Method and system for purifying water containing heavy metal ions and other substances.

Abstract

This invention provides a method for purifying water containing iron ions and dissolved arsenic, and a corresponding purification system, that can stably remove not only dissolved ions such as these heavy metal ions but also precipitates from the water over a long period of time using a simple method. [Solution] A method for purifying water to be treated that contains iron ions and dissolved arsenic, comprising the step of removing iron ions and dissolved arsenic from the water to be treated by passing the water to be treated through a reaction vessel having a packing layer containing fine grain husks and a water seal layer disposed above the water seal layer from above the water seal layer.

Description

[Technical Field]

[0001] This invention relates to a method and system for purifying water to be treated that contains iron ions (dissolved iron) as heavy metal ions and dissolved arsenic. In particular, this invention relates to a method for purifying water to be treated that contains iron ions and dissolved arsenic using a reaction vessel equipped with a packing layer containing grain husks, and a corresponding purification system. [Background technology]

[0002] Mining-derived wastewater, such as that from metal mines, and various types of wastewater, such as industrial wastewater, generally contain ions of various heavy metals such as Fe (iron), Zn (zinc), Cu (copper), Pb (lead), and Cd (cadmium), as well as sulfate ions (SO4). 2- These may also contain dissolved arsenic. Many of these heavy metal ions have harmful effects on the human body and the environment. Therefore, when discharging water containing these heavy metal ions, treatment is required to meet established wastewater standards.

[0003] A known method for removing iron ions and dissolved arsenic from treated water involves the action of iron-oxidizing bacteria and dissolved oxygen. For example, Patent Document 1 reports a method for treating acidic mine wastewater containing arsenic and iron, comprising an arsenic oxidation step of oxidizing trivalent arsenic-containing ions to pentavalent arsenic-containing ions, a dearsenic step of adsorbing arsenic-containing ions using an adsorbent made of an iron compound having arsenic-containing ion adsorption capacity supported on a silicate fiber material, an iron oxidation step of oxidizing divalent iron ions to trivalent iron ions using iron-oxidizing bacteria, a de-ironization step of precipitating trivalent iron ions as iron compounds on the surface of a silicate fibrous inorganic material, and a pH adjustment step. However, such methods for purifying treated water require multiple complicated steps to remove heavy metal ions such as iron ions, resulting in additional costs and effort for operation and management.

[0004] Furthermore, in contrast to active treatment, which directly removes heavy metal ions and other substances from treated water by administering chemicals, research has recently been conducted on so-called passive treatment technologies that utilize natural purification processes to treat water, minimizing the use of electricity-consuming equipment and chemicals, with the aim of reducing treatment costs and energy consumption. As an example of such passive treatment technology, Non-Patent Literature 1 reports a process for removing iron from mine wastewater using a vertical flow reactor equipped with a crushed stone layer at the bottom and a water seal layer above it. In the process described in this document, iron(II) ions (divalent iron ions) in the mine wastewater are oxidized to iron(III) ions (trivalent iron ions) by the action of iron-oxidizing bacteria and dissolved oxygen, and iron is removed by filtering the iron-containing precipitate (ferrihydrite or schwertmanite, depending on the liquid properties) through the crushed stone layer. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2005-058955 [Non-patent literature]

[0006] [Non-Patent Document 1] Ivan Blanco et al., “International Trials of Vertical Flow Reactors for Coal Mine Water Treatment”, Mine Water Environ(2018)37 pp.4-17 [Overview of the project] [Problems that the invention aims to solve]

[0007] Conventional methods for purifying water containing iron ions often make it difficult to adequately remove and purify iron-containing precipitates (sediments and suspended solids derived from precipitates) from the water over long periods. For example, while the method described in Non-Patent Literature 1 relating to passive treatment technology can remove iron ions, it is difficult to adequately prevent the outflow of iron-containing precipitates over long periods. Therefore, if the water to be treated also contains other harmful elements such as arsenic, there is a risk that these other harmful elements may also be adsorbed onto the iron-containing precipitates and leached out.

[0008] Therefore, the problem that the present invention aims to solve is to provide a method for purifying water containing iron ions and dissolved arsenic, and a corresponding purification system, that can stably remove these heavy metal ions and the like over a long period of time using a simple method that does not require complicated processes. [Means for solving the problem]

[0009] As a result of diligent research, the inventors have discovered that by passing water to be treated, which contains iron ions and dissolved arsenic, from above into a reaction vessel containing a packing layer of fine grain husks, it is possible to stably and sufficiently remove and purify iron ions and dissolved arsenic from the water to be treated over a long period of time by capturing the reaction products between iron ions and dissolved oxygen in the water, the products formed by the oxidation of iron-oxidizing bacteria, and the dissolved arsenic that is adsorbed or not adsorbed onto these products on the surface of the fine grain husks, thus completing the present invention.

[0010] One aspect of the present invention for achieving the above objective is as follows: A method for purifying water to be treated that contains iron ions and dissolved arsenic, A method comprising the step of removing iron ions and dissolved arsenic from water to be treated by passing the water to be treated through a reaction vessel having a packing layer containing fine grain husks and a water seal layer positioned above the water seal layer, from above the water seal layer.

[0011] Furthermore, another aspect of the present invention for achieving the above objective is as follows. A water purification system for water containing iron ions and dissolved arsenic, A reaction vessel comprising a packing layer containing fine grain husks and a water seal layer positioned above it, A supply system for supplying the water to be treated to the reaction tank, A system comprising a discharge system for discharging treated water, from which iron ions and dissolved arsenic have been removed from the treated water, from the reaction tank. [Effects of the Invention]

[0012] According to the method and system for purifying treated water of the present invention, by passing treated water containing iron ions and dissolved arsenic through a reaction vessel comprising a packing layer containing fine grain husks and a water seal layer positioned above it, from above the water seal layer, it is possible to stably and sufficiently remove and purify iron ions and dissolved arsenic from the treated water over a long period of time. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a schematic diagram of a water purification treatment device for water containing iron ions and dissolved arsenic, according to one embodiment of the present invention. [Figure 2] Figure 2(a) is a graph plotting the changes in the amount of soluble Fe (iron) in the water to be treated flowing into the reaction tank, the total amount of Fe (iron) in the treated water flowing out of the reaction tank, and the amount of soluble Fe (iron) against the number of days elapsed in the water treatment method of Example 1 according to the present invention. Figure 2(b) is a graph plotting the changes in the amount of soluble As (arsenic) in the water to be treated flowing into the reaction tank, the total amount of As (arsenic) in the treated water flowing out of the reaction tank, and the amount of soluble As (arsenic) against the number of days elapsed in the water treatment method of Example 1 according to the present invention. [Figure 3]Fig. 3(a) is a graph plotting the changes in the amount of dissolved Fe (iron) in the water to be treated flowing into the reaction tank, the total amount of Fe (iron) in the treated water flowing out of the reaction tank, and the amount of dissolved Fe (iron) with respect to the number of days elapsed in the method for purifying the water to be treated of Comparative Example 1 according to the prior art. Further, Fig. 3(b) is a graph plotting the changes in the amount of dissolved As (arsenic) in the water to be treated flowing into the reaction tank, the total amount of As (arsenic) in the treated water flowing out of the reaction tank, and the amount of dissolved As (arsenic) with respect to the number of days elapsed in the method for purifying the water to be treated of Comparative Example 1 according to the prior art.

Embodiments for Carrying out the Invention

[0014] Hereinafter, one embodiment for carrying out the present invention will be described. The present invention is not limited by the description of the following embodiments. The purification system for the water to be treated containing iron ions and dissolved arsenic according to the present invention is a system including components for realizing the method for purifying the water to be treated containing iron ions and dissolved arsenic according to the present invention. Since both have substantially common technical matters, hereinafter, the description will be mainly made from the viewpoint of the purification method.

[0015] The method for purifying treated water containing iron ions and dissolved arsenic according to this embodiment includes a step of removing iron ions and dissolved arsenic from the treated water by passing the treated water through a reaction tank including a packing layer containing fine grain husks and a water seal layer disposed above the packing layer from the upper side of the water seal layer (for example, as a vertical downward flow). That is, in the method for purifying treated water containing iron ions and dissolved arsenic according to this embodiment, by passing the treated water containing iron ions and dissolved arsenic from the upper side of the water seal layer (for example, as a vertical downward flow), the treated water passes through the water seal layer and the packing layer of the reaction tank from above to below. In the process of this passage, precipitates of products formed by the reaction product of dissolved oxygen in water and iron ions in the treated water (mainly understood to be iron hydroxide) and iron ions (III) formed by the oxidizing action of iron-oxidizing bacteria (mainly understood to be ferrihydrite and schwertmannite depending on the liquidity) are formed. These precipitates of products together with the dissolved arsenic adsorbed thereto or the dissolved arsenic not adsorbed thereto are directly filtered and captured on the surface of the grain husks filled in the reaction tank, thereby removing and purifying iron ions and dissolved arsenic. In this specification, "precipitate" refers to any solid substance precipitated from iron ions and the like in water, and includes both precipitates (also referred to as "sediments") and suspended substances. In addition, the passage of the treated water as a vertical downward flow in the reaction tank can usually be carried out only by the natural fall of the treated water due to gravity. The passage of the treated water as a vertical downward flow in the reaction tank may be advanced temporarily or intermittently by means of decompression and suction from the discharge side depending on the operating conditions of the device.

[0016] In this specification, when referring to "water to be treated containing iron ions and dissolved arsenic," this means water before purification treatment in the reaction tank (supply water to the reaction tank), or water during this purification treatment. Also, in this specification, "water after treatment" means water after it has been subjected to purification treatment in the reaction tank. The water to be treated containing iron ions and dissolved arsenic that is purified by the purification method according to this embodiment is not particularly limited as long as it contains iron ions and dissolved arsenic, but examples include wastewater from mines such as mine wastewater from metal mines, and industrial wastewater. Also, in this specification, "dissolved arsenic" refers to arsenic dissolved in the water to be treated that passes through a filter with a pore size of 0.45 μm.

[0017] In the water purification method according to this embodiment, the pH of the water to be treated, which contains iron ions and dissolved arsenic and is supplied to the reaction tank, is not particularly limited, but is usually about 2.5 to 8.0, about 3.0 to 7.8, about 3.5 to 7.6, about 4.0 to 7.4, about 4.5 to 7.2, or about 4.7 to 7.0 at room temperature.

[0018] In the water purification method according to this embodiment, the fine grain husks constituting the packing material of the reaction tank are not particularly limited as long as they are obtained from grains, but the following grain husks are examples: Grasses belonging to the Poaceae family, such as rice, wheat, barley, rye, oats, millet, barnyard millet, and foxtail millet; legumes belonging to the Poaceae family, such as soybeans, adzuki beans, mung beans, kidney beans, peanuts, and peas; buckwheat of the Polygonaceae family; quinoa of the Chenopodiaceae family; and senna of the Amaranthaceae family.

[0019] In the water purification method according to this embodiment, among these examples of grain husks that can be used as fine grain husks constituting the packing material of the reaction tank, rice husks are preferred because they are consumed in large quantities (i.e., readily available) and inexpensive. Rice husks usually have a flattened ellipsoid shape, and the average length of the major axis may be about 3 mm to 10 mm, or about 4 mm to 9 mm. The average length of the major axis of the rice husks can be determined by measuring the length of the major axis of 10 arbitrarily selected granular materials using a standard measuring tape and calculating the average value. By using rice husks of this size, precipitates containing iron ions formed as the water to be treated passes from top to bottom through the water seal layer and packing layer of the reaction tank can be filtered and captured on the surface of the grain husks packed in the reaction tank, along with dissolved arsenic adsorbed on them, or dissolved arsenic that is not adsorbed can be filtered and captured directly on the surface of the grain husks packed in the reaction tank, while ensuring a clear flow path for the water to be treated and suppressing clogging by precipitates.

[0020] In the water purification method according to this embodiment, the packing layer of the reaction tank is preferably composed of fine grain husks, but a mixture of grain husks and crushed stone may also be used. By mixing crushed stone with the fine grain husks that make up the packing layer, it is possible to prevent the fine grain husks, which have a low specific gravity, from floating upward in the water of the reaction tank. As a result, the fine grain husks are continuously fixed in a predetermined position at an appropriate packing density, thereby enabling stable and sufficient filtration and capture of precipitates over a long period of time.

[0021] The type of crushed stone used in the packing layer of the reaction vessel is not particularly limited, as long as it can adequately prevent the suspension of fine grain husks. The particle size of the crushed stone is usually about 10 mm to 50 mm, preferably about 15 mm to 45 mm, about 20 mm to 40 mm, or about 15 mm to 30 mm, from the viewpoint of adequately preventing the suspension of fine grain husks and ensuring a clear flow path for the treated water.

[0022] In the water purification method according to this embodiment, when the packing layer of the reaction tank contains a mixture of fine grain husks and crushed stone, the volume ratio of fine grain husks to crushed stone in this mixture is not particularly limited, but is preferably 1:0.5 or more and 1:100 or less. By adjusting the volume ratio of fine grain husks to crushed stone in the mixture of grain husks and crushed stone to within the above range, the fine grain husks, which have a low specific gravity, are effectively prevented from floating upward in the water of the reaction tank, allowing for sufficient filtration and capture of precipitates, and ensuring an adequate flow path for the water to be treated. This makes it possible to avoid stopping the operation of the purification treatment device for maintenance to remove blockages, and to continue stable operation over a long period of time. From the viewpoint of further enhancing the above effect, the volume ratio of fine grain husks to crushed stone in a mixture of grain husks and crushed stone may more preferably be 1:0.7 to 1:80, 1:1 to 1:50, 1:1.5 to 1:40, 1:2 to 1:30, 1:2.5 to 1:20, or 1:3 to 1:10.

[0023] In the water purification method according to this embodiment, when the packing layer of the reaction tank contains a mixture of fine grain husks and crushed stone, the packing layer may normally be a single layer, but alternatively, it may be formed from multiple layers with different volume ratios of fine grain husks to crushed stone. When the packing layer of the reaction tank is formed from multiple layers with different volume ratios of fine grain husks to crushed stone, from the viewpoint of balancing the effect of preventing the suspension of fine grain husks and the filtering and capturing effect of precipitates, it is preferable that the upper layers have a lower volume ratio of fine grain husks to crushed stone (i.e., the upper layers have a higher volume ratio of crushed stone to fine grain husks). Furthermore, when the packing layer is formed from multiple layers with different volume ratios of fine grain husks to crushed stone, it is desirable that the total volume ratio of fine grain husks to crushed stone of these multiple layers is within one of the above preferred ranges, and it is even more desirable that the volume ratio of fine grain husks to crushed stone of each of these multiple layers is within one of the above preferred ranges.

[0024] In the reaction tank used in the water purification method of this embodiment, the thickness of the packing layer can be appropriately set depending on the characteristics of the water to be treated and the treatment scale. The thickness of the packing layer in the reaction tank is not particularly limited, but may be, for example, 0.03 m to 5 m, 0.05 m to 4 m, 0.1 m to 3 m, 0.12 m to 2 m, or 0.15 m to 1 m. If the packing layer is formed from multiple layers with different volume ratios of grain husk particles to crushed stone, and the upper layer has a lower volume ratio of grain husk particles to crushed stone, it is preferable that the upper layer has a thinner thickness from the viewpoint of balancing the filtering and capturing action of precipitates with ensuring sufficient flow of water to be treated. Furthermore, if the packing layer is formed from multiple layers with different volume ratios of grain husk particles to crushed stone, it is desirable that the sum of the thicknesses of these multiple layers be within one of the ranges exemplified above.

[0025] In the water purification method according to this embodiment, a water seal layer of water to be treated, having a predetermined thickness, is placed above a packing layer containing fine grain husks in the reaction tank. The water seal layer is provided to prevent the packing layer in the reaction tank from being exposed to the outside air, and to further promote the reaction between dissolved oxygen and iron ions in the water to be treated, and the formation of products containing iron(III) ions by the oxidation action of iron-oxidizing bacteria. The thickness of the water seal layer is not particularly limited, but may be, for example, 0.01m to 3m, 0.01m to 2m, 0.02m to 1.5m, 0.02m to 1m, 0.03m to 0.5m, 0.03m to 0.3m, 0.04m to 0.2m, or 0.04m to 0.1m.

[0026] In the water purification method according to this embodiment, the volume ratio of the packing layer to the water seal layer in the reaction tank is preferably 1:0.1 or more and 1:50 or less. By adjusting the volume ratio of the packing layer to the water seal layer in the reaction tank to within the above range, exposure of the packing layer in the reaction tank to the outside air is reliably suppressed, deterioration due to drying of grain husks, etc., can be prevented, and filtration and capture of precipitates in the packing layer can be sufficiently achieved, thereby enabling stable operation to be continued for a long period of time. From the perspective of further enhancing the above effects, the volume ratio of the filler layer to the water seal layer in the reaction tank is more preferably 1:0.2 or more and 1:40 or less, 1:0.4 or more and 1:30 or less, 1:0.7 or more and 1:25 or less, 1:1 or more and 1:20 or less, 1:1.5 or more and 1:15 or less, or 1:2 or more and 1:10 or less.

[0027] In the method for purifying treated water according to this embodiment, a bottom gravel layer for supporting the filler layer may be provided at the bottom of the reaction tank. The gravel constituting the bottom gravel layer is not particularly limited as long as it exhibits the relevant function. The thickness of such a bottom gravel layer is not particularly limited, but may be, for example, 0.01 m or more and 2 m or less, 0.02 m or more and 1 m or less, 0.03 m or more and 0.5 m or less, or 0.04 m or more and 0.3 m or less.

[0028] The reaction tank that can be used in the method for purifying treated water of this embodiment is not particularly limited in terms of shape, material, capacity, etc., as long as the method can be implemented and the reaction treatment can proceed while the treated water moves within the reaction tank. The material of the reaction tank is not particularly limited, but may be mainly made of resin or concrete, for example, and some members may contain metal, ceramics, rock, clay, etc. The shape of the reaction tank is not particularly limited, and may be vertically long or flat, substantially rectangular parallelepiped, substantially cubic, substantially spherical, substantially cylindrical (substantially tubular), or a combination thereof, etc. Also, the reaction tank can be an artificial pond, a constructed wetland, a large tank, etc. The volume of the reaction tank for actual use outdoors is not limited, but may be, for example, 10 m 3 ~1×10 5 m 3 、50 m 3 ~5×10 4 m 3 Or 100 m 3 ~1×10 4 m 3 or so.

[0029] The reaction tank is equipped with an inlet for the water to be treated and an outlet for the treated water. Furthermore, if the water to be treated is groundwater, the reaction tank can be constructed with permeable reaction walls buried underground, and the groundwater flow can be utilized for the supply system of the water to be treated, the discharge system of the treated water, and the energy for supply and discharge.

[0030] In the water treatment purification method according to this embodiment, it is preferable to continuously introduce the water to be treated, such as mine wastewater, into the reaction tank as a vertical downward flow, and then continuously discharge the water that has remained in the reaction tank for a predetermined time. This allows for the continuous removal and purification of iron ions and dissolved arsenic from the water to be treated. Even when such a continuous flow process is adopted, it may be permissible to temporarily interrupt the continuous flow and perform batch processing for operational reasons, such as for periodic or irregular inspection, maintenance, or repair of the equipment.

[0031] In the case of continuous flow of treated water such as mine wastewater, the flow rate (supply and discharge flow rate of treated water to the reaction tank) is not particularly limited, as long as the treated water remains in the reaction tank for a period of time sufficient to adequately remove the desired iron ions and dissolved arsenic. In the case of continuous flow of treated water such as mine wastewater, the flow rate is not particularly limited as it depends on the reaction scale, the type and characteristics of the treated water, etc., but from the viewpoint of sufficient treatment, for example, the lower limit may be 0.01 L / min or more, 0.03 L / min or more, 0.1 L / min or more, 0.3 L / min or more, 1 L / min or more, 2 L / min or more, 4 L / min or more, or 5 L / min or more, and the upper limit may be 500 L / min or less, 300 L / min or less, 100 L / min or less, 50 L / min or less, 30 L / min or less, 20 L / min or less, or 10 L / min or less. These upper and lower limits for the continuous flow rate of the water to be treated may be combined in any way. Furthermore, the hydraulic residence time (HRT) in the case of continuous flow of treated water such as mine wastewater is not particularly limited, but from the viewpoint of sufficient treatment, it may be, for example, 1 hour to 100 hours, 2 hours to 60 hours, 3 hours to 40 hours, 4 hours to 30 hours, or 5 hours to 20 hours.

[0032] The water purification method according to this embodiment can be used with water at any temperature, as long as the reaction between iron ions and dissolved oxygen in the water and / or the oxidation by iron-oxidizing bacteria occur at that temperature. Therefore, this purification method can be implemented in environments where water at a wide range of temperatures is used (for example, in natural environments such as high-latitude regions or high-altitude regions where temperatures are low in winter).

[0033] The method of supplying the water to be treated in the purification method according to this embodiment is not particularly limited, as long as it can be set and adjusted so that it can be supplied to the reaction tank (treatment vessel for carrying out the purification method) at a desired constant flow rate. The water to be treated can be supplied to the reaction tank through a transfer path such as piping. When applying the water purification method according to this embodiment to a natural passive treatment system, it is desirable from the viewpoint of saving labor and reducing costs to configure the system so that it can move by gravity from the supply of the water to be treated to the treatment system and the discharge system, using as little electricity as possible. For this reason, it is preferable to set the height of the transfer path such as piping and the inlet of the reaction tank so that electric pumps are not used in actual application fields.

[0034] In the water purification method according to this embodiment, it is preferable to perform aeration, a treatment operation that increases the oxygen concentration in the water by supplying oxygen or air to the water to further promote the reaction between iron ions and dissolved oxygen in the water. Aeration may be performed by any known method. Examples of aeration methods are not particularly limited, but include a physical aeration method that supplies oxygen to the water to be treated through bubbles using diffusers or aeration devices in the reaction tank, a mechanical aeration method that forcibly mixes the water to be treated and air using an axial flow mixer or agitator in the reaction tank to supply oxygen, and an aeration method that mixes the water to be treated and air when the water falls in a multi-stage cascade channel (stepped channel) prior to supplying the water to be treated to the reaction tank. The aeration method using a multi-stage cascade channel prior to supplying the water to be treated to the reaction tank is advantageous in that it is highly compatible with a natural-use passive treatment system that does not use electricity, because it performs aeration using the energy of gravity due to the difference in altitude.

[0035] According to the water purification method of this embodiment, preferably, when the treated water is discharged after passing water through the water seal layer and packing layer of the reaction tank, the total amount of dissolved iron and precipitated iron in the treated water flowing out of the reaction tank is kept substantially at 1 mg / L or less, and / or the total amount of dissolved arsenic and precipitated arsenic in the treated water flowing out of the reaction tank is kept substantially at 0.1 mg / L or less, and the purification treatment by passing water can be carried out continuously for 30 days or more, more preferably 60 days or more, 90 days or more, 120 days or more, or 150 days or more, without performing maintenance work to remove blockages. Thus, the water purification method of this embodiment can stably remove most iron ions and dissolved arsenic for a considerably long period of time without interrupting the operation of the equipment for maintenance, and can therefore be suitably used for natural passive treatment systems.

[0036] The treated water discharged from the reaction tank by the water purification method of this embodiment, from which iron ions and dissolved arsenic have been removed, may, if necessary, be subjected to a secondary biological purification treatment using a biological purification agent containing grain husks carrying sulfate-reducing bacteria, which is housed in a treatment container where anaerobic conditions are maintained. There, heavy metal ions other than iron ions and dissolved arsenic can be removed by precipitation through the action of sulfate-reducing bacteria. Furthermore, the treated water discharged from the reaction tank using the water purification method of this embodiment, from which iron ions and dissolved arsenic have been removed, may, if necessary, be subsequently passed through an artificial wetland planted with reeds, cattails, water hyacinths, or Canadian pondweed before being released into the environment.

[0037] Figure 1 shows a non-limiting example of an apparatus for implementing the purification method / purification system for water to be treated containing iron ions and dissolved arsenic according to this embodiment. This apparatus is merely a preferred example of an apparatus for implementing the purification method / purification system, and the present invention is not limited in any way thereto. In Figure 1, the reference symbols are as follows: 1 is the reaction tank (a treatment vessel for purifying the water to be treated), 2 is the water seal layer inside the reaction tank, 3 is the packing layer inside the reaction tank, 3a is the fine grain husks (rice husks as a typical example) that make up packing layer 3, 3b is the crushed stone that makes up packing layer 3, 4 is the bottom crushed stone layer located below the packing layer in the reaction tank and supporting the packing layer, 4c is the crushed stone that makes up bottom crushed stone layer 4, 5 is the introduction pipe via a cascade channel for introducing (transferring) the water to be treated into the reaction tank, 6 is the discharge pipe for discharging (transferring) the treated water from the reaction tank, and 10 refers to the entire water purification treatment system.

[0038] In Figure 1, the reaction vessel 1 is a resin or concrete container with a roughly rectangular prism (cuboid or cube) or roughly cylindrical outline. In addition to resin or concrete, the material of the surrounding wall of the reaction vessel 1 may include metal, ceramics, rock, clay, glass, etc., in whole or in part.

[0039] In Figure 1, a bottom crushed stone layer 4 is formed inside the reaction vessel 1 so as to cover the area above the discharge port at the bottom, and a packing layer 3 is positioned above it, supported by the crushed stone layer. The packing layer 3 is composed of a uniform mixture of fine grain husks (rice husks as a typical example) 3a and crushed stone 3b. The bottom crushed stone layer 4 has the function of preventing the outflow of solid matter from the container, thereby preventing clogging of the drainage system.

[0040] In the purification apparatus shown schematicly in Figure 1, the water to be treated, containing iron ions and dissolved arsenic, is introduced into the reaction tank 1 continuously after being thoroughly aerated through an introduction pipe 5, which consists of a multi-stage cascade channel, along the direction of arrow F from the supply source (aeration by a cascade channel is preferable, but is not limiting to the invention). The water to be treated flows downward through the water seal layer 2 and packing layer 3 of the reaction tank 1 in a vertical downward flow due to gravity, and the reaction between iron ions in the water and dissolved oxygen in the water, as well as the formation of precipitates due to the oxidation by iron-oxidizing bacteria, gradually proceeds. The dissolved arsenic adsorbed on these precipitates, or dissolved arsenic that is not adsorbed, is directly filtered and captured on the surface of grain husks (rice husks as a typical example) 3b in the packing layer 3, thereby removing iron ions and dissolved arsenic from the water to be treated, and thus purifying the water to be treated. The water to be treated undergoes such reactions and filtration / capture treatment in the packing layer 3, then passes through the bottom crushed stone layer 4 and reaches the bottom of the reaction tank 1. After treatment, the water is discharged through the outlet and out the discharge pipe 6 in the direction of arrow D.

[0041] In the water purification method using the above apparatus, a uniform mixture of fine grain husks (typically rice husks) 3a and crushed stone 3b is stably arranged in the packing layer 3 of the reaction tank 1 without floating in the water. By passing the water to be treated, which contains iron ions and dissolved arsenic, through this in a vertical downward flow, precipitates are formed, consisting of reaction products between dissolved oxygen in the water and iron ions in the water to be treated, and iron(III) ions formed by the oxidation action of iron-oxidizing bacteria. Dissolved arsenic adsorbed onto these products, or unadsorbed dissolved arsenic, is directly captured in the packing layer 3, mainly by the fine grain husks 3a. By capturing these precipitates derived from iron ions mainly by the fine grain husks 3a, it is possible to prevent iron ions and dissolved arsenic from being discharged together from the system. Therefore, the above method and system offer the advantage of being able to stably and sufficiently remove and purify iron ions and dissolved arsenic from the treated water over a long period of time. Furthermore, since it is assumed that dissolved arsenic is removed by adsorption to the fine grain husks 3a when the precipitate containing iron ions is captured by them, under this assumption, it is expected that the removal rate of dissolved arsenic will be achieved insofar as it correlates with the ratio of iron ions to dissolved arsenic initially contained in the treated water. However, since the dissolved arsenic itself is also captured by the fine grain husks 3a (without adsorption to iron ion-derived substances), it is possible to remove dissolved arsenic with a higher efficiency than expected, which correlates with the ratio of iron ions to dissolved arsenic in the treated water. Furthermore, by using a uniform mixture of fine grain husks 3a and crushed stone 3b, precipitates adhere to the grain husks 3a, ensuring a clear flow path for the treated water and suppressing clogging. This reduces the need to interrupt operations for maintenance work to remove blockages, enabling continuous and stable operation over a considerably long period. [Examples]

[0042] The present invention will be further explained below with reference to examples, but there is no intention to limit the present invention to these examples. The following examples should be understood as illustrative.

[0043] Preparation of the reaction vessel A reaction tank was prepared inside the room, configured to allow the water to be treated to flow vertically downwards from above, with an outlet for the treated water at the bottom. The reaction tank was a roughly rectangular container with transparent plastic walls and an open top, measuring 39.5 cm wide x 49.5 cm deep x 29.5 cm high.

[0044] Example 1 A bottom layer of crushed stone, approximately 1.5 cm thick and with a particle size of 20-40 mm, was laid at the bottom of the reaction vessel. On top of the bottom layer of crushed stone, an 18 cm thick packing layer was placed, consisting of a homogeneous mixture of rice husks (grain husks) and crushed stone with a particle size of 15-30 mm in a 4:1 volume ratio. The rice husks used were procured from farmers. The average length of the long axis of the rice husks used was approximately 5 mm.

[0045] The water to be treated supplied to the above reaction vessel has an average pH of 6.5 and an average concentration of approximately 28.1 mg / L of Fe ions (Fe 2+ Neutral mine wastewater containing dissolved arsenic with an average concentration of approximately 6.3 mg / L was used. The concentrations of these heavy metal ions were measured using ICP-AES. pH was measured at room temperature using a glass electrode manufactured by Toa DKK Corporation. (The same procedure was followed for measuring the concentrations of heavy metal ions below.)

[0046] The water to be treated was continuously passed through the reaction tank from above at a flow rate of 39 mL / min, and allowed to descend vertically through an 18 cm thick packing layer consisting of a homogeneous mixture of rice husks and crushed stone in a 4:1 volume ratio. The water seal layer was 2 cm thick. The hydraulic residence time (HRT) of the water to be treated in the packing layer was adjusted to 10 hours according to the design. As the water to be treated descended through the packing layer of the reaction tank, the reaction between iron ions in the water and dissolved oxygen in the water, as well as the oxidation by iron-oxidizing bacteria, gradually led to the formation of precipitates. These iron ion-containing substances and the dissolved arsenic adsorbed thereon attached to the rice husks, thus purifying the water. After treatment, the water passed through the crushed stone layer at the bottom and reached the bottom of the reaction tank, where it was discharged through the outlet and out the discharge pipe.

[0047] Comparative Example 1 A bottom layer of crushed stone, approximately 3.5 cm thick and with a particle size of about 20-40 mm, was laid at the bottom of the reaction vessel. On top of the bottom layer of crushed stone, a 4.5 cm thick packing layer was provided, consisting only of crushed stone with a particle size of about 15-30 mm (i.e., unlike Example 1, it does not contain rice husks, which are grain husks).

[0048] In Comparative Example 1, the same water to be treated as described above for Example 1 was used as the water to be treated supplied to the reaction tank. In Comparative Example 1, the water to be treated was continuously passed through the reaction tank from above at a flow rate of 180 mL / min, allowing it to naturally descend vertically through a 4.5 cm thick packing layer consisting solely of crushed stone. The thickness of the water seal layer was 19.5 cm. The hydraulic residence time (HRT) of the water to be treated in the packing layer was adjusted to 10 hours according to the design. Six months after the start of treatment, in order to improve the continuously poor results in the removal of iron ions and dissolved arsenic, the flow rate of the water to be treated was changed to 60 mL / min, and the hydraulic residence time (HRT) of the water to be treated in the packing layer was adjusted to 30 hours according to the design.

[0049] In the water purification method of Example 1, the changes in the amount of soluble Fe (iron) in the water to be treated flowing into the reaction tank, the total amount of Fe (iron) in the treated water flowing out of the reaction tank, and the amount of soluble Fe (iron) were measured and recorded with respect to the number of days elapsed, and the resulting graphs are shown in Figure 2(a). Furthermore, in the water purification method of Example 1, the changes in the amount of soluble As (arsenic) in the water to be treated flowing into the reaction tank, the total amount of As (arsenic) in the treated water flowing out of the reaction tank, and the amount of soluble As (arsenic) were measured and recorded with respect to the number of days elapsed, and the resulting graphs are shown in Figure 2(b). Here, "soluble Fe (iron) content" refers to the amount of iron measured from the liquid portion (dissolved portion) after treatment by filtration using a filter with a pore size of 0.45 μm, while "total Fe (iron) content" refers to the amount of iron measured from all of the solid and liquid (dissolved portion) in the treated water. (The same applies to the amount of arsenic measured.) Furthermore, in the water purification method of Comparative Example 1, the changes in the amount of soluble Fe (iron) in the water to be treated flowing into the reaction tank, the total amount of Fe (iron) in the treated water flowing out of the reaction tank, and the amount of soluble Fe (iron) were measured and recorded with respect to the number of days elapsed, and the graphs plotting these results are shown in Figure 3(a). In addition, in the water purification method of Comparative Example 1, the changes in the amount of soluble As (arsenic) in the water to be treated flowing into the reaction tank, the total amount of As (arsenic) in the treated water flowing out of the reaction tank, and the amount of soluble As (arsenic) were measured and recorded with respect to the number of days elapsed, and the graphs plotting these results are shown in Figure 3(b).

[0050] Based on the matters described above for Example 1 and Comparative Example 1, and the comparative results of the graphs shown in Figures 2(a), 2(b), and 3(a) and 3(b), it can be seen that Example 1, according to the embodiment of the present invention, was able to reliably and stably remove and purify Fe (iron) ions and As (arsenic) ions precipitated and dissolved in the treated water over a long period of time, compared to Comparative Example 1. [Explanation of symbols]

[0051] 1: Reaction vessel 2: Hydraulic layer 3: Filling layer 3a: Fine grain husks (rice husks are a typical example) 3b: crushed stone 4: Bottom crushed stone layer 4c: Crushed stone 5: Inlet pipe for treated water via cascade channel 6: Discharge pipe for treated water F: Direction of introduction of water to be treated D: Discharge direction of treated water 10: Purification treatment equipment (overall)

Claims

1. A method for purifying water to be treated that contains iron ions and dissolved arsenic, A method comprising the step of removing iron ions and dissolved arsenic from water to be treated by passing the water to be treated through a reaction vessel having a packing layer containing fine grain husks and a water seal layer positioned above the water seal layer, from above the water seal layer.

2. The method according to claim 1, wherein the fine grain husks include rice husks.

3. The method according to claim 1 or 2, wherein the packing layer comprises a mixture of the fine grain husks and crushed stone.

4. The method according to claim 3, wherein the volume ratio of fine grains of grain husk to crushed stone in the mixture is 1:0.5 or more and 1:100 or less.

5. The method according to claim 1 or claim 2, wherein the volume ratio of the packing layer to the water seal layer in the reaction vessel is 1:0.1 or more and 1:50 or less.

6. The method according to claim 1, wherein the water to be treated is passed through as a vertical downward flow.

7. A water purification system for water containing iron ions and dissolved arsenic, A reaction vessel comprising a packing layer containing fine grain husks and a water seal layer positioned above it, A supply system for supplying the water to be treated to the reaction tank, A system comprising a discharge system for discharging treated water, from which iron ions and dissolved arsenic have been removed from the treated water, from the reaction tank.

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