Method and system for the biological purification of treated water

Abstract

This invention provides a biological purification method for treated water that can effectively suppress the rise in water level caused by biofilm formation in an anaerobic reactor over a long period of time. [Solution] A method for biologically purifying water to be treated is disclosed, which includes a removal step of removing metal ions from the water to be treated using an anaerobic reactor, wherein the anaerobic reactor comprises a first packing layer made of a mixture of solid granular material including grain husks and an organic material containing rice bran, and a second packing layer having metal ion removal ability, and the first packing layer is positioned above the second packing layer. Preferably, the mass ratio of the organic material to the solid granular material in the first packing layer is 0.2 or more and 5 or less. Preferably, the method may include a step of replenishing the first packing layer with the organic material containing rice bran after a predetermined period of time has elapsed.

Description

[Technical Field]

[0001] This invention relates to a method and a biological purification system for treating water. In particular, this invention relates to a method and a corresponding biological purification system for biologically purifying water while suppressing a decrease in water permeability, using an anaerobic reaction tank comprising a plurality of packing layers containing specific components. [Background technology]

[0002] Mining-derived wastewater, such as that from metal mines, and various types of wastewater, such as industrial wastewater, generally contain various heavy metal ions such as Fe, Zn, Cu, Pb, Cd, and As, as well as sulfate ions (SO4). 2- These heavy metal ions may also be present. 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 the wastewater standards set by each country.

[0003] Active treatment, which directly treats water by administering chemicals that react with heavy metal ions, requires chemicals, electricity, and maintenance personnel at all times, and demands frequent maintenance, resulting in high costs. Therefore, in recent years, research has been conducted on passive treatment technologies that utilize natural purification processes to treat water, such as precipitating and removing heavy metals with microorganisms or filtering and absorbing heavy metals with plants, with the aim of reducing treatment costs and energy consumption.

[0004] As an example of such passive treatment technology, Patent Document 1 describes a method for biologically purifying water to be treated, which includes pre-sealing a biological purification agent containing grain husks carrying sulfate-reducing bacteria with the water to be treated and allowing it to stand to cultivate the sulfate-reducing bacteria attached to the grain husks under anaerobic conditions, and then continuously passing the water to be treated through the biological purification agent under anaerobic conditions to precipitate and release heavy metal ions using the sulfate-reducing bacteria, thereby removing heavy metal ions from the water to be treated. Furthermore, Patent Document 2 discloses a biological purification method that can sufficiently remove heavy metal ions from water to be treated over a long period of time even in a wide temperature range including low temperatures of 15°C or below, and is characterized by using an organic matter-containing material selected from sake lees, okara, rice bran, tea leaves, lotus, timothy, and clover as a nutrient source for sulfate-reducing bacteria supported on grain husks. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 5773541 [Patent Document 2] Patent No. 5761884 [Overview of the project] [Problems that the invention aims to solve]

[0006] In the conventional biological purification methods for treated water described above, organic matter-containing materials, including rice bran, are generally used as nutrients for sulfate-reducing bacteria. However, in order to carry out the purification treatment by sulfate-reducing bacteria for as long as possible, it is necessary to add a large amount of rice bran, etc., in batches (for example, a year's supply) to the anaerobic reaction tank at the start of treatment. By adding a large amount of rice bran, etc., in batches to the anaerobic reaction tank in this way, biofilms frequently form due to the excessive decomposition of the rice bran, etc., by sulfate-reducing bacteria. When a large amount of biofilm forms in the anaerobic reaction tank, operational problems such as deterioration of water permeability, rise in the water level of the treated water, and blockage of the treated water flow path and discharge system occur, and maintenance, including stirring the inside of the tank, is often required, making it difficult to continue stable purification treatment of the treated water over a long period of time.

[0007] Therefore, the object of the present invention is to provide a biological purification method for treated water that can continuously remove metal ions from the treated water while effectively suppressing the rise in the water level of the treated water caused by the formation of biofilm in the anaerobic reaction tank over a long period of time, and a corresponding purification system. [Means for solving the problem]

[0008] As a result of diligent research, the inventors unexpectedly discovered that when using an organic material containing rice bran as a nutrient source for sulfate-reducing bacteria in an anaerobic reaction tank, it is possible to perform biological purification suitable for the above purpose by forming a packing layer by mixing solid granular material containing grain hulls with the organic material containing rice bran, and by arranging this packing layer above a packing layer having metal ion removal ability. Thus, the inventors completed the present invention.

[0009] One aspect of the present invention for achieving the above objective is as follows: A biological purification method for water to be treated, The process includes a removal step of removing metal ions from the water to be treated using an anaerobic reactor, The anaerobic reactor comprises a first packing layer made of a mixture of solid granular material containing grain husks and organic material containing rice bran, and a second packing layer having the ability to remove metal ions. A method in which the first packing layer is located above the second packing layer.

[0010] Furthermore, another aspect of the present invention for achieving the above objective is as follows. A biological purification system for water to be treated, An anaerobic reactor comprising a first packing layer made of a mixture of solid granular material containing grain husks and an organic material containing rice bran, and a second packing layer having metal ion removal capabilities, wherein the water to be treated is passed through the first and second packing layers to remove metal ions from the water to be treated. A supply system for supplying the treated water into the anaerobic reaction tank, and A discharge system for discharging the treated water from the anaerobic reaction tank including A system in which, in the anaerobic reaction tank, the first packing layer forms an upper layer than the second packing layer.

Advantages of the Invention

[0011] According to the biological purification method and system of the water to be treated according to the present invention, when using an organic matter-containing material containing rice bran as a nutrient source for sulfate-reducing bacteria in an anaerobic reaction tank, a first packing layer mixed with solid granular materials containing cereal husks is formed, and this first packing layer forms an upper layer than the second packing layer having the ability to remove metal ions. Thus, even when a large amount of the organic matter-containing material containing rice bran as a nutrient source for sulfate-reducing bacteria is batch-fed into the anaerobic reaction tank, it is possible to effectively suppress the rise in the water level of the water to be treated resulting from the generation of biofilm due to excessive decomposition of the nutrient source over a long period of time, and at the same time, continuously remove metal ions in the water to be treated, obtaining the advantage. Further, according to a preferred embodiment of the biological purification method and system of the water to be treated according to the present invention, in the first packing layer in which solid granular materials containing cereal husks are mixed with the organic matter-containing material containing rice bran, by adjusting the mass ratio of the organic matter-containing material containing rice bran to the solid granular materials containing cereal husks within a predetermined range, it is possible to maintain the removal performance of metal ions in the water to be treated and more effectively suppress the generation of biofilm and the rise in the water level of the water to be treated.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a schematic view of a biological purification treatment apparatus according to an embodiment of the present invention. [Figure 2]Figure 2(a) is a photograph showing the state in which the first packing layer is placed on top of the second packing layer (before the water sealing operation) during the process of setting up the biological purification treatment apparatus according to Example 1 described later. Figure 2(b) is a photograph showing the state in which the first packing layer has been placed and the water sealing operation has been performed on it during the process of setting up the biological purification treatment apparatus according to Example 1. Figure 2(c) is a photograph showing the rice husks used in the first packing layer and the second packing layer of the biological purification treatment apparatus according to Example 1. [Figure 3] Figure 3 is a graph plotting the change over time of the head difference (water level rise: cm) on the vertical axis against the number of days elapsed (days) on the horizontal axis for the biological purification treatment of the treated water in each of Examples 1 to 3 and Comparative Example 1, which will be described later. [Modes for carrying out the invention]

[0013] The following describes one embodiment for carrying out the present invention. The present invention is not limited by the description of the following embodiment. The biological purification system for treated water according to the present invention is a system that includes components for realizing the biological purification method for treated water according to the present invention, and the two have substantially common technical matters, so the following description will mainly be from the perspective of the biological purification method.

[0014] The biological purification method for water to be treated according to this embodiment includes a removal step of removing metal ions from the water to be treated using an anaerobic reaction tank equipped with a first packing layer made of a mixture of solid granular material including grain husks and an organic matter-containing material including rice bran, and a second packing layer having metal ion removal ability. In the first packing layer of the anaerobic reaction tank, the solid granular material including grain husks and the organic matter-containing material including rice bran usually exist in a substantially uniform mixed state. Here, the biological purification method for water to be treated according to this embodiment is characterized in that the first packing layer is above the second packing layer (and therefore the second packing layer is below the first packing layer), and water to be treated is passed through the first packing layer and the second packing layer in this order, that is, from top to bottom in the anaerobic reaction tank. The water to be treated passing through the anaerobic reaction tank forms a vertical downward flow due to gravity. The first packing layer is not particularly limited, but usually forms the uppermost layer in the anaerobic reaction tank.

[0015] In this specification, when "water to be treated" refers to water before treatment with a biological purification agent (in particular, including solid granular material on which sulfate-reducing bacteria are supported contained in the second packing layer), or water during biological purification treatment in an anaerobic reactor. In this specification, "water after treatment" refers to water after it has been subjected to biological purification treatment in an anaerobic reactor. Examples of treated water include mine-derived wastewater such as mine wastewater from metal mines, and industrial wastewater. Treated water can also typically contain sulfate ions. For example, mine wastewater from metal mines in Japan generally contains heavy metal ions such as Fe, Zn, Cu, Pb, Cd, and As, as well as sulfate ions (SO4). 2-) also contains approximately 50 to 3000 mg / L. Therefore, the metal ions to be removed from the treated water by the biological purification method according to this embodiment usually include one or more of these heavy metal ions. The pH of the treated water is not particularly limited, but may usually be about 2.5 to 8.0, about 2.5 to 7.5, about 3.0 to 7.0, about 3.0 to 6.5, about 3.3 to 6.0, about 3.3 to 5.5, or about 3.5 to 5.0. These upper and lower limits of the pH of the treated water may be combined in any way.

[0016] The water to be treated in this embodiment may be water that has been previously subjected to a treatment for precipitation and removal of iron ions in a reaction tank separate from the anaerobic reaction tank, for example, using the action of iron-oxidizing bacteria (iron removal treatment, which is a pretreatment for the biological purification method according to this embodiment). In this case, it is preferable that the concentration of iron(II) ions is reduced to, for example, 1000 mg / L or less, 700 mg / L or less, 500 mg / L or less, 300 mg / L or less, 200 mg / L or less, 100 mg / L or less, 70 mg / L or less, or 50 mg / L or less.

[0017] The organic material containing rice bran that constitutes the first packing layer of the anaerobic reactor can function as a nutrient source for sulfate-reducing bacteria contained in the second packing layer, which is located below the first packing layer. The organic material containing rice bran is not particularly limited as long as it contains rice bran in at least a portion, but it is preferable that it contains rice bran in an amount of 50% or more due to its ease of availability, low cost, and high efficiency in providing nutrients to sulfate-reducing bacteria. The proportion of rice bran in the organic material containing rice bran that constitutes the first packing layer is not particularly limited, but is usually 50% by mass or more, preferably 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or substantially 100% by mass. Rice bran, like rice husks, is generated in large quantities as agricultural waste and can therefore be easily and inexpensively obtained.

[0018] The organic material containing rice bran that constitutes the first packing layer of the anaerobic reactor may contain, or may not contain, one or more of the following as a nutrient source for sulfate-reducing bacteria contained in the second packing layer: hay, livestock manure, sake lees, okara, tea leaves, lotus, timothy, and clover molten or dispersed media. The proportion of substances other than rice bran in the organic material containing rice bran that constitutes the first packing layer is usually 50% by mass or less, preferably 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or substantially 0% by mass. In the biological purification method for treated water according to this embodiment, in addition to the organic matter-containing material (solid matter) including rice bran that constitutes the first packing layer, liquid organic matter such as alcohol containing one or more of methanol, ethanol, and propanol may be used as a nutrient source for sulfate-reducing bacteria contained in the second packing layer, or it may not be used.

[0019] The solid granular material containing grain husks that constitutes the first packing layer of the anaerobic reactor is not particularly limited as long as it contains grain husks in at least a portion, but it is preferable that it contains grain husks in an amount of 50% or more due to its availability and low cost, and because the amorphous shape of the individual particles provides sufficient voids for the water being treated to pass through. The proportion of grain husks in the solid granular material constituting the first packing layer is not particularly limited, but is usually 50% by mass or more, preferably 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or substantially 100% by mass.

[0020] The grain husks that can be used in the solid granular material containing grain husks that constitute the first packing layer are not particularly limited, but examples include the following grain husks: 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.

[0021] Among these examples of grain hulls that can be used as solid granular materials containing grain hulls, rice hulls are preferred because they are widely consumed (i.e., readily available) and inexpensive. Rice hulls typically have a flattened ellipsoidal shape, with an average length of the major axis being approximately 3 mm to 10 mm, or approximately 4 mm to 9 mm. This average length of the major axis of the rice hulls promotes substantially uniform mixing with organic matter-containing materials including rice bran, thereby ensuring sufficient flow of the treated water. The average length of the major axis of the rice hulls 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.

[0022] The solid granular material containing grain husks that constitutes the first packing layer of the anaerobic reactor may contain woody materials other than grain husks, such as wood chips. For example, chips from coniferous trees such as cedar and pine can be used. Waste wood chips can also be reused. The size of the wood chips is not particularly limited, but the average length of the longest size of each particle (the average length of 10 arbitrarily selected particles) may be about 3 mm to 10 mm, or about 4 mm to 9 mm. The proportion of material other than grain husks in the solid granular material containing grain husks that constitutes the first packing layer is usually 50% by mass or less, preferably 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or substantially 0% by mass.

[0023] In the biological purification method for treated water according to this embodiment, an organic material containing rice bran is used in the first packing layer of the anaerobic reaction tank as a nutrient source for sulfate-reducing bacteria contained in the second packing layer below it. Furthermore, solid granular material containing grain hulls is coexisted with this material in a substantially uniform manner. This ensures that even when a large amount of the organic material containing rice bran is introduced into the anaerobic reaction tank in batches, sufficient flow path for the treated water is maintained. This effectively suppresses the generation of biofilm due to excessive decomposition of the nutrient source and the resulting rise in the water level of the treated water over a long period, while continuously removing metal ions from the treated water. In other words, the biological purification method for treated water according to this embodiment fully fulfills the fundamentally required role of continuously removing metal ions from the treated water, while also offering the excellent advantage of effectively suppressing the rise in the water level of the treated water over a long period. This reduces the need for cumbersome maintenance, such as regularly or sometimes irregularly stirring the anaerobic reaction tank, and enables the continued stable purification of the treated water over a long period, thereby further enhancing its applicability to passive treatment technology.

[0024] In the first packing layer of the anaerobic reactor, the mass ratio of organic matter-containing material (typically rice bran) containing rice bran to solid granular material (typically rice husks) containing grain husks is usually 0.2 to 5, preferably 0.3 to 4, 0.4 to 3, 0.5 to 2, or 0.6 to 2, most preferably 0.7 to 2. By adjusting the mass ratio of organic matter-containing material to solid granular material in the first packing layer to within such a specific range, it becomes possible to further effectively suppress the generation of biofilm and the rise in the water level of the treated water while maintaining the metal ion removal performance in the treated water in the second packing layer.

[0025] The first packing layer of the anaerobic reactor may be supplemented with organic matter-containing material, including rice bran, after a predetermined period has elapsed since the start of the flow of water to be treated. The "predetermined period" referred to here is the period (time interval) until the replenishment of nutrients for sulfate-reducing bacteria contained in the second packing layer, which is below the first packing layer, is carried out. This "predetermined period" can be appropriately set depending on the treatment scale and the characteristics of the water to be treated, and is not particularly limited, but from the viewpoint of extending the period of stable operation of the passive treatment technology, it is preferably 30 days or more, 60 days or more, 120 days or more, 180 days or more, 240 days or more, 320 days or more, or 360 days or more. When an organic material containing rice bran is added to the first packing layer after a predetermined period has elapsed since the start of water flow to be treated, the mass ratio of the organic material (total mass of the previous batch added and the additional batch added) to the solid granular material in the first packing layer may, as described above, be typically 0.2 or more and 5 or less, preferably 0.4 or more and 4 or less, 0.6 or more and 3 or less, 0.8 or more and 3 or less, or 0.9 or more and 3 or less, most preferably 1 or more and 3 or less.

[0026] In the anaerobic reaction tank used in the biological purification method of treated water according to this embodiment, the first packing layer, which is a mixture of solid granular material including grain husks and organic material including rice bran, is located above the second packing layer which has metal ion removal ability, and therefore the second packing layer is located below the first packing layer. The first packing layer and the second packing layer may be adjacent to each other, or at least one other layer may be interposed between them, as long as they are relatively upper and lower layers. From the viewpoint of further enhancing the effect of effectively suppressing the rise in the water level of the treated water over a long period of time, it is preferable that the first packing layer and the second packing layer are adjacent to each other. If at least one other layer is interposed between the first packing layer and the second packing layer, such other layer is not particularly limited, but may be, for example, a layer mainly consisting of solid granular material other than grain husks, or a layer mainly consisting of granular neutralizing agent.

[0027] The solid granular material on which sulfate-reducing bacteria are supported in the second packing layer of the anaerobic reactor is not particularly limited, as long as it can impart to the second packing layer the ability to remove one or more metal ions, particularly heavy metal ions as exemplified above. From the viewpoint of providing the second packing layer with a sufficiently high metal ion removal ability, the solid granular material on which sulfate-reducing bacteria are supported may preferably include grain husks on which sulfate-reducing bacteria are supported.

[0028] The grain husks constituting the second packing layer include, as exemplified above for the grain husks constituting the first packing layer, grains belonging to the grass family such as rice, wheat, barley, rye, oats, millet, barnyard millet, and foxtail millet; legumes such as soybeans, adzuki beans, mung beans, kidney beans, peanuts, and peas; and grain husks such as buckwheat of the Polygonaceae family, quinoa of the Chenopodiaceae family, and sorghum of the Amaranthaceae family. Among these, rice husks are preferred. The solid granular material on which sulfate-reducing bacteria are supported may include not only grain husks on which sulfate-reducing bacteria are supported, but also granular material of biological or plant origin other than grain husks on which sulfate-reducing bacteria are supported. In the second packing layer having metal ion removal ability, the possibility of using bacteria other than sulfate-reducing bacteria (e.g., sulfur disproportionating bacteria) in place of and / or in addition to sulfate-reducing bacteria is not excluded, but the use of sulfate-reducing bacteria is preferred.

[0029] The sulfate-reducing bacteria supported on solid granular material such as grain husks in the second packing layer of the anaerobic reactor are anaerobic bacteria that are mainly active in the neutral range (pH 5-8), and that use organic matter-containing materials, including rice bran, which constitute the first packing layer, as a nutrient source (energy source) and reduce sulfate. Any known sulfate-reducing bacteria may be used, and there are no particular limitations, but examples include Desulfovibrio vulgaris and Desulfosporosinus sp.

[0030] Generally, since sulfate-reducing bacteria existing in the wastewater to be treated, such as pit wastewater and industrial wastewater, are supported on solid granular materials such as cereal husks, there is no need to add sulfate-reducing bacteria from outside the system. Alternatively, sulfate-reducing bacteria may be further added from outside the system.

[0031] Sulfate-reducing bacteria act to reduce sulfate ions in the wastewater to be treated to produce hydrogen sulfide ions. The hydrogen sulfide ions react with metal ions (mainly heavy metal ions) in the wastewater to be treated, and sulfides of the metal ions precipitate. As a result, the metal ions in the wastewater to be treated precipitate and settle as sulfides, thereby removing the metal ions. That is, in the present embodiment, sulfate-reducing bacteria take in an organic matter-containing material and sulfate ions (SO4 2- ), reduce the sulfate ions as shown in the following reaction formula (A), and have the action of discharging hydrogen sulfide ions (HS - ). 2CH2O + SO4 2- → 2HCO3 - + HS - + H + ···(A) (However, CH2O represents an organic matter-containing material that is a nutrient source.)

[0032] When the reduction reaction (the reaction in the right direction) of the above reaction formula (A) proceeds, hydrogen sulfide ions (HS - ) are generated, and the generated hydrogen sulfide ions (HS - ) combine with metal ions in the wastewater to be treated, and as shown in the following reaction formula (B), the metal ions can be precipitated as sulfides and rendered harmless. Me 2+ + HS - → MeS↓ + H + ····(B) (However, Me 2+ represents a metal ion (illustrated as a divalent ion).)

[0033] Prior to the purification treatment, the sulfate-reducing bacteria may be pre-acclimatized (or cultured / activated) by placing solid granular material (typically grain husks on which sulfate-reducing bacteria are carried) together with the water to be treated under anaerobic conditions, for example, at a water temperature of about 15-30°C. Alternatively, such pre-acclimatization of sulfate-reducing bacteria may be omitted.

[0034] While the optimal pH for sulfate-reducing bacteria to act is considered to be around 6.5 to 8, treated water such as mine wastewater from metal mines and industrial wastewater is often moderate to strongly acidic. Therefore, the second packing layer of the anaerobic reaction tank preferably contains granular neutralizing agent material in addition to solid granular material on which sulfate-reducing bacteria are supported, in order to lower the pH of the treated water to the above range. In this case, preferably, the solid granular material on which sulfate-reducing bacteria are supported and the granular neutralizing agent material may form a substantially homogeneous mixture.

[0035] The granular neutralizing agent is not particularly limited as long as it provides a neutralizing function, but may be formed from a material containing at least one alkaline earth metal-containing compound. Examples of alkaline earth metal-containing compounds include calcium and magnesium oxides, hydroxides, carbonates and silicates, limestone mainly composed of calcium carbonate, calcium silicate minerals or artificial materials, dolomite, tobermorite, xonotlite, rock wool, glass wool, nickel slag wool, cement, cement clinker, concrete, mortar, iron slag, non-ferrous slag, fly ash, zeolite, and their waste materials. These granular neutralizing agents may be used individually or as a mixture of any two or more.

[0036] Among these examples of alkaline earth metal-containing compounds that can be used as granular neutralizing agents, limestone and / or concrete waste can be preferably used, and limestone is particularly preferred, from the viewpoint of availability, ease of handling, and providing sufficient neutralizing performance. Limestone is a natural mineral mainly composed of calcium carbonate (CaCO3), and may contain small amounts of magnesium oxide (MgO), silica (SiO2), aluminum oxide (Al2O3), iron oxide (Fe2O3), etc. Concrete waste is usually formed by containing coarse aggregate, fine aggregate, and cement. The average size of the limestone and / or concrete waste used in this embodiment is not particularly limited, but may be about 5mm to 60mm, about 10mm to 50mm, or about 20mm to 40mm. Having such an average size for the limestone and / or concrete waste promotes uniform mixing with the solid granular material on which the sulfate-reducing bacteria are supported, thereby ensuring proper neutralization and adequate flow of the treated water.

[0037] In the biological purification method for treated water of this embodiment, the mass ratio of granular neutralizing agent to solid granular material on which sulfate-reducing bacteria are supported in the second packing layer of the anaerobic reaction tank depends on the characteristics of the treated water, but is usually 1 to 40, preferably 2 to 30, 3 to 25, or 4 to 20. By setting the mass ratio of granular neutralizing agent to solid granular material on which sulfate-reducing bacteria are supported in the packing layer of the anaerobic reaction tank to be 1 to 40, preferably any of the above ranges, for example, 2 to 30, excessive neutralization reactions are suppressed when the treated water is passed through, making it possible to stably maintain the liquid properties of the treated water in the anaerobic reaction tank within a desirable range for a long period of time (for example, for a period of 30 days or more, 60 days or more, 120 days or more, 180 days or more, 240 days or more, 320 days or more, or 360 days or more), and sufficient void space is provided for the flow of the treated water, ensuring a sufficient flow path for the treated water.

[0038] When the second packing layer of the anaerobic reaction tank contains granular neutralizing agent material in addition to solid granular material on which sulfate-reducing bacteria are supported (preferably forming a substantially homogeneous mixture), the second packing layer may contain multiple layers (two or more layers) with different mass ratios of granular neutralizing agent material to solid granular material on which sulfate-reducing bacteria are supported. In this case, the multiple layers with different mass ratios of granular neutralizing agent material to solid granular material on which sulfate-reducing bacteria are supported are not particularly limited, but from the viewpoint of balancing the ability to sufficiently remove metal ions from the water to be treated, to stably maintain the liquid properties of the water to be treated within a desired range, and to ensure sufficient flow of the water to be treated, it is preferable to arrange the multiple layers so that the upper layers have a higher mass ratio of granular neutralizing agent material.

[0039] The anaerobic reactor that can be used in the biological purification treatment of the water to be treated in this embodiment is not particularly limited in shape, material, or capacity, as long as it is possible to carry out the treatment and the biological treatment (reactions according to reaction equations (A) and (B) above) can proceed while the water to be treated moves downward within the anaerobic reactor due to gravity. The material of the anaerobic reactor is not particularly limited, but for example, it may be mainly made of resin or concrete, and some components may include metal, ceramics, rock, clay, etc. The shape of the anaerobic reactor is not particularly limited, but it may be elongated or flattened, approximately rectangular, approximately cubic, approximately spherical, approximately cylindrical (approximately cylindrical), or a combination thereof. The anaerobic reactor can also be an artificial pond, artificial wetland, large tank, etc. The volume of the anaerobic reactor used in practice outside the laboratory is not limited, but for example 10 m³ 3 ~1 × 10 5 m 3 , 50m 3 ~5×10 4 m 3 or 100m 3 ~1 × 10 4 m 3 It can be to a certain extent.

[0040] An anaerobic reactor is equipped with an inlet for the water to be treated and an outlet for the treated water. If the anaerobic reactor is an artificial pond, artificial wetland, large tank, etc., the inlet for the water to be treated can be connected to the bottom of the anaerobic reactor to maintain an anaerobic environment, and the water can be made to flow underground within the tank. Furthermore, if the water to be treated is groundwater, the anaerobic 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.

[0041] In the anaerobic reaction tank used in the biological purification method of the treated water of this embodiment, the thicknesses of the first packing layer and the second packing layer can be appropriately set depending on the characteristics of the treated water and the treatment scale. The thickness of the first packing layer is not particularly limited, but may be, for example, 0.01m to 2m, 0.02m to 1.5m, 0.03m to 1m, 0.04m to 0.8m, or 0.05m to 0.6m. The thickness of the second packing layer (total thickness in the case of multiple layers) is not particularly limited, but may be, for example, 0.2m to 5m, 0.4m to 4m, 0.5m to 3.5m, 0.6m to 3m, or 0.7m to 2.5m. When the second packing layer consists of multiple layers, the thickness of each layer may be the same or different. If the second packing layer consists of multiple layers with different mass ratios of granular neutralizing agent to solid granular material on which sulfate-reducing bacteria are supported, and the upper layers have a higher mass ratio of granular neutralizing agent, it is preferable that the upper layers have a thinner thickness from the viewpoint of balancing the ability to fully exhibit the metal ion removal capacity from the treated water, to stably maintain the liquid properties of the treated water within a desirable range, and to ensure sufficient flow of the treated water.

[0042] To prevent the first packing layer (typically the uppermost layer) in the anaerobic reaction tank from being exposed to the outside air when the water to be treated is passed through, it is preferable to provide a water seal layer for the water to be treated with a predetermined thickness above the first packing layer. The thickness of the water seal layer is not particularly limited, but may be, for example, 0.01 m to 2 m, 0.02 m to 1 m, 0.03 m to 0.5 m, or 0.04 m to 0.4 m.

[0043] Furthermore, it is preferable to provide a crushed stone layer at the bottom of the anaerobic reaction tank to support the first and second packing layers, and as a means to prevent the outflow of solid matter from the anaerobic reaction tank and to prevent clogging of the drainage system. The crushed stone is not particularly limited as long as it performs the function, but for example, the second packing layer may be limestone as exemplified above. The thickness of such a crushed stone layer at the bottom is not particularly limited, but for example, it may be 0.01m to 2m, 0.02m to 1m, 0.03m to 0.5m, or 0.04m to 0.3m.

[0044] In the biological purification method for treated water according to this embodiment, it is preferable that the flow of treated water, such as mine wastewater, be a continuous flow process in which the treated water is continuously introduced into the anaerobic reaction tank and then continuously discharged from the anaerobic reaction tank as a natural downward flow after remaining there for a predetermined time. This allows for the continuous removal and purification of metal ions from the treated water. 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.

[0045] 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 anaerobic reaction tank) is not particularly limited, as long as the treated water remains in the reaction tank for a period of time sufficient to remove the desired metal ions. The flow rate in the case of continuous flow of treated water such as mine wastewater is not particularly limited, as it depends on the reaction scale, the type and characteristics of the treated water, etc., but for example, the lower limit may be 1 L / min or more, 3 L / min or more, 5 L / min or more, 10 L / min or more, or 20 L / min or more, and the upper limit may be 1000 L / min or less, 500 L / min or less, 300 L / min or less, 200 L / min or less, 100 L / min or less, or 50 L / min or less. These upper and lower limits of the continuous flow rate of treated water may be combined arbitrarily.

[0046] The biological purification method for treated water according to this embodiment can be used with treated water at any temperature, as long as the temperature in the anaerobic reactor allows sulfate-reducing bacteria to act. Sulfate-reducing bacteria generally exhibit activity that varies with temperature. When biological treatment is performed at room temperature or ambient temperature, their activity tends to be higher in the summer when the treated water is hotter and lower in the winter when the treated water is colder. Generally, the optimal temperature for sulfate-reducing bacteria is said to be around 15°C to 45°C, but they can maintain a certain level of activity even in environments where treated water is used at a wide range of temperatures, including those outside this range (for example, in natural environments such as high-latitude or high-altitude regions where temperatures are low in winter). In addition, to maintain the activity of sulfate-reducing bacteria, the anaerobic reactor can be configured to be kept warm or heated, for example.

[0047] The method of supplying the water to be treated is not particularly limited, as long as it can be set and adjusted so that it can be supplied to the anaerobic reactor (treatment vessel for biological purification) at a desired constant flow rate. The water to be treated can be supplied to the anaerobic reactor through a transfer path such as piping. If the transfer path such as piping for supplying the water to be treated is located at a higher position than the inlet of the anaerobic reactor, an electric pump may be used, for example, to provide supply energy (lift). In natural-use passive treatment systems, it is desirable to configure the system so that the water to be treated, from the supply to the treatment and discharge systems, can move by gravity, minimizing the use of electricity from the perspective of labor saving and cost reduction. Therefore, in actual application fields, it is preferable to set the height of the transfer paths such as piping and the inlet of the anaerobic reaction tank so that electric pumps are not used. Furthermore, if electric pumps are used, it is possible to create a natural-use passive treatment system by utilizing solar cells or the like for power supply.

[0048] Figure 1 shows a non-exclusive example of an apparatus for implementing the biological purification method / purification system for treated water according to this embodiment. In Figure 1, the reference symbols are as follows: 1 is the anaerobic reactor (a treatment vessel for biological purification), 2 is the water seal layer in the anaerobic reactor, 3 is the first packing layer in the anaerobic reactor, 3a is the solid granular material containing grain husks that constitutes the first packing layer 3 (rice husks being a typical example), 3b is the organic matter-containing material containing rice bran that constitutes the first packing layer 3 (rice bran being a typical example), 4 is the second packing layer in the anaerobic reactor, and 4a is the solid material on which sulfate-reducing bacteria are supported that constitutes the second packing layer 4. 4b is a granular material (typically rice husks on which sulfate-reducing bacteria are supported), 4b is a granular neutralizing agent constituting the second packing layer 4 (typically limestone), 5 is a crushed stone layer placed below the first and second packing layers in the anaerobic reaction tank to support these packing layers, 6 is an inlet pipe for introducing (transferring) the water to be treated to the anaerobic reaction tank, 7 is an outlet pipe for discharging (transferring) the treated water in the anaerobic reaction tank, and 10 refers to the entire biological purification treatment device.

[0049] In Figure 1, the anaerobic reactor 1 is a resin or concrete container with a roughly prism-like (rectangular or cubic) or cylindrical outline. The material of the surrounding wall of the anaerobic reactor 1 may include metal, ceramics, rock, clay, etc., in whole or in part, in addition to resin or concrete. In a natural passive treatment system, the anaerobic reactor 1 can be replaced by an artificial pond, artificial wetland, large tank, etc.

[0050] Furthermore, in Figure 1, a crushed stone layer 5 is formed at the bottom of the anaerobic reaction tank 1 so as to cover the area above the discharge port, and above the crushed stone layer, a second packing layer 4 is arranged supported by the crushed stone layer, with the first packing layer 3 above that. The first packing layer 3 is composed of a homogeneous mixture of solid granular material containing grain husks (typically rice husks) 3a and organic matter-containing material containing rice bran (typically rice bran) 3b. The second packing layer 4 is composed of a homogeneous mixture of solid granular material on which sulfate-reducing bacteria are supported (typically rice husks on which sulfate-reducing bacteria are supported) 4a and granular neutralizing agent material constituting the second packing layer 4 (typically limestone) 4b. The crushed stone layer 5 has the function of preventing the outflow of solid matter from the container, thereby preventing clogging of the drainage system. The crushed stone in the crushed stone layer 5 may typically be limestone.

[0051] In the biological purification apparatus shown schematicly in Figure 1, the water to be treated (which may be wastewater from an iron removal treatment device) containing metal ions is continuously introduced into the anaerobic reactor 1 from the supply source along the direction of arrow A through the introduction pipe 6. The water to be treated descends through the water seal layer 2, the first packing layer 3, and then the second packing layer 4 of the anaerobic reactor 1 by gravity, and biological treatment by sulfate-reducing bacteria gradually progresses. In other words, as the water to be treated descends from the first packing layer 3 to the second packing layer 4 of the anaerobic reactor 1, it undergoes a neutralization reaction with granular neutralizing agent (typically limestone) 4b, and is purified by the precipitation and removal of metal ions by sulfide ions produced by the action of sulfate-reducing bacteria supported on solid granular material (typically rice husks) 4a. The water to be treated undergoes this biological treatment by sulfate-reducing bacteria in the second packing layer 4, passes through the crushed stone layer 5, and reaches the bottom of the anaerobic reaction tank 1. After treatment, the water is discharged through the outlet and out the discharge pipe 7 in the direction of arrow B.

[0052] In this biological purification method for treated water using such a device, an organic material containing rice bran 3b is used in the first packing layer 3 of the anaerobic reactor 1 as a nutrient source for sulfate-reducing bacteria supported on solid granular material 4a contained in the second packing layer 4 below it. Furthermore, solid granular material 3a containing grain hulls is coexisted with this material in a substantially uniformly mixed manner. This ensures that even when a large batch of the organic material containing rice bran 3b is introduced into the anaerobic reactor 1, sufficient flow path for the treated water is maintained, effectively suppressing the generation of biofilm due to excessive decomposition of the nutrient source and the resulting rise in the water level of the treated water over a long period, while continuously removing metal ions from the treated water. Therefore, this biological purification method for treated water using the above device is suitable for application as a so-called passive treatment technology that reduces the number of personnel, treatment costs, and energy consumption. [Examples]

[0053] The present invention will be further described 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.

[0054] Preparation of the anaerobic reactor An anaerobic reaction tank was prepared in the surrounding environment, configured to allow the introduction of water to be treated from above, and having an outlet for treated water at the bottom. The anaerobic reaction tank was a roughly rectangular container with its top open to the atmosphere, and its perimeter walls and bottom were made of concrete. Two anaerobic reaction tanks were used: Anaerobic Reaction Tank A, measuring 5m in length, 16m in width, and 3.5m in height, and Anaerobic Reaction Tank B, measuring 2m in length, 3m in width, and 2.5m in height.

[0055] Example 1 In anaerobic reactor A, a 0.15 m thick layer of crushed stone made of limestone with a particle size of approximately 20-40 mm was laid at the bottom (the same limestone was used in the second packing layer described later). On top of the crushed stone layer, a 1 m thick packing layer (the lower of the multiple layers of the second packing layer) made of a homogeneous mixture of rice husks and limestone in a 1:4 mass ratio was laid, above this was a 0.5 m thick packing layer (the upper of the multiple layers of the second packing layer) made of a homogeneous mixture of rice husks and limestone in a 1:16 mass ratio, and above this was a 0.3 m thick packing layer (the first packing layer) made of a homogeneous mixture of rice husks and rice bran in a 1:0.66 mass ratio. The rice husks and rice bran were procured from farmers. The average length of the long axis of the rice husks used was approximately 5 mm. The composition of each layer in anaerobic reactor A in this example, along with the continuous water flow rate described later, is shown in Table 1 below. Figure 2(a) is a photograph showing the state in which the first packing layer is placed on top of the second packing layer (before the water sealing operation is performed) during the setup of the biological purification treatment apparatus according to this embodiment. Figure 2(b) is a photograph showing the state in which the first packing layer has been placed and the water sealing operation has been performed on it during the setup of the biological purification treatment apparatus according to this embodiment. Figure 2(c) is a photograph showing the rice husks used in the first packing layer and the second packing layer of the biological purification treatment apparatus according to this embodiment.

[0056] The water to be treated in this experiment contains Zn ions (Zn) with an average concentration of approximately 16.7 mg / L (which fluctuates over time). 2+ Acidic mine wastewater (iron-removed) containing ) was used. The concentration of Zn ions was measured using ICP-AES. The water to be treated was continuously passed through the top of anaerobic reaction tank A at a flow rate of 50 L / min and allowed to descend naturally within the tank. The thickness of the water seal layer was 0.3 m (the same thickness was used for subsequent examples). The residence time of the water to be treated in the second packing layer, which consisted of upper and lower layers of a mixture of rice husks and limestone, was adjusted to 22.5 hours. As the water to be treated descended from the first packing layer to the second packing layer in anaerobic reaction tank A, it underwent a neutralization reaction with limestone, and metal ions were purified by the precipitation and removal of sulfide ions produced by the action of sulfate-reducing bacteria supported on the rice husks. After treatment, the water passed through the crushed stone layer and reached the bottom of anaerobic reaction tank A, and was then discharged through the outlet and discharge pipe.

[0057] In this example, the Zn ion concentration of the treated water, measured using ICP-AES, reached approximately 0 mg / L immediately after discharge began (within 24 hours of discharge), meaning that virtually all Zn ions had been removed. Subsequently, the Zn ion concentration of the treated water remained continuously maintained between approximately 0 mg / L and approximately 0.035 mg / L for a period of more than 300 days. In this example, the rise in water level from the water level at the start of water flow (reference water level) at the top of the anaerobic reaction tank, i.e., the head difference (cm), was measured at predetermined intervals over a period of 300 days or more after the start of water flow. Figure 3 shows a graph plotting the change over time with the number of days elapsed since the start of water flow (days) on the horizontal axis and the head difference (cm) on the vertical axis (see the ■ marks in Figure 3). The rise in water level was gradual, remaining below 8 cm even after 250 days from the start of water flow, and below 12 cm even after 300 days.

[0058] Example 2 Except for changing the mass ratio of rice husks and limestone in the upper and lower layers of the second packing layer to that shown in Table 1 below, the water to be treated / biological purification treatment was carried out continuously for one year as described above for Example 1. Then, 2,000 kg of rice bran was added to the first packing layer to a thickness of 0.12 m (total amount of rice bran added with the first layer was 4,000 kg), and this first packing layer was homogenized by stirring, and then the water to be treated / biological purification treatment was carried out continuously for more than 300 days. The water to be treated after the addition of rice bran contained an average concentration of Zn ions (Zn) of approximately 16.6 mg / L (which fluctuated over time). 2+ Acidic mine wastewater containing ) (after iron removal treatment) was used.

[0059] In this example, the Zn ion concentration of the treated water, measured using ICP-AES, reached approximately 0 mg / L immediately after the addition of rice bran and the start of discharge (within 24 hours of the start of discharge), meaning that virtually all Zn ions had been removed. Subsequently, the Zn ion concentration of the treated water remained continuously maintained between approximately 0 mg / L and approximately 0.135 mg / L for a period of more than 300 days. In this example, after adding rice bran and starting water flow, the rise in water level from the initial water level (reference water level) at the top of the anaerobic reaction tank, i.e., the head difference (cm), was measured at predetermined intervals for a period of 300 days or more. Figure 3 shows a graph plotting the change over time with the number of days elapsed since adding rice bran and starting water flow (days) on the horizontal axis and the head difference (cm) on the vertical axis (see the ▲ marks in the figure). The rise in water level was gradual, remaining below 6.5 cm even after 250 days from the start of water flow, and below 8 cm even after 300 days.

[0060] Example 3 Using anaerobic reactor B (small tank), the first and second packing layers were constructed as shown in Table 1 below. Specifically, on top of the crushed stone layer, a 1m thick packing layer (the lower of the multiple layers of the second packing layer) made of a homogeneous mixture of rice husks and limestone in a 1:4 mass ratio was placed, above this a 0.5m thick packing layer (the upper of the multiple layers of the second packing layer) made of a homogeneous mixture of rice husks and limestone in a 1:8 mass ratio was placed, and above this a 0.15m thick packing layer (the first packing layer) made of a homogeneous mixture of rice husks and rice bran in a 1:1.32 mass ratio was placed. The water to be treated in this experiment contains Zn ions (Zn) with an average concentration of approximately 17.0 mg / L (which fluctuates over time). 2+ Acidic mine wastewater (iron-removed) containing ) was used. The water to be treated was continuously passed through the top of anaerobic reaction tank B at a flow rate of 3.4 L / min and allowed to descend naturally within the tank. The residence time of the water to be treated in the second packing layer, which consisted of upper and lower layers of a mixture of rice husks and limestone, was adjusted to 24.8 hours.

[0061] In this example, the Zn ion concentration of the treated water, measured using ICP-AES, fell below approximately 5 mg / L 14 days after the start of discharge. Subsequently, although there were occasional relatively large fluctuations due to the small scale of the treatment, the Zn ion concentration of the treated water remained generally below 5-6 mg / L, and for most observation points from 150 days to over 300 days, it remained at approximately 0 mg / L, meaning that virtually all Zn ions had been removed. In this example, the rise in water level from the water level at the start of water flow (reference water level) at the top of the anaerobic reaction tank, i.e., the head difference (cm), was measured at predetermined intervals over a period of 300 days or more after the start of water flow. Figure 3 shows a graph plotting the change over time with the number of days elapsed since the start of water flow (days) on the horizontal axis and the head difference (cm) on the vertical axis (see the diamond marks in Figure 3). The rise in water level was very gradual and remained stably suppressed to 4 cm or less even 300 days after the start of water flow.

[0062] Comparative Example 1 Using anaerobic reactor A, the first and second packing layers were constructed as shown in Table 1 below. Specifically, on top of the crushed stone layer, a 1m thick packing layer (the lower of the multiple layers of the second packing layer) made of a homogeneous mixture of rice husks and limestone in a 1:4 mass ratio was placed, above this a 0.5m thick packing layer (the upper of the multiple layers of the second packing layer) made of a homogeneous mixture of rice husks and limestone in a 1:8 mass ratio was placed, and above this a 0.12m thick packing layer (the first packing layer) made of rice bran in a 0:1 mass ratio, i.e., rice bran only (without the addition of rice husks). The water to be treated in this experiment contains Zn ions (Zn) with an average concentration of approximately 16.5 mg / L (which fluctuates over time). 2+ Acidic mine wastewater (iron-removed) containing ) was used. The water to be treated was continuously passed through the top of anaerobic reaction tank A at a flow rate of 50 L / min and allowed to descend naturally within the tank. The residence time of the water to be treated in the second packing layer, which consisted of upper and lower layers of a mixture of rice husks and limestone, was adjusted to 22.5 hours.

[0063] In this example, the Zn ion concentration of the treated water, measured using ICP-AES, reached approximately 0 mg / L two days after the start of discharge, meaning that virtually all Zn ions had been removed. Subsequently, the Zn ion concentration of the treated water remained consistently low for a period of more than 170 days, with the exception of temporary fluctuations. In this example, the rise in water level from the water level at the start of water flow (reference water level) at the top of the anaerobic reaction tank, i.e., the head difference (cm), was measured at predetermined intervals over a period of 300 days or more after the start of water flow. Figure 3 shows a graph plotting the change over time with the number of days elapsed since the start of water flow (days) on the horizontal axis and the head difference (cm) on the vertical axis (see the ● marks in Figure 3). The rise in water level gradually increased over time, reaching 10 cm after 95 days, 20 cm after 135 days, and 24 cm after 175 days.

[0064] [Table 1]

[0065] From the above-mentioned findings and the results shown in the graph in Figure 3, it can be understood that, according to the embodiment of the present invention, even when rice bran is added in large batches to an anaerobic reaction tank as a nutrient source for sulfate-reducing bacteria, the rise in the water level of the treated water resulting from the formation of biofilm due to excessive decomposition of the nutrient source can be effectively suppressed over a long period of time, and metal ions (zinc ions) in the treated water can be continuously removed. [Explanation of symbols]

[0066] 1: Anaerobic reactor 2: Hydraulic layer 3: 1st filling layer 3a: Solid granular material containing grain husks (rice husks are a typical example) 3b: Materials containing organic matter, including rice bran (rice bran is a typical example) 4:Second filling layer 4a: Solid granular material on which sulfate-reducing bacteria are supported (a typical example is rice husks on which sulfate-reducing bacteria are supported). 4b: Granular neutralizing agent (limestone is a typical example) 5: Crushed stone layer 6: Inlet pipe for treated water 7: Discharge pipe for treated water A: Direction of introduction of water to be treated B: Discharge direction of treated water 10: Biological purification treatment equipment (overall)

Claims

1. A biological purification method for water to be treated, The process includes a removal step of removing metal ions from the water to be treated using an anaerobic reactor, The anaerobic reactor comprises a first packing layer made of a mixture of solid granular material containing grain husks and an organic material containing rice bran, and a second packing layer having the ability to remove metal ions. A method in which the first packing layer is located above the second packing layer.

2. In the first packing layer, The method according to claim 1, wherein the mass ratio of the organic matter-containing material to the solid granular material is 0.2 or more and 5 or less.

3. The method according to claim 1 or claim 2, further comprising the step of replenishing the first packing layer with an organic material containing rice bran after a predetermined period of time has elapsed.

4. The method according to claim 1 or 2, wherein the second packing layer includes grain husks on which sulfate-reducing bacteria are supported.

5. The method according to claim 1 or claim 2, wherein the second packing layer further comprises granular neutralizing agent.

6. The method according to claim 1 or claim 2, further comprising, in a step prior to the removal step, a step of precipitating and removing the iron ions in the water to be treated using iron-oxidizing bacteria.

7. A biological system of treated water, An anaerobic reactor comprising a first packing layer made of a mixture of solid granular material containing grain husks and an organic material containing rice bran, and a second packing layer having metal ion removal capabilities, wherein the water to be treated is removed from the water to be treated by passing through the first packing layer and the second packing layer. A supply system for supplying the treated water into the anaerobic reaction tank, and Discharge system for discharging treated water from the anaerobic reaction tank. Includes, In the anaerobic reaction tank, the first packing layer is located above the second packing layer.

Images