Groundwater treatment method and groundwater treatment system

Abstract

A lift pipe 2 is inserted into a ground G where alkaline groundwater W having a hydrogen ion concentration index of equal to greater than pH 9 is present. A gas C, having a carbon dioxide volume percentage greater than that of air at a construction site, is injected by an air feed device 4 into the pipe of the lift pipe 2 from an injection port 2b that is disposed at the lower part of the lift pipe 2, causing the generation of a large number of bubbles in the pipe of the lift pipe 2; the groundwater W is raised and lifted due to a reduction in the apparent specific gravity by the large number of bubbles; and during or after lifting a neutralization reaction is brought about between the carbon dioxide contained in the bubbles and the alkaline groundwater W, thereby lowering the hydrogen ion concentration index of this groundwater NW. As a result, an alkaline groundwater W having a high hydrogen ion concentration index can be easily and efficiently neutralized by effectively utilizing carbon dioxide, the discharge of which into the atmosphere is undesirable.

Description

Groundwater treatment method and treatment system

[0001] The present invention relates to a groundwater treatment method and treatment system, and more particularly to a groundwater treatment method and treatment system that can easily and efficiently neutralize alkaline groundwater with a high hydrogen ion concentration index by effectively utilizing carbon dioxide, which is not desired to be emitted into the atmosphere.

[0002] Land near bodies of water containing buried tailings may contain alkaline groundwater with a high pH. If left untreated, this type of land could potentially leak into the water. Conventionally, when altering such land, the alkaline soil and sand containing the tailings are excavated. Then, the alkaline groundwater is pumped to the surface by an airlift using a water pumping pipe and an air supply system (see, for example, Patent Document 1).

[0003] Pumped alkaline groundwater cannot be discharged directly into public water bodies or sewer systems. For this reason, in the past, a neutralizing agent was added to the pumped alkaline groundwater to neutralize it. Because large amounts of groundwater must be pumped during land modification work, it took a relatively long time and costly process to neutralize the groundwater after pumping. Furthermore, alkaline soil and sand cannot be backfilled in its original state in the excavated area. For this reason, in the past, alkaline soil and sand were often disposed of by landfill at waste disposal sites, and the disposal of alkaline soil and sand also required a relatively large cost.

[0004] Furthermore, the construction industry is facing challenges such as reducing carbon dioxide (CO2) emissions during construction, which is believed to be a cause of global warming, and effectively utilizing the carbon dioxide contained in the exhaust of power generation equipment, construction machinery, etc. Therefore, it would be extremely beneficial if the carbon dioxide generated at construction sites, which should not be released into the atmosphere, could be effectively utilized to modify alkaline soil and fix the carbon dioxide in groundwater or underground.

[0005] Japanese Patent Application Publication No. 2005-307575

[0006] An object of the present invention is to provide a groundwater treatment method and treatment system that can easily and efficiently neutralize alkaline groundwater with a high hydrogen ion concentration index by effectively utilizing carbon dioxide, which is not desired to be emitted into the atmosphere.

[0007] In order to achieve the above object, the groundwater treatment method of the present invention is characterized in that a lifting pipe is inserted into the ground where alkaline groundwater with a pH of 9 or more exists, and an air supply device is used to inject gas containing a higher volumetric proportion of carbon dioxide than the air at the construction site into the lifting pipe through an injection port provided at the bottom of the lifting pipe, thereby generating a large number of bubbles inside the lifting pipe, causing the groundwater to rise and be pumped up due to a decrease in apparent specific gravity caused by the large number of bubbles, and causing a neutralization reaction between the carbon dioxide contained in the bubbles and the alkaline groundwater during or after pumping, thereby lowering the hydrogen ion concentration index of the groundwater.

[0008] In order to achieve the above-mentioned object, the groundwater treatment system of the present invention comprises a pumping pipe inserted into the ground where alkaline groundwater with a pH of 9 or higher is present, and an air supply device connected to an inlet provided at the bottom of the pumping pipe, wherein the air supply device injects gas with a higher volumetric proportion of carbon dioxide than the air at the construction site into the pumping pipe through the inlet, generating a large number of bubbles in the pumping pipe, and the apparent specific gravity of the groundwater decreases due to the large number of bubbles, causing the groundwater to rise and be pumped up, and the carbon dioxide contained in the bubbles neutralizes the alkaline groundwater during or after pumping, thereby reducing the hydrogen ion concentration index of the groundwater.

[0009] According to the present invention, when alkaline groundwater is pumped using a pumping pipe, a gas with a high volumetric ratio of carbon dioxide is injected into the pumping pipe to generate a large number of bubbles within the pipe. The large number of bubbles then causes the groundwater to rise and be pumped due to a decrease in apparent specific gravity, and the carbon dioxide contained in the bubbles neutralizes the alkaline groundwater during or after pumping. This effectively utilizes carbon dioxide, which is not desired to be released into the atmosphere, to effectively lower the pH of the groundwater, thereby easily and efficiently neutralizing the alkaline groundwater.

[0010] Fig. 1 is an explanatory diagram illustrating a groundwater treatment system according to an embodiment of the present invention. Fig. 2 is an explanatory diagram illustrating a groundwater treatment system according to another embodiment of the present invention.

[0011] Hereinafter, a groundwater treatment method and treatment system according to the present invention will be described based on an embodiment shown in the drawings.

[0012] The groundwater treatment method and treatment system of the present invention are a method and system for neutralizing alkaline groundwater with a pH of 9 or higher that is present underground, such as in land where slag is buried. Note that the neutralization of groundwater referred to in this invention does not necessarily mean changing the pH of the groundwater to completely neutral (pH 7), but also means lowering a strongly alkaline pH to bring the pH closer to weakly alkaline or neutral.

[0013] As shown in Figure 1, the groundwater treatment system 1 of the present invention comprises a lift pipe 2 inserted into ground G containing alkaline groundwater W with a hydrogen ion concentration index of pH 9 or higher, and an air supply device 4 connected to an inlet 2b provided at the bottom of the lift pipe 2. The treatment system 1 of this embodiment further comprises a high-pressure container (cylinder) 8 connected to the air supply device 4, a water tank 9 to which an outlet 2c of the lift pipe 2 is connected, and a mixing pipe 11 connected to the water tank 9.

[0014] The riser pipe 2 extends vertically while inserted into the ground G. An inlet 2a at the lower end of the riser pipe 2 is located in an aquifer containing groundwater W. An inlet 2b is provided on the sidewall of the lower part of the riser pipe 2, and an air pipe 5 is connected to the inlet 2b. The upper part of the riser pipe 2 is located above ground, and the outlet 2c of the riser pipe 2 is connected to a water tank 9 also located above ground. In this embodiment, the riser pipe 2 is bent above ground, and the portion of the riser pipe 2 located above ground extends laterally toward the water tank 9. The downstream end of the riser pipe 2 is bent downward, and the outlet 2c of the riser pipe 2 is located inside the water tank 9. In this embodiment, an outer pipe 3 is inserted into the ground G, and the portion of the riser pipe 2 inserted into the ground G and the air pipe 5 are located inside the outer pipe 3. The outer pipe 3 can be installed as needed.

[0015] The air supply device 4 injects gas C, which has a higher volumetric ratio of carbon dioxide (CO2) than the air at the construction site, into the inlet 2b of the riser pipe 2. The air supply device 4 is composed of, for example, an air compressor. The rear end of the air supply pipe 5 is connected to the air supply port of the air supply device 4, and the tip of the air supply pipe 5 is connected to the inlet 2b of the riser pipe 2.

[0016] In this embodiment, a circulation mechanism 7 is provided that supplies the exhaust gas discharged from the air supply device 4 to the air inlet of the air supply device 4. The circulation mechanism 7 may be provided with a filter that removes pollutants such as particulate matter (PM), nitrogen oxides (e.g., NO and NO2), and volatile organic compounds (VOCs) contained in the exhaust gas from the air supply device 4. In this embodiment, a high-pressure container 8 that has carbon dioxide stored in advance is also connected to the air inlet of the air supply device 4. It is preferable to store carbon dioxide that is not desired to be released into the atmosphere, such as carbon dioxide generated at other construction sites or carbon dioxide generated at power plants, in the high-pressure container 8.

[0017] In this embodiment, exhaust gas containing carbon dioxide discharged from the air supply device 4 is supplied to the air inlet of the air supply device 4 by the circulation mechanism 7, and gas with a high volumetric percentage of carbon dioxide is supplied to the air inlet of the air supply device 4 by the high-pressure container 8. As a result, gas C with a higher volumetric percentage of carbon dioxide than the air at the construction site is supplied from the air supply port of the air supply device 4, and the supplied gas C is supplied to the inlet 2b of the lifting pipe 2 via the air supply pipe 5.

[0018] Although the volumetric percentage of carbon dioxide contained in the air (atmosphere near the ground surface) varies slightly depending on the altitude of the construction site, the volumetric percentage of carbon dioxide in normal air is approximately 0.03 to 0.04%. The gas C having a higher volumetric percentage of carbon dioxide than the air at the construction site referred to in the present invention is, for example, gas C having a volumetric percentage of carbon dioxide of 0.4% or more, more preferably 2% or more, and even more preferably 10% or more. Alternatively, it is gas C having a volumetric percentage of carbon dioxide that is, for example, 10 times or more, more preferably 50 times or more, and even more preferably 250 times or more of the volumetric percentage of carbon dioxide contained in the air at the construction site. The upper limit of the volumetric percentage of carbon dioxide contained in gas C having an increased volumetric percentage of carbon dioxide is, for example, 99% or less. The volumetric percentage of carbon dioxide contained in the exhaust gas from the air supply device 4 is, for example, approximately 1% to 10%.

[0019] As shown in Figure 1, the water tank 9 is provided with an exhaust port 10. A mixing pipe 11 is connected to the downstream side of the water tank 9. The mixing pipe 11 is a pipe used for in-pipe mixing of liquid and gas, and is composed of, for example, an in-line mixer such as a static mixer. In this embodiment, the rear end of a branch pipe 6 is connected midway through the air supply pipe 5, and the tip of the branch pipe 6 is connected to the upstream side of the mixing pipe 11. A portion of the gas C, which has a higher volumetric proportion of carbon dioxide than the air at the construction site and is supplied from the air supply device 4 to the air supply pipe 5, is supplied to the mixing pipe 11 via the branch pipe 6.

[0020] A discharge pipe 12 is connected to the downstream side of the mixing pipe 11. The tip side of the discharge pipe 12 is inserted into the ground, and the discharge outlet of the discharge pipe 12 is located in an aquifer where groundwater W exists.

[0021] Next, a method for treating groundwater W using this treatment system 1 will be described.

[0022] As shown in Fig. 1, a lift pipe 2 is inserted into ground G where alkaline groundwater W with a hydrogen ion concentration index of pH 9 or higher exists. In this embodiment, an outer pipe 3 is inserted into the ground G, and sediment is removed from inside the outer pipe 3. Then, the lift pipe 2 and air pipe 5 are installed inside the outer pipe 3. An inlet 2a provided at the lower end of the lift pipe 2 is placed in an aquifer where groundwater W exists. An outlet 2c of the lift pipe 2 is connected to a water tank 9 placed above ground.

[0023] Then, the air supply device 4 is driven to supply gas C having a higher volumetric percentage of carbon dioxide than the air at the construction site from the air supply port of the air supply device 4 to the air supply pipe 5, and the gas C is injected into the riser pipe 2 from the inlet 2b provided at the bottom of the riser pipe 2, thereby generating a large number of bubbles in the riser pipe 2. In this embodiment, the exhaust gas discharged from the exhaust port of the air supply device 4 is supplied to the air inlet of the air supply device 4 by the circulation mechanism 7, thereby increasing the volumetric percentage of carbon dioxide in the gas C supplied from the air supply port of the air supply device 4. Furthermore, the volumetric percentage of carbon dioxide in the gas C supplied from the air supply port of the air supply device 4 is increased by supplying carbon dioxide stored in advance in the high-pressure container 8 to the air inlet of the air supply device 4.

[0024] The groundwater W rises within the riser pipe 2 due to a decrease in apparent specific gravity caused by the numerous bubbles generated within the riser pipe 2, and is then pumped up. Downstream of the inlet 2a of the riser pipe 2, a neutralization reaction occurs between the carbon dioxide contained in the bubbles (gas C) and the alkaline groundwater W during or after pumping, thereby lowering the pH of the groundwater W during or after pumping. In this embodiment, the groundwater W flowing in from the inlet 2a of the riser pipe 2 undergoes a neutralization reaction while ascending within the riser pipe 2 to the ground (during pumping), and further undergoes a neutralization reaction while passing through the riser pipe 2 above ground (after pumping). The groundwater NW after the neutralization reaction, with its pH lowered due to the neutralization reaction within the riser pipe 2, is discharged from the outlet 2c of the riser pipe 2.

[0025] Specifically, for example, in the ground where slag is buried, the groundwater W contains a large amount of calcium hydroxide (Ca(OH)2). Therefore, when the groundwater W comes into contact with the air bubbles inside the pumping pipe 2, the carbon dioxide (CO2) contained in the air bubbles reacts with the water (H2O) contained in the groundwater W to produce carbon dioxide (H2CO3) (CO2 + H2O → H2CO3). Furthermore, the produced carbon dioxide (H2CO3) reacts with calcium hydroxide (Ca(OH)2) contained in the groundwater W, resulting in a neutralization reaction that produces calcium carbonate (CaCO3) (Ca(OH)2 + H2CO3 → CaCO3 + 2H2O). As a result, the hydrogen ion concentration index of the groundwater W decreases during the pumping process, and the carbon dioxide contained in the air bubbles (gas C) also decreases.

[0026] In this embodiment, the groundwater NW after the neutralization reaction discharged from the discharge port 2c of the lifting pipe 2 and the gas NC after the neutralization reaction remaining after the neutralization reaction flow into the water tank 9, and the groundwater NW stored in the water tank 9 sequentially flows into the mixing pipe 11. The gas NC after the neutralization reaction is exhausted to the outside of the water tank 9 from an exhaust port 10 provided in the water tank 9. Since the volume ratio of carbon dioxide in the gas NC after the neutralization reaction is about the same as that of the air at the construction site, there is no problem with venting it as is.

[0027] In this embodiment, gas C, which has a higher volumetric ratio of carbon dioxide than the air at the construction site, is injected from the air supply device 4 into the mixing pipe 11 via the branch pipe 6, causing a large number of bubbles to form inside the mixing pipe 11, through which the groundwater NW flows. The groundwater NW and the large number of bubbles then flow in a swirling manner inside the mixing pipe 11, and a neutralization reaction occurs between the carbon dioxide contained in the bubbles (gas C) and the groundwater NW inside the mixing pipe 11. This further reduces the pH of the groundwater NW that has passed through the mixing pipe 11.

[0028] In this embodiment, the groundwater NW after the neutralization reaction that has passed through the mixing pipe 11 is returned to the ground G via the discharge pipe 12. It is preferable that the groundwater NW after the neutralization reaction be discharged at a position away from the insertion position of the lifting pipe 2. In this embodiment, when the groundwater W is pumped by the treatment system 1, the groundwater level near the inlet 2a of the lifting pipe 2 temporarily drops, and the surrounding groundwater W flows into the area near the inlet 2a of the lifting pipe 2 where the groundwater level has dropped. The groundwater W that has flowed into the area near the inlet 2a of the lifting pipe 2 is then pumped by the treatment system 1, neutralized, and returned to the ground G. As this circulation is repeated, the average value of the hydrogen ion concentration index of the groundwater W present in the ground approaches neutrality (pH 7).

[0029] In the present invention, the hydrogen ion concentration index of the neutralized groundwater NW finally discharged from the treatment system 1 is, for example, pH 5.0 or higher and pH 9.0 or lower, more preferably pH 5.8 or higher and pH 8.6 or lower, and even more preferably pH 6.0 or higher and pH 8.0 or lower. In other words, the volume fraction and supply amount of carbon dioxide in the gas C injected into the lifting pipe 2 and the mixing pipe 11, the lengths of the lifting pipe 2 and the mixing pipe 11, and the like are set under conditions such that the hydrogen ion concentration index of the neutralized groundwater NW finally discharged from the treatment system 1 is, for example, pH 5.0 or higher and pH 9.0 or lower, more preferably pH 5.8 or higher and pH 8.6 or lower, and even more preferably pH 6.0 or higher and pH 8.0 or lower.

[0030] Thus, in the present invention, when alkaline groundwater W having a pH of 9 or higher is pumped using the pumping pipe 2, gas C containing a higher volumetric carbon dioxide content than the air at the construction site is injected into the pumping pipe 2, generating numerous bubbles within the pumping pipe 2. The numerous bubbles then lower the apparent specific gravity of the groundwater W, causing it to rise and be pumped. The carbon dioxide contained in the bubbles neutralizes the alkaline groundwater W during or after pumping. This effectively utilizes carbon dioxide that is not desired to be released into the atmosphere to effectively lower the pH of the groundwater NW during or after pumping, allowing for simple and efficient neutralization of the alkaline groundwater W. In other words, the present invention effectively utilizes carbon dioxide that is not desired to be released into the atmosphere for both pumping and neutralization of the groundwater W. Furthermore, the addition of a neutralizing agent to the groundwater NW neutralized by the present invention is unnecessary, or, if a neutralizing agent is added, the amount of neutralizing agent added can be reduced compared to conventional methods. Therefore, the present invention is advantageous in reducing the cost and time required for neutralizing groundwater W.

[0031] Furthermore, in the present invention, carbon dioxide, which is a cause of global warming, can be effectively utilized to neutralize alkaline groundwater W. Therefore, it can also contribute to reducing carbon dioxide, which is extremely beneficial. As in this embodiment, by providing a circulation mechanism 7 and configuring the gas C supplied from the air supply device 4 to include the exhaust gas discharged from the air supply device 4, the carbon dioxide contained in the exhaust gas from the air supply device 4 can be effectively utilized to neutralize the groundwater W, and the amount of carbon dioxide emitted during construction can also be reduced.

[0032] As in this embodiment, by configuring carbon dioxide stored in advance in a high-pressure container 8 or the like to be supplied to the air inlet of the air supply device 4, it is possible to easily increase the volumetric ratio of carbon dioxide in the gas C supplied into the pumping pipe 2. Furthermore, by storing carbon dioxide generated outside the construction site and which is not desired to be released into the atmosphere in a high-pressure container 8 or the like, it becomes possible to use the carbon dioxide generated outside the construction site for neutralizing the groundwater W. In particular, by using the high-pressure container 8, carbon dioxide can be easily stored in a stable state and the amount of carbon dioxide supplied to the air inlet of the air supply device 4 can be easily adjusted, making it extremely convenient.

[0033] In the present invention, the method of supplying gas C having a higher volumetric carbon dioxide content than the air at the construction site from the air supply device 4 is not limited to using the circulation mechanism 7 or the high-pressure container 8, and various other methods can be adopted. Specifically, for example, a configuration can be adopted in which exhaust gas emitted from machines other than the air supply device 4 used at the construction site is supplied to the air inlet of the air supply device 4. Examples of machines used at the construction site include construction machinery, vehicles, and various machines used in construction work (e.g., generators, etc.). When using carbon dioxide emitted by machines at the construction site, it is recommended to remove pollutants contained in the exhaust gas emitted by the machines at the construction site before supplying the exhaust gas to the air inlet of the air supply device 4. That is, in the present invention, the circulation mechanism 7 and the high-pressure container 8 can be provided as needed. The volumetric carbon dioxide content of the exhaust gas from construction machinery and vehicles is, for example, approximately 1% to 10%.

[0034] The size of the bubbles generated in the riser pipe 2 from the inlet 2b of the riser pipe 2 may be set to, for example, 0.1 mm or more and 5 mm or less. As the bubbles rise in the riser pipe 2, a neutralization reaction occurs between the carbon dioxide contained in the bubbles and the groundwater W, causing the bubbles to gradually shrink. However, by setting the size of the generated bubbles to 0.1 mm or more, the lift effect of the bubbles, which causes the groundwater W to rise, can be maintained up to the surface, which is advantageous for stable pumping while neutralizing the groundwater W. Furthermore, the larger the size of the generated bubbles, the fewer the number of bubbles generated in the riser pipe 2 relative to the amount of gas supplied. However, setting the size of the bubbles generated from the inlet 2b to 0.5 mm or less can prevent the number of bubbles generated in the riser pipe 2 from becoming too small, which is advantageous for stable pumping of the groundwater W.

[0035] Although not provided in this embodiment, for example, a porous filter with many tiny holes can be provided at the inlet 2b of the water lift pipe 2. The size of the holes in the porous filter can be set to, for example, 0.01 mm or more and 0.1 mm or less. Providing a porous filter is advantageous in making the size of bubbles generated inside the water lift pipe 2 uniform. Furthermore, providing a porous filter makes it easier to easily generate many tiny bubbles.

[0036] As in this embodiment, when the groundwater NW after the neutralization reaction discharged from the outlet 2c of the lifting pipe 2 is configured to be returned to the ground G, the process of neutralizing the groundwater W while pumping it using the treatment system 1 and the process of returning the neutralized groundwater NW to the ground G are performed in parallel, thereby efficiently bringing the average value of the hydrogen ion concentration index of the groundwater W present in the ground G closer to neutrality. Note that the present invention can also be configured, for example, to discharge the groundwater NW after the neutralization reaction discharged from the outlet 2c of the lifting pipe 2 into a public water area or a sewer, or to use the groundwater NW after the neutralization reaction for another purpose.

[0037] As in this embodiment, by injecting gas C having a higher volumetric carbon dioxide content than the air at the construction site into the groundwater NW discharged from the discharge port 2c of the pumping pipe 2 and mixing the groundwater NW with the gas C, further neutralization of the groundwater NW after pumping can bring the hydrogen ion concentration index of the groundwater NW closer to neutral. As in this embodiment, by injecting gas C sent from the air supply device 4 into the groundwater NW discharged from the discharge port 2c of the pumping pipe 2, the treatment system 1 for further neutralization after pumping can be configured very simply. As in this embodiment, when the mixing pipe 11 is used for neutralization after pumping, the groundwater NW and air bubbles flow in a swirling manner within the mixing pipe 11, making it easier for the groundwater NW and the air bubbles to come into contact with each other, thereby very efficiently neutralizing the groundwater NW.

[0038] Next, a processing system 1 according to another embodiment of the present invention will be described, which is shown in Fig. 2. The following mainly focuses on the differences from the processing system 1 according to the embodiment shown in Fig. 1.

[0039] In land near a water area where tailings is buried, not only is alkaline groundwater W with a high pH value held underground, but in most cases the pH value of the soil S in the ground G is also high. When altering (neutralizing) such alkaline land, it is necessary to neutralize not only the groundwater W but also the soil S in the ground G. Therefore, the treatment system 1 of this embodiment is configured to neutralize the groundwater W and the soil S in the ground G.

[0040] As shown in Figure 2, this embodiment uses a crushing device 14 for crushing soil and sand S, a barge 15 equipped with a mud shed, and a work boat 16 equipped with a mud transport device 18. The barge 15 and work boat 16 are moored, for example, in waters near the land to be altered.

[0041] In this embodiment, a shielding plate 13 (e.g., a sheet pile) is driven into the ground G, and a portion of the ground G is partitioned by the shielding plate 13. A lifting pipe 2 is inserted into the inner area partitioned by the shielding plate 13. In this embodiment, a mixing pipe 11 is connected to the discharge port 2c of the lifting pipe 2. A discharge pipe 12 is connected to the downstream side of the mixing pipe 11, and the discharge pipe 12 extends to a barge 15. In this embodiment, the water tank 9 of the embodiment illustrated in FIG. 1 is not used.

[0042] The work boat 16 is equipped with a backhoe 17, a mud feeding device 18, and an air feeding device 20. The air feeding device 20 installed on the work boat 16 is provided separately from the air feeding device 4 connected to the lift pipe 2. A mud feeding pipe 19 is connected to the downstream side of the mud feeding device 18, and the mud discharge port of the mud feeding pipe 19 is located in the inner area partitioned by the shielding plate 13. An expansion pipe 19a having an inner diameter larger than the pipe diameter of the remaining part of the mud feeding pipe 19 is provided midway in the mud feeding pipe 19.

[0043] In this embodiment, a circulation mechanism 22 is provided that supplies the exhaust gas discharged from the exhaust port of the gas supply device 20 to the air inlet of the gas supply device 20. Furthermore, a high-pressure container 23 in which carbon dioxide is stored in advance is connected to the air inlet of the gas supply device 20. The other configurations are the same as those of the treatment system 1 of the embodiment illustrated in FIG.

[0044] In the method for treating groundwater W using the treatment system 1 of this embodiment, a shielding plate 13 is cast into ground G containing alkaline groundwater W with a pH of 9 or more and alkaline soil and sand S with a pH of 9 or more, and a portion of the ground G is partitioned off by the shielding plate 13. An outer pipe 3 and a lifting pipe 2 are then inserted into the inner area partitioned off by the shielding plate 13.

[0045] Then, as in the previously described embodiment, the air supply device 4 is driven to supply gas C having a higher volumetric percentage of carbon dioxide than the air at the construction site from the air supply port of the air supply device 4 to the air supply pipe 5, and the gas C is injected into the riser pipe 2 from the inlet 2b provided at the bottom of the riser pipe 2, thereby generating a large number of bubbles in the riser pipe 2. In this embodiment, the exhaust gas discharged from the exhaust port of the air supply device 4 is supplied to the air inlet of the air supply device 4 by the circulation mechanism 7, thereby increasing the volumetric percentage of carbon dioxide in the gas C supplied from the air supply port of the air supply device 4. In this embodiment, carbon dioxide previously stored in the high-pressure container 8 is further supplied to the air inlet of the air supply device 4, thereby increasing the volumetric percentage of carbon dioxide in the gas C supplied from the air supply port of the air supply device 4.

[0046] The groundwater W is then raised and pumped up by a decrease in apparent specific gravity due to the large number of bubbles generated inside the rise pipe 2, and a neutralization reaction occurs downstream of the inlet 2a of the rise pipe 2 between the carbon dioxide contained in the bubbles (gas C) and the alkaline groundwater W during or after pumping, thereby lowering the hydrogen ion concentration index of the groundwater W during or after pumping. The pumped groundwater NW after the neutralization reaction flows into the mixing pipe 11 from the outlet 2c of the rise pipe 2.

[0047] Gas C, which has a higher volumetric carbon dioxide content than the air at the construction site, is injected from the air supply device 4 into the mixing pipe 11 via the branch pipe 6, generating a large number of bubbles inside the mixing pipe 11, through which the groundwater NW flows. The groundwater NW and the large number of bubbles then flow in a vortex inside the mixing pipe 11, causing a neutralization reaction between the carbon dioxide contained in the bubbles (gas C) and the groundwater NW inside the mixing pipe 11. As a result, the hydrogen ion concentration index of the groundwater NW that has passed through the mixing pipe 11 further decreases, approaching neutrality. The groundwater NW that has passed through the mixing pipe 11 and undergoes the neutralization reaction is discharged through the discharge pipe 12 into the mud storehouse of the barge 15.

[0048] In this embodiment, in parallel with the process of pumping and neutralizing the groundwater W, the soil S is excavated from the area inside the partition wall 13. The soil S is excavated using construction machinery such as a backhoe. The excavated soil S is transported to a crushing device 14 using a large vehicle or the like, and the soil S is crushed into fine particles by the crushing device 14. The particle size of the crushed soil DS is preferably 40 mm or less, more preferably 20 mm or less.

[0049] The crushed soil DS, which has been refined by the crushing device 14, is then transported to the barge 15 using a large vehicle or the like, and the crushed soil DS is dumped into the mud bin of the barge 15. As a result, the crushed soil DS and the neutralized groundwater NW are stored in the mud bin of the barge 15. In this embodiment, a backhoe 17 installed on the work boat 16 is used to stir and mix the crushed soil DS and the neutralized groundwater NW in the mud bin of the barge 15 to produce a slurry soil SS. Since the crushed soil DS is alkaline, the slurry soil SS is also alkaline. The backhoe 17 is then used to dump the slurry soil SS produced in the mud bin of the barge 15 into the inlet (hopper) of the mud transport device 18. The slurry soil SS dumped into the inlet of the mud transport device 18 flows into the mud transport pipe 19.

[0050] In this embodiment, the air supply device 20 mounted on the work vessel 16 is driven to supply gas C having a higher volumetric proportion of carbon dioxide than the air at the construction site from the air supply port of the air supply device 20 to the air supply pipe 21, and the gas C is injected into the mud supply pipe 19. In this embodiment, exhaust gas discharged from the exhaust port of the air supply device 20 is supplied to the air supply port of the air supply device 20 by the circulation mechanism 22, thereby increasing the volumetric proportion of carbon dioxide in the gas C supplied from the air supply port of the air supply device 20. In this embodiment, carbon dioxide previously stored in a high-pressure container 23 is further supplied to the air supply port of the air supply device 20, thereby increasing the volumetric proportion of carbon dioxide in the gas C supplied from the air supply port of the air supply device 20.

[0051] As the slurry sediment SS and the gas C flow into the sludge transport pipe 19, the slurry sediment SS and the gas C are mixed as they pass through the sludge transport pipe 19. Then, within the sludge transport pipe 19, a neutralization reaction occurs between the alkaline slurry sediment SS and the carbon dioxide contained in the gas C, thereby reducing the pH of the slurry sediment SS. In this embodiment, the slurry sediment SS and the gas C are mixed particularly effectively in the diffuser pipe 19a provided midway through the sludge transport pipe 19, and the neutralization reaction between the slurry sediment SS and the carbon dioxide contained in the gas C is effectively carried out. Then, the slurry-like post-neutralization sediment NS, whose pH has been reduced as a result of the neutralization reaction, flows out of the discharge port of the sludge transport pipe 19 into the inner area partitioned by the shielding plate 13. That is, the post-neutralization sediment NS is transported to the excavation site in the ground G, and the excavation site is backfilled with the post-neutralization sediment NS.

[0052] In the present invention, the hydrogen ion concentration index of the neutralized sediment NS finally discharged from the treatment system 1 is, for example, pH 5.0 or higher and pH 9.0 or lower, more preferably pH 5.8 or higher and pH 8.6 or lower, and even more preferably pH 6.0 or higher and pH 8.0 or lower. In other words, the volume fraction and supply amount of carbon dioxide in the gas C injected into the sludge transport pipe 19, the length of the sludge transport pipe 19, and the like are set under conditions such that the hydrogen ion concentration index of the neutralized sediment NS finally discharged from the treatment system 1 is, for example, pH 5.0 or higher and pH 9.0 or lower, more preferably pH 5.8 or higher and pH 8.6 or lower, and even more preferably pH 6.0 or higher and pH 8.0 or lower.

[0053] As in this embodiment, alkaline soil S with a pH of 9 or higher is excavated from the ground G, and the soil S is stirred and mixed with groundwater NW whose pH has been reduced to produce slurry soil SS. The slurry soil SS is then mixed with gas C, which has a higher volumetric carbon dioxide content than the air at the construction site, to neutralize the carbon dioxide contained in the gas C and the alkaline soil SS. The resulting slurry soil NS, which has a lower pH due to the neutralization reaction, is then pumped to the excavated area of ​​the ground G and backfilled. By effectively utilizing the groundwater NW neutralized in the pumping pipe 2, the alkaline soil S excavated from the ground G can be easily and efficiently neutralized. In other words, by neutralizing the groundwater W in the ground G and neutralizing the soil S excavated from the ground G and backfilling the excavated area, alkaline land with a high pH can be efficiently modified (neutralized).

[0054] As in this embodiment, when the crushing device 14 or the like is used to crush and refine the soil S excavated from the ground G and the refined crushed soil DS is used to produce slurry soil SS, crushing the excavated soil S to make each particle finer increases the surface area of ​​the soil S particles that come into contact with the gas C in the slurry soil SS. This makes it easier for the carbon dioxide contained in the gas C to neutralize the alkaline soil SS, which is advantageous for efficiently neutralizing the alkaline soil SS. Furthermore, crushing and refined the soil S excavated from the ground G also reduces the risk of the slurry soil SS clogging the mud transport device 18 and the mud transport pipe 19.

[0055] As in this embodiment, by configuring the slurry soil SS and the gas C to be mixed in the mud transport pipe 19 that transports the slurry soil SS to the excavation site of the ground G, the slurry soil SS and the gas C can be efficiently mixed with a simple configuration, and the alkaline soil SS and the carbon dioxide contained in the gas C can be efficiently neutralized. In particular, by providing a diffuser 19a in the middle of the mud transport pipe 19 as in this embodiment, the slurry soil SS and the gas C can be efficiently mixed in the diffuser 19a, and the alkaline soil SS and the carbon dioxide contained in the gas C can be efficiently neutralized. Note that the diffuser 19a can be provided as desired.

[0056] As in this embodiment, by configuring carbon dioxide stored in advance in a high-pressure container 23 or the like to be supplied to the air inlet of the air supply device 20 mounted on the work vessel 16, the volumetric ratio of carbon dioxide in the gas C supplied into the mud transport pipe 19 can be easily increased. Also, by storing carbon dioxide generated outside the construction site in a high-pressure container 23 or the like, it becomes possible to effectively use the carbon dioxide generated outside the construction site for neutralizing slurry-like soil and sand SS. In particular, by using the high-pressure container 23, carbon dioxide can be easily stored in a stable state and the amount of carbon dioxide supplied to the air inlet of the air supply device 20 can be easily adjusted, making it extremely convenient.

[0057] In the present invention, the method of supplying carbon dioxide to the air inlet of the air supply device 20 mounted on the work boat 16 is not limited to using the circulation mechanism 22 or the high-pressure container 23, and various other methods can be used to supply carbon dioxide to the air inlet of the air supply device 20. Specifically, for example, a configuration can be used in which exhaust gas emitted from machines other than the air supply device 20 used at the construction site is supplied to the air inlet of the air supply device 20. Examples of machines used at the construction site mentioned above include the work boat 16, the backhoe 17, and various machines used in construction (e.g., generators, etc.). When using carbon dioxide emitted by machines at the construction site, it is advisable to remove pollutants contained in the exhaust gas emitted by the machines at the construction site before supplying the exhaust gas to the air inlet of the air supply device 20. In other words, in the present invention, the circulation mechanism 22 and the high-pressure container 23 can be provided as needed.

[0058] In this embodiment, if the soil S excavated from the ground G and crushed and the groundwater NW after the neutralization reaction are stored in the mud storehouse of the barge 15, there is no need to set up a temporary soil storage area on land to store the soil S and groundwater NW, which is advantageous for reducing the cost required for land modification. Furthermore, if a work boat 16 equipped with a mud transport device 18 is used, there is no need to set up a work area on land to place the mud transport device 18 and the backhoe 17, which is advantageous for reducing the cost required for land modification.

[0059] The system for transporting the slurry soil SS stored in the mud shed of the work vessel 16 to the excavation site of the ground G is not limited to the configuration of this embodiment, and various other configurations are possible. For example, the system may be configured so that the slurry soil SS stored in the mud shed of the work vessel 16 is sucked up by a soil lifting pump and transported to the excavation site of the ground G by water.

[0060] Furthermore, in the present invention, for example, a temporary soil and sand storage area may be provided on land instead of the barge 15. For example, the mud transport device 18 and the backhoe 17 may be located on land. Furthermore, in the present invention, the shape and arrangement of each pipe (such as the lifting pipe 2 and the air transport pipe 5), the connection positions between the pipes, and the arrangement of each device (such as the air transport device 4 and the crushing device 14) are not limited to the above-described exemplary embodiment, and various other configurations may be used depending on the construction conditions. Furthermore, the present invention is not limited to the modification of land where tailings is buried, but can also be used in the modification of land where groundwater W or soil S with a high pH index due to factors other than tailings is present, or in other construction work.

[0061] REFERENCE SIGNS LIST 1 Treatment system 2 Lifting pipe 2a Inlet 2b Inlet 2c Outlet 3 Outer pipe 4 Air supply device 5 Air supply pipe 6 Branch pipe 7 Circulation mechanism 8 High-pressure vessel 9 Water tank 10 Exhaust port 11 Mixing pipe 12 Discharge pipe 13 Shielding plate 14 Crushing device 15 Barge 16 Work boat 17 Backhoe 18 Mud supply device 19 Mud supply pipe 19a Expander pipe 20 Air supply device 21 Air supply pipe 22 Circulation mechanism 23 High-pressure vessel C Gas (having a higher volumetric proportion of carbon dioxide than air) NC Gas after neutralization reaction W Groundwater NW Groundwater after neutralization reaction S Soil and sand in the ground DS Soil and sand after crushing SS Slurry-like soil and sand NS Soil and sand after neutralization reaction G Ground

Claims

1. A method for treating groundwater, comprising inserting a lift pipe into ground where alkaline groundwater with a hydrogen ion concentration index of pH 9 or higher exists, injecting gas containing a higher volumetric proportion of carbon dioxide than the air at the construction site into the pipe of the lift pipe through an injection port provided at the bottom of the lift pipe using an air supply device, generating a large number of air bubbles inside the pipe, causing the groundwater to rise and be pumped up due to a decrease in apparent specific gravity caused by the large number of air bubbles, and causing a neutralization reaction between the carbon dioxide contained in the air bubbles and the alkaline groundwater during or after pumping, thereby lowering the hydrogen ion concentration index of the groundwater.

2. A method for treating groundwater as described in claim 1, wherein the gas includes exhaust gas discharged from the gas supply device.

3. A method for treating groundwater according to claim 1 or 2, wherein the groundwater having a reduced hydrogen ion concentration index is returned to the ground.

4. A method for treating groundwater as described in claim 1 or 2, which comprises excavating alkaline soil and sand having a pH of 9 or more from the ground, stirring and mixing the excavated soil and the groundwater whose hydrogen ion concentration index has been reduced to produce a slurry of the soil and sand, mixing the slurry of the soil and sand with a gas having a higher volumetric proportion of carbon dioxide than the air at the construction site to neutralize the carbon dioxide contained in the gas with the alkaline soil and sand, and transporting the slurry of the soil and sand with a reduced hydrogen ion concentration index to an excavated area of ​​the ground to backfill the excavated area.

5. A method for treating groundwater as described in claim 4, wherein the soil excavated from the ground is crushed into fine particles, and the fine particles are used to prepare a slurry of the soil.

6. A method for treating groundwater as claimed in claim 4, wherein the slurry-like soil and the gas are mixed in a sludge transport pipe which transports the slurry-like soil to the excavation site.

7. A groundwater treatment system comprising a rise pipe inserted into ground where alkaline groundwater with a hydrogen ion concentration index of pH 9 or more exists, and an air supply device connected to an injection port provided at the bottom of the rise pipe, wherein the air supply device injects gas containing a higher volumetric proportion of carbon dioxide than the air at the construction site into the rise pipe from the injection port, generating a large number of air bubbles inside the rise pipe, causing the groundwater to rise and be pumped up due to a decrease in apparent specific gravity caused by the large number of air bubbles, and a neutralization reaction occurs between the carbon dioxide contained in the air bubbles and the alkaline groundwater during or after pumping, thereby lowering the hydrogen ion concentration index of the groundwater.

Images