Features
3 Action to Remediation of Soil contamination(2) - Volatile organic compound 
Keisuke Funaishi

Ataka construction & engineering co., ltd.
Environmental technology R&D center

 1. Introduction
In soil and groundwater contamination due to volatile organic compound (VOC), the conventional procedure of removing VOC is through the activated carbon adsorption method. The activated carbon adsorption method operates at low initial cost and there are many actual achievements noted using the process. However, the exchange and regeneration of activated carbon, which is used to adsorb VOC, and the treatment of VOC adsorbed to activated carbon as a specially controlled waste, are necessary. Furthermore, when the VOC concentration is high, activated carbon needs replacement frequently. The high VOC concentration then results in a greater running cost in relation to industrial waste disposal expense.
In this study, the mechanism of decomposing and oxidizing VOC extracted by gas suction or pumping treatment on-site is described. The operation is performed at a lesser running cost and it does not include any complicated process. The method does not produce industrial wastes, such as breakdown products of activated carbon. Especially in the presence of high VOC, the method is more cost-effective as compared to activated carbon adsorption.

2. Principle of harmless degradation system
The VOC decomposition process of this system is shown in Fig. 1.

Fig. 1 Decomposition process of VOC

2.1 UVPLUS system
The tetrachloroethylene (PCE) in soil gas is oxidized by ultraviolet rays at 185nm wavelength. It is converted to gas which consists primarily of trichloroacetyl chloride. When the gas is absorbed with alkali liquid using an absorber, trichloroacetic acid is generated in the absorption liquid. This absorption liquid is biodegraded by biological activated carbon into carbon dioxide and sodium chloride in order to detoxify the liquid.
In the case of trichloroethylene (TCE), dichloroacetic acid is generated via dichloroacetyl chloride. This compound can also be purified by using the same system as described above.

2.2 AOPLUS system
During the process of advanced oxidation, OH radicals are emitted together with ozone and hydrogen peroxide. The PCE in groundwater is decomposed to carbon dioxide and sodium chloride through the action of OH radicals. In this system, trichloroacetic acid is partly yielded, but the compound is decomposed and detoxified by biological activated carbon.
In like manner as in the UVPLUS system, TCE is partly converted to dichloroacetic acid. However, it can be also purified by the AOPLUS system.

3. Application sample of decontamination
3.1 Treatment flow chart of application sample
In areas where soil and groundwater are contaminated, purification using UVPLUS and AOPLUS systems was conducted. The flow chart of this case is shown in Fig. 2. The external view of the system is also presented (Fig. 3).

 
Fig. 2 Treatment flow chart of soil gas and groundwater
Fig. 3 External view of UVPLUS system and AOPLUS system
Fig. 3 External view of UVPLUS system and AOPLUS system

The VOC in soil gas extracted from the suction well is purified by ultraviolet oxidation and absorption. The treatment process is then complete. However, VOC is emitted into the air through activated carbon as by-product to be utilized for further decontamination. A certain amount of suction liquid from the absorption train was obtained. Then, the liquid is decomposed and detoxified through the action of biological activated carbon. Finally, it is discharged into a sewer. The disposal capacity of soil gas is 5m3/min at maximum.
On the other hand, if the pumped-up polluted groundwater contains many turbid substances, it is decomposed and detoxified with advanced oxidization and biological activated carbon after removing the turbid substances through coagulating sedimentation and sand filter. Finally, the purified groundwater is discharged to a sewer. The system is performed systematically. The disposal capacity of groundwater is 1?3m3/hr.
In this treatment system, when the VOC concentration in groundwater is high, the load in advanced oxidization is reduced by vaporizing VOC in water through aeration. Furthermore, it is through the complex system of the unstable VOC that it is decomposed and detoxified by ultraviolet rays oxidization with soil gas.

3.2 Past records
The PCE concentration of contaminated groundwater was detected to be high at 43mg/L during the early stages of suction. In addition, TCE, which is a decomposition product of PCE, and cis-1,2-dichloroethylene (cis-1,2-DCE) were also discovered from raw water at a maximum of 4mg/L. Whereas, 1-1- dichloroethylene (1,1-DCE) was hardly detected. This groundwater was decomposed and detoxified by AOPLUS system. Consequently, the VOC concentration of treated water was achieved below the targeted value for treatment (PCE<0.1mg/L, TCE<0.3mg/L, cis-1,2-DCE<0.2mg/L, 1,1-DCE<0.4mg/L) during the operation period. A stable purification could be performed below a detection threshold.
Moreover, the PCE concentration in the soil gas is as high as that found in groundwater. During the early stages of suction, the concentration skyrocketed at 1720 mg/m3. Similarly, both TCE and cis-1,2-DCE were detected, whereas 1,1-DCE was hardly noted. This soil gas was decomposed and detoxified by UVPLUS system. Consequently, the VOC concentration of treated gas was achieved below the targeted value for treatment (PCE<300mg/m3, TCE<300mg/m3, cis-1,2-DCE<300mg/m3, 1,1-DCE<300mg/m3) during the operation period. Likewise, a stable purification could be performed below a detection threshold.


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