Colloid transport in a geochemically heterogeneous porous medium: aquifer tank experiment and modeling

To examine colloid transport in geochemically heterogeneous porous media at a scale comparable to field experiments, we monitored the migration of silica-coated zirconia colloids in a two-dimensional layered porous media containing sand coated to three different extents by ferric oxyhydroxides. Transport of the colloids was measured over 1.65 m and 95 days. Colloid transport was modeled by an advection-dispersion-deposition equation incorporating geochemical heterogeneity and colloid deposition dynamics (blocking). Geochemical heterogeneity was represented as favorable (ferric oxyhydroxide-coated) and unfavorable (uncoated sand) deposition surface areas. Blocking was modeled as random sequential adsorption (RSA). Release of deposited colloids was negligible. The time to colloid breakthrough after the onset of blocking increased with increasing ferric oxyhydroxide-coated surface area. As the ferric oxyhydroxide surface area increased, the concentration of colloids in the breakthrough decreased. Model-fits to the experimental data were made by inverse solutions to determine the fraction of surface area favorable for deposition and the deposition rate coefficients for the favorable (ferric oxyhydroxide-coated) and unfavorable sites. The favorable deposition rate coefficient was also calculated by colloid filtration theory. The model described the time to colloid breakthrough and the blocking effect reasonably well and estimated the favorable surface area fraction very well for the two layers with more than 1% ferric oxyhydroxide coating. If mica edges in the uncoated sand were considered as favorable surface area in addition to the ferric oxyhydroxide coatings, the model predicted the favorable surface area fraction accurately for the layer with less than 1% ferric oxyhydroxide coating.     

倒置AAO工艺中物料及能量流向的研究

通过对北京某城市污水厂的曝气池各单元及二沉池营养物质变化进行跟踪试验,分析该污水厂的活性污泥单元倒置AAO工艺部分碳、氮、磷的物料流向,并且基于物料平衡分析了工艺中的能量流向.     

Modelling radionuclide transport for time varying flow in a channel network

Water flowrates and flow directions may change over time in the subsurface for a number of reasons. In fractured rocks flow takes place in channels within fractures. Solutes are carried by the advective flow. In addition, solutes may diffuse in and out of stagnant waters in the rock matrix and other stagnant water regions. Sorbing species may sorb on fracture surfaces and on the micropore surfaces in the rock matrix. We present a method by which solute particles can be traced in flowing water undergoing changes in flowrate and direction in a complex channel network where the solutes can also interact with the rock by diffusion in the rock matrix. The novelty of this paper is handling of diffusion in the rock matrix under transient flow conditions. The diffusive processes are stochastic and it is not possible to follow a particle deterministically. The method therefore utilises the properties of a probability distribution function for a tracer moving in a fracture where matrix diffusion is active. The method is incorporated in a three dimensional channel network model. Particle tracking is used to trace out a multitude of flowpaths, each of which consists of a large number of channels within fractures. Along each channel the aperture and velocity as well as the matrix sorption properties can vary. An efficient method is presented whereby a particle can be followed along the variable property flowpath. For stationary flow conditions and a network of channels with advective flow and matrix diffusion, a simple analytical solution for the residence time distribution along each pathway can be used. Only two parameter groups need to be integrated along each path. For transient flow conditions, a time stepping procedure that incorporates a stochastic Monte-Carlo like method to follow the particles along the paths when flow conditions change is used. The method is fast and an example is used for illustrative purposes. It is exemplified by a case where land rises due to glacial rebound. It is shown that the effects of changing flowrates and directions can be considerable and that the diffusive migration in the matrix can have a dominating effect on the results.     

水平渗滤系统的污染物去除效果及动力学分析

基于人工快渗(CRIS)和水平潜流人工湿地(HSSFCWs)构建了水平流人工渗滤系统(HFCIS),研究了该系统对耗氧有机物(以COD计)、氨氮(NH₄⁺-N)的沿程去除情况和污染物在系统内的垂向分布情况,并进行了动力学分析。结果表明,在水力负荷为0.083 m·d⁻¹、进水耗氧有机物(以COD计)浓度为220~630 mg·L⁻¹、NH₄⁺-N质量浓度为13~47 mg·L⁻¹时,COD、NH₄⁺-N的去除率分别为88.6%和91.9%以上。在水力负荷为0.25 m·d⁻¹的条件下,进水耗氧有机物(以COD计)和NH₄⁺-N质量浓度分别为613~690 mg·L⁻¹和36~48 mg·L⁻¹时,总去除率分别为95.5%和78.2%以上。水平方向沿程污染物质量浓度呈现逐渐衰减的趋势,污染物降解符合一阶动力学模型,去除速率常数在CRIS和HSSFCWs的速率常数范围内并处于较高水平。该HFCI系统填料简单,复氧效果好,污染物去除性能优异,提高了土地利用率,建造位置选择较为灵活,在分散式污水处理中有独特的优势。     

旋流除砂系统排砂管路改造与优化运行

针对南方某污水处理厂旋流除砂系统由于设计及设备选型不合理,造成排砂管路反复堵塞,使系统长期处于"瘫痪"状态,通过水力学分析结合实际运行工况提出改进措施,并对系统除砂性能进行测试,依据测试结果提出沉砂池优化运行方案。结果表明,旋流除砂系统排砂管路高度应满足气提扬程要求,经改造后,系统未发生堵塞现象,优化运行后的系统出砂量由平均240 kg/d增加到300 kg/d,且出砂中粒径小于0.3 mm砂粒所占比例由40%提高至78%,砂粒表面的附着有机物含量由前期的10%下降到4%。     

Analytical model of solute transport by unsteady unsaturated gravitational infiltration

Penetration of reactive solute into a soil during a cycle of water infiltration and redistribution is investigated by deriving analytical closed form solutions for fluid flux, moisture content and contaminant concentration. The solution is developed for gravitational flow and advective transport and is applied to two scenarios of solute applications encountered in the applications: a finite pulse of solute dissolved in irrigation water and an instantaneous pulse broadcasted onto the soil surface. Through comparison to simulations of Richards' flow, capillary suction is shown to have contrasting effects on the upper and lower boundaries of the fluid pulse, speeding penetration of the wetting front and reducing the rate of drying. This leads to agreement between the analytical and numerical solutions for typical field and experimental conditions. The analytical solution is further incorporated into a stochastic column model of flow and transport to compute mean solute concentration in a heterogeneous field. An unusual phenomenon of plume contraction is observed at long times of solute propagation during the drying stage. The mean concentration profiles match those of the Monte-Carlo simulations for capillary length scales typical of sandy soils.     

Transport and degradation of pesticides in a biopurification system under variable flux, part I: A microcosm study

The efficiency of a biopurification system, developed to treat pesticide contaminated water, is to a large extent determined by the chemical and hydraulic load. Insight into the behaviour of pesticides under different fluxes is necessary. The behaviour of metalaxyl, bentazone, linuron, isoproturon and metamitron was studied under three different fluxes with or without the presence of pesticide-primed soil in column experiments. Due to the time-dependent sorption process, retention of the pesticides with intermediate mobility was significantly influenced by the flux. The higher the flux, the slower pesticides will be sorbed, which resulted in a lower retention. Degradation of the intermediate mobile pesticides was also submissive to variations in flux. An increase in flux, led to a decrease in retention, which in turn decreased the opportunity time for biodegradation. Finally, the presence of pesticide-primed soil was only beneficial for the degradation of metalaxyl.     

Operator-splitting errors in coupled reactive transport codes for transient variably saturated flow and contaminant transport in layered soil profiles

One possible way of integrating subsurface flow and transport processes with (bio)geochemical reactions is to couple by means of an operator-splitting approach two completely separate codes, one for variably-saturated flow and solute transport and one for equilibrium and kinetic biogeochemical reactions. This paper evaluates the accuracy of the operator-splitting approach for multicomponent systems for typical soil environmental problems involving transient atmospheric boundary conditions (precipitation, evapotranspiration) and layered soil profiles. The recently developed HP1 code was used to solve the coupled transport and chemical equations. For steady-state flow conditions, the accuracy was found to be mainly a function of the adopted spatial discretization and to a lesser extent of the temporal discretization. For transient flow situations, the accuracy depended in a complex manner on grid discretization, time stepping and the main flow conditions (infiltration versus evaporation). Whereas a finer grid size reduced the numerical errors during steady-state flow or the main infiltration periods, the errors sometimes slightly increased (generally less than 50%) when a finer grid size was used during periods with a high evapotranspiration demand (leading to high pressure head gradients near the soil surface). This indicates that operator-splitting errors are most significant during periods with high evaporative boundary conditions. The operator-splitting errors could be decreased by constraining the time step using the performance index (the product of the grid Peclet and Courant numbers) during infiltration, or the maximum time step during evapotranspiration. Several test problems were used to provide guidance for optimal spatial and temporal discretization.     

Modelling tritium and phosphorus transport by preferential flow in structured soil

Subsurface solute transport through structured soil is studied by model interpretation of experimental breakthrough curves from tritium and phosphorus tracer tests in three intact soil monoliths. Similar geochemical conditions, with nearly neutral pH, were maintained in all the experiments. Observed transport differences for the same tracer are thus mainly due to differences in the physical transport process between the different monoliths. The modelling is based on a probabilistic Lagrangian approach that decouples physical and chemical mass transfer and transformation processes from pure and stochastic advection. Thereby, it enables explicit quantification of the physical transport process through preferential flow paths, honouring all independently available experimental information. Modelling of the tritium breakthrough curves yields a probability density function of non-reactive solute travel time that is coupled with a reaction model for linear, non-equilibrium sorption–desorption to describe the phosphorus transport. The tritium model results indicate that significant preferential flow occurs in all the experimental soil monoliths, ranging from 60–100% of the total water flow moving through only 25–40% of the total water content. In agreement with the fact that geochemical conditions were similar in all experiments, phosphorus model results yield consistent first-order kinetic parameter values for the sorption–desorption process in two of the three soil monoliths; phosphorus transport through the third monolith cannot be modelled because the apparent mean transport rate of phosphorus is anomalously rapid relative to the non-adsorptive tritium transport. The occurrence of preferential flow alters the whole shape of the phosphorus breakthrough curve, not least the peak mass flux and concentration values, and increases the transported phosphorus mass by 2–3 times relative to the estimated mass transport without preferential flow in the two modelled monoliths.     

Surfactant-induced changes in gravity fingering of water through a light oil

Gravity-driven preferential flow (fingering) can greatly affect how one fluid displaces another in the subsurface. We have studied the internal properties of these preferential flow paths for water, with and without surfactants, infiltrating into oil saturated porous media using synchrotron X-rays, and miniature tensiometers to characterize fluid content and pressure relationships. We also used a light transmission technique to visualize overall flow pattern. Capillary pressure and water content decrease behind the front, similar to fingers in air-dry sand, with quantitative differences for five different surfactants with surface tensions ranging from 4–21 g/s². Using unstable flow theory, the finger widths, capillary pressure drops within the fingers, finger tip lengths, and finger splitting dynamics were scaled successfully with interfacial tension, fluid density, and the contact angle using the fingers in air–water systems as the reference.     

Modeling of non-reactive solute transport in fractured clayey till during variable flow rate and time

Fractures and biopores can act as preferential flow paths in clay aquitards and may rapidly transmit contaminants into underlying aquifers. Reliable numerical models for assessment of groundwater contamination from such aquitards are needed for planning, regulatory and remediation purposes. In this investigation, high resolution preferential water-saturated flow and bromide transport data were used to evaluate the suitability of equivalent porous medium (EPM), dual porosity (DP) and discrete fracture/matrix diffusion (DFMD) numerical modeling approaches for assessment of flow and non-reactive solute transport in clayey till. The experimental data were obtained from four large undisturbed soil columns (taken from 1.5 to 3.5 m depth) in which biopores and channels along fractures controlled 96-99% of water-saturated flow. Simulating the transport data with the EPM effective porosity model (FRACTRAN in EPM mode) was not successful because calibrated effective porosity for the same column had to be varied up to 1 order of magnitude in order to simulate solute breakthrough for the applied flow rates between 11 and 49 mm/day. Attempts to simulate the same data with the DP models CXTFIT and MODFLOW/MT3D were also unsuccessful because fitted values for dispersion, mobile zone porosity, and mass transfer coefficient between mobile and immobile zones varied several orders of magnitude for the different flow rates, and because dispersion values were furthermore not physically realistic. Only the DFMD modeling approach (FRACTRAN in DFMD mode) was capable to simulate the observed changes in solute transport behavior during alternating flow rate without changing values of calibrated fracture spacing and fracture aperture to represent the macropores.     

SCR脱硝系统烟道内流场优化

为了改善选择性催化还原法(SCR)催化剂上游烟气的流场、温度场和浓度场分布,基于CFD仿真分析技术,建立了整个脱硝系统三维模型,并通过优化导流板及整流格栅,使得速度标准偏差、浓度标准偏差、温度偏差、速度偏转角以及压降等性能指标都满足设计要求,在满足脱硝效率和氨逃逸率前提下,大大降低了烟气对催化剂层的冲刷,保证催化剂的使用寿命。最后,为了合理布置吹灰器,对优化结构下粉尘颗粒在烟道内的运动情况进行了仿真分析研究,为设计提供了依据。     

Incorporating layer- and local-scale heterogeneities in numerical simulation of unsaturated flow and tracer transport

This study characterizes layer- and local-scale heterogeneities in hydraulic parameters (i.e., matrix permeability and porosity) and investigates the relative effect of layer- and local-scale heterogeneities on the uncertainty assessment of unsaturated flow and tracer transport in the unsaturated zone of Yucca Mountain, USA. The layer-scale heterogeneity is specific to hydrogeologic layers with layerwise properties, while the local-scale heterogeneity refers to the spatial variation of hydraulic properties within a layer. A Monte Carlo method is used to estimate mean, variance, and 5th, and 95th percentiles for the quantities of interest (e.g., matrix saturation and normalized cumulative mass arrival). Model simulations of unsaturated flow are evaluated by comparing the simulated and observed matrix saturations. Local-scale heterogeneity is examined by comparing the results of this study with those of the previous study that only considers layer-scale heterogeneity. We find that local-scale heterogeneity significantly increases predictive uncertainty in the percolation fluxes and tracer plumes, whereas the mean predictions are only slightly affected by the local-scale heterogeneity. The mean travel time of the conservative and reactive tracers to the water table in the early stage increases significantly due to the local-scale heterogeneity, while the influence of local-scale heterogeneity on travel time gradually decreases over time. Layer-scale heterogeneity is more important than local-scale heterogeneity for simulating overall tracer travel time, suggesting that it would be more cost-effective to reduce the layer-scale parameter uncertainty in order to reduce predictive uncertainty in tracer transport.     

Fluid flow and solute transport in a network of channels

A new model to describe flow and transort in fractured rocks is proposed. It is based on the concept of a network of channels. The individual channel members are given stochastically selected conductances and volumes. Flow-rate calculations have been performed. For large standard deviations in conductances, channeling becomes pronounced with most of the water flowing in a few paths. The effluent patterns and flow-rate distributions obtained in the simulations have been compared to three field measurements in drifts and tunnels of flow-rate distributions. Standard deviations of channel conductances were between 1.6 and 2.4 in some cases. A particle-following technique was used to simulate solute transport in the network. Non-sorbing as well as sorbing solute transport can be simulated. By using a special technique, solutes that diffuse into the rock matrix can also be simulated.     

The influence of system complexity on bacterial transport in saturated porous media

A series of miscible-displacement column experiments were conducted under saturated flow conditions to systematically investigate the influence of physical and biological complexity on bacterial activity and fate in the presence and absence of a non-sorbing growth substrate, salicylate. Bacterial elution was monitored for three different systems; System I--a sterilized, inoculated, well-sorted sand, System II--a sterilized, inoculated, heterogeneous loamy sand (Hayhook), and System III--two different unsterilized loamy sands (Hayhook and Vinton) each with their associated indigenous microbial community. Results show that System I behaved ideally with respect to both cell and substrate transport, wherein: (1) growth occurred in response to substrate addition, (2) cell elution increased in response to the substrate pulse, and (3) breakthrough curves were reproducible for both substrate and cell elution. In contrast, System II showed ideal behavior with respect to substrate transport but showed variable behavior for cell transport. Further, there was no measurable growth in response to substrate addition and no increase in cell elution during the salicylate pulse. System III exhibited non-ideal behavior for both substrate and cell transport. Of particular interest is the fact that the indigenous communities of the two soils behaved differently. Specifically, for the Hayhook soil, an increased elution response was observed for the heterotrophic population while the salicylate-degrading community was preferentially retained in the column. In contrast for the Vinton soil, the substrate pulse did not elicit an elution response from either the heterotrophic or salicylate-degrading community from the culturable, indigenous Vinton microorganisms. For Systems II and III, the observed variability appears to be associated with the biological component of the system, since sterile controls were reproducible. This type of systematic study is critical for understanding cell and substrate transport behavior in complex, heterogeneous systems, and illustrates the potential uncertainty associated with measurements in such systems.     

Sorption and transport of trichloroethylene in caliche soil

Sorption of TCE to the caliche soil exhibited linear isotherm at the high TCE concentrations (Co = 122-1300 mg L⁻¹) but Freundlich isotherm at the low concentration range (1-122 mg L⁻¹). Sorption strength of the carbonate fraction of the soil was about 100-fold lower than the sorption strength of soil organic matter (SOM) in the caliche soil, indicating weak affinity of TCE for the carbonate fraction of the soil. Desorption of TCE from the caliche soil was initially rapid (7.6 × 10⁻⁴ s⁻¹), then continued at a 100-fold slower rate (7.7 × 10⁻⁶ s⁻¹). Predominant calcium carbonate fraction of the soil (96%) was responsible for the fast desorption of TCE while the SOM fraction (0.97%) controlled the rate-limited desorption of TCE. Transport of TCE in the caliche soil was moderately retarded with respect to the water (R = 1.75-2.95). Flow interruption tests in the column experiments indicated that the rate-limited desorption of TCE controlled the non-ideal transport of TCE in the soil. Modeling studies showed that both linear and non-linear nonequilibrium transport models provided reasonably good match to the TCE breakthrough curves (r² = 0.95-0.98). Non-linear sorption had a negligible impact on both the breakthrough curve shape and the values of sorption kinetics parameters at the high TCE concentration (Co = 1300 mg L⁻¹). However, rate-limited sorption/desorption processes dominated at this concentration. For the low TCE concentration case (110 mg L⁻¹), in addition to the rate-limited sorption/desorption, contribution of the non-linear sorption to the values of sorption kinetics became fairly noticeable.     

不同碳源下缺氧/好氧连续流系统生物除磷效果及其机理

为了考察前期发现的以淀粉为唯一碳源、缺氧好氧生物脱氮系统对含多种有机物的废水中磷的脱除,在以淀粉为唯一碳源、已能稳定生物除磷(除磷率达72％)的缺氧好氧连续流生物脱氮系统中,改变进水碳源组成及浓度,测定了系统对磷去除的变化、分析了系统除磷与进水碳源的关系.结果 表明,在进水中淀粉浓度保持为400 mg·L-1(以COD...     

Darcian preferential water flow and solute transport through bimodal porous systems: Experiments and modelling

Soils often exhibit a variety of small-scale heterogeneities such as inter-aggregate pores and voids which partition flow into separate regions. In this paper a methodological approach is discussed for characterizing the hydrological behaviour of a heterogeneous clayey–sandy soil in the presence of structural inter-aggregate pores. For the clay soil examined, it was demonstrated that, coupling the transfer function approach for analyzing BTCs and water retention data obtained with different methods from laboratory studies captures the bimodal geometry of the porous system along with the related existence of fast and slow flow paths. To be effectively and reliably applied this approach requires that the predominant effects of the soil hydrological behaviour near saturation be supported by accurate experimental data of both breakthrough curves (BTCs) and hydraulic functions for high water content values. This would allow the separation of flow phases and hence accurate identification of the processes and related parameters.     

半导体存储介质破碎-分选系统的因子优化设计

废弃半导体存储介质一方面载有隐私信息,需要被安全销毁,另一方面富含金属导体、半导体、绝缘体材料,极具回收价值。提出用破碎-风力-高压静电分选系统对其进行安全销毁和物料分选,其中多级破碎部分使信息半导体硅存储材料破碎至0.80 mm以下,而风力-高压静电分选部分能实现半导体、导体、绝缘体的分选。为确定系统工艺参数,实验对风机蝶阀角度（α）,风机频率（f）,电选电压（U）,电选机转辊转速（n）等因素对半导体收集量（M）的影响进行研究,通过因子设计建立了非线性数学模型：M=1.943 8-0.418 7α*+0.306 2f*-0.193 7 f*U*n*+0.181 3α*f*U。进而通过响应优化得出最优工艺参数设置为：α=30°,f=45 Hz,U=30 kV,n=40 r·min-1。在此参数下,金属导体收集率达90.3%,半导体硅收集率达61.0%,实现了半导体存储介质破碎销毁和物料分选。