地下水曝气修复过程的三维数值模拟

地下水曝气法是去除挥发性有机污染物的重要原位修复方法之一,目前已得到广泛应用,但其现场设计主要依据经验,缺乏系统的设计标准.为深入了解曝气去除污染物过程,并为现场设计提供重要参考依据,针对地下水曝气过程开展了数值模拟研究.水气两相渗流数值模型以水压力和气压力作为基本未知量,利用达西定律和质量守恒原理可以建立水气两相渗流过程的控制方程.利用Van Genuchten(VG)模型及Mualem公式,建立渗透系数-饱和度-基质吸力(K-S-P)三者之间的关系.污染物的去除过程则是在水气两相渗流的基础上,引入污染物的溶质运移、相间交换及生物降解模型.采用开发的有限元数值模型,对地下水曝气过程及污染物去除过程进行三维数值模拟,并将三维数值模拟的结果与二维数值模拟的结果进行对比.结果表明,三维模型的曝气影响区域偏小,在曝气口附近,水有效饱和度最小;在曝气口上方,水饱和度先增大后减小.考虑气体所受的浮力作用或不考虑气体可压缩性均会使计算得到的曝气影响区域偏小.污染物去除边界与曝气影响区域的边界基本一致,在曝气区域内,溶质交换过程大大促进了污染物的去除速率;在曝气区域外,污染物的去除主要通过生物降解作用,去除较慢.结果表明实际工程地下水曝气修复系统设计时,应使得曝气影响区域覆盖污染区域以得到较好的修复效果.研究结果表明,两相渗流模型结合污染物迁移转化模型的三维有限元数值模拟可以较好地模拟地下水曝气法去除污染物的全过程,对地下水曝气的设计、应用与效果评价具有重要指导意义.

3D Numerical Simulation of Air Sparging Remediation Process

Air sparging (AS) is an important in situ remedial technology which was widely used in groundwater remediation for volatile organic compounds (VOCs). However, lack of standards, the field design of air sparging system was mainly based on experience. In order to further explore the process of contaminant removal by AS and provide principles for field design, numerical simulation of air sparging process was conducted. In the process of water-air two phase flow, Darcy's law and mass conservation law were employed to get the control equations of water and air transport. The relationship between permeability-saturation-suction (K-S-P) was described by the Van Genuchten (VG) model and Mualem formula. Based on the water-air two phase flow, numerical simulation of contaminant removal was performed regarding the solute transport, inter-phase mass transfer and biodegradation model. 3D FEM model was developed to simulate the air sparging process and compared with the 2D simulation results. As shown in the simulation results, the zone of influence (ZOI) calculated by the 3D model is much smaller, and the lowest water saturation occurs near the sparging point while above the point the water saturation goes higher then lower. Either considering buoyant force or neglecting the air compressibility makes the ZOI smaller. The contaminant removal zone fits well with sparging influence zone. Within the ZOI, solute transfer enhances removal process while biodegradation which is the main removal factor outside the ZOI makes the removal process slow. The results suggest a better remediation effect is achieved when the contaminant zone is covered by the air sparging ZOI. It is shown that two phase flow model combined with contaminant removal could be applied to well simulate the whole process of air sparging, instructive and meaningful to the in-situ air sparging design, application and effectiveness evaluation.