Air Sparging Process Research Articles

Enhanced air sparging for groundwater remediation using alginate gel-based removable hydraulic barriers

The objective of this study was to investigate the effect of a removable physical barrier on the air sparging performance using a lab-scale aquifer model was investigated. The barrier was installed in water-saturated porous media, prior to the air sparging, by injecting calcium chloride aqueous solution into the aquifer with pre-applied alginate solution. Changes in the air flow direction and air flux at the media surface during air sparging were evaluated. With a hydrogel barrier set at the center of the media, the airflow detoured the barrier resulting in a bimodal air flux distribution at the media surface. While employing two gel-formed barriers positioned away from the media's center, the airflow concentrated specifically on the gap between the barriers. The hydrogel was successfully removed using a sodium bicarbonate solution (1.0 mol/L). Using the hydrogel barrier, the performance of air sparging was significantly enhanced for removing contaminants [tetrachloroethene (PCE) and n-hexane mixture] due to increased air flux; 9.8% of PCE applied (7.8 g) was removed during 120 min air sprging for the gel barrier system whereas no PCE was removed for the control. Alginate gel did not show significant sorption capacity for PCE. It was stable in the contaminant up to 68 days with reasonable loss of its mass. Findings of this study present a promising option for air sparging process specifically targeting the contaminant source zone in the aquifer.

Effect of increased groundwater viscosity on the remedial performance of surfactant-enhanced air sparging

The effect of groundwater viscosity control on the performance of surfactant-enhanced air sparging (SEAS) was investigated using 1- and 2-dimensional (1-D and 2-D) bench-scale physical models. The viscosity of groundwater was controlled by a thickener, sodium carboxymethylcellulose (SCMC), while an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), was used to control the surface tension of groundwater. When resident DI water was displaced with a SCMC solution (500 mg/L), a SDBS solution (200 mg/L), and a solution with both SCMC (500 mg/L) and SDBS (200 mg/L), the air saturation for sand-packed columns achieved by air sparging increased by 9.5%, 128%, and 154%, respectively, (compared to that of the DI water-saturated column). When the resident water contained SCMC, the minimum air pressure necessary for air sparging processes increased, which is considered to be responsible for the increased air saturation. The extent of the sparging influence zone achieved during the air sparging process using the 2-D model was also affected by viscosity control. Larger sparging influence zones (de-saturated zone due to air injection) were observed for the air sparging processes using the 2-D model initially saturated with high-viscosity solutions, than those without a thickener in the aqueous solution. The enhanced air saturations using SCMC for the 1-D air sparging experiment improved the degradative performance of gaseous oxidation agent (ozone) during air sparging, as measured by the disappearance of fluorescence (fluorescein sodium salt). Based on the experimental evidence generated in this study, the addition of a thickener in the aqueous solution prior to air sparging increased the degree of air saturation and the sparging influence zone, and enhanced the remedial potential of SEAS for contaminated aquifers.

A mechanism study of airflow rate distribution within the zone of influence during air sparging remediation

In this study, an improved laboratory two-dimensional airflow visualization device was developed for the quantitative analysis of airflow distribution at different heights from the sparger (20, 30, and 40cm) within the zone of influence (ZOI). The results indicated that the measured airflow rate distribution appeared Trapezium when the height was 20cm; however, the airflow rate matched a Gaussian distribution when the heights became 30cm and 40cm. The conical shape of the ZOI was observed in the experimental processes. The experimental results verified that the airflow distribution within the ZOI conformed to turbulent jet theory. According to turbulent jet theory, the distribution of the airflow rate changes from Trapezium to Gaussian, and the jet boundary mixed layer is a linear extension in the processes of jets. Through our study, it was found that this theory could be applied to airflow distribution and predictive models for the ZOI in air sparging remediation. The shape of the ZOI should be cone-like and the boundary layer of the ZOI is a linear extension in air sparging process. All the results from this study can provide theoretical support for the design and prediction of air sparging remediation for groundwater pollution.

Changes in air flow patterns using surfactants and thickeners during air sparging: Bench-scale experiments

Air injected into an aquifer during air sparging normally flows upward according to the pressure gradients and buoyancy, and the direction of air flow depends on the natural hydrogeologic setting. In this study, a new method for controlling air flow paths in the saturated zone during air sparging processes is presented. Two hydrodynamic parameters, viscosity and surface tension of the aqueous phase in the aquifer, were altered using appropriate water-soluble reagents distributed before initiating air sparging. Increased viscosity retarded the travel velocity of the air front during air sparging by modifying the viscosity ratio. Using a one-dimensional column packed with water-saturated sand, the velocity of air intrusion into the saturated region under a constant pressure gradient was inversely proportional to the viscosity of the aqueous solution. The air flow direction, and thus the air flux distribution was measured using gaseous flux meters placed at the sand surface during air sparging experiments using both two-, and three-dimensional physical models. Air flow was found to be influenced by the presence of an aqueous patch of high viscosity or suppressed surface tension in the aquifer. Air flow was selective through the low-surface tension (46.5dyn/cm) region, whereas an aqueous patch of high viscosity (2.77cP) was as an effective air flow barrier. Formation of a low-surface tension region in the target contaminated zone in the aquifer, before the air sparging process is inaugurated, may induce air flow through the target zone maximizing the contaminant removal efficiency of the injected air. In contrast, a region with high viscosity in the air sparging influence zone may minimize air flow through the region prohibiting the region from de-saturating.

Remediation of saturated Shanghai sandy silt contaminated with p-xylene using air sparging

Air sparging is an effective technique for the remediation of soil and groundwater polluted by volatile organic compounds. In this paper, this technique was investigated by conducting air-sparging test in the laboratory on the Shanghai sandy silt that was artificially contaminated with p-xylene. A test tank was designed for this purpose. During the air-sparging process, aqueous p-xylene solutions were extracted from the observation holes, and their concentrations were quantified by the spectrophotometric detection method. The mechanism of mass transfer process of p-xylene in the vicinity of sparging well and the remediation of the contaminated groundwater by air sparging were explored. The results showed that the removal zone of the p-xylene was mainly located within a radius of about 20 cm around the air injection well, with 90 % p-xylene removed after 20-day air sparging. Within the initial 5-day sparging, the concentration of p-xylene decreased rapidly in the mass transfer zone. By contrast, in the area far from the injection well, the p-xylene concentration decreased evenly and slowly. Thus, the remediation of contaminated soil and groundwater by air sparging is space–time dependent. For further analysis, the adsorption of silt was taken into account, and the distribution coefficient, Kd, was introduced to the modified Shackleford’s mass transfer model. The comparison between the simulated and measured results indicates that the modified model can satisfactorily describe the p-xylene mass transfer observed in this study.

Numerical Simulation of Air Flow during Air Sparging Remediation

A 2D numerical model is used to modeling the movement of air during air sparging process for groundwater remediation. The zone of influence (ZOI) and the water saturation distribution can be obtained from the calculations. The results agree well with the centrifuge test data, indicating the two-phase flow model is reasonable for numerical simulation of air sparging process. It was also shown that air compressibility has a significant influence on the extent of ZOI.

Centrifuge Modeling and Numerical Simulation of Air Sparging Process

Air sparging (AS) is one of the in-situ groundwater remediation techniques for remediating volatile organic compounds (VOCs) contaminated soil, and the knowledge of air flow features is essential in designing air sparging system for soil remediation. The centrifuge modeling technique was employed to simulate the in-situ conditions and to investigate air follow characteristics during air sparging by using glass beads as soils. Several centrifugal modeling tests were performed under various g-levels. According to the test results, the zone of influence (ZOI) during air sparging is in a truncated-cone shape under various g-levels, which can be expressed by the lateral expansion around the air injection point and the cone angle between the vertical axis and the boundary of ZOI. A 2D numerical model is used to model the air movement during air sparging process. The ZOI and the water saturation distribution were obtained. The results agree well with the centrifuge test data, which indicates the two phase flow model is reasonable to simulate the air sparging process. It was also shown that air compressibility and buoyant force have a significant influence on the extent of ZOI and should be adequately considered. Hence centrifuge modeling and numerical simulation can be used as a design tool for in-situ air sparging system

AIR SPARGING TRACER TEST TO INVESTIGATE THE FLOW PATH

The paper presents a useful method for investigating the flow path of the injected air, estimating the radius of influence, and assisting the design of well spacing of sparging wells. The method applies both tracer and modeling approach. A field test using helium as a tracer was conducted on an air sparging pilot. A 3-dimensional, 3-phase compositional model was utilized to simulate the air sparging process at the pilot. The model was first calibrated with the tracer concentrations observed in the field test at various locations between the air sparging well and the soil vapor extraction well. The calibrated model was then used to predict the flow path of the injected air and to simulate the water saturation distributions in the aquifer. The steady-state water saturation distributions was found useful for estimating the radius of influence of air sparging and for designing the well spacing of sparging well.

Air-Sparging Remediation: A Study on Heterogeneity and Air Mobility Reduction

Contaminated groundwater is a widespread problem often requiring innovative technologies to remediate. In this study, physical models of the air-sparging process are developed, and contaminant recoveries are compared with predictions from a mathematical model. Initial tests used a very fine, porous media (a glass bead-packed column) to represent relatively homogeneous soil samples. Subsequent testing employed budded core samples taken from an actual site to represent more realistic, heterogeneous samples. 1,1,1 trichloroethane (TCA) was used as the dissolved contaminant to represent benzene, toluene, ethyl benzene, and xylene (BTEX), considered to be gasoline contamination in water. The results obtained, however, can be applied to any nonaqueous-phase liquid (NAPL) dissolved phase. A technique based on foam injection is proposed and is demonstrated to reduce air mobility. This reduction in air mobility has the potential to improve contaminant removal due to increased contact area, and contact time between the air and contaminant. Laboratory results are compared with predictions from an advection – diffusion, air-sparge numerical simulation model. Sensitivity analysis of the numerical model provides the range of key parameters used to evaluate air sparging. Eventual scaleup of the model to an actual site can be justified by the favorable results presented in this paper.

Air distribution during in situ air sparging: an overview of mathematical modeling

The performance of in situ air sparging is controlled by the distribution of air pathways in the subsurface, which is in turn controlled by the structure of the medium to be sparged. The specific pathways that the air follows are determined, at the grain scale, by the distribution of air entry pressures of the pores. At the field scale, pore size distributions are usually correlated with heterogeneous structures (e.g. layers) within the medium, which control the macroscopic distribution of the air. The processes that produce an observed air distribution at a particular site are complicated, and are potentially well suited to modeling with multiphase flow models. Recent numerical modeling of heterogeneous media appears to successfully represent expected distributions of air; however, current models do not provide a tool to predict sparging performance. For this to be the case, the models need to represent the detailed structure of the medium at the site to be studied, as well as to capture the relevant aspects of the discrete air/water distribution (i.e., represent air channels at the centimeter or smaller scale). This will, in general, require a level of site data that is not available and numerical models that require many millions of computational elements. As a consequence, at least for the foreseeable future, numerical modeling of the air sparging process will continue to play a vital role as a conceptual tool with limited predictive capability at sites.

Mathematical modeling of air sparging for subsurface remediation: state of the art

A review of published mathematical models used to simulate air sparging is provided. Applicability of the models, efforts to test the models using experimental data and contributions of modeling efforts to the practice of air sparging are also discussed. Compartmentalized lumped-parameter models and multiphase flow models have dominated air-sparging modeling efforts. In essence, each class of models requires the assumption of a continuum over some model domain. Each approach has significant benefits as well as some inherent disadvantages. Based on the literature, both lumped-parameter modeling and multiphase-flow modeling have been successful in improving our theoretical understanding of the air-sparging process and in facilitating practical development of sparging systems. Lumped-parameter models are simpler to use, and can lend considerable insight to sparging operations. Multiphase flow models have the potential to offer a more realistic simulation of the airflow process, but may require a considerable amount of data collection for model input. The literature suggests that for any air-sparging model to be useful for field applications, detailed model calibration is necessary. It is recommended that models incorporate, in some fashion, the diffusion and dispersion of contaminants to macro-scale air channels, and nonequilibrium interphase mass transfer of contaminants. These mass-transfer-limited processes are frequently listed as causes for the “tailing” of vapor-extraction effluent contaminant concentrations that are frequently observed during field applications. However, time-varying mixing of relatively clean and contaminated vapors in the extraction system may also explain this tailing. Geophysical imaging techniques and inverse modeling combined with air-sparging pilot tests and measurement of traditional hydrogeologic parameters may allow for successful modeling efforts.

Mechanisms controlling toluene removal from saturated soils during in situ air sparging

In situ air sparging is an effective method for removing volatile organic compounds from saturated soils and groundwater. Removal efficiency levels as high as 98% are often reported, and the remediation time is significantly less than that required for conventional pump and treat technology. However, predictions of the time required for contaminant mass removal by air sparging have been approximate at best due to a lack of understanding of the relative importance of the various mechanisms that are responsible for this contaminant removal. Volatilization is considered the most dominant mass transfer mechanism during the air sparging process. Dissolution, desorption and biodegradation are the other major mechanisms that determine the rate at which contaminants are partitioned into different phases or transformed into nonhazardous substances. Additionally, advection, dispersion and diffusion are the transport mechanisms that dictate the overall contaminant removal efficiency. This paper first describes these different mechanisms along with the factors that affect these mechanisms. Then, experimental data is presented for toluene removal from Ottawa sand and fine gravel by means of air sparging. The tests performed included batch tests to characterize the adsorption characteristics of toluene on the Ottawa sand, and air sparging column tests on both the sand and the gravel to provide information on the effects of soil type and injected air flow rate on the overall air sparging remedial efficiency. These test results are assessed in light of the mechanisms affecting contaminant removal during air sparging.

A Review of In-Situ Air Sparging for the Remediation of VOC-Contaminated Saturated Soils and Groundwater

In-situ air sparging is a developing remediation technique that has significant potential for use in VOC-contaminated saturated soils and groundwater. This technique consists of injecting air below the contaminated area to partition the dissolved, sorbed and free phase VOCs into the gas phase and to enhance the aerobic biodegradation of the VOCs. Because of the buoyancy effect, the VOCs in the gas phase are transported by air to the vadose zone where they are removed and subsequently treated by a soil vapor extraction system. Currently, the design, operation, and monitoring of air sparging systems is based mainly on an empirical approach with limited field experience. Extreme care must be exercised in designing and implementing the air sparging system so that the contaminants are removed efficiently and without adverse effects on the subsurface environment, particularly the spread of the contaminants to the clean areas. The current state of knowledge is inadequate for the design of effective air sparging systems which also prevent the spreading of contaminants into the clean areas. This paper first outlines the fundamentals of air sparging and then presents an overview of previous air sparging field and laboratory investigations. The paper then details a critical assessment of modeling studies which predict contaminant transport during the air sparging process. Finally, the paper outlines an on-going comprehensive research program that involves developing the most efficient and economical air sparging systems. This research program, which is being conducted at the University of Illinois at Chicago (UIC), includes performing laboratory aquifer simulation tests to characterize the basic mechanisms of air sparging, developing a contaminant transport model to optimize the different design variables in a typical air sparging system, and conducting a field demonstration of the optimal air sparging system.