JP7639381B2 - Soil or groundwater purification method - Google Patents

Description

The present invention relates to a method for purifying soil or groundwater, and in particular to a method for treating volatile organic compounds (VOCs) in soil or groundwater using an electric heating method.

There is an electric heating method in which electrodes are inserted at intervals into soil contaminated with volatile compounds such as organochlorine compounds, a voltage is applied between the electrodes to pass electricity, and the volatile compounds are dissolved and removed by Joule heat generated by the current (Patent Document 1).

In a method for purifying soil or groundwater using electrical heating, multiple electrodes are inserted into the soil to be purified, and a voltage is applied between the electrodes to pass an electric current through the soil between them, generating Joule heat due to the electrical resistance of the soil and heating it. When the temperature of the soil rises, VOCs (volatile organic compounds) such as tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylene (DCE), and trichloroethane that are adsorbed to the soil particles are released from the soil and eluted into the groundwater. The VOCs are removed from the soil or groundwater, and the soil or groundwater is purified by performing a pumping process in which the groundwater is pumped to the surface, or a gas suction process in which underground gas containing VOCs that has been gasified from the groundwater is sucked in.

JP 2016-159259 A

When applying the electric heating method to the unsaturated layer of soil, the water content is lower than in the saturated layer, and the amount of electricity passing through is small, so the temperature does not rise easily and may only rise to around 40°C. Since VOCs often have a boiling point of 50°C or higher (PCE: 121°C, TCE: 87°C, DCE: 55°C), there was a problem that the amount of VOCs recovered was small even when the soil gas suction method was applied.

The present invention aims to provide a method for purifying soil or groundwater that can efficiently purify unsaturated soil or groundwater using an electric heating method.

One aspect of the present invention is a method for purifying soil or groundwater by using an electric heating method to purify an unsaturated layer contaminated with volatile organic compounds, and is characterized in that water is poured into the unsaturated layer.

In one embodiment of the present invention, the unsaturated layer is heated to 60°C or higher.

In one embodiment of the present invention, the volatile organic compound includes at least one of tetrachloroethylene, trichloroethylene, dichloroethylene, and trichloroethane.

According to one embodiment of the soil or groundwater purification method of the present invention, water is injected into the unsaturated layer from an injection well or the like, and the water content of the unsaturated layer is increased, and then an electric heating method is applied. As a result, the amount of electricity flow increases and the temperature rises compared to when water is not injected into the unsaturated layer. This makes it possible to increase the amount of VOCs recovered. It also makes it possible to shorten the purification process time.

1 is a cross-sectional view of ground illustrating a soil purification method according to an embodiment of the present invention. 1 is a graph showing measurement results in an example.

Below, one embodiment of the present invention will be described in detail with reference to the drawings.

In one embodiment of the present invention, as shown in FIG. 1, in soil 10 in which an unsaturated layer 12 exists above a saturated layer 11, VOCs in the unsaturated layer 12 are removed by an electrical heating method.

A pipe 1 with numerous holes 1a drilled therein and multiple electrodes 2 are driven into the unsaturated layer 12 contaminated with VOCs. A power supply unit 3 is connected to the electrodes 2. Water in a tank 4 can be supplied to the pipe 1 via a water supply pump 5 and piping 6. In addition, a suction piping 7 is connected to the upper end of the pipe 1, and VOCs gas can be sent to a gas treatment facility 9 via a suction pump 8.

The depth of the interface between the unsaturated layer 12 and the saturated layer 11 is determined in advance by a boring survey or an electrical conductivity survey conducted by driving a probe with an electrical conductivity meter into the soil. The moisture content of the unsaturated layer 12 is also measured based on the electrical conductivity detected by the boring survey or the electrical conductivity meter.

It is preferable to drive the pipe 1 and electrode 2 up to just above the interface between the unsaturated layer 12 and the saturated layer 11.

When starting the VOCs removal work in the unsaturated layer 12, first operate the water supply pump 5 to supply (pressurize) water into the unsaturated layer 12, increasing the water content of the unsaturated layer 12 and increasing the electrical conductivity.

The amount of water injected into the unsaturated layer 12 may be determined based on the water content measured in a boring survey or a probe driving survey conducted in advance, or the electrical conductivity may be monitored using an electrical conductivity meter of a probe driven into the unsaturated layer 12 at a point a specified distance away from the pipe 1, and water may be injected until the electrical conductivity reaches a specified value or higher.

After that, or before the start of water injection, a current is applied between the electrodes 2, 2 by the power supply unit 3, and the unsaturated layer 12 is heated by Joule heat. By applying current, the temperature of the unsaturated layer 12 contaminated with VOCs is raised to preferably 50°C or higher, for example, 50 to 90°C, and particularly preferably 60 to 80°C. Note that the water supply pump 5 may be stopped before the start of current application to the electrode 2, or the water supply pump 5 may be stopped halfway through the start of current application to the electrode 2, or water may be continuously injected during current application.

After starting the current supply, the suction pump 8 is operated to suck the VOCs-containing gas from inside the pipe 1 and send it to the gas treatment facility 9 for treatment. This removes the VOCs from inside the unsaturated layer 12. Note that the process of applying current and suction may be repeated after water is poured in. It is preferable to suction the gas after the temperature of the unsaturated layer 12 has risen sufficiently, but this is not a limitation.

In this embodiment, the amount of electricity flowing to the unsaturated layer 12 is increased by injecting water into the unsaturated layer 12, and the temperature of the unsaturated layer 12 is raised sufficiently, so that the VOCs in the unsaturated layer 12 can be sufficiently removed in a short period of time.

In the above explanation, water is injected by driving pipe 1 into the soil, but water may also be injected using a suction well installed in the soil.

The above embodiment is an example of the present invention, and the present invention is not limited to the above embodiment.

[Example 1]
In the VOCs-contaminated unsaturated layer where a soil gas suction well was installed (2 m deep from the ground surface at the interface with the saturated layer, 2 m deep in the well), nine electrodes were driven in and an electric heating method was applied. The electrodes were spaced 2.5 m apart.

The temperature of the unsaturated layer before the start of energization was about 15°C. After the start of energization, the temperature of the unsaturated layer before the start of water injection into the unsaturated layer was about 35°C. After the start of energization, 10 ^m3 of tap water was injected into the unsaturated layer from the suction well over a period of 7 hours. As shown in Figure 2, the temperature of the unsaturated layer gradually increased, reaching 60°C after 38 days. When gas suction was started on the 43rd day, the temperature of the unsaturated layer gradually decreased, reaching 45°C on the 51st day, and gas suction was stopped.

On the 85th day after the start of the previous energization, 10 ^m3 of tap water was injected again over 7 hours, and the unsaturated layer temperature rose to 53°C on the 102nd day, so gas suction from the suction well was started. After that, the unsaturated layer temperature remained at about 50°C until the 143rd day.

The above experiments confirmed that injecting water into the unsaturated layer sufficiently raises the temperature of the layer.

Reference Signs List 1 Pipe 1a Hole 2 Electrode 4 Tank 10 Soil 11 Saturated layer 12 Unsaturated layer

Claims (4)

1. A method for electrothermal purification of an unsaturated layer contaminated with volatile organic compounds, comprising:
The depth of the interface between the unsaturated layer and the saturated layer is identified in advance by a boring survey or an electrical conductivity survey by driving a probe having an electrical conductivity meter into the soil.
A method for purifying soil or groundwater , comprising measuring the water content of the unsaturated layer, determining the amount of water to be injected into the unsaturated layer, and injecting the water into the unsaturated layer. 1. A method for electrothermal purification of an unsaturated layer contaminated with volatile organic compounds, comprising:
The depth of the interface between the unsaturated layer and the saturated layer is identified in advance by a boring survey or an electrical conductivity survey by driving a probe having an electrical conductivity meter into the soil.
A method for purifying soil or groundwater, comprising monitoring the electrical conductivity of the unsaturated layer and pouring water into the unsaturated layer until the electrical conductivity reaches a predetermined value or higher . 3. The method for purifying soil or groundwater according to claim 1 , wherein the temperature of the unsaturated layer is raised to 60° C. or higher. 4. The method for purifying soil or groundwater according to claim 1 , wherein the volatile organic compound comprises at least one of tetrachloroethylene, trichloroethylene, dichloroethylene, and trichloroethane.

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