Filtration medium for removal or inactivation of microorganisms from water

Abstract

A filtration medium useful to remove bacteria and/or inactivate virus in water. Examples of the medium include two outer layers made from cellulose fibers and an inner layer made of packed ceramic granules. Methods for producing the medium are also provided.

Claims

1. A filtration medium in the form of a flexible sheet, comprising: a first layer of fibrous material; a second layer of fibrous material; and a third layer disposed between the first layer and the second layer, the third layer comprising porous ceramic granules having pores of about 20 nm to about 140 nm in diameter, wherein at least 60% of the pores have a diameter between 20 nm and about 70 nm, and wherein the porous ceramic granules comprise zero valent iron deposited in the pores in-situ, and the porous ceramic granules comprise about 90 wt % or more aluminum oxide which is obtained from desilicication of a clay material.

2. The medium of claim 1, wherein at least one of the first layer and the second layer of fibrous material comprises cellulose fibers.

3. The medium of claim 1, wherein the medium has a total thickness of about 1 mm or less.

4. The medium of claim 1, wherein the granules of the third layer have an outer diameter of greater than zero and not exceeding 50 microns.

5. The medium of claim 1, wherein the porous ceramic granules of the third layer have a packing density of 100 to 200 gsm.

6. The medium of claim 1, wherein the medium is effective for removal of bacteria in water.

7. The medium of claim 1, wherein the medium is effective for inactivating viruses in water when virus-containing water is passed through the medium.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts the layered structure of a filtration medium according to some embodiments of the present invention.

(2) FIGS. 2A-2D are SEM micrographs of captured E. coli by a filtration medium according to some embodiments of the present invention.

(3) FIG. 3 is an SEM micrograph of a tobacco mosaic virus having passed a filtration medium according to some embodiments of the present invention.

DETAILED DESCRIPTION

(4) Embodiments of the invention are disclosed herein, in some cases in exemplary form or by reference to one or more Figures. However, any such disclosure of a particular embodiment is for illustration purpose only, and is not indicative of the full scope of the invention.

(5) In one aspect of the present invention, a filtration medium is provided. In some embodiments, the filtration medium is effective to remove bacteria from water. In certain embodiments, the filtration medium is effective to inactivate or kill viruses. The medium can also remove metallic ions contaminants from water, e.g., arsenic (including arsenates, arsenites), and lead (Pb) ions. Thus, the filtration medium can treat water to remove many species of harmful substances all at once. Moreover, the filtration medium can perform all these functions by gravity filtration, i.e., the medium can be self-standing or placed on top of a container and water is passed through it from above and flows to the container thereunder. No vacuum pump or external pressure is required; water can pass through the layers just by gravity. Thus, the filter medium is particularly suitable for point of use devices for treating water of questionable quality.

(6) As illustrated in FIG. 1, filtration medium 100 includes a first layer 110 made from a fibrous material, a second layer 120 made from a fibrous material, and a third layer 130 disposed between the first layer 110 and the second layer 120. The third layer includes porous ceramic granules having pores of about 20 nm to about 140 nm in diameter. For convenience and illustration below, the first layer 110 is also referred to as the bottom layer, and the second layer 120 is also referred to as the top layer, and the third layer 130 is also referred to as the intermediate layer.

(7) The bottom layer 110 and the top layer 120 can each be made from nonwoven fibers, such as natural cellulose fibers, which can be cotton fibers or fibers derived from other plants. Synthetic fibers can also be included, such as polyethylene, polypropylene, polyesters, polyamides, acrylics, fiber glass, etc. Manufacture of the filtration medium can be accomplished by a procedure similar to that for making nonwoven fabric. For example, cellulose fibers, e.g., in pulp form, can be laid on a surface, and then the ceramic granules are laid on top of the cellulose fibers with predetermined density (g/m.sup.2, or gsm). Finally, another layer of cellulose fibers is laid on top of the ceramic granules. The layered structure is compacted while being subject heat treatment, e.g., by heated rollers, for the bonding of the fibers and the tight integration of the layers, as well as for removing moisture. The ceramic granules may partially aggregate during the process. The residual moisture can be further heated by microwave heater. The finished medium product is sheet-like, and can be flexible and foldable like regular paper. While generally porous (with pore sizes in tens or hundreds of microns), the top and bottom fiber layers provide strength and structural integrity for the medium. The medium can be made in varying thickness, e.g., about 2 mm or less, about 1 mm or less, or even thinner.

(8) The ceramic granules for the intermediate layer include a plurality of pores having a diameter from 20 nm to 140 nm. In some embodiments, at least 50% of the pores of the medium have a diameter between about 20 nm and about 70 nm. In other embodiments, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, or at least 90% of the pores of the medium have a diameter between 20 nm to 70 nm. In some embodiments, at least 70% of the plurality of pores have a diameter between 40 nm and about 60 nm. The pores form open structure so that water can permeate through the medium. If the pores sizes are too large, microorganisms like bacteria and virus can pass freely through the pores without being trapped or otherwise impaired.

(9) The ceramic granules can be based on aluminum oxide and contains about 90 wt % or more aluminum oxide, and may further include carbon. In some embodiments, the ceramic granules include zero valent iron (or ZVI or Fe(0)). In exemplary embodiments, the granules can be made according to the procedure described in U.S. provisional application 62/349,022 filed Jun. 12, 2016 as well as in PCT Application PCT/US2017/036922 filed on Jun. 11, 2017, the disclosure of which is incorporated herein by reference in its entirety. Briefly, a structuring material (such as a clay material containing 70 wt % or more of aluminum oxide, e.g., diatomaceous earth, which can be desiliciated first to reduce SiO.sub.2 content) is mixed with a carbon source material (such as a carbohydrate, e.g., sugar or starch) and water to obtain a raw pottery or ceramic granule, which contains carbon (at least some of which is believed to be activated carbon) adhering to the walls of the pores comprising mostly aluminum oxide. The raw pottery or ceramic granule is then heated or fired in an anoxic atmosphere or chamber to produce a porous granule, which is then put into contact first with a solution containing Fe.sup.2+ and then a reductant (such as NaBH.sub.4 or KBH.sub.4) that reduces Fe.sup.2+ to ZVI in the pores of the porous granule in-situ. The ZVI containing porous granule is again heated in an anoxic atmosphere to produce the granules for use in the intermediate layer. Such ZVI containing granules are capable of removing arsenates, arsenites, Pb, Cd, Hg, and other heavy metal ions in water.

(10) To reduce the chance of the microorganisms escaping through the interstices of the granules, the size of the ceramic granules should be sufficiently small to ensure close packing of the granules. For example, the granules can have an outer diameter of not exceeding about 50 microns (or the granules can be screened by size 320 mesh). In some embodiments, the packing density of the granules in terms of basis weight can be about 50 to about 300 gsm, or about 100 to about 200 gsm. The granules should be evenly distributed within the outer fibrous layers without crevices or holes when examined in spotlight devices.

(11) The filtration medium of the present invention can be effective for removal of bacteria in water. As the size of the pores of the granules are generally smaller than the size of bacteria, bacteria in water will be intercepted by the filtration medium, usually at the interface between the granules and the fibrous layer.

(12) Furthermore, the filtration medium of the present invention can be effective for inactivating or killing viruses in water when virus-containing water is passed through the filtration medium by gravity. While not wishing to be bound by any particular theory, it is believed that when a virus is small enough to get into the pores to the ceramic granules, when it migrates through the interconnected porous structure, its protective envelope or capsid (the protein shell) can be damaged by the jagged pore surface of the ceramic granules, which causes the genetic material of the virus to degrade or otherwise lose its activity.

(13) The following examples are by way of illustration and not by way of limitation.

Example 1: Manufacture of Granules of the Intermediate Layer

(14) Diatomaceous earth powders from bauxite mining site treated by desilicication were grinded into 1200 standard mesh by air blow selection and separation, and mixed with 5% of starch as carbon source. The mixture powder was granulated in size of 0.5 mm to 1.0 mm raw pottery by adding about 12% to about 15% pure water (on the basis of the weight of the raw pottery). The raw pottery granule thus formed was fired in 500° C. for three hours with a temperature increase rate of 2° C./min. The fired media was submerged in 2% FeSO.sub.4 solution for 15 minutes, taken out to naturally leach out the water, and then put into 2% of NaBH.sub.4 solution for 30 minutes for zero valent iron crystallization to occur inside the pores of the media. The ZVI solution treated media was fired again in an oven at 480-500° C. for 3 hours with protection of nitrogen during the entire firing process. The treated media was then cooled down to room temperature and stored for future use.

Example 2: E. Coli Capture

(15) A sheet-like filtration medium of the present invention containing an intermediate layer including granules prepared in accordance with Example 1 is used to filter a solution containing E. coli. FIGS. 2A-2D includes certain SEM pictures of captured E. coli by the filtration medium. It is shown that the bacteria are generally retained on the edges of the ceramic granules, near the interface at the fibrous layer (FIG. 2D shows a dividing E. coli as being immobilized on ceramic granules). It is surmised that the absorptive properties of the ceramic granules play a role in retaining the bacteria on their surfaces.

Example 3: Removal of Bacillus cereus

(16) 50 μl Bacillus cereus solution having a concentration of 10,000/ml was added to 500 ml sterile distilled water to prepare a Bacillus stock solution. 20 ml of the stock solution was taken with a graduated cylinder, and filtered with one sheet of a filtration medium of the present invention. Another 20 ml of the stock solution was filtered with two sheets of the filtration medium. 400 μL of the unfiltered bacilli bacteria solution as well as 400 μL of the two filtered solutions were added and spread in separate sterile agar plates. The plates were incubated at 37° C. for 24 h. The number of colonies in each plate was visually observed. It was found that one sheet of filtration medium was already sufficiently effective to remove all Bacillus cereus from the stock solution. Two sheets of the filtration medium also removed 100% of the bacillus from the stock solution.

Example 4: Killing Virus by Filtration Medium

(17) A solution containing tobacco mosaic virus was passed through a filtration medium of the present invention. The virus was determined inactivated. As seen in FIG. 3, which is a SEM picture of a tobacco mosaic virus having passed the medium. It appeared that the envelope of the virus was cut open along its length direction, which apparently led to the demise of the virus.

(18) It will be apparent to one skilled in the art that varying modifications can be made to the invention disclosed herein without departing from the scope and spirit of the invention.