JP7674838B2 - Methods for purifying contaminated soil - Google Patents

Description

The present invention relates to a method for purifying contaminated soil.

A soil purification device is known that uses an electrical heating method in which an electric current is passed between multiple electrodes buried in the contaminated soil to heat the contaminated soil to, for example, 50°C or higher (see, for example, Patent Document 1).

In addition, a method for purifying contaminated soil is known in which nutrients such as nutrient salts, organic matter, and oxygen are added, or microorganisms capable of decomposing pollutants and nutrients are added, and hot water heated to, for example, 30°C is injected into the contaminated soil (see, for example, Patent Document 2).

JP 2014-231050 A JP 2014-205086 A

The technology disclosed in Patent Document 1 makes it possible to efficiently remove contaminants such as VOCs (volatile organic compounds) that are attached to soil particles in contaminated soil by heating the contaminated soil to 50°C or higher.

Here, contaminants that are detached from soil particles in the contaminated soil dissolve into groundwater, etc. In this case, it is conceivable that the contaminants that have dissolved into the groundwater, etc., can be decomposed and purified by microorganisms.

However, heating soil to above 50°C can easily kill microorganisms capable of breaking down pollutants, and it may take longer to purify the pollutants.

Taking the above facts into consideration, the present invention aims to improve the purification efficiency of pollutants detached from soil particles in contaminated soil.

The method for purifying contaminated soil according to the first embodiment includes a heating process for heating the temperature of the contaminated soil from room temperature to a first temperature, a temperature adjustment process for adjusting the temperature of the contaminated soil to a second temperature that is lower than the first temperature and higher than the room temperature, and a microorganism injection process for injecting microorganisms capable of decomposing pollutants into the contaminated soil whose temperature has been adjusted to the second temperature.

According to the first aspect of the method for purifying contaminated soil, first, in the heating step, the temperature of the contaminated soil is raised from room temperature to a first temperature. This first temperature is set to a temperature (e.g., 40°C or higher) at which contaminants attached to soil particles in the contaminated soil are easily detached. This allows the contaminants to be efficiently eluted from the soil particles in the contaminated soil into groundwater or the like.

Here, when the temperature of the contaminated soil is raised to a first temperature in the heating process, microorganisms in the contaminated soil that are capable of decomposing the contaminants eluted into groundwater, etc., are more likely to be killed.

As a countermeasure to this, in the present invention, in the temperature adjustment process, the temperature of the contaminated soil is adjusted to a second temperature that is lower than the first temperature and higher than room temperature. This second temperature is set, for example, to a temperature at which microorganisms capable of decomposing the contaminants can live and at which the decomposition of the contaminants by the microorganisms is activated.

Next, in the microorganism injection process, microorganisms capable of decomposing pollutants are injected into the contaminated soil whose temperature has been adjusted to the second temperature. This allows the microorganisms to efficiently decompose and purify the pollutants that have been dissolved into groundwater, etc., in the heating process.

In this way, the present invention can improve the purification efficiency of pollutants detached from soil particles in contaminated soil.

The method for purifying contaminated soil according to the second aspect includes a heating step of heating the temperature of a clayey soil layer of the contaminated soil from room temperature to a first temperature, a temperature adjustment step of adjusting the temperature of a sandy soil layer above the clayey soil layer to a second temperature lower than the first temperature and higher than the room temperature, and a microorganism injection step of injecting microorganisms capable of decomposing contaminants into the sandy soil layer whose temperature has been adjusted to the second temperature.

According to the second aspect of the method for purifying contaminated soil, first, in the heating step, the temperature of the contaminated clayey soil layer is raised from room temperature to a first temperature. This first temperature is set, for example, to a temperature (e.g., 40°C or higher) at which contaminants attached to soil particles in the clayey soil layer are easily detached. This allows the contaminants attached to the soil particles in the clayey soil layer to be efficiently eluted into groundwater or the like. At this time, contaminants such as VOCs (volatile organic compounds) are eluted into the clayey soil layer and the sandy soil layer above the clayey soil layer.

Here, when the clayey soil layer is heated to a first temperature in the heating process, the sandy soil layer is also heated to a predetermined temperature. This makes it easier for microorganisms in the sandy soil layer that can decompose pollutants to die.

As a countermeasure to this, in the present invention, in the temperature adjustment process, the temperature of the sandy soil layer is adjusted to a second temperature that is lower than the first temperature and higher than room temperature. This second temperature is set, for example, to a temperature at which microorganisms capable of decomposing pollutants can live and at which the decomposition of the pollutants by the microorganisms is activated.

Next, in the microorganism injection process, microorganisms capable of decomposing pollutants are injected into the sandy soil layer whose temperature has been adjusted to the second temperature. This allows the microorganisms to efficiently decompose and purify the pollutants that have dissolved into the sandy soil layer in the heating process.

Therefore, the present invention can improve the purification efficiency of pollutants detached from soil particles in a clayey soil layer.

A third aspect of the method for remediating contaminated soil is the method for remediating contaminated soil of the second aspect , wherein in the heating step, the temperature of the sandy soil layer is raised from the room temperature to a third temperature which is lower than the first temperature.

According to the method for purifying contaminated soil according to the third aspect , in the heating step, the temperature of the sandy soil layer is raised from room temperature to a third temperature lower than the first temperature. This makes it difficult for indigenous microorganisms in the sandy soil layer to be killed. Therefore, in the heating step, the contaminants eluted into the sandy soil layer can be decomposed early by the indigenous microorganisms in the sandy soil layer. Therefore, the purification period of the sandy soil layer is shortened.

Furthermore, the amount of microorganisms added to the sandy soil layer during the microorganism injection process is reduced, which can lead to cost savings.

A contaminated soil purification method according to a fourth aspect is the contaminated soil purification method according to the second or third aspect , in which, in the temperature adjustment step, the temperatures of the clayey soil layer and the sandy soil layer are adjusted to the second temperature, and in the microorganism injection step, the microorganisms are injected into the clayey soil layer and the sandy soil layer.

According to the fourth aspect of the method for purifying contaminated soil, in the temperature adjustment step, the temperatures of the clayey soil layer and the sandy soil layer are adjusted to a second temperature. Next, in the microorganism injection step, microorganisms are injected into the clayey soil layer and the sandy soil layer. This allows the contaminants eluted into the clayey soil layer and the sandy soil layer in the temperature increase step to be efficiently decomposed and purified by the microorganisms.

Therefore, the present invention can further improve the purification efficiency of contaminants detached from soil particles in a clayey soil layer.

As described above, the present invention can improve the purification efficiency of contaminants detached from soil particles in contaminated soil.

1 is an elevational cross-sectional view showing ground to which a contaminated soil remediation system according to one embodiment is applied. FIG. 2 is a plan view of the heating well, observation well, and injection well shown in FIG. 1 . 1 is a graph showing experimental results of a method for remediating contaminated soil according to an embodiment of the present invention. 1 is a graph showing experimental results of a method for remediating contaminated soil according to a comparative example.

Below, a method for purifying contaminated soil according to one embodiment will be described with reference to the drawings.

(ground)
Fig. 1 shows a ground 10 to which a soil purification system 20 according to this embodiment is applied. The ground 10 has, as an example, a buried soil layer 10A, a clayey soil layer 10B, a sandy soil layer 10C, and a clayey soil layer 10D, in that order from the surface of the ground. The symbol S shown in Fig. 1 indicates an example of the water level of groundwater.

The sandy soil layer 10C and the clayey soil layer 10D contain contaminated soil P that contains contaminants such as VOCs (volatile organic compounds). Examples of contaminants include organic compounds (toluene contained in paints, printing inks, adhesives, cleaning agents, gasoline, thinners, etc., and volatile organic compounds such as benzene, xylene, tetrachloroethylene, trichloroethylene, cis-1,2-dichloroethylene, and chloroethylene (vinyl chloride monomer)), heavy metal compounds, inorganic compounds, oils, etc.

(Soil purification system)
The soil remediation system 20 includes a heating device 30, an observation well 40, a control device 38, an injection well 50, an injection tank 60, an activator adjustment tank 80, and a microorganism adjustment tank 70.

(Warming device)
The heating device 30 is a device that heats (increases the temperature of) the ground 10 by an electrical heating method (electrical heat generation method). The heating device 30 has a plurality of heating electrodes 32 and a power source 36.

The heating electrodes 32 are buried in the ground 10. The heating electrodes 32 extend from the ground surface through the buried soil layer 10A, the clayey soil layer 10B, and the sandy soil layer 10C to the clayey soil layer 10D. A power source 36 installed on the ground is electrically connected to these heating electrodes 32.

As shown in FIG. 1, the power source 36 is an AC (three-phase AC) power source device. An AC voltage is applied from this power source 36 to each heating electrode 32. In addition, the power source 36 is electrically connected to a control device 38, which will be described later.

As shown in FIG. 2, the heating electrodes 32 (three in this embodiment) are arranged at the vertices of a triangle (approximately equilateral triangle) in a plan view. When an AC voltage is applied to the heating electrodes 32 from a power source 36 (see FIG. 1), an AC current flows through the clayey soil layer 10B, the sandy soil layer 10C, and the clayey soil layer 10D between the heating electrodes 32, as shown in FIG. 1. As a result, Joule heat is generated by the electrical resistance of the clayey soil layer 10B, the sandy soil layer 10C, and the clayey soil layer 10D, and the clayey soil layer 10B, the sandy soil layer 10C, and the clayey soil layer 10D are heated.

Here, the electrical resistance value (specific resistance value) of the clayey soil layer 10D is generally lower than the electrical resistance value (specific resistance value) of the sandy soil layer 10C. Therefore, the alternating current flowing through the clayey soil layer 10D is greater than the alternating current flowing through the sandy soil layer 10C. As a result, the Joule heat generated in the clayey soil layer 10D is greater than the Joule heat generated in the sandy soil layer 10C, and the temperature of the contaminated soil P in the clayey soil layer 10D reaches a high temperature earlier than the contaminated soil P in the sandy soil layer 10C.

In addition, in this embodiment, by partially insulating the surface of the heating electrode 32, the surface area (exposed area) of the heating electrode 32 that is in electrical contact with the viscous soil layer 10D is larger than the surface area (exposed area) that is in electrical contact with the sandy soil layer 10C. This makes the AC current flowing through the viscous soil layer 10D larger than the AC current flowing through the sandy soil layer 10C. In other words, the Joule heat generated in the viscous soil layer 10D is larger than the Joule heat generated in the sandy soil layer 10C, and the temperature of the contaminated soil P in the viscous soil layer 10D reaches a higher temperature earlier than the contaminated soil P in the sandy soil layer 10C.

In this embodiment, the heating device 30 also heats the clayey soil layer 10B above the sandy soil layer 10C. This heats the upper part of the sandy soil layer 10C through the clayey soil layer 10B. However, the clayey soil layer 10B above the sandy soil layer 10C can be heated only if necessary, and does not have to be heated.

(Observation well)
An observation well 40 is provided in the center of the multiple heating electrodes 32. The observation well 40 is a well for observing (detecting) the temperature in the contaminated soil P (groundwater), the concentration of contaminants, and the concentrations of activators and microorganisms described below. The observation well 40 is formed, for example, by burying a cylindrical pipe having an opening in the ground 10. The observation well 40 penetrates from the ground surface through the buried soil layer 10A, the clayey soil layer 10B, and the sandy soil layer 10C to reach the clayey soil layer 10D.

A first temperature sensor 42 and a second temperature sensor 44 are provided in the observation well 40. The first temperature sensor 42 is placed in the contaminated soil P in the clayey soil layer 10D and detects the temperature of the groundwater in the contaminated soil P. On the other hand, the second temperature sensor 44 is placed in the contaminated soil P in the sandy soil layer 10C and detects the temperature of the groundwater in the contaminated soil P. A control device 38, which will be described later, is electrically connected to the first temperature sensor 42 and the second temperature sensor 44.

It is also possible to detect the temperature of the groundwater of the contaminated soil P pumped from the observation well 40 using a temperature sensor without providing the first temperature sensor 42 and the second temperature sensor 44 in the observation well 40. The first temperature sensor 42 is an example of a first temperature detection unit, and the second temperature sensor 44 is an example of a second temperature detection unit.

Two pumping pipes are provided in the observation well 40 to pump groundwater from the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D. A pumping pump (not shown) is connected to each of the two pumping pipes. These pumping pipes pump up the groundwater from the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D. The concentrations of the activator and microorganisms in the pumped up groundwater are measured, for example, by a concentration measuring device.

It is also possible to measure the concentrations of the activator and microorganisms in the contaminated soil P using a concentration measuring device or the like installed in the observation well 40, without pumping groundwater from the contaminated soil P from the observation well 40. The number and arrangement of the observation wells 40 can also be changed as appropriate.

(Control device)
The control device 38 is realized, for example, by a computer. The computer includes a CPU, a memory as a temporary storage area, and a non-volatile storage unit. Furthermore, temperature information of the contaminated soil detected by the first temperature sensor 42 and the second temperature sensor 44 is input to the control device 38. This control device 38 controls the operation of the power source 36 of the heating device 30 based on the temperature information of the contaminated soil detected by the first temperature sensor 42 and the second temperature sensor 44.

(Injection well)
A plurality of injection wells 50 are provided around the contaminated soil P. The plurality of injection wells 50 are used to inject an injection liquid supplied from an injection tank 60 (described later) into the sandy soil layer 10C and the clayey soil layer 10D of the contaminated soil P. Each injection well 50 is formed, for example, by burying a cylindrical pipe in the ground.

The injection wells 50 penetrate from the ground surface through the buried soil layer 10A, the clayey soil layer 10B, and the sandy soil layer 10C to the clayey soil layer 10D. The cylindrical pipe of each injection well 50 has a plurality of openings 50H that penetrate the peripheral wall of the cylindrical pipe. The plurality of openings 50H are respectively arranged in the sandy soil layer 10C and the clayey soil layer 10D.

As shown in FIG. 2, the multiple injection wells 50 are arranged at the vertices of a triangle (equilateral triangle) in a plan view. From these injection wells 50, the injection liquid is injected into the contaminated soil P that has been heated by the heating device 30. In this embodiment, the injection liquid is injected from the multiple injection wells 50 into the contaminated soil P surrounded by the multiple heating electrodes 32.

(Injection tank)
1, an injection tank 60 is provided on the ground and stores the injection liquid to be injected into the contaminated soil P of the sandy soil layer 10C and the clayey soil layer 10D from a plurality of injection wells 50. Water is supplied to the injection tank 60 as the injection liquid from a water source such as a water supply (not shown).

The injection tank 60 is connected to a plurality of injection wells 50 via piping 62. Then, by operating an injection pump (not shown) provided on the piping 62, the injection liquid stored in the injection tank 60 is injected into the contaminated soil P of the sandy soil layer 10C and the clayey soil layer 10D via the piping 62 and the plurality of injection wells 50. Here, microorganisms are added to the injection liquid stored in the injection tank 60 from the microorganism adjustment tank 70, and an activator is added to the injection liquid from the activator adjustment tank 80.

(Microbial Adjustment Tank)
The microbial adjustment tank 70 is provided above ground and serves to adjust the type and concentration of microorganisms to be added to the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D. A solvent such as water is supplied to the microbial adjustment tank 70 from a water source such as a water supply (not shown). In addition, predetermined microorganisms capable of decomposing contaminants are added to the solvent stored in the microbial adjustment tank 70. This produces an additive liquid containing microorganisms (microorganism-added liquid).

The microbial adjustment tank 70 is connected to the injection tank 60 via piping 72. The microbial additive liquid stored in the microbial adjustment tank 70 is added to the injection liquid in the injection tank 60 by operating a pump (not shown) installed in the piping 72. The type and amount of microorganisms are appropriately adjusted based on the type and concentration of contaminants in the contaminated soil P observed in the observation well 40.

(Surfactant adjustment tank)
The activator adjustment tank 80 is provided on the ground and serves to adjust the type and concentration of an activator to be added to the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D. A solvent such as water is supplied to the activator adjustment tank 80 from a water source such as a water supply (not shown). An activator that activates the microorganisms added from the microorganism adjustment tank 70 to the injection tank 60 is added to the solvent stored in the activator adjustment tank 80. This produces an additive liquid (activator-added liquid) containing the activator. Examples of activators that can be used include nutrients, organic matter, and oxygen.

The activator adjustment tank 80 is connected to the injection tank 60 via a pipe 82. The activator-added liquid stored in the activator adjustment tank 80 is added to the injection liquid in the injection tank 60 by operating a pump (not shown) provided in the pipe 82. The type and amount of the activator are adjusted based on the type and concentration of the microorganisms added from the microorganism adjustment tank 70 to the injection tank 60.

(Methods for remediating contaminated soil)
Next, an example of a method for purifying contaminated soil will be described.

(Heating process)
First, the heating process will be described. In the heating process, the temperature of the contaminated soil P in the clayey soil layer 10D is raised from room temperature to a first temperature, and the temperature of the contaminated soil P in the sandy soil layer 10C is raised from room temperature to a third temperature by the heating device 30. The room temperature of the sandy soil layer 10C and the clayey soil layer 10D is, for example, 10°C to 20°C.

Specifically, when an AC voltage is applied from the power source 36 to the heating electrodes 32, an AC current flows between the heating electrodes 32. This causes an AC current to flow through the contaminated soil P in the clayey soil layer 10B, the sandy soil layer 10C, and the clayey soil layer 10D that are between the heating electrodes 32. As a result, Joule heat is generated by the electrical resistance of the contaminated soil P in the clayey soil layer 10B, the sandy soil layer 10C, and the clayey soil layer 10D, and the contaminated soil P is heated.

At this time, the temperature of the contaminated soil P in the clayey soil layer 10D is detected by the first temperature sensor 42 in the observation well 40, and the detected temperature information is output to the control device 38. The control device 38 stops the power supply 36 when the temperature information of the contaminated soil P in the clayey soil layer 10D input from the first temperature sensor 42 is equal to or higher than the first temperature.

On the other hand, when the temperature information of the contaminated soil in the clayey soil layer 10D input from the first temperature sensor 42 is less than the first temperature, the control device 38 activates the power source 36 to heat the contaminated soil P in the clayey soil layer 10D. This maintains the temperature of the contaminated soil P in the clayey soil layer 10D at the first temperature.

In addition, in the heating process, the temperature of the contaminated soil P in the clayey soil layer 10D is maintained at the first temperature by the heating device 30 until the concentration of the contaminant in the contaminated soil P in the clayey soil layer 10D observed in the observation well 40 becomes equal to or lower than a predetermined value (target value).

Here, the first temperature is set, for example, to a temperature at which contaminants adhering to the soil particles of the clayey soil layer 10D can be peeled off. As a result, when the temperature of the clayey soil layer 10D reaches the first temperature, the contaminants are efficiently peeled off from the soil particles in the contaminated soil P of the clayey soil layer 10D and dissolved into groundwater, etc. At this time, as shown by the arrow F in Figure 1, when the contaminants such as VOCs volatilize and rise, they are dissolved into the sandy soil layer 10C above the clayey soil layer 10B.

The first temperature is preferably 40°C or higher and 80°C or lower (40°C to 80°C), and more preferably 50°C or higher and 70°C or lower (50°C to 70°C).

Incidentally, the electrical resistance value (specific resistance value) of the clayey soil layer 10D is generally lower than the electrical resistance value (specific resistance value) of the sandy soil layer 10C. Also, as described above, in this embodiment, the surface area (exposed area) of the heating electrode 32 that is in electrical contact with the clayey soil layer 10D is larger than the surface area (exposed area) of the heating electrode 32 that is in electrical contact with the clayey soil layer 10B and the sandy soil layer 10C.

As a result, the alternating current flowing through the clayey soil layer 10D becomes greater than the alternating current flowing through the clayey soil layer 10B and the sandy soil layer 10C. In other words, the Joule heat generated in the clayey soil layer 10D becomes greater than the Joule heat generated in the clayey soil layer 10B and the sandy soil layer 10C. As a result, the temperature of the clayey soil layer 10D reaches the first temperature earlier than the clayey soil layer 10B and the sandy soil layer 10C.

As a result of the above, when the temperature of the clayey soil layer 10D reaches the first temperature, the temperature of the sandy soil layer 10C is raised to a third temperature that is higher than room temperature and lower than the first temperature. Then, when the temperature of the clayey soil layer 10D reaches the third temperature, the contaminants are detached from the soil particles in the contaminated soil P of the clayey soil layer 10D and eluted into the groundwater in the clayey soil layer 10D, the sandy soil layer 10C above the clayey soil layer 10D, etc.

(Temperature control process)
Next, the temperature adjustment step will be described. In the temperature raising step, when the temperature of the contaminated soil P in the clay soil layer 10B is raised to a first temperature, microorganisms indigenous to the contaminated soil P are likely to be killed. Similarly, when the temperature of the contaminated soil P in the sandy soil layer 10C is raised to a third temperature, microorganisms indigenous to the contaminated soil P are likely to be killed. As a result, it may take a long time to purify the contaminated soil P.

Therefore, in this embodiment, in the temperature adjustment process, the temperature of the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D is adjusted to a second temperature that is higher than room temperature and lower than the first temperature. Specifically, the heating device 30 is stopped, and the temperature of the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D is lowered. The third temperature in this embodiment is higher than the second temperature and lower than the first temperature.

At this time, the temperature of the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D is monitored by the first temperature sensor 42 and the second temperature sensor 44 in the observation well 40. Then, when the temperature of the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D reaches the second temperature, the temperature adjustment process is terminated.

Here, the second temperature is set to a temperature that causes the proliferation and activation of microorganisms that decompose pollutants. This second temperature is preferably higher than the normal temperature of the contaminated soil P and lower than 40°C, and more preferably 25°C or higher and 30°C or lower (25°C to 30°C).

(Microorganism injection process)
Next, the microorganism injection step will be described. In the microorganism injection step, microorganisms and an activator are injected from a plurality of injection wells 50 into the contaminated soil P of the sandy soil layer 10C and the clayey soil layer 10D whose temperature has been adjusted to the second temperature.

Specifically, first, the type and concentration of microorganisms in the microorganism adjustment tank 70 are adjusted based on the concentration of contaminants and microorganisms in the contaminated soil P observed in the observation well 40, and the type and concentration of the activator in the activator adjustment tank 80 are adjusted.

Next, a pump (not shown) provided on the piping 72 of the microorganism adjustment tank 70 is operated to supply the microorganism-added liquid stored in the microorganism adjustment tank 70 to the injection tank 60. Similarly, a pump (not shown) provided on the piping 82 of the activator adjustment tank 80 is operated to supply the activator-added liquid stored in the activator adjustment tank 80 to the injection tank 60.

Next, an injection pump (not shown) attached to the piping 62 of the injection tank 60 is operated, and the injection liquid stored in the injection tank 60 is injected from the multiple injection wells 50 into the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D. As a result, the contaminants in the contaminated soil P are decomposed and purified by the microorganisms injected into the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D.

At this time, the temperature of the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D is monitored by a first temperature sensor 42 and a second temperature sensor 44 installed in the observation well 40. Then, when the temperature of the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D drops below the second temperature, the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D is reheated by the heating device 30 to maintain the second temperature. This causes the microorganisms in the contaminated soil P to grow and become activated, thereby increasing the efficiency of decomposing contaminants by the microorganisms.

In addition, the concentrations of contaminants and microorganisms in the contaminated soil P are periodically observed in the observation wells 40. Then, based on the observed concentrations of contaminants and microorganisms in the contaminated soil P, the concentration of the microorganisms in the microorganism adjustment tank 70 is increased or decreased, and the concentration of the activator in the activator adjustment tank 80 is increased or decreased, thereby adjusting the concentrations of the microorganisms and activator to be injected from the injection tank 60 into the contaminated soil P via the multiple injection wells 50. This further improves the efficiency of decomposing contaminants by microorganisms.

(effect)
Next, the effects of this embodiment will be described.

Known methods for biopurifying contaminated soil include biostimulation, which involves injecting an activator that activates indigenous microorganisms into the contaminated soil, and bioaugmentation, which involves injecting cultured microorganisms together with an activator. These biostimulation and bioaugmentation methods are suitable for purifying contaminants in, for example, sandy soil layer 10C, which has high permeability, but are difficult to apply to purifying contaminants in clayey soil layer 10D, which has low permeability.

In this embodiment, therefore, first, in the heating process, the temperature of the contaminated soil P in the clayey soil layer 10D is raised from room temperature to a first temperature. This first temperature is set, for example, to a temperature (e.g., 40°C to 80°C) at which contaminants attached to the soil particles in the clayey soil layer are easily detached. This allows the contaminants attached to the soil particles in the contaminated soil P in the clayey soil layer 10D to be efficiently eluted into groundwater, etc. At this time, when contaminants such as VOCs (volatile organic compounds) volatilize, the contaminants are eluted into the sandy soil layer 10C above the clayey soil layer 10D, as shown by the arrow F in FIG. 1.

In addition, in the heating process, the temperature of the contaminated soil P in the sandy soil layer 10C is raised from room temperature to a third temperature that is higher than the second temperature and lower than the first temperature. This allows the contaminants attached to the soil particles in the contaminated soil P in the sandy soil layer 10C to be dissolved into groundwater, etc.

In addition, the clayey soil layer 10D generally has a lower electrical resistance (resistivity) than the sandy soil layer 10C, and therefore generates a larger amount of heat due to Joule heat than the sandy soil layer 10C. Therefore, by heating the clayey soil layer 10D using an electrical heating method, the temperature of the clayey soil layer 10D can be raised to the first temperature efficiently and quickly.

Here, in the heating process, if the temperature of the contaminated soil P in the clay soil layer 10D is raised to a first temperature, there is a possibility that microorganisms in the contaminated soil P will be killed. In addition, in this embodiment, the temperature of the contaminated soil P in the sandy soil layer 10C is raised to a third temperature. This third temperature is lower than the first temperature but higher than the second temperature, so that microorganisms in the contaminated soil P in the sandy soil layer 10C will also be more likely to be killed.

As a countermeasure to this problem, in this embodiment, in the temperature adjustment process, the temperatures of the sandy soil layer 10C and the clayey soil layer 10D are adjusted to a second temperature that is lower than the first temperature and higher than room temperature, and then in the microorganism injection process, microorganisms are injected into the sandy soil layer 10C and the clayey soil layer 10D.

The second temperature is set, for example, to a temperature at which microorganisms capable of decomposing pollutants can live and at which the decomposition of the pollutants by the microorganisms is activated. This allows the pollutants such as VOCs that have dissolved into the sandy soil layer 10C and the clayey soil layer 10D during the heating process to be efficiently decomposed and purified by the microorganisms.

In addition, in this embodiment, in the microorganism injection process, an activator that activates microorganisms is injected into the sandy soil layer 10C and the clayey soil layer 10D whose temperatures have been adjusted to the second temperature. This allows the contaminants eluted into the sandy soil layer 10C and the clayey soil layer 10D to be decomposed and purified more efficiently by the microorganisms.

In this way, this embodiment can improve the purification efficiency of contaminants detached from soil particles in the sandy soil layer 10C and the clayey soil layer 10D.

In addition, in this embodiment, as described above, in the heating process, the temperature of the contaminated soil P in the sandy soil layer 10C is raised to a third temperature lower than the first temperature. This makes it more difficult for microorganisms native to the contaminated soil P in the sandy soil layer 10C to die than microorganisms native to the contaminated soil P in the clayey soil layer 10D. Therefore, in the heating process, the contaminants eluted into the sandy soil layer 10C can be decomposed early by the native microorganisms in the sandy soil layer 10C. This shortens the purification period of the sandy soil layer 10C.

In addition, the amount of microorganisms added to the sandy soil layer 10C during the microorganism injection process is reduced. This allows for cost reduction.

Furthermore, in this embodiment, the surface area (exposed area) of the heating electrode 32 that is in electrical contact with the viscous soil layer 10D is larger than the surface area (exposed area) that is in electrical contact with the sandy soil layer 10C. As a result, in the heating process, the alternating current flowing through the contaminated soil P in the sandy soil layer 10C is smaller than the alternating current flowing through the contaminated soil P in the viscous soil layer 10D. In other words, the Joule heat generated in the contaminated soil P in the sandy soil layer 10C is smaller than the Joule heat generated in the contaminated soil P in the viscous soil layer 10D.

Therefore, the temperature of the sandy soil layer 10C can be adjusted to a third temperature that is lower than the first temperature, while the temperature of the contaminated soil P in the clayey soil layer 10D can be efficiently raised to the first temperature.

(Comparative Experiment)
Next, a comparative experiment will be described.

Figure 3 shows the experimental results of the method for purifying contaminated soil according to the comparative example, and Figure 4 shows the experimental results of the method for purifying contaminated soil according to the embodiment.

In the comparative example of the method for purifying contaminated soil, the contaminated soil was heated from 17°C to 50°C, and then the temperature of the contaminated soil was lowered from 50°C to 30°C. In this state, an activator was added to the contaminated soil, and the concentration of contaminants in the contaminated soil was measured.

In the method for purifying contaminated soil according to the embodiment, the contaminated soil was heated from 17°C to 50°C, and then the temperature of the contaminated soil was lowered from 50°C to 30°C. In this state, microorganisms (c-DCE decomposing bacteria) and an activator were added to the contaminated soil.

The vertical axis of the graph shown in FIG. 3 indicates the concentration of the pollutant, and the horizontal axis of the graph indicates the number of days that have passed since the activator was added to the contaminated soil. The vertical axis of the graph shown in FIG. 4 indicates the concentration of the pollutant, and the horizontal axis of the graph indicates the number of days that have passed since the microorganisms and the activator were added to the contaminated soil. Furthermore, the pollutants in FIG. 3 and FIG. 4 are VC (chloroethylene (vinyl chloride monomer)), c-DCE (dichloroethylene), TCE (trichloroethylene), and PCE (tetrachloroethylene).

As shown in Figure 3, in the comparative example of the method for purifying contaminated soil, after adding an activator to the contaminated soil, the concentrations of VC, TCE, and PCE decreased, but the concentration of c-DCE did not decrease. This is thought to be because the microorganisms that decompose c-DCE (c-DCE-decomposing bacteria) were killed when the contaminated soil was heated from 17°C to 50°C.

On the other hand, as shown in Figure 4, in the contaminated soil purification method according to the embodiment, after the microorganisms and activator were added to the contaminated soil, the concentrations of not only VC, TCE, and PCE but also c-DCE decreased. This is thought to be because the decomposition of c-DCE was promoted by adding the microorganisms (c-DCE-degrading bacteria) and activator to the contaminated soil while the temperature of the contaminated soil was lowered from 50°C to 30°C.

(Modification)
Next, a modification of the above embodiment will be described.

In the above embodiment, no pumping well is provided in the ground 10. However, it is also possible to provide a pumping well in the ground 10 and recover contaminants that have leached from the contaminated soil P into groundwater, etc., during the heating process or microbial injection process of the above embodiment, for example, in the pumping well.

In the above embodiment, the surface area (exposed area) of the heating electrode 32 in electrical contact with the sandy soil layer 10C is made smaller than the surface area (exposed area) of the heating electrode 32 in electrical contact with the clayey soil layer 10D in the heating process, thereby making the amount of heating of the sandy soil layer 10C smaller than the amount of heating of the clayey soil layer 10D. However, for example, the amount of electrodes placed in the sandy soil layer 10C may be made smaller than the amount of heating of the clayey soil layer 10D.

Also, for example, the sandy soil layer 10C and the clayey soil layer 10D may be heated in separate heating wells, so that the amount of heating of the sandy soil layer 10C is less than the amount of heating of the clayey soil layer 10D.

In the above embodiment, the sandy soil layer 10C and the clayey soil layer 10D are heated in the heating process. However, in the heating process, it is sufficient to heat at least the clayey soil layer 10D to the first temperature, and the sandy soil layer 10C does not have to be heated.

In the above embodiment, in the microorganism injection process, both microorganisms and an activator are injected into the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D. However, in the microorganism injection process, it is sufficient to inject at least microorganisms into the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D.

In addition, for example, if the concentration of contaminants in the contaminated soil P in the clayey soil layer 10D falls below a predetermined value (a specified value or a standard value) in the heating process, microorganisms and activators may be injected only into the contaminated soil P in the sandy soil layer 10C in the microorganism injection process.

In the above embodiment, the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D is heated by an electrical heating method in the heating process. However, the heating process is not limited to the electrical heating method, and for example, the contaminated soil P in the sandy soil layer 10C and the clayey soil layer 10D may be heated by a submersible heater inserted in a well formed in the ground 10. The contaminated soil P may also be heated by injecting hot water heated by a heater or the like on the ground into the sandy soil layer 10C and the clayey soil layer 10D.

In addition, in the above embodiment, the multiple heating electrodes 32 and multiple injection wells 50 are arranged at the vertices of a triangle. However, the number and arrangement of the heating electrodes 32 and injection wells 50 can be changed as appropriate, and for example, the multiple heating electrodes 32 may be arranged in a polygonal or circular shape when viewed in a plan view. Similarly, for example, the multiple injection wells 50 may be arranged in a polygonal or circular shape when viewed in a plan view.

In the above embodiment, the heating electrode 32 is buried in the ground 10. However, the heating electrode 32 may be immersed in stored water, such as groundwater, stored in a well formed in the ground 10. In this case, when an AC voltage is applied from the power source 36 to the heating electrode 32 in the well, an AC current flows through the stored water in the well. As a result, Joule heat is generated due to the electrical resistance of the stored water, and the ground 10 around the well is heated.

In addition, as described above, when the heating electrode 32 is immersed in the stored water in the well, the opening ratio of the opening formed in the well placed in the clayey soil layer 10D may be made larger than the opening ratio of the opening formed in the well placed in the sandy soil layer 10C, so that the alternating current flowing in the clayey soil layer 10D is made larger than the alternating current flowing in the sandy soil layer 10C.

In the above embodiment, the contaminated soil P is present in both the sandy soil layer 10C and the clayey soil layer 10D. However, the contaminated soil P may be present, for example, only in the sandy soil layer 10C or only in the clayey soil layer 10D.

The above embodiment is also applicable to various types of ground. Therefore, the purification area of the ground may be only in the clayey soil layer, or only in the sandy soil layer. Furthermore, the purification area of the ground may be in a layer other than the clayey soil layer and the sandy soil layer.

Although one embodiment of the present invention has been described above, the present invention is not limited to this embodiment, and one embodiment and various modified examples may be used in appropriate combination, and the present invention may of course be embodied in various forms without departing from the spirit of the present invention.

10C Sandy soil layer 10D Clayey soil layer P Contaminated soil

Claims (2)

a heating process for heating a clayey soil layer of the contaminated soil from room temperature to a first temperature and maintaining the temperature of the clayey soil layer at the first temperature until a concentration of a contaminant in the clayey soil layer becomes equal to or lower than a predetermined value;
A temperature control step of controlling the temperature of the sandy soil layer above the clayey soil layer to a second temperature lower than the first temperature and higher than the room temperature;
a microorganism injection step of injecting microorganisms capable of decomposing pollutants into the sandy soil layer whose temperature has been adjusted to the second temperature;
Equipped with
In the temperature increasing step, the temperature of the sandy soil layer is increased from the room temperature to a third temperature that is lower than the first temperature and higher than the second temperature.
Methods for remediating contaminated soil. In the temperature adjustment step, the temperatures of the clayey soil layer and the sandy soil layer are adjusted to the second temperature,
In the microorganism injection step, the microorganisms are injected into the clayey soil layer and the sandy soil layer.
The method for purifying contaminated soil according to claim 1.