현장규모의 해안가 고농도 유류오염 부지 정화 연구: 토양경작법과 토양세척법을 연계한 방법 적용 사례

Abstract

In this study, laboratory and pilot scale experiments for soil landfarming and soil washing were performed to clean up the coastal site highly contaminated by diesel and bunker C oil and to support the optimal operation conditions according to the pollution characteristics of the research area. In addition, the feasibility of soil washing in conjunction with soil landfarming to the coastal area was evaluated through a pilot scale test and this conjugated remediation process was applied to a contaminated coastal site in field scale. Limitations and problems arising from the field-scale application for soil landfarming and soil washing were reviewed and their counterplans were also presented, supporting technical information and data for cleaning up the TPH contaminated coastal area similar to this site in the future. From results of batch experiments for the TPH removal efficiency evaluation in soil landfarming, the optimal TPH removal efficiency for the soil was observed while the microorganism concentration injected to soil was 109 CFU and the moisture content in soil was 20%, which were applied as one of the soil landfarming conditions in the field scale study. The pilot scale test at three soil landfarming conditions (only moisture content control; only nutrient support; nutrient + introduced oil-degrading microorganism support) was conducted and the TPH concentration of the soil by the injection of nutrient + introduced oil-degrading microorganism decreased below the tolerance limit (2,000 mg/kg) within 36 days of soil landfarming. The average operation time of the soil landfarming for the contaminated soil was determined to 45 days based on the TPH degradation coefficient calculated from the pilot test results. As a result of soil washing batch experiments, while the washing ratio(soil: solution wt.) was 1:2, it showed the highest TPH removal efficiency of 86.0% and it was applied to the field study, considering both of removal efficiency and economic feasibility. The TPH removal efficiency of the soil washing for three different particle size ranges (> 2 mm; 0.075 ~ 2.00 mm; < 0.075 mm) was 100%, 80%, and 61%, respectively, suggesting that the soil washing process could be successfully appled to this coastal site. The field scale remediation process was performed for this coastal site by applying conditions and data determined from laboratory and pilot scale experiments. The soil landfarming process was applied to oil contaminated soils (the initial TPH concentration range: 5,669 ~ 13,612 mg/kg) in the field and 5 landfarming buildings (total landfarming capacity: 9,000 m2) was used for 6 batch intervals. The highest operation time of landfarming needed for the satisfactory remediation in the field site was 156 days, which was more than 3 times of expected operation time (45 days) based on the pilot scale test. It resulted from two main reasons: ① more than 40% of the treated soil had very high initial TPH concentration of soil (> 10,000 mg/kg) and ② the ratio of fine particles originated from the wide mud flat of the coastal area was relatively high in treated soil medium, leading to the decrease of TPH removal efficiency for the soil landfarming process. To reduce the operation time of soil landfarming and to increase the TPH removal efficiency, the surfactant solution and H2O2 5 ~ 35% solution was additionally used to landfarming soils. When the surfactant solution was injected to landfarming soils at high injection pressure, the operation time of soil decreased to 40 ~ 50 days. It resulted from the physical desorption of free NAPLs phases due to high pressurized injection process and the decrease of interfacial tension due to surfactant. The effect of H2O2 5 ~ 35% solution injection on the removal efficiency was negligible because H2O2 might hinder the degradation activity of microorganism and/or have the lower chance of reaction with oil according to limited surface sparging of H2O2 solution. The soil in this coastal area contained lot of fine particles due to mud flat beach layers and the free DNAPL phases were broadly located at below the sea level. In order to clean up soil highly contaminated by heavy oils (> 10,000 mg/L) and composed of relatively high clay contents in this coastal area, the soil landfarming conjugated with soil washing was applied in the field and total 14,000 m3 of contaminated soil was successfully remediated by the conjugated remediation process in this coastal site. When high volumes of oil were spilled in the coastal area, more free phases of NAPLs easily spread all over joints or cracks of bedrock at the lower part of the oil storage tank due to sea level variation and various escarpment shapes. The underwater excavation process to obtain contaminated soil below the sea level was necessary for the application of the soil landfarming. If it is not possible to completely prevent the inflow of sea water to the site during the underwater excavation process, the vertical cut-off walls to prevent the spread of pollution by the sea water flow and/or the high performance shortcrete process to isolate NAPLs presented in cracks of bedrock are necessary for the satisfactory TPH remediation in the coastal area.