Predicting suitability of different scale-dependent dispersivities for reactive solute transport through stratified porous media

In this paper,the behavior of breakthrough curves(BTCs) for reactive solute transport through stratified porous media is investigated.A physical laboratory model for layered porous media was constructed,in which thin layer of gravel was sandwiched in between two thick layers of natural soil.Gravel layer and natural soil layers were hydraulically connected as single porous continuum.A constant source of tracer was connected through gravel layer and elucidated at different sampling points in the direction of flow.Flexible multiprocess non-equilibrium(MPNE) transport equation with scale-dependent dispersivity function was used to simulate experimental BTCs of reactive solute transport through layered porous media.The values of equilibrium sorption coefficient and other input parameters were obtained experimentally.The simulation of BTC was performed using MPNE model with scale-dependent dispersivity.The simulation of different scale-dependent dispersivities was then compared and it was found that for field scale of estimation of dispersivity,asymptotic and exponential dispersivity functions performed better.In continuation to the comparison of simulated BTCs obtained using different models,spatial moment analysis of each aforesaid scale-dependent dispersivity model was also done.Spatial moment analysis provides the information related to mean solute mass,rate of mass travel,and mean plume dispersion.Linear and constant dispersivities showed higher variance as compared to asymptotic and exponential dispersion functions.This supports the field applicability of asymptotic and exponential dispersivity functions.The BTCs were also found to elucidate a nonzero concentration with time,which was clearly affected by physical non-equilibrium.In natural condition,such information is required in effective aquifer remediation process.     

多孔介质中化学–热–水力–力学耦合过程本构模型和数值模拟

本文发展了一个多孔介质中的化学–热–水力–力学(CTHM)耦合本构模型。该模型基于文献[1]中多孔介质热–水力–力学(THM)本构模型、文献[2,3]中的污染物传输本构模型和文献[4]提出的化学–力学本构模型。在本构模型中引入化学软化函数以模拟孔隙水中有机污染物对多孔介质力学性质的影响。考虑了温度对污染物传输机制的影响。本文CTHM耦合本构模型已在多孔介质的热–水力–力学–污染物传输数学模型中建立。在CTHM本构模拟框架中对计及化学塑性效应、描述在热–水力–污染物传输耦合作用下多孔介质力学行为、在应力–吸力–温度四维空间中包含了五重屈服面的耦合本构模型发展了一致性切线模量矩阵。数值结果突出显示了污染物浓度在多孔介质化学–热–水力–力学(CTHM)耦合系统中的影响。     

Non-equilibrium partitioning tracer transport in porous media: 2-D physical modelling and imaging using a partitioning fluorescent dye

This paper describes an investigation into non-equilibrium partitioning tracer transport and interaction with non-aqueous-phase liquid (NAPL) contaminated water-saturated porous media using a two-dimensional (2-D) physical modelling methodology. A fluorescent partitioning tracer is employed within a transparent porous model which when imaged by a CCD digital camera can provide full spatial tracer concentrations and tracer breakthrough curves. Quasi one-dimensional (1-D) benchmarking tests in models packed with various combinations of clean quartz sand and NAPL are described. These modelled residual NAPL saturations, S_n, of 0–15%. Results demonstrated that the fluorescent partitioning tracer was able to detect and quantify the presence of NAPL at low flow rates. At larger flow rates and/or higher NAPL saturations, the tracer increasingly underpredicted the NAPL volume as expected and this is attributed primarily to non-equilibrium partitioning. Despite little change in permeability, change in NAPL saturations from 4% to 8% resulted in significant NAPL saturation underestimates at the same flow rates implying coalescence of NAPL into wider separated but larger ganglia. A 2-D investigation of an idealised heterogeneous residual NAPL contaminated flow field indicated little permeability change in the NAPL contaminated zone and thus little flow bypassing, leading to reduced underpredictions of NAPL saturations than for equivalent quasi 1-D cases. This was attributed to increased ‘sampling’ of the NAPL by the tracer. The process is clearly visually identifiable from the experimental images. This rapid and relatively inexpensive experimental method is of value in laboratory studies of partitioning tracer behaviour in porous media; in particular, the ability to observe full field concentrations makes it valuable for the study of complex heterogeneous systems.     

Numerical modeling of contaminant advection impact on hydrodynamic diffusion in a deformable medium

The self-weight of solid waste or machine-rolled compaction can induce or trigger contaminant migration in the landfill. Although the consolidation-induced hydraulic gradient driving solution transport has been extensively investigated, little attention has been paid to ion migration caused by its concentration gradient variation. It is necessary to more precisely predict the multi-stage contaminant transports in deforming porous material. Based on the modified Cam-clay model, the proposed fluid-solid coupled model can simulate the elastoplastic deformation behavior of layered kaolinite and KBr solution transport/sorption, and its modeling results were validated by published laboratory data. The solid-fluid interactions were analyzed by comparing various transport manners of K⁺ and Br^? from excess pore pressure generation to dissipation. Results reveal that the consolidation process can accelerate KBr solute advection from the contaminated layer into the uncontaminated layer, and then affects the subsequent diffusion, mechanical dispersion and sorption for K⁺ and Br^?. The simulations also indicate that consolidation-induced solute transport is time-dependent, and therefore the ion diffusion and mechanical dispersion should receive more attention.     

地下水中石油污染物运移的耦合模型及其应用研究

本文采用氯化钠作示踪剂测定含水层的弥散系数,并根据实测的溶解油吸附等温线来确定含水介质对石油污染物运移的阻滞系数,从而把反应性石油污染物在地下水中运移化为示踪剂的对流－弥散迁移和溶解油在水相与固相之间转移的叠加,最后作者把所得的耦合模型应用于评价大庆油田排污干渠对地下水污染问题。     

Quantitative salt-water modeling of fire-induced flow

The dispersion of salt water carefully introduced into fresh water closely simulates the dispersion of hot exhaust gases in fire plumes. The salt concentration downstream of the source and exhaust gas temperature downstream of the flame zone behave like passive scalars and are transported by similar turbulent convective and diffusive processes. Transport properties of buoyant plumes are investigated through quantitative visualization of the distinctive turbulent structures responsible for dispersion in these flows. Turbulent mixing patterns are visualized via Planar Laser Induced Fluorescence (PLIF). This technique uses a laser light sheet to excite a fluorescent dye tracer suspended in the salt-water solution allowing visualization of a particular plane of the flow. The intensity of the fluorescence is proportional to the concentration of the salt water providing quantitative instantaneous field measurements of flow dispersion. Canonical flows including the unconfined plume and the impinging plume are carefully characterized and compared with fire scaling laws and fire measurements to validate the quantitative implementation of the PLIF salt-water technique.     

裂隙介质溶质运移试验研究

介绍了自行研制的裂隙介质溶质运移试验测试系统,与国内外同类试验装置相比,该测试系统可以对不同尺寸的裂隙介质进行溶质运移试验,在裂隙介质中布设了测试流体压力、温度、浓度的传感器,可以对原样裂隙中各点的溶液浓度进行直接测量,以此来获得溶质在裂隙介质中的运移规律。应用该测试系统进行了NaCl示踪剂溶液在裂隙介质中运移的试验,并用解析–优化方法求解了裂隙介质中NaCl示踪剂溶液运移的水动力弥散参数。     

Modeling and testing of reactive contaminant transport in drinking water pipes: chlorine response and implications for online contaminant detection

A modified one-dimensional Danckwerts convection-dispersion-reaction (CDR) model is numerically simulated to explain the observed chlorine residual loss for a "slug" of reactive contaminants instantaneously introduced into a drinking water pipe of assumed no or negligible wall demand. In response to longitudinal dispersion, a contaminant propagates into the bulk phase where it reacts with disinfectants in the water. This process generates a U-shaped pattern of chlorine residual loss in a time-series concentration plot. Numerical modeling indicates that the residual loss curve geometry (i.e., slope, depth, and width) is a function of several variables such as axial Péclet number, reaction rate constants, molar fraction of the fast- and slow-reacting contaminants, and the quasi-steady-state chlorine decay inside the "slug" which serves as a boundary condition of the CDR model. Longitudinal dispersion becomes dominant for less reactive contaminants. Pilot-scale pipe flow experiments for a non-reactive sodium fluoride tracer and the fast-reacting aldicarb, a pesticide, were conducted under turbulent flow conditions (Re=9020 and 25,000). Both the experimental results and the CDR modeling are in agreement showing a close relationship among the aldicarb contaminant "slug", chlorine residual loss and its variations, and a concentration increase of chloride as the final reaction product. Based on these findings, the residual loss curve and its geometry are useful tools to identify the presence of a contaminant "slug" and infer its reactive properties in adaptive contaminant detections.     

考虑降解时变扩散系数污染物迁移模型解析解

降解作用是污染物在介质中运移的主要控制进程之一。假设扩散系数为深度的线性函数,建立了污染物在多孔介质中的一维扩散模型,同时考虑降解作用的影响,针对零浓度的下边界条件,推求了模型的解析解。将常扩散系数模型解与相关文献进行了对比,说明了本文所提方法的正确性及可靠性。基于变系数计算模型,讨论了降解作用对污染物运移规律的影响,并分析了相关参数的敏感性。结果表明,扩散系数对计算结果存在较大影响,降解作用能够有效降低污染物的浓度,且降解越强,污染物浓度达到稳定状态需要的时间越短。     

Photocatalytic degradation of Acid Red 4 using a titanium dioxide membrane supported on a porous ceramic tube

A photocatalytic membrane supported on a porous ceramic tube was described, in which permeation of solutes through the membrane and tube and photocatalytic reaction occur simultaneously. In this photocatalytic membrane reactor, TiO(2) catalyst was coated on the surface of a porous ceramic tube and all experiments were conducted in one pass dead-end system. The objectives of this study are to demonstrate the predominance of dead-end operation and to determine the reaction kinetics model of the photocatalytic reaction. Acid Red 4 (AR 4) dye was used as a model pollutant. A detailed study of physical parameters including flow configurations (dead-end and cross-flow), flow rate, initial dye concentration, light intensity and catalyst loading has been performed to obtain the reaction kinetics. The simultaneous effect of light intensity and catalyst loading was also determined experimentally. Experiments were also conducted to compare the photocatalytic degradation of AR 4 in the dead-end and cross-flow system. The major findings of this study are: (1) the decomposition ratios for dead-end system were three and five times higher than cross-flow system at flow rates of 6.67x10(-8) and 4.00x10(-7)m(3)/s, respectively. (2) The decomposition ratio increased with increasing catalyst loading and light intensity, but remained constant at higher catalyst loading. (3) The decomposition ratio was found to be decreased with increasing flow rate.     

A comparison of several nanoscale photocatalysts in the degradation of a common pollutant using LEDs and conventional UV light

A comparative study on the photocatalytic activities of four different catalysts, P-25 TiO₂, TiO₂ nanofibers, tin-doped TiO₂ nanofibers under UV light irradiation at 350 nm, and coumarin (C-343) coated TiO₂ nanofibers at 436 nm light emitting diodes (LED) is reported. Catalysts performance has been compared based on their reflectance spectrum and activity. A common water contaminant 4-chlorophenol was used as a substrate to compare the activity of the different catalysts under both direct and dye sensitized conditions. Results indicated that amongst the four different catalysts the activity of P-25 was the highest. However the activity of C-343 coated TiO₂ nanofibers in the LED (436 nm) based reactor was competitive. Identification of reaction intermediates implied that the reaction pathways under UV (band gap) and visible (dye sensitized) irradiation were different. Nonetheless, ring opening took place in all reactions with both maleic and dihydroxymaleic have been identified as intermediates. The study indicates that ordered arrays of TiO₂ irradiated by panels of arrays of low cost high intensity LEDs might be used for the design of reactors. The near monochromaticity, long life, and operation under direct currents are advantages of using LEDs.     

Field measurement of transverse diffusion in unidirectional flow in a wide, straight channel

A knowledge of the magnitude of the transverse diffusion coefficient for open channel flow is of importance in water quality studies of rivers and estuaries. Measurements of the coefficient have been made in a straight reach of an urban stream. Rhodamine WT was used as the tracer and good dye recovery ratios and data reproducibility were obtained. The variation of the transverse diffusion coefficient and the dimensionless parameter (β) with river flow showed a linear trend. The dimensionless coefficient had values similar to those for other prototype studies.     

Transport of solutes through unsaturated fractured media

A numerical model is presented to represent the transport of solutes through a highly fractured, unsaturated, porous medium. To accomplish this, the solute is tracked separately in two flow systems, a matrix pore flow system and a fracture network, with interaction terms. Compatible hydraulic equations for such a dual system are also presented to enable solution of the solute transport. The hydraulic equations chosen use the equivalent porous media concept. These equations can also be applied to a saturated medium without modification; however, many of the transport terms will be negligible for such an application. A brief sample calculation illustrates the method.     

Residence time distribution in submerged biofilters

A mathematical model is presented for the residence time distribution in a submerged denitrification filter, in which pronounced tailing is experienced due to hold-up in the biofilm. Experiments with a dye tracer, Eosine-Y, and with Tritium show that adsorption of the dye makes it impossible to determine biofilm characteristics. The same is true for any tracer if nitrogen bubbles are present in the filter. An experiment with Tritium in the filter without bubbles indicates that the surface of the media is only partly covered with biofilm. Biofilm characteristics can be determined only with great uncertainty, due to uneven distribution.     

Subsurface multiphase flow of organic contaminants: Model development and validation

A numerical model has been developed to predict the migration of organic contaminants in the subsurface. The formulations take a multiphase approach in describing the flow of organic contaminants in saturated and unsaturated porous media. In a three-phase fluid system of contaminant, gas, and water, simultaneous flow of the water and the contaminant phases is formulated by applying mass conservation principles to each of the phases under the condition of no interphase mass exchange. For each phase, the formulations incorporate the spatial variability of the relative permeability and its direction dependency. The complex formulations are solved numerically using an implicit finite difference scheme. The accuracy of the numerical model is determined against experimental data from the literature. The predicted migration pattern in both homogeneous and stratified media agrees well with the experimental data. Numerical simulations illustrate the strong effects of the medium permeability and the water distribution profiles on the flow pattern and the pressure distribution of the contaminant.     

Fluorescent dye labeled bacteriophages--a new tracer for the investigation of viral transport in porous media: 1. Introduction and characterization

A new method for the study of pathogen transport in porous media is presented. The method is based on conjugation of fluorescent dyes to target bacteriophages and application of the modified bacteriophages for tracer studies. We demonstrate that the relevant transport determining properties of Rhodamine and several fluorescein-labeled phages are practically identical to those of the native bacteriophages. The advantages of the proposed method relative to direct enumeration of bacteriophages by plaque forming unit method, turbidity, fluorescent microspheres, and other alternative tracers are discussed. Notable advantages include simple quantitation by optical methods, unbiased signals even when virus aggregates are formed, and the ability to decouple inactivation kinetics from transport phenomena. Additionally, the signal reflects the removal and transport of the studied microorganism and not a surrogate.     

城市生活垃圾淋滤液运移模型问题

本文通过对淋滤液运移模型的研究，给出了多孔介质和双重介质淋滤液运移模型的定解问题，采用广义迎风格式求解淋滤液运移模型，消除数值弥散。运用模型预测垃圾填埋场污染晕，为防治地下水污染提供依据。     

土体固结与溶质运移三维耦合模型

考虑堆场上覆堆积体自重引起的土体固结与溶质运移的耦合效应已成为环境土工领域中的热点问题。在 Biot 固结理论和溶质运移理论相结合的基础上,以孔隙率作为耦合参量,将固结方程与运移方程相联系,考虑土体固结对溶质运移模型参数的影响,建立了溶质在固结变形土体中运移的三维耦合模型。采用多场耦合有限元软件 COMSOL Multiphysics 对模型进行了数值求解。数值模拟结果表明,土体固结变形对溶质运移过程具有较强和持久的阻滞作用,导致溶质运移深度以及污染羽范围的减小,在模拟运移20 a 时,运移距离减小率已高达27．75％,且随着时间的增加而逐渐增大;溶质在水平方向的迁移距离不可忽略,随着横向水力梯度的增大而增大。     

The effect of UV irradiation on human-derived Giardia lamblia cysts

The effect of UV irradiation on human-derived Giardia lamblia cysts (WB isolate, cultivated using the gerbil model, and purified to homogeneity) was assessed using a 254nm collimated beam protocol. The infection kinetics of UV-exposed cysts were directly compared to the infection kinetics of control (non-UV-exposed) cysts. This demonstrated that a UV dose at 254 nm resulted in significant inactivation of the Giardia cysts. Up to 2 log (99%) inactivation was observed at a UV dose of approximately 10 mJ cm(-2) (range 9.3-11.7 mJ cm(-2)). Higher UV doses (between 20 and 40 mJ cm(-2)) resulted in up to 3 log (99.9%) inactivation of the cysts. The actual inactivation at these dose levels could be higher, but for this study the maximum quantifiable limit of cyst inactivation was up to 3 logs (99.9%). Chemical actinometry using the Rahn potassium iodide actinometer was used to confirm the UV dose delivered. No correlation between a PI/DAPI vital dye viability assay and the infectious dose assay was observed. The vital dye viability assay demonstrated no inactivation. Future work using an alternative UV delivery systems including a demonstration-scale device is recommended and more work with a variety of isolates is warranted.     

UV inactivation and characteristics after photoreactivation of Escherichia coli with plasmid: health safety concern about UV disinfection

Occurrence and degree of photoreactivation after ultraviolet (UV) exposure have been widely studied. However, the characteristics of photoreactivated microorganisms were rarely investigated. Hence, in this study, Escherichia coli with plasmids of ampicillin (amp)-resistance or fluorescence was used as indicators to examine the UV inactivation efficiencies and variations of characteristics of E. coli after subsequent photoreactivation.The experimental results indicate that the amp-resistant bacteria and the fluorescent bacteria used in this study had similar trends of UV dose-response curves. 3.5-log₁₀ and 3-log₁₀ reductions were achieved with a UV dose of 5 mJ/cm² for the amp-resistant and fluorescent E. coli, respectively. There was no significant difference in the UV inactivation behavior, as compared with common strains of E. coli.For the amp-resistant E. coli and the fluorescent E. coli, after exposures with UV doses of 5, 15, 25, 40 and 80 mJ/cm², the corresponding percent photoreactivations after a 4 h exposure to photoreactivating light were 1% and 46% respectively for a UV dose of 5 mJ/cm², and essentially negligible for all other UV doses. Furthermore, the photoreactivated amp-resistant bacteria still have the ability of amp-resistance. And the revived fluorescent E. coli showed similar fluorescent behavior, compared with the untreated bacteria. The experimental results imply that after UV inactivation and subsequent photoreactivation, the bacteria retained the initial characteristics coded in the plasmid. This reveals a possibility that some characteristics of bacteria can retain or recover through photoreactivation, and a safety concern about pathogenicity revival might need to be considered with UV disinfection and photoreactivation.