Fibrils” are tiny, minute irregular threadlike elements of a fiber that has been fibrillated by a fibrillation process.“Non-fibrillated” means unprocessed fibers having essentially no fibrils and which exhibit a Canadian Standard Freeness of greater than about 500 mL.Synthetic cellulosic fibers are made by direct dissolution and spinning of wood pulp in an organic solvent, such as an amine oxide.The term homogeneous refers to a blend of two or more ingredients that is preferably uniform in composition or character throughout. In different contexts, it can describe mixtures, groups, or systems that are consistent and similar in nature. It can include small local variations in the composition such as local fluctuations, gradients, and the like.As used herein, positively charged binder refers to binders which contain an electropositive charge, for example as measured using zeta potential, at a pH of 6 to 8.5 (the pH of drinking water), preferably at a pH of 6.5 to 8.0.SUMMARY OF THE INVENTION

[0011] The present invention relates to a filter medium for filtering fluids, such as water, comprising, based on the total weight of the filter medium,(A) up to 60 wt.% of fibrillated fibers(B) up to 65% of positively charged nanofibers, and(C) at least 1.0 wt.% of a positively charged binder.

[0012] The content of all compounds constituting the filter medium - including the optional components listed below - are always chosen to sum up to 100 wt.%.

[0013] In the filter medium according to the present invention the fibrillated fibers and the positively charged nanofibers are in the form of a nonwoven or fibrous material, in which the fibers have a random fiber orientation. The fibers are preferably laid down without any specific direction, resulting in an irregular, multi-directional structure. The fibers - and any further component, if present, - are adhered by the cationic binder.

[0014] The filter medium of the present invention specifically can be used to remove biological contaminants, such as viruses, microbes and bacteria from fluids, especially drinking water.

[0015] Surprisingly, it has been discovered that filter medium according to the present invention that contain both positively charged nanofibers and at least 1.0 wt.% positively charged binder (or synonymously: cationic binder) showed much longer lifetime, being able to maintain maximum removal efficiency for a longer time.

[0016] Further, the filter medium can operate at increased flow rates. Simply changing furnish components to get a larger pore size and thus higher flow medium leads to a loss in microbial removal. It has been found that a combination of positively charged nanofibers and at least 1.0 wt.% positively charged binder allows for using larger and more open fibers to help the final filter medium achieve increased pore size and flow rate without sacrificing microbial removal, especially virus removal efficiency and capacity

[0017] The present invention allows to reduce the amount of nano-alumina while at the same time keeping the same virus removal or better. Reducing the amount of a non-biodegradable material like alumina is better for the environment and also ismore cost-effective. It has been reported by some customers that reduction in nano-alu-mina in the filter could lead to better taste and smell of the filtered water.

[0018] The filter medium according to the invention can have a thickness of 600 to 1000 micrometers, preferably 650 to 900 micrometers, and more preferably 675 to 875 micrometers, measured according to ISO 9073-2:1997.

[0019] In an embodiment the filter medium according to the invention can have a maximum pore size of less than 5 micrometers, preferably less than 4.5 micrometers and / or a mean flow pore size less than 2.5 micrometers, preferably less than 2 micrometers.

[0020] The filter medium according to the invention has an effective minimum of 4-log virus removal (tested with the MS2 virus) and a minimum of 6-log for bacterial removal from water.

[0021] The filter medium according to the invention can operate at both low-flow and high-flow applications, and preferably at flow rates above 3 mL / min, more preferably over 5 mL / min, and even preferably over 7 mL / min.DESCRIPTION OF COMPONENTS IN THE FILTER MEDIUM:1) Fibrillated Fibers (component (A))

[0022] The filter medium according to the present invention comprises fibrillated fibers which may be cellulosic fibers. Fibrillation is a process where the fibers are mechanically or chemically treated to create fine, hair-like strands or fibrils on their surface. The fibrillated fibers, preferably fibrillated cellulosic fibers, have fibrillation level less than 300 ml Canadian Standard Freeness (CSF), preferably less than 200 ml CSF, preferably less than 100 ml CSF, preferably less than 60 ml CSF, and more preferably less than 50 ml CSF as specified by TAPPI test method T 227 om-9494 (Freeness of Pulp). The “fibrillation level” refers to the degree or extent to which the fibers have been split or fibrillated into smaller fibrils. Canadian Standard Freeness is a measure of the drainage rate of a fiber suspension, often used in the pulp and paper industry to evaluate the fibrillation level of fibers. Lower CSF values (below 100 ml) indicate a higher degree of fibrillation.

[0023] The fibrillated fibers may include parent fibers and fibrils. The parentfiber is the original, larger fiber from which the fibrils are derived. These fibers are typically long and continuous, forming the primary structural component before the fibrillation process begins. The parent fibers provide the main strength and bulk of the material. They are responsible for the overall integrity of the fiber mat or fabric.

[0024] The fibrillated fibers preferably have an average parent fiber diameter of 5 to 30 micrometers, more preferably 10 to 20 micrometers and most preferably around 10 to 15 micrometers. The fibrils preferably have an average diameter of 1000 nanometers or less, preferably 400 nanometers or less, and most preferably 100 nanometers or less. As the diameter of the fibrils is much smaller than the diameter of the parent fibers, the diameter of the fibrillated fibers and the diameter of the parent fibers is used synonymously. Some preferred embodiments will include fibrils having average diameters of 250 to 350 nanometers. Average fiber diameter can be measured using Scanning Electron Microscopy (SEM) by averaging the measurements of over 50, preferably over 100 fibers. In this method, handsheet samples made exclusively of the fibers are placed in the SEM chamber, where an electron beam scans the samples, producing high-resolution images. These images are analyzed using specialized software to measure the diameters of individual fibers. By averaging these measurements, a representative average fiber diameter is obtained. Alternatively, the average fiber diameter can be calculated using the dtex / denier, fiber length and material density of the fiber, all of which are provided by the supplier.

[0025] The preferred average length of the fibrillated fibers is 0.4 to 8 millimeters, more preferably 1 to 6 millimeters. In addition, in preferred embodiments, less than 15 wt.% of the fibrillated fibers have a length-weighted average fiber length (LWAFL) of greater than 2 millimeters. Fiber length is measured with any suitable equipment that follows ISO 16065-1:2014 or ISO 16065-2:2014 and expressed as average fiber length. A suitable equipment can be, for example, Morfi TECHPAP which follows the ISO 16065-2:2014 standard.

[0026] The fibrillated cellulose fibers can be naturally occurring fibers, synthetic fibers or a combination of both.

[0027] In an embodiment, preferred fibrillated fibers include fibrillated cellulosic fibers. Preferred fibrillated cellulosic fibers are those made by a synthetic process, such as viscose, rayon, lyocell, or the like.

[0028] The fibrillated fibers may be fibrillated either during the fiber production process, or by being subjected to standard mechanical beaters, refiners and the like employed in the paper-making industry.

[0029] The fibrillated fibers preferably are present in an amount of 2 to 60 wt.%, preferably 3 to 55 wt.%, and more preferably 4 to 50 wt.%, based on the total weight of the filter medium.2) Positively charged nanofibers (component (B))

[0030] The filter medium according to the present invention comprises positively charged nanofibers. In a preferred embodiment at least a part of the positively charged nanofibers preferably is disposed on the surface at least some of the fibrillated fibers.

[0031] In a preferred embodiment the positively charged nanofibers are nano alumina fibers. The positively charged nano alumina fibers are primarily composed of the mineral boehmite, is produced by reacting aluminum powder (in metallic form) with a base (usually 50% NaOH in water) in the pulper under heat (for example, between 130- 170F (equal to 327.6 - 349.8 K)). Details of the reaction are described below.

[0032] The nano alumina fibers also are referred to as “alumina” in the prior art. They are made of a mix of primarily boehmite (A100H) and small amounts of gibbsite (Al(0H)3) and aluminum oxide (AI2O3). The high surface area of the fibers and the electropositive charge allows the material to adsorb large amounts of negatively charged micro-organisms like viruses.

[0033] The surface of the nano alumina fibers is positively charged. To be specific, the nano alumina fibers are positively charged at the pH of drinking water of 6 to 8.5 (usually measured by the zeta potential). Specifically, and especially preferred, the filter medium has a zeta potential > 20 mV at the pH of drinking water.

[0034] Exemplary nano alumina fibers are disclosed in WO 2008 / 073507 Al, the disclosure of which - as far as nano alumina fibers are concerned - is herewith incorporated by reference. The nano alumina fibers produced according to this patent application are especially suitable for the purposes of the present invention.

[0035] Further exemplary nano alumina fibers which can be used for the purpose of the present invention as well as their synthesis are described by Ridzuan M and Yong K (2010) Synthesis of Alumina Nanofibers and Composites. Nanofibers. InTech.Available at: h tt : / / d do i . or g / 10.5772 / 8165 , the disclosure of which is herewith incorporated by reference.

[0036] Positively charged nanofibers (Component B), such as nano-alumina especially preferred can be formed by reacting aluminum metal with a base, such as NaOH under heat. The nanofibers are primarily made of aluminum oxide hydroxide, or boehmite (A100H). There may also be small amounts of aluminum trihyroxide [A1(OH)3] and / or aluminum oxide (AI2O3). The nano-alumina fibers are about 2 nm in diameter and 200-300 nm in length. The nanofiber has a surface area from 350-500 m2 / g.

[0037] Nano-alumina is primarily composed of the mineral boehmite and can be produced by reacting aluminum powder (in metallic form) with a base (usually 50% NaOH in water) in a pulper. All fibers (i.e., both fibrillated fibers (A) and if used other fibers (D) are added to the pulper. The pulper is a machine or device used to mix and / or break materials into a pulp-like consistency. Due to the reaction between aluminum and the base in the pulper, positively charged nano-alumina is formed and is disposed on the surface of the fibers. Once the reaction is complete, sulfuric acid or another suitable acid is added to the pulper to neutralize the reaction.

[0038] In an embodiment, at least a part of the nano-alumina may be disposed onto at least some of the fibrillated fibers. In another embodiment, the nano-alumina may be disposed on the fibrillated fibers and at least one other optional non-fibrillated fiber. In yet another embodiment, the nano-alumina may be disposed on all of the fibers present in the filter media.

[0039] The positively charged nanofibers thus are disposed on the surface of the fibrillated fibers present in the filter medium. In another embodiment, the positively charged nanofibers may be disposed on any type of fibers present in the filter medium, such as e.g., also the non-fibrillated fibers discussed below.

[0040] Typically, the filter medium can comprise up to 65 wt.% positively charged nanofibers, preferably 15 to 60 wt.%, e.g., 25 to 55 wt.%, based on the total weight of the filter medium.

[0041] The total fibrous content (the total weight of the fibrillated and - if present - non-fibrillated fibers, see detailed description below) in the filter medium may be 25 to 90 wt.%, preferably 30 to 85 wt.%, preferably 45 to 70 wt.%, and more preferably 55 to 65 wt.%, based on the total weight of the filter medium.3) Positively charged binder (component (C))

[0042] The filter medium according to the present invention comprises at least a positively charged binder which also is referenced to as cationic binder. The binder(s) play a useful role in holding various components of the filter medium together, ensuring structural integrity and enhancing the performance of the filter. The binders may be bonding agents that help maintain the form and functionality of the filter medium while allowing the filter medium to remain porous and effective in capturing particles or contaminants.

[0043] The positively charged binder may be a cationic polymer. For example, the cationic binder may comprise a thermoplastic polymer, a blend of thermoplastic polymers, a thermosetting polymer (e.g., a crosslinkable or crosslinked polymer), or a blend of a thermoplastic polymer with thermosetting polymers.

[0044] In an embodiment, the positively charged binder may comprise a thermoplastic polymer, a blend of thermoplastic polymers, a thermosetting polymer (e.g., a crosslinkable or crosslinked polymer), or a blend of a thermoplastic polymer with thermosetting polymers. These polymers may comprise an oligomer, a homopolymer, a copolymer, a block copolymer, an alternating block copolymer, a random copolymer, a graft copolymer, a star block copolymer, a dendrimer, a polyelectrolyte (polymers that have some repeat groups that contain electrolytes), a polyampholyte (a polyelectrolyte having both cationic and anionic repeat groups), an ionomer, or the like, or a combination thereof.

[0045] In an embodiment, the positively charged binder is a crosslinkable polymer that undergoes crosslinking during the process of manufacturing the filter medium. The positively charged binder in the final filter medium product is typically crosslinked.

[0046] The positively charged binder may be any polymer bearing an overall positive charge, such as, for example, a polyethyleneimine (PEI), a polyamidoamine, a polylysine, a poly(allylamine), a poly(diallyldimethylammonium chloride), a poly(N-isopropyl acrylamide-co-acrylamide), a poly(N-isopropyl acrylamide-co-acrylic acid), a diethylaminoethyl-dextran copolymer, a poly-(N-ethyl-vinylpyridinium bromide), a poly(dimethylamino)ethyl methacrylate), a polyethylene glycol)-co-poly(trimethyla-minoethylmethacrylate chloride), an epichlorohydrin based polymer, a polyamide, an epichlorohydrin-dimethylamine copolymer, a polyamide-epichlorohydrin (PAE), or thelike, or a combination thereof.

[0047] Preferred positively charged binders may include epichlorohydrin containing polymers. Epichlorohydrin is advantageous because it reacts with hydroxyl groups, making it useful in crosslinking polysaccharides such as starch, cellulose, or polyvinyl alcohol (PVA). In polymers or biomaterials, epichlorohydrin can be used to introduce crosslinks that enhance mechanical strength, chemical resistance, and stability.

[0048] In an embodiment, the preferred positively charged binders include a crosslinkable epichlorohydrin based polymer, a crosslinkable polyamide, a crosslinkable epichlorohydrin-dimethylamine copolymer, a crosslinkable polyamide-epichlorohydrin (PAE), or a combination thereof. A preferred positively charged binder is polyamideepichlorohydrin (PAE).

[0049] The positively charged binder may be bound covalently and / or electrostatically to the surface of the cellulosic fibers. In some embodiments, the positively charged binder is electrostatically bound to the surface of the cellulosic fibers. For example, the positively charged binder may bind electrostatically to the surface of the cellulosic fibers, which have an overall negative charge. In an embodiment, hydrogen bonding may occur between the positively charged binder and the biodegradable fibers. The electropositive nature of the filter medium is at least, in part, due to the impregnation of the fibers by the positively charged binder.

[0050] The positively charged binder and / or the non-positively charged binder may have a number average molecular weight (Mn) greater than 10,000 grams per mole, preferably greater than 20,000 g / mole and more preferably greater than 50,000 g / mole. In an embodiment, the positively charged binder or the non-positively charged binder may have a number average molecular weight (Mn) of less than 2,000,000 g / mole, preferably less than 1,000,000 g / mole.

[0051] The positively charged binder is preferably used in an amount of less than or equal to 5 wt.%, preferably less than or equal to 2.5 wt.%, and more preferably less than or equal to 2.0 wt.%, based on the total weight of the filter medium. In an embodiment, the positively charged binder is present in an amount of greater than or equal to 1.0 wt.%, preferably greater than 1.25 wt.%, based on the total weight of the filter medium.

[0052] The total amount of binders (positively charged binders and - if present, see detailed description below - non-positively charged binders) in the medium is typically less than 10 wt.%, preferably less than 6 wt.%, and more preferably less than 4 wt.%, based on the total weight of the filter medium.4) Non-fibrillated fibers (optional component (D))

[0053] In an embodiment the filter medium according to the present invention additionally can comprise non-fibrillated fibers (component (D)). The non-fibrillated fibers may be at least one of non-fibrillated cellulosic fibers, non-fibrillated non-cellulosic fibers such as synthetic polymeric fibers and / or glass fibers.

[0054] The non-fibrillated fibers may preferably comprise cellulosic fibers. The non-fibrillated cellulosic fibers can be from woody and non-woody plants. They can include hardwood and / or softwood fibers. Woody plants include, for example, deciduous (hardwood), coniferous trees (softwood), or the like, or a combination thereof. Non-woody plants include, for example, cotton, flax, esparto grass, kenaf, sisal, abaca, milkweed, straw, jute, hemp, bagasse, or the like, or a combination thereof.

[0055] Examples of softwood fibers include fibers obtained from chemical treatments like mercerization (e.g., HPZ fibers), northern bleached softwood kraft (e.g., fibers from Redwoods, Douglas firs, Western red cedar, or the like), southern bleached softwood kraft (e.g., fibers from slash pine, loblolly pine, or the like), or chemically treated mechanical pulps (e.g., CTMP fibers), or the like, or a combination thereof. Exemplary hardwood fibers include fibers obtained from eucalyptus, birch, aspen, beech, oaks, gum trees, or the like, or a combination thereof.

[0056] The non-fibrillated cellulosic fibers are preferably cellulose pulp fibers and more preferably comprise large diameter softwood fibers (e.g., Northern Bleached Softwood Kraft (NBSK), Southern Bleached Softwood Kraft (SBSK),), or a combination thereof). The non-fibrillated cellulosic fibers open up the material and improve drainage during the wet-laying process.

[0057] The non-fibrillated cellulosic fibers, such as the cellulose pulp fibers may have a larger average diameter than the fibrillated fibers, which can add rigidity and stiffness to the filter medium. Further, this combination of fibrillated fibers and larger diameter non-fibrillated fibers assists to achieve increased pore size and flow rate without sacrificing microbial removal. The medium can contain up to 10wt% larger diameternon-fibrillated fibers. The larger diameter non-fibrillated fibers are preferably cellulose fibers, and more preferably softwood fibers.

[0058] The dimensions of the “parent fiber” are generally understood to be the same dimensions as the fibrillated fiber and can be used interchangeably as the fibrillated fiber’s diameter.

[0059] The non-fibrillated cellulosic fibers preferably have an average fiber diameter of 10 to 55 micrometers, more preferably 15 to 50 micrometers. The non-fibril-lated cellulosic fibers preferably have an average fiber length, as measured with any suitable equipment that follows ISO 16065-1:2014 or ISO 16065-2:2014, of 0.3 to 8 millimeters, more preferably from 1 to 6 millimeters, more preferably from 3 to 6 millimeters.

[0060] In an embodiment, the non-fibrillated cellulosic fibers have an average aspect ratio (calculated as the ratio of average fiber length to average fiber diameter) of 4 to 800, preferably 10 to 500.

[0061] The non-fibrillated cellulosic fibers are present in the filter medium in an amount of up to 75 wt.%, preferably 3 to 70 wt.% and more preferably 4 to 60 wt.%, preferably 1 to 30 wt%, based on the total weight of the filter medium. It is to be noted that if desired, the filter medium may be devoid of the non-fibrillated cellulosic fibers.

[0062] The filter medium may also contain other non-fibrillated fibers that may include non-cellulosic synthetic polymeric fibers and glass fibers. The non-cellulosic synthetic polymeric fibers are preferably insoluble in water. They can reinforce the filter medium and provide it with strength when in the presence of a liquid that is to be filtered. These fibers are often selected for their durability, resistance to chemicals or their structural dimensions depending upon specific filtration needs.

[0063] Non-cellulosic synthetics polymeric fibers may comprise thermoplastic or thermoset polymers. Examples of thermoplastic or thermoset polymers for use in the filter medium include a polyolefin, a polyester, a polyamide, a polyaramid, a polyamideimide, a polyarylate, a polyurethane, a polysiloxane, a polyimide, a polyetherimide, a polytetrafluoroethylene, a polyetherketone, or the like, or a combination thereof.

[0064] Preferred non-cellulosic synthetic polymeric fibers include polyesters such as polyethylene terephthalate, polybutylene terephthalate, polyethylene naph-thalate, or the like, or a combination thereof. The polyester fibers may be used in theform of spunbond or meltblown fibers. The non-cellulosic non-fibrillated fibers may be single component or bicomponent core / sheath type fibers.

[0065] The non-cellulosic synthetic polymeric fibers may have an average diameter of less than less than 25 micrometers, preferably less than 20 micrometers, and preferably less than 18 micrometers. The non-cellulosic polymeric fibers may have an average diameter of greater than 1 micrometer, preferably greater than 2 micrometers and more preferably greater 5 micrometers.

[0066] The non-cellulosic synthetic polymeric fibers may have an average length of less than 15 mm, preferably less than 12 mm, preferably less than 10 mm, preferably less than 8 mm. For example, the average length of the non-cellulosic synthetic polymeric fibers may be 0.1 to 15 mm, between 1 and 10 mm, between 2 and 8 mm, between 3 and 6 mm.

[0067] The non-cellulosic synthetic polymeric fibers may optionally be used in amounts of 2 to 38 wt%, preferably 3 to 35 wt%, preferably 4 to 30 wt.%, preferably 5 to 25 wt.%, based on the total weight of the filter medium.

[0068] In another embodiment, the non-fibrillated fibers may be glass fibers. The glass fiber can comprise E, A, B, C, ECR, R, S, D, or NE glass, or the like, or a combination thereof. In an embodiment, two or more different types of glass fibers may be used in the filter medium. When two or more types of glass fibers are used, each type may have a different average length and / or a different average diameter prior to incorporation in the filter medium. Preferably, the glass fibers are micro-glass fibers with diameters indicated below.

[0069] The glass fibers detailed above can be made by standard processes, e.g., by steam or air blowing, flame blowing, and mechanical pulling. Exemplary glass fibers are made by mechanical pulling.

[0070] The glass fiber can be chopped fibers. Glass fibers in the form of chopped strands can have a length of 0.3 millimeters (mm) to 10 centimeters (cm), preferably 0.5 mm to 5 cm, and more preferably 1 mm to 2.5 cm. For example, the glass fiber can be a microglass fiber. The microglass fibers can have a length from 0.2 to 20 mm, preferably 0.25 to 10 mm, and more preferably 0.3 to 6 mm.

[0071] The glass fibers can have any cross-section, such as a round (or circular), flat, or irregular cross-section. For example, the glass fiber has a circular cross-section.The diameter of the glass fiber can be from 0.1 to 25 micrometers (gm), preferably 0.2 to 20 pm, preferably 0.3 to 15 pm. For example, the glass fiber can be a microglass fiber, with a diameter from 0.1 to 10pm, preferably 0.2 to 8pm and more preferably 0.3 to 6pm. Flat glass or bilobe glass fibers can be used to provide, for example, low warp-high strength articles.

[0072] The glass fibers may optionally be used in amounts of 5 to 40 wt.%, preferably 8 to 35 wt.%, and more preferably 10 to 30 wt.%, based on the total weight of the filter medium. When two or more types of glass fibers are simultaneously used in the filter medium, the first type may be used in an amount of 5 to 15 wt.%, preferably 6 to 12 wt.%, based on the total weight of the filter medium. The second type may be used in an amount of 10 to 20 wt.%, preferably 11 to 16 wt.%, based on the total weight of the filter medium.

[0073] The total fibrous content (the total weight of the fibrillated and non-fi-brillated fibers) in the filter medium may be 25 to 90 wt.%, preferably 40 to 85 wt.%, preferably 45 to 70 wt.%, and more preferably 55 to 65 wt.%, based on the total weight of the filter medium.Non-positively charged binder (component (F))

[0074] In an embodiment, the filter medium may contain at least a positively charged binder and preferably also a non-positively charged binder. The term “neutral” means that this binder is not non-positively charged as the cationic binder (component (Q).

[0075] In an embodiment, the non-positively charged binder may comprise a thermoplastic polymer, a blend of thermoplastic polymers, a thermosetting polymer (e.g., a crosslinkable or crosslinked polymer), or a blend of a thermoplastic polymer with thermosetting polymers. These polymers may comprise an oligomer, a homopolymer, a copolymer, a block copolymer, an alternating block copolymer, a random copolymer, a graft copolymer, a star block copolymer, a dendrimer, a polyelectrolyte (polymers that have some repeat groups that contain electrolytes), a polyampholyte (a polyelectrolyte having both cationic and anionic repeat groups), an ionomer, or the like, or a combination thereof.

[0076] The non-positively charged binders are typically in emulsion form (i.e., they are latex-based binders). Emulsion polymers are a class of polymers producedthrough a process called emulsion polymerization, where monomers (small molecules) are polymerized in an emulsion containing water, surfactants (emulsifiers), and initiators. This technique is widely used in the production of polymers that are water-borne. In an emulsion, water typically forms the continuous phase. Surfactants are used to stabilize the polymer particles in the water.

[0077] Examples of non-positively charged binders (that may be used in conjunction with the positively charged polymeric binders) that may be used in emulsion form include polyethylene, polypropylene, polyester, polyurethane, phenolic resin, acrylic resin, silicone resin, nitrile rubber (acrylonitrile-butadiene rubber), or the like, or a combination thereof.

[0078] The non-positively charged binders preferably in latex form are included in the filter material in amounts of up to 5 wt.%, preferably in amounts of 0.5 to 3 wt.%, and more preferably in amounts of 1.0 to 2 wt.%, based on the total weight of the filter medium.

[0079] The total amount of binders (positively charged binders and non-positively charged binders) in the medium is typically less than 10 wt.%, preferably less than 6 wt.%, and more preferably less than 4 wt.%, based on the total weight of the filter medium.

[0080] In a preferred embodiment, the positively charged binders and non-positively charged binders are both present in the filter medium. In an embodiment, it is desirable to add the positively charged binders and non-positively charged binders sequentially during the manufacturing of the filter medium. When sequential addition is used, the positively charged binder is first added before the non-positively charged binder is added. Surprisingly, the electropositive charge on fiber surfaces in the filter medium can be maximized by this sequence of addition steps.5) Adsorbent particles (Optional component (F))

[0081] The filter medium may also contain adsorbents. Adsorbents are added to filter medium to enhance the filtration process by removing specific contaminants and improving the filters effectiveness in various applications. They play a valuable role in capturing and retaining substances primarily through adsorption (the adherence of molecules to the surface of the adsorbent) but may also include some level of absorption (the uptake of substances into the material itself).

[0082] Examples of adsorbent particles include activated carbon, diatomaceous earth, silica, porous glass, zeolites, alumina, activated alumina, ion exchange resins, silica gel, molecular sieves, chitosan, rice husk, straw, crystalline materials such as y-alu-mina, zirconia, cellulose nanocrystals (CNC), cyclic oligosaccharides such as cyclodextrin, titania, clay minerals, or the like, or a combination thereof. Adsorbent particles with high surface area (e.g., granular activated carbon) and / or modified surface chemistry (e.g., catalytic activated carbon) are preferred. Activated carbons are preferred, with catalytic activated carbon being an especially preferred option. In some embodiments, a mixture of adsorbent particles may be used in combination.

[0083] Surface area is defined by iodine numbers greater than 700mg / g, preferably greater than 900mg / g, and more preferably greater than HOOmg / g. This method measures the amount of iodine adsorbed by the adsorbent particle from an iodine solution. The iodine number is an indicator of the micropore content of the adsorbent particle and is often used as a quick test for surface area.

[0084] In an embodiment, the BET surface area of adsorbent particles may be greater than 900m2 / g, preferably greater than 1000m2 / g, and more preferably greater than 1200 m2 / g. BET (Brunauer-Emmett-Teller) is used to measure the surface area of material. It involves measuring the amount of nitrogen gas adsorbed onto the surface of the adsorbent particles at various pressures. The data is then used to calculate the surface area based on the BET theory.

[0085] Activated carbon (also known as activated charcoal) is a highly porous form of carbon with a large surface area, making it an effective material for adsorption. Catalytic activated carbon (CAC) is a specially treated form of activated carbon that has enhanced catalytic properties, allowing it to not only adsorb contaminants but also chemically react with them to break them down into less harmful or non-toxic substances. CAC’s contain copper manganese, manganese oxide, iron, iron oxide, cobalt, silver, nitrogen functional groups, oxygen functional groups (e.g., carboxyl, hydroxyl, or carbonyl groups), and halogens to facilitate catalytic action during absorption.

[0086] Adsorbents are added to the filter medium either at the pulper or added at the headbox during the manufacturing of the filter medium. The filter medium may contain adsorbent particles in an amount of up to 60 wt.%, preferably 10 to 55 wt.% and more preferably 12 to 40 wt.%, based on the total weight of the filter medium.METHOD OF MAKING THE FILTER MEDIUM AND FILTERING WATER:

[0087] According to another aspect the present invention discloses a method for the production of a filter medium, comprising the following steps:- mixing fibrillated fibers (A) and optionally - if present - non fibrillated fibers (D) with water to form a fiber mixture- subsequently, producing positively charged nanofibers (B) in the fiber mixture and / or adding positively charged nanofibers (B) to the fiber mixture,- adding a positively charged binder (C) to the fiber mixture to form a slurry,- applying the slurry onto a wire conveyor in a papermaking machine and removing water to form a fibrous web,- drying the fibrous web to form the filter medium.

[0088] According to the present invention it is possible to add the positively charged nanofibers to the fiber mixture. According to a specific embodiment, the positively charged nanofibers are in situ produced within the fiber mixture comprising at least the fibrillated fibers, or - if present - also the non-fibrillated fibers and / or adsorbent particles. According to this specific embodiment the production of the positively charged fibers is preferably carried out by- adding a reactive precursor material (Bl) for positively charged nanofibers (B) and a reactant (B2) capable to react with the reactive precursor (Bl) to form positively charged nanofibers (B) on the surface of at least some of the fibrillated fibers (A) and optionally- if present - on the surface of at least some of the non-fibrillated fibers (D), and- reacting the reactive precursor material (Bl) with the reactant (B2) to form positively charged nanofibers (B) on at least some of the surface of the fibrillated fibers (A) and optionally - if present - on at least some of the surface of the non- fibrillated fibers (B).

[0089] The components used in the method are chosen to arrive at the weight ratios - including their preferred embodiments - as described for the filter medium earlier above.

[0090] According to a preferred embodiment, the reactive precursor material(Bl) is selected from aluminum metal, preferably in the form of a powder.

[0091] A preferred reactant (B2) is selected from Bronsted bases, such as NaOH or KOH. A preferred reactant (B2) is 50 wt.% NaOH solution.

[0092] The reaction can be carried out under elevated temperatures, i.e., temperatures above normal temperature (298 K), such as e.g. 130- 170F (equal to 328 - 350 K), more preferably 140F to 165F K (equal to 333 - 347 K).

[0093] A typical and exemplary process in the pulper can be as follows:a. Fibrillated fibers A and optionally other fibers D are added to the pulper. b. Al metal and 50% NaOH are added. Reaction occurs to form nano-alu- mina (component B).c. Acid (e.g., sulfuric acid) is added in sufficient quantity to neutralize the mixture after the reaction is complete, such that the pH is between 6.0 to 8.5.d. A positively charged binder is addede. Other binder (i.e., non-positively charged binder) is added f. Mixture is transferred to machine chest, where optionally adsorbents may be added.g. Mixture is then applied to the wire conveyor of a papermaking machine.

[0094] Adsorbent particles - if used - can be added before or after the fibrillated fibers A and optionally other fibers D.

[0095] A very specific process, which also includes the addition of adsorbent particles can be carried as follows (this process usually is carried out in the lab).a. Fibrillated Fibers A and optionally other fibers are added to the disintegrator. The suspension is disintegrated for e.g., 10000 revolutions. b. The reaction components (Aluminum powder and 50 wt.% NaOH) are added after the fibers are added. Reaction takes place under heat to form nano-alumina, which is disposed onto the surface of at least some of the fibrillated fibers A.c. Sulfuric acid is added in sufficient amount to neutralize the suspension after the reaction is complete, such that the pH is between 6.0 and 8.5 d. A positively charged binder is added.e. A non-positively charged binder is added.f. Adsorbent particles are added to a Labtech Standard Disintegrator Model 500-1.g. The suspension (also called slurry) is then added to an 8x8 inch Williams Apparatus handsheet mold.h. The suspension is then diluted further with water, agitated additionally by hand and drained onto a wire to form a fibrous web. The fibrous web is then removed from the wire by hand.i. Excess moisture is removed by pressing with blotting papers and the examples are subsequently dried on a laboratory hot plate to form the example filter medium.

[0096] All the examples in the application are handsheets made in lab, using a handsheet mold to mimic the wet-laying machine process described above.

[0097] In an embodiment, non-fibrillated fibers (D), selected from non-fibril-lated cellulosic fibers and non-fibrillated non-cellulosic fibers such as synthetic polymeric fibers and / or glass fibers can also be mixed with the fibrillated fibers and subsequently mixed with water to form the fiber mixture or are added to the fiber mixture containing the fibrillated fibers (A).

[0098] If necessary, adsorbent particles (E) can also be included, which preferably are added in a subsequent step after the positively charged nanofibers and the positively charged binder are added.

[0099] It is also possible that in addition to the positively charged binder (C) a non-positively charged binder (F) is added, wherein the positively charged binder (C) is added to the fiber mixture and the non-positively charged binder (F) is added subsequently to the positively charged binder (C), preferably to the slurry, especially preferred to the slurry instantly before the slurry is applied onto the wire conveyor. The non-positively charged binder can e.g., be applied to the pulper used for producing the fiber mixture. When the cationic binder is added prior to the non-cationic latex binder, the virus removal performance is better than comparative example.

[0100] As far as the fibrillated fibers (A), positively charged nanofibers (B), the positively charged binder (C) non-fibrillated fibers (D), adsorbent particles (E) and non-positively charged binder (F) is concerned, reference is made to the specification above. The respective components can be included in the amounts described above.

[0101] In one embodiment, in one method of producing the filter medium, the fibrillated fibers and optionally the non-fibrillated fibers, are mixed together in a pulper (in a paper machine) along with water to form a fiber mixture. The positively charged particles or fibers (e.g., nanoalumina), and the binders (e.g., positively charged binders and / or non-positively charged binders) may also be added to the fiber mixture in the pulper to form a slurry. In an embodiment, the positively charged binder is first added to the pulper before the non-positively charged binder is added.

[0102] There is no refining necessary for the fiber mixture. The slurry and the constituents can be allowed to mix together before the slurry is transferred to the headbox and preferably further diluted. The diluted slurry is applied onto a wire conveyor in a papermaking machine (e.g., a fourdrinier or a rotoformer) to form the filter medium. Vacuum is continuously applied during the aforementioned processes to remove the solvent from the mixture and / or the slurry, thereby producing a fibrous web. The web thus formed, is subsequently dried to form the filter medium and wound into a roll.

[0103] In an embodiment, some of the ingredients may be added to the fiber mixture in the headbox instead of at the pulper. For example, the positively charged binder may be added to the fiber mixture in the pulper, while the non-positively charged binder may be added to the slurry in the headbox. In a similar manner, some of the adsorbents (e.g., the ion exchange resin or the activated carbon) may be added to the slurry at the headbox instead of at the pulper.

[0104] The slurry and the constituents contained therein are allowed to mix together before the slurry is transferred to the headbox and further diluted. The diluted slurry is applied onto a wire conveyor in a papermaking machine (e.g., a fourdrinier or a rotoformer) to form the filter medium. Vacuum is continuously applied during the aforementioned processes to remove the solvent from the mixture and / or the slurry, thereby producing a fibrous web. The web thus formed, is dried to form a filter medium and wound into a roll.

[0105] An exemplary embodiment to produce a filter medium according to the invention is:- Fibrillated fibers (A) and optionally non-fibrillated fibers (D) are added to a pulper and mixed with water,- Al metal and NaOH are added and reacted in order to form thepositively charged nanoalumina under heat.- A positively charged binder is added.- In a separate step and after the positively charged binder is added, the non-positively charged binder optionally can be added.- The mixture is transferred over to a machine chest, where optionally adsorbent particles like activated carbon can be added.- The mixture is then applied to the wire conveyor of the papermaking machine to form a fibrous web- The fibrous web is dried to form the filter medium.

[0106] Adsorbent particles can be added before or after the first step of the method mentioned above.

[0107] In an embodiment, this filter medium may be used in combination with other layers, wherein the filter medium serves either as a pre-filter or a post-filter to the other layer(s). Optionally, an additional layer may be laminated on one or both sides of the filter medium. The additional layer may function as a mechanical support, a filtration layer and / or may also be a protective layer to prevent adsorbent particles from leaching out. In an embodiment, one or more of the layers contains nano-alumina.

[0108] The resulting filter medium has a maximum pore size of less than 10 micrometers, preferably less than 5 micrometers. In an embodiment, the resulting filter medium has a mean flow pore size less than 8 micrometers, preferably less than 4 micrometers.

[0109] The present invention further relates to a method of reducing the concentration of bacteria and / or viruses in water by passing water containing bacteria and / or viruses through a filter medium according to the present invention.

[0110] In another aspect, the filter medium can remove up to at least 40 wt%, preferably at least 50 wt%, more preferably at least 60 wt%, more preferably at least 66 wt%, more preferably at least 70 wt%, more preferably at least 75 wt%, and more preferably at least 80 wt%, based on a total weight of the chlorine present in a solution.

[0111] In yet another aspect the present invention is directed to the use of a filter medium according to the present invention for reducing the concentration of bacteria and / or viruses in water containing bacteria and / or viruses.EXAMPLES:Examples 1 - 8

[0112] The examples described below were formed in the laboratory, using the lab process described above. The composition of the respective filter media, the permeability and the pore sizes were measured and are listed in Table 1 below:Table 1

[0113] The virus capacities of the samples were measured as follows:

[0114] Three 25 mm samples were cut for each filter media and then placed in Millipore filter holders. The samples were then flushed with 40 mL of RO water (with conductivity adjusted to ~35pS / cm and pH adjusted to between 7.5-7.8). In general, capacity tests are conducted in quarter strength ringer’s solution. Virus removal was tested with MS2 virus contaminants present in the test solution at a concentration of 850,000-950,000 viruses (also known as plaque forming units, pfu) per mL of effluent. Sample points were typically taken at 50, 200, 400, 600, 800, and 1000 mL of effluent (with some variance). The results are expressed in a graph of virus removal vs pfu / m2, wherein pfu / m2represents the total amount of plaque forming units (i.e. viruses) per unit area.

[0115] Influent concentrations were measured after dilution by 1000. The effluent samples were also diluted to 1000X (100 pL of the challenge was added to 99 mL of phosphate buffered saline (with 1 mL of tryptic soy broth added to avoid lysis of the virus). Effluent samples were then plated properly for each microorganism in order to provide filtration percentage for each. To determine virus removal, effluent samples were plated using double layer agar method described in Kropinski AM, Mazzocco A, Waddell TE, Lingohr E, Johnson RP. Enumeration of bacteriophages by double agar overlay plaque assay. Methods Mol Biol.2009;501:69-76. doi: 10.1007 / 978-l-60327-164-6_7. PMID: 19066811. The total volume of effluent and agar was 8mL. Plates were allowed to dry, inverted, and then incubated for 17-19 hours at 37 C before viruses were counted.

[0116] The results of the tests are shown below in Figure 1, where the removal efficiency of MS2 virus is plotted for the various viral loads (expressed as pfu / m2). From Figure 1, it can be seen that a combination of positively charged nanofibers and least 1.0 wt.% of a positively charged binder is needed to maintain high viral removal efficiency at higher viral loads, where the comparative media start to show a drop in performance. This is evident when comparing Comparative Example 2 with Working Example 2, where Working Example 2 maintains virus removal above 3-log (99.9%) even at high viral loads of up to 2.0 X 1012viruses (pfu) per unit area while the Comparative Example 2 dropped below 3-log removal at viral loads above 1.75 X 1012viruses (pfu) per unit area. Thus, the working examples show higher virus removal capacities.

[0117] Additionally, the impact of using non-fibrillated fibers, preferably large diameter non-fibrillated fibers like softwood pulp was also studied. Lab samples, where the cationic binder is added at 2wt%, were made containing 5 wt.%, 7.5 wt.% and 10 wt.% of Alabama River Pine softwood, based on the weight of the filter media. A further lab sample was alsomade with 2wt% cationic binder and no additional larger diameter non-fibrillated fibers. The examples were formed in the laboratory, using the lab process described above. The results are summarized below in Table 2 and Figure 2.Table 2

[0118] From Figure 2, it can be seen that adding up to 10wt% of non-fibrillated fibers, and more specifically large diameter non-fibrillated fibers such as softwood pulp, allowed the sheets to have a better performance than the comparative media while also improving the flow-rate or permeability to water (i.e. water flows through the media at a higher flowrate, ensuring filtration is faster). This allows one to design more permeable material for high-flow applications without compromising the virus removal. Working Example 5, which contained 7.5wt% Alabama Pine, and the Comparative Example, were tested for an extended time, for viral loads up to almost 3.5 X 1012pfu / m2. Working Example 5 maintained virus removal above 3-log (99.9%) even at high viral loads of up to 3.5 X 1012viruses (pfu) per unit area while the Comparative Example 1 went well below 3-log removal at viral loads above 1.75 X 1012viruses (pfu) per unit area. Working Example 3, 4 and 5 showed better virus removal capacities than the Comparative Example 1.

[0119] Lastly, the amount of positively charged nano-fibers was varied. Lab samples, where the cationic binder is added at 2wt%, were made containing 55 wt.%, 40wt% and 35wt% of positively charged nanofibers, more specifically nano-alumina fibers, based on the weight of the filter media. The examples were formed in the laboratory, using the lab process described above. The results are summarized below in Table 3 and Figure 3.Table 3

[0120] From Figure 3, it can be seen that when the filer media contains cationic binder amounts above 1 wt.%, a lower amount of positively charged nanofibers can be used. Working Example 7, Working Example 8 and the Comparative Example 1, were tested for an extended time, for viral loads up to almost 3.5 X 1012pfu / m2. All working examples showed better virus removal capacities than the control media. In particular, Working Example 3 and Working Example 7 showed virus removal of above 5-log (99.999%) throughout the duration of the test while Comparative Example dropped to below 3-log virus removal at viral loads above 1.75 X 1012pfu / m2. Working Example 8, which contains about 19wt% less nano-alumina than the comparative example, showed much better virus removal capacity than the Comparative example 1.Example 9

[0121] This example was conducted to demonstrate the chlorine removal capabilities of the filter medium. The filter medium contains activated carbon adsorbent and has chlorine removal of 87% by weight, based on the original chlorine content of the chlorine containing solution.

[0122] The example described below in Table 4 was manufactured on a paper machine, using the lab process described as follows. Fibrillated fibers (A) and non-fibrillated fibers (D) were added to a pulper and mixed with water. Aluminum (Al) metal and sodium hydroxide (NaOH) was added and reacted in order to form the positively charged nanoalumina under heat. A positively charged binder was then added. In a separate step and after the positively charged binder was added, the non-positively charged binder was added. The mixture was transferred over to a machine chest, where adsorbent particles (in this case, activated carbon) was added. The mixture was then applied to the wire conveyor of the papermaking machine to form a fibrous web. The fibrous web was dried to form the filter medium.

[0123] Chlorine Removal test method: The chlorine removal capability of the filter media is evaluated by preparing a chlorine solution at approximately 2 ppm (verified to be between 1.8 and 2.0 ppm) using diluted bleach in reverse osmosis water and maintaining the solution at 23 °C ± 2 °C. The initial chlorine concentration is verified to be between 1.8 and 2.0 ppm using a spectrophotometric method with DPD reagent (HACH Method 8021 -Free Chlorine, DPD Colorimetric Method). The filter sample is mounted in a 25 mm syringe filter holder and connected to a pressurized system delivering the chlorine solution at a controlled flow rate of about 121 mL / min. A total of 1000 mL of solution is passed through the filter, and the final effluent sample is analyzed for free chlorine using the same spectrophotometric method. Chlorine removal efficiency is calculated as the percentage reduction from the initial concentration.Table 4

[0124] While the invention has been described with reference to some embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. CLAIMS1. Filter medium for filtering fluids, comprising, based on the total weight of the filter medium,3.(A) up to 60 wt% of fibrillated fibers4.(B) up to 65 wt.% of positively charged nanofibers and5.(C) at least 1.0 wt.% of a positively charged binder.

2. Filter medium according to claim 1, characterized in that the fibrillated fibers (A) are comprised in an amount of 2 to 60 wt%, preferably 3 to 55 wt%, and more preferably 4 to 50 wt%, based on the total weight of the filter medium.

3. Filter medium according to one of the preceding claims, characterized in that the fibrillated fibers (A) are selected from naturally occurring cellulosic fibers, synthetic cellulosic fibers (such as viscose, rayon, lyocell) and combinations thereof.

4. Filter medium according to one of the preceding claims, characterized in that the fibrillated fibers (A) have a fibrillation level less than 300 ml Canadian Standard Freeness (CSF), preferably less than 200 ml CSF, preferably less than 100 ml CSF, preferably less than 60 ml CSF, and more preferably less than 50 ml CSF as specified by TAPPI test method T 227 om-9494 (Freeness of Pulp).

5. Filter medium according to one of the preceding claims, characterized in that at least a part of the positively charged nanofibers (B) is disposed on at least some of the fibrillated fibers (A).

6. Filter medium according to one of the preceding claims, characterized in that the positively charged nanofibers (B) are nano alumina fibers.

7. Filter medium according to one of the preceding claims, characterized in that the positively charged nanofibers (B) are comprised by 15 to 60 wt.%, e.g., 25 to 55 wt.%, based on the total weight of the filter medium.

8. Filter medium according to one of the preceding claims, characterized in that the positively charged binder (C) is selected from cationic polymers, preferably cationic polymers selected from the group consisting of a polyamide-epichlorohydrin (PAE), a polyethyleneimine (PEI), a polyamidoamine, a polylysine, a poly(allyla-mine), a poly(diallyldimethylammonium chloride), a poly(N-isopropyl aery 1 ami de-co-acrylamide), a poly(N-isopropyl acrylamide-co-acrylic acid), a diethylaminoethyl-dextran copolymer, a poly-(N-ethyl-vinylpyridinium bromide), a poly(dimethyla-mino)ethyl methacrylate), a poly(ethylene glycol)-co-poly(trimethylaminoethyl-methacrylate chloride), an epichlorohydrin based polymer, a polyamide, an epichloro-hydrin-dimethylamine copolymer, and a combination thereof.

9. Filter medium according to one of the preceding claims, characterized in that the positively charged binder (C) is comprised in an amount of 1.0 to 5.0 wt%, preferably 1.1 to 2.5 wt%, more preferred 1.25 to 1.25 wt.%, based on the total weight of the filter medium.

10. Filter medium according to one of the preceding claims, characterized in that additionally non-fibrillated fibers (D) selected from non-fibrillated cellulosic fibers and non-fibrillated non-cellulosic fibers such as synthetic polymeric fibers and / or glass fibers are comprised.

11. Filter medium according to one of the preceding claims, characterized by the total fibrous content being the total weight of the fibrillated fibers (A) and non-fibrillated fibers (D) in the filter medium is between 25 to 90 wt%, preferably 30 to 85 wt%, based on the total weight of the filter medium.

12. Filter medium according to one of the preceding claims, characterized in that additionally adsorbent particles (E) are comprised which preferably are selected from the group consisting of activated carbon, diatomaceous earth, silica, porous glass, zeolites, alumina, activated alumina, ion exchange resins, silica gel, molecular sieves, chitosan, rice husk, straw, crystalline materials such as y-alumina, zirconia, cellulose nanocrystals (CNC), cyclic oligosaccharides such as cyclodextrin, titania, clay minerals, and a combination thereof, which preferably are comprised in an amount of up to 60 wt%, preferably 10 to 55 wt% and more preferably 12 to 40 wt%, based on the total weight of the filter medium.

13. Filter medium according to one of the preceding claims, characterized in that additionally at least one non-positively charged binder (F) is comprised, preferably in amounts of up to 5 wt%, preferably in amounts of 0.5 to 3 wt%, and more preferably in amounts of 1.0 to 2 wt.%, based on the total weight of the filter medium which preferably is selected from the group of emulsion polymers, preferably emul-sion polymers selected from the group consisting of polyethylene, polypropylene, polyester, polyurethane, phenolic resin, acrylic resin, silicone resin, nitrile rubber (e.g. acrylonitrile-butadiene rubber) and a combination thereof.

14. Filter medium according to one of the preceding claims, characterized in that the total amount of positively charged binders (C) and non-positively charged binders (F) in the medium is less than 10 wt%, preferably less than 6 wt%, and more preferably less than 4 wt.%, based on the total weight of the filter medium.

15. Filter medium according to one of the preceding claims, characterized by a thickness of 600 to 1000 micrometers, preferably 650 to 900 micrometers, and more preferably 675 to 875 micrometers, measured according to ISO 9073-2:1997.

16. Filter medium according to one of the preceding claims, characterized by a maximum pore size of less than 5 micrometers, preferably less than 4.5 micrometers and / or a mean flow pore size less than 2.5 micrometers, preferably less than 2 micrometers.

17. Filter medium according to one of the preceding claims, characterized by a minimum virus removal from water of 4-log and a minimum bacterial removal of 6-log from water.

18. Filter medium according to one of the preceding claims, characterized by a chlorine removal capability of at least 40 wt%, preferably at least 50 wt%, preferably at least 60 wt%, preferably at least 66 wt%, preferably at least 70 wt%, preferably at least 75 wt%, and preferably at least 80 wt%, based on a total weight of the chlorine present in a solution.

19. Method for the production of a filter medium according to one of the preceding claims, comprising the following steps:24.- mixing fibrillated fibers (A) and optionally - if present - non fibrillated fibers (D) with water to form a fiber mixture25.- subsequently, producing positively charged nanofibers (B) in the fiber mixture and / or adding positively charged nanofibers (B) to the fiber mixture,26.- adding a positively charged binder (C) to the fiber mixture to form a slurry,27.- applying the slurry onto a wire conveyor in a papermaking machine and removing water to form a fibrous web, - drying the fibrous web to form the filter medium.

20. Method according to the preceding claim, characterized in that the producing of the positively charged nanofibers (B) by29.- adding a reactive precursor material (Bl) for positively charged nano-fibers (B) and a reactant (B2) capable to react with the reactive precursor (Bl) to form positively charged nanofibers (B) on the surface of at least some of the fibrillated fibers (A) and optionally - if present - on the surface of at least some of the non-fibrillated fibers (D), and30.- reacting the reactive precursor material (Bl) with the reactant (B2) to form positively charged nanofibers (B) on at least some of the surface of the fibrillated fibers (A) and optionally - if present - on the surface of at least some of the non-fibril-lated fibers (B).

21. Method according to one of two preceding claims, characterized in that in addition to the fibrillated fibers (A) at least one of32.non-fibrillated fibers (D), selected from non-fibrillated cellulosic fibers and non-fibrillated non-cellulosic fibers such as synthetic polymeric fibers and / or glass fibers are mixed with the fibrillated fibers and subsequently mixed with water to form the fiber mixture or are added to the fiber mixture containing the fibrillated fibers (A), adsorbent particles (E) are added in a subsequent step after the positively charged nanofibers (B) and the positively charged binder (C) are added and33.in addition to the positively charged binder (C) a non-positively charged binder (F) is added, wherein the positively charged binder (C) is added to the fiber mixture and the non-positively charged binder (F) is added subsequently to the positively charged binder (C), preferably to the slurry, especially preferred to the slurry before the slurry is applied onto the wire conveyor.

22. Method according to one of the two preceding claims, characterized in that Method of reducing the concentration of bacteria and / or viruses in water by passing water containing bacteria and / or viruses through a filter medium according to one of the claims 1 to 17.

23. Use of a filter medium according to one of the claims 1 to 17 for reducing the concentration of bacteria and / or viruses in water containing bacteria and / or viruses.

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