School of Environment, Tsinghua University, Beijing 100084, China.
School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, United States.
Sci Total Environ. 2020 May 20;718:137309. doi: 10.1016/j.scitotenv.2020.137309. Epub 2020 Feb 14.
Light non-aqueous phase liquid (LNAPL) contaminated sites pose a risk to human health and the natural environment. Multi-phase extraction (MPE) is one of the most widely used technologies to remediate these sites. Thus, it is important to optimize MPE systems to improve their effectiveness and cost-efficiency. In this study, we developed a numerical model to optimize LNAPL mass removal by MPE, in which the aquifer domain was simplified as a cylinder with a single MPE extraction well located at the center. A dual-pump extraction system was applied to the model, which involved vacuum enhanced recovery to remove volatilized gaseous phase contaminants and a submerged pump to remove NAPL and contaminants in groundwater. After the model was validated with field data, the results showed that the contaminant extraction rate varied with the LNAPL thickness and submerged pump position. For benzene selected as the contaminant of concern, greater LNAPL extraction rates were achieved when the initial LNAPL thickness was large (>1.5 m) or in aquifers of high permeability (>2.45 × 10 m). Importantly, it was discovered that in highly permeable coarse sand and gravel, the submerged pump ought to be placed within the LNAPL layer, whereas the pump should be placed below the water-NAPL interface in fine to medium sand aquifers. It was also found that an optimal liquid pumping rates exist, beyond which contaminant mass removal rates do not increase. Furthermore, it was found that in aquifers contaminated with thin LNAPL layers, mass transfer modelling that assumes equilibrium between the phases may greatly overestimate the accumulated mass of contaminants removed and, therefore, non-equilibrium modelling should be adopted. Finally, a cost analysis was carried out to compare the costs of remediating a contaminated site with MPE and by an alternative chemical oxidation approach. The MPE technology was found to be more cost effective when the initial thickness of LNAPL was relatively thin. In summary, the numerical model developed in this study is a useful tool for optimizing MPE system design.
轻质非水相液体 (LNAPL) 污染场地对人类健康和自然环境构成威胁。多相抽提 (MPE) 是修复这些场地最广泛使用的技术之一。因此，优化 MPE 系统以提高其效率和成本效益非常重要。在本研究中，我们开发了一种数值模型，通过 MPE 优化 LNAPL 质量去除，其中含水层域简化为一个带有位于中心的单个 MPE 抽提井的圆柱体。该模型应用了双泵抽提系统，包括真空强化回收去除挥发性气相污染物和潜水泵去除地下水的 NAPL 和污染物。在模型经过现场数据验证后，结果表明污染物抽提率随 LNAPL 厚度和潜水泵位置而变化。对于选择的苯作为关注污染物，当初始 LNAPL 厚度较大 (>1.5 m) 或在高渗透性含水层 (>2.45×10-3 m) 中时，实现了更大的 LNAPL 抽提率。重要的是，发现高渗透性粗砂和砾石中，潜水泵应放置在 LNAPL 层内，而在细至中砂含水层中，泵应放置在水-NAPL 界面以下。还发现存在最佳液体泵送率，超过该值后，污染物质量去除率不会增加。此外，发现对于 LNAPL 薄层污染的含水层，假设相间达到平衡的传质模型可能会大大高估去除的污染物累积质量，因此应采用非平衡模型。最后，进行了成本分析，比较了使用 MPE 和替代化学氧化方法修复受污染场地的成本。当 LNAPL 的初始厚度相对较薄时，MPE 技术被发现更具成本效益。总之，本研究开发的数值模型是优化 MPE 系统设计的有用工具。
