Method for treating contaminated water

Abstract

Provided is a useful method for treating waste water that contains a heavy metal or an organic pigment as a contaminant, wherein waste water from a factory or the like is continuously and gradually supplied, via an introduction path (21), to a treatment tank (2) which has been charged with thermophilic cyanobacteria such as Thermosynechococcus elongatus, and carbon dioxide gas is ejected from a gas supply pipe (22). In a photosynthetic environment which is, for example, irradiated with solar light, the waste water from the introduction path (21) is treated by being stirred with the thermophilic cyanobacteria by the ejected carbon dioxide gas, the water level in the treatment tank (2) rises, and the waste water flows out to a precipitation tank (3) through a liquid feed pipe (31). In the waste water that has flowed out to the precipitation tank (3), thermophilic bacteria contained therein precipitates to a bottom part as sludge, and a supernatant liquid flows out through a waste water pipe (32) due to a rise in water level. When waste water containing a heavy metal is treated, a supernatant liquid from which the heavy metal has been separated flows out, and when waste water containing an organic pigment is treated, a supernatant liquid from which the pigment has been removed flows out.

Description

How to treat contaminated water

[0001] The present invention relates to a method for treating contaminated water, and more particularly to a method for treating water containing heavy metals or organic dyes as contaminants.

[0002] Heavy metals are taken into the body through contact with the environment, such as the air, soil, seawater, rivers, lakes, and marshes, or through food. Some of them may be essential or beneficial to the body in small amounts, but as a whole, they accumulate in the body and cause various toxic symptoms, so they are treated as regulated substances from the perspectives of health, hygiene, and environmental conservation. Therefore, water that may contain heavy metals, such as wastewater from various factories, mine wastewater from metal mines, and runoff from dredged soil, i.e., water contaminated by heavy metals, requires water treatment to remove the heavy metals.

[0003] Water treatment methods for removing heavy metals are generally physical or chemical. For example, Non-Patent Document 1 presents a method of adding a chemical to water to precipitate heavy metals as sparingly soluble compounds and then separating the precipitate (e.g., coprecipitation, substitution, sulfide, ferrite, or iron powder method), as well as a method of capturing heavy metals using an adsorbent such as an ion exchange resin. However, these methods require complex and cumbersome treatment steps and large amounts of chemicals or adsorbents, resulting in high treatment costs. Furthermore, since it is not easy to recover heavy metals from the resulting precipitates or adsorbents, environmentally friendly disposal of the precipitates and used adsorbents is required.

[0004] On the other hand, Non-Patent Document 2 reports that a mesophilic cyanobacterium called Synechocystis 6803 can grow in a culture solution containing zinc. Synechocystis 6803 has a protein called metallothionein that can bind to heavy metal ions, and can grow by incorporating zinc ions in the culture medium into this protein. Since the zinc incorporated into Synechocystis 6803 can be recovered by acid treatment, Synechocystis 6803 may also be used as a method for removing and recovering zinc from water. However, Non-Patent Document 2 reports that Synechocystis 6803 can grow in a culture solution containing a low concentration of zinc, but cannot grow in a culture solution containing a high concentration of zinc. That is, since Synechocystis 6803 lacks tolerance to high concentrations of zinc, it is practically difficult to apply a method for removing zinc from water using Synechocystis 6803 to industrial wastewater containing high concentrations of heavy metals such as zinc, and therefore the method lacks practicality.

[0005] Furthermore, organic dyes such as azo compounds are used in a variety of industries, including papermaking, textiles, leather, printing, food, cosmetics, and pharmaceuticals, and industrial wastewater from these industries often contains organic dyes. While industrial wastewater colored by organic dyes is subject to general legal regulatory standards, such as COD and BOD, in Japan, there are no legal regulatory standards targeting the coloring itself; only a few municipalities, such as Wakayama City and Kawasaki City, have established their own regulatory standards. Consequently, industrial wastewater is often discharged into rivers and irrigation channels while still colored by organic dyes, resulting in various problems. For example, colored wastewater has a strong appearance of pollution, and its use as agricultural or drinking water raises physiological aversions and health concerns. Furthermore, colored wastewater flowing into rivers and lakes can block sunlight, inhibiting photosynthesis in aquatic plants and potentially adversely affecting ecosystems.

[0006] Therefore, methods for treating water contaminated by organic dyes, such as those in wastewater, have been investigated. Non-Patent Document 3 introduces methods for decolorizing textile dye wastewater, such as using coagulants or adsorbents to separate organic dyes, and methods for decomposing organic dyes in wastewater through oxidation treatments such as ozone oxidation and electrolytic oxidation. However, methods using coagulants or adsorbents generate large amounts of secondary waste, such as coagulated sludge and used adsorbents, which must be disposed of. Oxidation treatment methods require complex wastewater treatment equipment depending on the oxidation method, and require a large amount of energy for operation. Furthermore, Non-Patent Document 3 introduces an example of dye decomposition using microorganisms, but points out that this is at the experimental level and further study is needed before it can be applied to practical applications such as decolorizing wastewater.

[0007] "New Pollution Prevention Technology and Regulations 2020 IV.1.4 General Treatment Technology for Heavy Metal Wastewater" Japan Environmental Management Association for Industry

[0008] Thelwell, C. et al., Proc. Natl. Acad. Sci. U.S.A. (1998) 95, 10728-10733. Futoshi Sugimoto, Teizo Isono, Decolorization Treatment Technology of Textile Dyeing Wastewater and Its Trends, Environmental Technology, 29(9), 71-81(2000).

[0009] The present invention aims to realize a practical method for treating water containing heavy metals or organic pigments as pollutants.

[0010] More specifically, one aspect of the present invention relates to a method for separating heavy metals, and in particular to a method for separating heavy metals from water containing heavy metals, which aims to efficiently separate heavy metals from water containing heavy metals in a state that makes it easy to recover the heavy metals.

[0011] Another aspect of the present invention relates to a method for treating water, particularly a method for treating water containing an organic dye, which method uses microorganisms to easily treat the organic dye in the water.

[0012] The present invention relates to a method for treating contaminated water, which comprises step 1 of treating contaminated water with thermophilic cyanobacteria. The thermophilic cyanobacteria used in the present invention is, for example, Thermosynechococcus elongatus. In the treatment method of the present invention, it is generally preferred to carry out step 1 in an environment that promotes photosynthesis in the thermophilic cyanobacteria.

[0013] An example of contaminated water to be treated by the treatment method of the present invention is water containing heavy metals. In this case, the contaminated water is, for example, industrial wastewater or soil washing water. When contaminated water containing heavy metals is to be treated, the treatment method of the present invention usually further includes step 2, in which the precipitate formed in step 1 is separated from the contaminated water. In this case, the treatment method of the present invention may further include step 3, in which the precipitate separated in step 2 is treated with an acid.

[0014] Another example of contaminated water that can be treated by the treatment method of the present invention is water containing organic pigments, such as industrial wastewater.

[0015] When the contaminated water contains heavy metals, the present invention can also be regarded as a method for separating heavy metals from the water containing the heavy metals. This separation method includes step 1 of treating the water containing the heavy metals with thermophilic cyanobacteria, and step 2 of separating the precipitate produced in step 1 from the water.

[0016] An example of a thermophilic cyanobacterium used herein is Thermosynechococcus elongatus.

[0017] In one form of this isolation method, step 1 is carried out in an environment that promotes photosynthesis in thermophilic cyanobacteria.

[0018] An example of water that can be subjected to this separation method is industrial wastewater containing heavy metals. Another example of water that can be subjected to this separation method is soil washing water that contains heavy metals.

[0019] Furthermore, when the contaminated water contains heavy metals, the separation method of the present invention can also be regarded from a more advanced perspective as a method for recovering heavy metals from the water containing the heavy metals. This recovery method includes step 1 of treating the water containing the heavy metals with thermophilic cyanobacteria, step 2 of separating the precipitate produced in step 1 from the water, and step 3 of treating the precipitate separated in step 2 with an acid.

[0020] When the contaminated water contains an organic pigment, the present invention can also be understood as a method for treating water containing an organic pigment, which includes a step of treating water with thermophilic cyanobacteria.

[0021] The thermophilic cyanobacteria used in this treatment method is, for example, Thermosynechococcus elongatus.

[0022] In one form of this treatment method, the treatment step is carried out in an environment that promotes photosynthesis in thermophilic cyanobacteria.

[0023] The water that is the target of this treatment method is, for example, industrial wastewater containing organic pigments.

[0024] The method for treating contaminated water according to the present invention uses specific microorganisms, and therefore can practically treat water containing heavy metals or organic pigments as contaminants.

[0025] For example, a method for separating heavy metals according to one aspect of the present invention uses specific microorganisms, which allows for efficient separation of heavy metals from water in a state that makes recovery easy. A method for recovering heavy metals, which is a further development of the method for separating heavy metals according to the present invention according to this aspect, uses specific microorganisms, which allows for efficient recovery of heavy metals from water.

[0026] Furthermore, the method for treating water containing an organic dye according to another aspect of the present invention uses specific microorganisms, and therefore, can easily treat organic dyes in the water.

[0027] 1 is a schematic diagram of an example of a wastewater treatment device capable of carrying out the treatment method of the present invention. A diagram showing the results of Experimental Example IA. A diagram showing the results of Experimental Example IC. An absorbance spectrum in the wavelength range of 340 to 800 nm measured for methylene blue in Experimental Example II. An absorbance spectrum in the wavelength range of 180 to 800 nm measured for methylene blue in Experimental Example II. An absorbance spectrum in the wavelength range of 340 to 800 nm measured for Remazol Brilliant Blue RN in Experimental Example II. An absorbance spectrum in the wavelength range of 180 to 800 nm measured for Remazol Brilliant Blue RN in Experimental Example II. An absorbance spectrum in the wavelength range of 340 to 800 nm measured for Remazol Brilliant Red BB 150% in Experimental Example II. An absorbance spectrum in the wavelength range of 180 to 800 nm measured for Remazol Brilliant Red BB 150% in Experimental Example II. Absorbance spectrum in the wavelength range of 340 to 800 nm measured for Remazol Brilliant Yellow 4GL gray in Experimental Example II. Absorbance spectrum in the wavelength range of 180 to 800 nm measured for Remazol Brilliant Yellow 4GL gray in Experimental Example II. Absorbance spectrum in the wavelength range of 340 to 800 nm measured for Remazol Yellow GR 133% in Experimental Example II. Absorbance spectrum in the wavelength range of 180 to 800 nm measured for Remazol Yellow GR 133% in Experimental Example II.

[0028] The present invention relates to a method for treating contaminated water. The contaminated water to which this treatment method is applicable is water that contains or may contain heavy metals or organic pigments as contaminants. The contaminated water treatment method according to the present invention includes a step of treating the contaminated water with thermophilic cyanobacteria. Specifically, it includes a step of contacting the contaminated water with thermophilic cyanobacteria.

[0029] In the case of contaminated water that contains or may contain heavy metals as contaminants, the thermophilic cyanobacteria in this process take up the heavy metals in the water and cause them to precipitate in the water. Therefore, by separating the precipitate from the water after this process, purified water from which the heavy metals have been removed can be obtained.

[0030] If the contaminated water contains or may contain organic pigments as contaminants, in this process the thermophilic cyanobacteria take up the organic pigments in the water and metabolically decompose them, thereby decolorizing the contaminated water.

[0031] Thermophilic cyanobacteria used in this step include, for example, Thermosynechococcus elongatus (T. elongatus) and Thermosynechococcus vulcanus (T. vulcanus). T. elongatus can be collected from Beppu Hot Springs in Oita Prefecture, Japan, and is preserved at the National Institute for Environmental Studies' Microbial Culture Collection (16-2 Onogawa, Tsukuba City, Ibaraki Prefecture, Japan) under the strain number NIES-2133, and is available from the Microbial Culture Collection. For example, it can be obtained by ordering through the Microbial Culture Collection's website (Japanese page: https: / / mcc.nies.go.jp / aboutOnlineOrder.do / English page: https: / / mcc.nies.go.jp / aboutOnlineOrder.do?lang=en). Note that some researchers use T. In some cases, T. elongatus is referred to as Thermosynechococcus vestitus, Thermosynechococcus vestitus BP-1, Thermosynechococcus elongatus BP-1, etc. In addition, T. vulcanus can be collected from Yunomine Hot Spring in Wakayama Prefecture, Japan, and is preserved at the National Institute for Environmental Studies' Microbial Culture Collection (16-2 Onogawa, Tsukuba City, Ibaraki Prefecture, Japan) under the strain number NIES-2134, and is available from the Microbial Culture Collection. For example, they can be obtained by ordering through the website of the Microbial Culture Collection (Japanese page: https: / / mcc.nies.go.jp / aboutOnlineOrder.do / English page: https: / / mcc.nies.go.jp / aboutOnlineOrder.do?lang=en).

[0032] The method for treating contaminated water according to the present invention will be specifically explained below for each type of contaminated water.

[0033] When contaminated water contains or may contain heavy metals as contaminants, the treatment method of the present invention can be understood as a method for separating heavy metals from the water (hereinafter, sometimes referred to as heavy metal-containing water). Therefore, the treatment method of the present invention will be described as a method for separating heavy metals from heavy metal-containing water in a form in which heavy metal-containing water is the contaminated water to which the method is applied.

[0034] The heavy metal-containing water, i.e., contaminated water, to which the heavy metal separation method is applicable is not particularly limited as long as it contains or is likely to contain the heavy metals described below, and examples thereof include wastewater from various factories such as plating factories and paint factories, environmental water such as rivers, lakes and groundwater, mine drainage from metal mines, soil washing wastewater, leachate from dredged soil, incineration ash washing wastewater, sewage, and wastewater generated in research facilities.

[0035] The heavy metals to be separated are usually metals with a specific gravity of 4 or more, and typical examples include iron, lead, copper, chromium, cadmium, mercury, zinc, arsenic, manganese, cobalt, nickel, molybdenum, tungsten, tin, bismuth, uranium, and plutonium. These heavy metals are usually in an ionic state, resulting from the ionization of inorganic or organic salts of the heavy metals in water.

[0036] In this separation method, first, heavy metal-containing water is treated with the above-mentioned thermophilic cyanobacteria (Step 1). Specifically, the heavy metal-containing water is brought into contact with the above-mentioned thermophilic cyanobacteria.

[0037] Thermophilic cyanobacteria contain metallothionein, a protein capable of binding to many heavy metal ions, within their cells. Furthermore, their gene expression is induced in the presence of heavy metals, resulting in increased production of this metallothionein. This allows them to efficiently take up and accumulate more heavy metals within their cells than the mesophilic cyanobacteria reported in Non-Patent Document 2. Furthermore, thermophilic cyanobacteria have high photosynthetic efficiency and high protein thermal stability. Therefore, they can actively take up carbon dioxide from the atmosphere and grow under sunlight at temperatures ranging from near room temperature (around 20°C) to 90°C. They can rapidly grow using the sugars produced by their own photosynthesis as nutrients, even without external nutrient supply. For example, they grow rapidly even in a laboratory lit by fluorescent lamps at near room temperature, with a doubling time of approximately 14 hours. Therefore, the synergistic effect of their unique metallothionein and high proliferation rate allows thermophilic cyanobacteria to take up significantly more heavy metals from heavy metal-containing water than the mesophilic cyanobacteria reported in Non-Patent Document 2.

[0038] Therefore, from the viewpoint of enhancing the above-mentioned synergistic effect, this step is preferably carried out in an environment that promotes photosynthesis in the thermophilic cyanobacteria. For example, it is preferable to maintain the heavy metal-containing water containing the thermophilic cyanobacteria at at least near room temperature or heated to 90°C, and then maintain the heavy metal-containing water under sunlight or under artificial lighting such as an incandescent lamp, fluorescent lamp, or LED while blowing in air or carbon dioxide gas. In particular, from the viewpoint of promoting photosynthesis in the thermophilic cyanobacteria, it is preferable to maintain the pH of the heavy metal-containing water at a pH between near neutral and weakly alkaline, for example, between pH 6 and 10. Note that if the pH of the heavy metal-containing water is 5 or less, the photosynthetic rate of the thermophilic cyanobacteria decreases, resulting in difficulty in the growth of the thermophilic cyanobacteria, which may reduce the efficiency of heavy metal uptake. The pH of the heavy metal-containing water can usually be adjusted by adding a pH adjuster such as a carbonate such as sodium carbonate or potassium carbonate, a bicarbonate such as sodium bicarbonate or potassium bicarbonate, a hydroxide such as sodium hydroxide or potassium hydroxide, a mineral acid such as dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid, phosphoric acid, a phosphate such as sodium phosphate or potassium phosphate, or an organic acid such as acetic acid or citric acid.

[0039] In this process, the thermophilic cyanobacteria that have taken up heavy metals are precipitated in the heavy metal-containing water.

[0040] Next, the precipitate formed in step 1 is separated from the heavy metal-containing water (step 2). This separates the heavy metals from the heavy metal-containing water, yielding purified water from which the heavy metals have been removed. Common solid-liquid separation methods, such as filtration, squeezing, decantation, or suction of the supernatant, can be used to separate the precipitate. When filtration is used, the MBR (membrane bioreactor) method can also be used.

[0041] The heavy metals separated from the heavy metal-containing water in this separation method can be easily recovered. In the recovery of heavy metals, the precipitate separated in step 2 is treated with an acid (step 3). For example, the precipitate is washed with an inorganic acid such as sulfuric acid, hydrochloric acid, or nitric acid, or an organic acid such as acetic acid, oxalic acid, succinic acid, or citric acid, and the solids are removed by a technique such as filtration, whereby the heavy metals in the precipitate can be recovered as a solution of a salt of the inorganic acid or organic acid.

[0042] When the contaminated water contains or may contain an organic dye as a contaminant, the treatment method of the present invention can be regarded as a method for treating the water (hereinafter, sometimes referred to as dye-containing water). Therefore, the treatment method of the present invention will be described as a method for treating dye-containing water in a form in which dye-containing water is the contaminated water to which the treatment method of the present invention is applied.

[0043] The dye-containing water, i.e., contaminated water, to which the method for treating water containing organic dyes can be applied is not particularly limited as long as it contains or is likely to contain organic dyes, and examples thereof include wastewater from various factories such as paper, textile, food, cosmetics, and pharmaceutical factories, environmental water such as rivers, lakes, and groundwater, soil washing wastewater, seepage water from dredged soil, and wastewater generated in research facilities.

[0044] The organic dyes to be treated are not limited in terms of chemical structure as long as they are water-soluble, and may be, for example, various types such as azo, quinone, triarylmethane, cyanine, phthalocyanine, and indigo dyes, and may also be fluorescent, but water-insoluble organic pigments and lake pigments, which are water-soluble organic dyes that have been made insoluble in water, are not treated.

[0045] In the method for treating dye-containing water, the dye-containing water is treated with the thermophilic cyanobacteria. Specifically, the dye-containing water is brought into contact with the thermophilic cyanobacteria. Through this treatment, the thermophilic cyanobacteria take in the organic pigments in the dye-containing water and metabolically decompose them, thereby decolorizing the dye-containing water.

[0046] As mentioned above in the explanation of the heavy metal separation method, thermophilic cyanobacteria, particularly the thermophilic cyanobacteria exemplified above, all have high photosynthetic efficiency and high protein thermostability, and therefore can grow by actively absorbing carbon dioxide gas from the atmosphere under sunlight at temperatures ranging from near room temperature (around 20°C) to high temperatures of 90°C, and can grow rapidly using the sugars produced by their own photosynthesis as nutrients without the need for external nutrient supply. For example, they grow rapidly even in a laboratory lit by fluorescent lights at near room temperature, with a doubling time of approximately 14 hours.

[0047] Therefore, when bringing dye-containing water into contact with thermophilic cyanobacteria, it is preferable to set the contact environment to one that promotes photosynthesis in the thermophilic cyanobacteria, as this can inexpensively and effectively promote the decomposition of organic dyes. For example, it is preferable to maintain the dye-containing water containing thermophilic cyanobacteria at at least near room temperature or heated to 90°C, and then maintain the dye-containing water under sunlight or artificial lighting such as an incandescent lamp, fluorescent lamp, or LED while blowing in air or carbon dioxide gas. In particular, from the perspective of promoting photosynthesis in thermophilic cyanobacteria, it is preferable to maintain the dye-containing water at a pH between near neutral and weakly alkaline, for example, between pH 6 and 10. Note that if the pH of the dye-containing water is 5 or less, the photosynthetic rate of the thermophilic cyanobacteria may decrease, potentially reducing the decomposition efficiency of the organic dye. The pH of the dye-containing water can usually be adjusted by adding a pH adjuster such as a carbonate such as sodium carbonate or potassium carbonate, a bicarbonate such as sodium bicarbonate or potassium bicarbonate, a hydroxide such as sodium hydroxide or potassium hydroxide, a mineral acid such as dilute hydrochloric acid, dilute nitric acid or dilute sulfuric acid, phosphoric acid, a phosphate such as sodium phosphate or potassium phosphate, or an organic acid such as acetic acid or citric acid.

[0048] Referring to Figure 1, one embodiment of a wastewater treatment device capable of implementing the contaminated water treatment method of the present invention, particularly the heavy metal separation method according to one aspect of the present invention or the organic dye-containing water treatment method according to another aspect of the present invention, is described. In the figure, wastewater treatment device 1 is for treating wastewater discharged from a factory and mainly comprises a treatment tank 2 and a settling tank 3. When wastewater treatment device 1 is used to implement the heavy metal separation method, the wastewater to be treated is contaminated water containing heavy metals discharged from factories such as plating factories. When wastewater treatment device 1 is used to implement the organic dye-containing water treatment method, the wastewater to be treated is contaminated water containing organic dyes discharged from factories such as textile factories.

[0049] The treatment tank 2 stores wastewater and is made of transparent glass or resin that allows external light to pass through. A wastewater inlet 21 from a factory is connected to the treatment tank 2. A gas supply pipe 22 is installed across the entire bottom of the tank 2. The gas supply pipe 22 supplies carbon dioxide-containing gas (hereinafter referred to as carbon dioxide) to the wastewater stored in the treatment tank 2. The gas supply pipe 22 has multiple nozzles capable of ejecting carbon dioxide bubbles to uniformly distribute the carbon dioxide throughout the wastewater. The carbon dioxide supply source for the gas supply pipe 22 is, for example, a carbon dioxide supply device such as a carbon dioxide tank or a pump for pumping atmospheric air. Alternatively, purified exhaust gas from a factory, which has been treated to remove nitrogen oxides, sulfur oxides, and other harmful substances, can also be used as a carbon dioxide supply source.

[0050] Illuminators 23 are installed at the top and bottom of the treatment tank 2 to irradiate light into the inside of the treatment tank 2. The illuminators 23 are light bulbs, fluorescent lamps, LEDs, or the like.

[0051] The settling tank 3 is a container with a conical bottom, and is connected to a liquid supply pipe 31 extending from the treatment tank 2. The liquid supply pipe 31 is an overflow path that regulates the wastewater accumulated in the treatment tank 2 so that it does not exceed a certain water level, and is connected to the top of the settling tank 3. In addition, a drain pipe 32 extends from the settling tank 3 for sending the wastewater from the treatment tank 2 to the outside. The drain pipe 32 is an overflow path that regulates the wastewater from the treatment tank 2 so that it does not exceed a certain water level in the settling tank 3.

[0052] A sludge recovery path 33 is connected to the lower end of the settling tank 3. The recovery path 33 branches into a sludge disposal path 34 and a sludge circulation path 35. The recovery path 33 also has a selector valve (not shown) at the branch point. The selector valve opens and closes the recovery path 33 and selects either the disposal path 34 or the circulation path 35. The circulation path 35 has a sludge pump (not shown) and can return sludge from the recovery path 33 to the treatment tank 2.

[0053] The wastewater treatment device 1 is installed in a greenhouse 10 made of transparent glass or resin material so that sunlight can enter inside.

[0054] In wastewater treatment by the wastewater treatment system 1, wastewater is continuously and gradually supplied through an inlet line 21 to a treatment tank 2 containing thermophilic cyanobacteria. At the same time, carbon dioxide gas is supplied through a gas supply pipe 22, and this carbon dioxide gas is sprayed into the wastewater in the treatment tank 2. The water level of the wastewater from the inlet line 21 rises in the treatment tank 2 while it comes into contact with the thermophilic cyanobacteria, and the wastewater flows out through a liquid supply pipe 31 to a settling tank 3. At this time, the wastewater in the treatment tank 2 is agitated together with the thermophilic cyanobacteria by the carbon dioxide gas sprayed out, particularly near the gas supply pipe 22, thereby increasing the efficiency of contact with the thermophilic cyanobacteria. When the wastewater flows into the settling tank 3, the thermophilic bacteria contained therein settle to the bottom as sludge, and the supernatant liquid flows out through a drain pipe 32 as the water level rises.

[0055] If the wastewater to be treated is contaminated water containing heavy metals, the thermophilic cyanobacteria introduced into the treatment tank 2 in the above process will take up the heavy metals contained in the wastewater into their cells. Therefore, the supernatant liquid flowing out of the drain pipe 32 of the settling tank 3 can be disposed of as wastewater from which heavy metals have been separated. If this wastewater does not meet the required legal water quality standards, it can be subjected to additional treatment before being disposed of in the environment.

[0056] Furthermore, if the wastewater to be treated is contaminated water containing organic pigments, the thermophilic cyanobacteria introduced into the treatment tank 2 during the above process will ingest and metabolically decompose the organic pigments contained in the wastewater. Therefore, the supernatant liquid flowing out of the drain pipe 32 of the settling tank 3 can be disposed of as decolorized wastewater containing some of the decomposition products of the organic pigments. If this wastewater does not meet the required legal water quality standards, it can be disposed of in the environment after undergoing additional treatment.

[0057] During the wastewater treatment process using the wastewater treatment device 1, the thermophilic cyanobacteria introduced into the treatment tank 2 photosynthesize and rapidly grow in the warm environment of the greenhouse 10, where the temperature rises due to sunlight entering during the daytime, and also by receiving sunlight and carbon dioxide gas emitted from the gas supply pipe 22. Therefore, over time, the thermophilic cyanobacteria's efficiency in capturing heavy metals in the wastewater or decomposing organic pigments increases, allowing for continuous treatment of the wastewater continuously supplied to the treatment tank 2. In other words, the heavy metals contained in the wastewater continuously supplied to the treatment tank 2 can be continuously separated, or the wastewater continuously supplied to the treatment tank 2 can be continuously decolorized. At night or in winter when sunlight is weak, it is preferable to turn on the illuminator 23 and heat the greenhouse 10 to maintain a photosynthetic environment for the thermophilic cyanobacteria. From the standpoints of energy efficiency and carbon dioxide emission reduction, it is preferable to heat the greenhouse 10 using waste heat from the factory where the wastewater is generated.

[0058] In the wastewater treatment system 1, the amount of sludge settling in the settling tank 3 increases as the treatment of the wastewater progresses. The settled sludge is preferably disposed of appropriately through the waste line 34 by operating the switching valve of the recovery line 33, or a portion of it is circulated to the treatment tank 2 through the circulation line 35. In particular, at night or during bad weather when it is difficult for thermophilic cyanobacteria to grow, it is preferable to increase the amount of sludge circulated to the treatment tank 2 to prevent a decrease in the thermophilic cyanobacteria in the treatment tank 2, thereby preventing a decrease in the efficiency of uptake of heavy metals by the thermophilic cyanobacteria or a decrease in the efficiency of decomposition of organic pigments.

[0059] A factory equipped with the wastewater treatment device 1 can use the exhaust heat to adjust the temperature inside the greenhouse 10 to create an environment suitable for the growth of thermophilic cyanobacteria, and can use the exhaust gas as a source of carbon dioxide, thereby reducing energy consumption and aiming for carbon negativity in wastewater treatment, i.e., in treating water containing heavy metals or organic pigments in wastewater.

[0060] When the wastewater to be treated is contaminated water containing heavy metals, the heavy metals separated from the wastewater in the wastewater treatment device 1 can be recovered by acid treating the sludge discarded through the disposal path 34 .

[0061] The present invention is further explained by the following experimental examples, which are not intended to limit the invention.

[0062] [Experimental Example I] Experimental Example IA: T. elongatus was added to a culture medium (DTN medium) so that the cell turbidity (OD) was approximately 0.01, and the mixture was incubated at 45°C under 40 μmol photons m －２ S －１ The culture was conducted under visible light irradiation at a light intensity of 1000 u / s. Separately, zinc chloride was added to a similar culture medium containing T. elongatus to give an OD of approximately 0.01, and zinc chloride was added to a zinc-equivalent concentration of 0.25 mM, 0.5 mM, 1 mM, or 2 mM, and the medium was cultured under the same temperature and light intensity conditions.

[0063] The O.D. of the culture medium was measured for approximately 110 hours from the start of culture, and the results are shown in Figure 2. According to Figure 2, the O.D. of T. elongatus was 0.2 (1 x 10７ The cells are tolerant to zinc at a concentration of at least 1 mM (65 mg / L) in a 1000 kJ / mL (corresponding to a cell concentration of 1000 kJ / mL).

[0064] Non-Patent Document 2 conducted a similar experiment using mesophilic cyanobacteria Synechocystis 6803 and found that the OD was 0.9 (7 × 10 ８ reported that bacteria were tolerant to zinc concentrations up to 0.002 mM (0.13 mg / L) at cell concentrations of 0.002 mM (0.13 mg / L) at 0.002 mM (0.13 mg / L) of zinc. Most bacteria generally have an O.D. of 10 (approximately 1 x 10 １０ It is known that the cells can continue to grow up to a cell concentration of approximately 1 × 10 １０ The zinc resistance in cells / mL was converted based on the above resistance of Synechocystis 6803 and the above experimental results of T. elongatus, and the results were compared as follows:

[0065] The cell concentration is approximately 1 x 10 １０ Zinc tolerance in cells / mL: Synechocystis 6803 0.029 mM T. elongatus 1,000 mM

[0066] Experiment IB: An experiment similar to Experiment IA was carried out except that cadmium chloride was added to the culture solution instead of zinc chloride. As a result, T. elongatus grew at an O.D. of 0.2 (1 x 10 ７ It was found that the strain exhibited tolerance to 0.66 mM (32 mg / L) cadmium at a concentration of 1.57 cells / mL.

[0067] Experimental Example IC: The results of time-of-flight mass spectrometry of metallothionein purified from T. elongatus cultured in Experimental Example IA without the addition of zinc and that purified from T. elongatus cultured in Experimental Example IA with the addition of 1 mM zinc for 24 hours are shown in Figure 3.

[0068] 3, when metallothionein was cultured without zinc, only a peak for the protein and a peak for metallothionein bound to one zinc atom were observed, whereas when metallothionein was cultured with zinc, at least 12 zinc atoms were bound to one protein molecule. Considering that metallothionein from the mesophilic cyanobacterium Synechocystis 6803 reported in Non-Patent Document 2 has been reported to bind four zinc atoms per molecule (Blindauer, C. et al., Proc. Natl. Acad. Sci. U.S.A. (2001) 98, 9593-9598), the number of zinc atoms bound to one metallothionein molecule in T. elongatus is significantly higher.

[0069] Evaluation of Experimental Example I: Based on the above experimental examples, T. elongatus exhibits a high level of heavy metal tolerance, is able to internalize and accumulate large amounts of heavy metals from heavy metal-containing water, and is able to efficiently separate heavy metals from heavy metal-containing water. There are two possible reasons why T. elongatus, a thermophilic cyanobacterium, has a higher heavy metal tolerance than mesophilic cyanobacteria. One is that the metallothionein in T. elongatus has a different amino acid sequence from mesophilic cyanobacteria, allowing it to bind more heavy metals than mesophilic cyanobacteria. The other is that metallothionein gene expression is induced in the presence of heavy metals, increasing the number of metallothionein molecules in T. elongatus. This is thought to be due to the promoter sequence of the metallothionein gene.

[0070] Experimental Example II T. elongatus was added to a solution containing 50 mg / L of methylene blue ("DTN medium" reported in Mulenhoff, U. and Chauvat, F. (1996) Gene transfer and manipulation in the thermophilic cyanobacterium Synechococcus elongatus. Mol. Gen. Genet. 252: 93-100) so that the cell turbidity (OD) reached 0.5, and the culture was then incubated at 45°C. The solution was sampled over time, and the cells were precipitated by centrifugation, and the UV-visible absorption spectrum of the supernatant was measured.

[0071] As shown in Figure 4, the absorbance of methylene blue at its maximum absorption wavelength of 663.5 nm decreased over time, becoming half in about 4 hours and reaching zero in 100 hours. Meanwhile, as shown in Figure 5, the absorbance in the ultraviolet region around 220 nm increased as the absorbance at 663.5 nm decreased. The results in Figures 4 and 5 indicate that methylene blue was decomposed by T. elongatus and converted into a substance with a maximum absorption wavelength around 220 nm, resulting in decolorization of the solution.

[0072] The same procedure was repeated except that Remazol Br Blue RN, Remazol Br Red BB 150%, Remazol Br Yellow 4GL gray, and Remazol Yellow GR 133% were used instead of methylene blue, and UV-visible absorption spectra were measured. The results are shown in Figures 6 to 13.

[0073] Figures 6 and 7, Figures 8 and 9, Figures 10 and 11, and Figures 12 and 13 all show trends similar to those of Figures 4 and 5. Therefore, it can be seen that the solutions containing each organic dye were decolorized as the organic dye was decomposed and converted into a substance with a maximum absorption wavelength in the ultraviolet region.

Claims

1. A method for treating contaminated water, comprising step 1 of treating the contaminated water with thermophilic cyanobacteria.

2. The method for treating contaminated water according to claim 1, wherein the thermophilic cyanobacterium is Thermosynechococcus elongatus.

3. The method for treating contaminated water according to claim 1, wherein step 1 is carried out in an environment promoting photosynthesis in the thermophilic cyanobacteria.

4. A method for treating contaminated water as described in any one of claims 1 to 3, wherein the contaminated water contains heavy metals.

5. The method for treating contaminated water according to claim 4, wherein the contaminated water is one of industrial wastewater and soil washing water.

6. The method for treating contaminated water according to claim 4, further comprising step 2 of separating the precipitate formed in step 1 from the contaminated water.

7. The method for treating contaminated water according to claim 6, further comprising step 3 of treating the precipitate separated in step 2 with an acid.

8. A method for treating contaminated water as described in any one of claims 1 to 3, wherein the contaminated water contains an organic dye.

9. The method for treating contaminated water according to claim 8, wherein the contaminated water is industrial wastewater.

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