Mathematical Modeling of Encapsulated Buffer Performance in Sand Columns

A one-dimensional mathematical model was developed to simulate pH control using an encapsulated phosphate buffer during denitrification in a sand column. The parameters required for the model were obtained from direct physical measurement, from a tracer study to characterize the dispersion coefficient in the column, and from batch experiments designed to obtain an empirical expression describing the variation of the first-order rate constant for the encapsulated buffer core release with pH. First-order kinetic constants describing the rates of denitrification and ethanol biodegradation were obtained by fitting the model to column runs without the encapsulated buffer. With these parameters, the model was subsequently used to predict the performance of column runs containing the encapsulated buffer. Since denitrification was essentially complete in the sand columns, an increase in the effluent pH was observed. This pH increase was counteracted by the controlled release of the acidic core of the encapsulated buffers added in the columns. The model reasonably predicted the release of the encapsulated buffer core and the performance of the encapsulated buffer for controlling pH in the column.     

Impact of Storm-Water Runoff on Clogging and Fecal Bacteria Reduction in Sand Columns

Storm-water runoff has been identified as a major cause of coastal water quality degradation. Storm-water outfalls, common in many coastal towns, convey bacteria and other pollutants into the ocean and estuaries. In an effort to minimize this impact, the Town of Kure Beach, North Carolina, installed Dune Infiltration Systems (DIS) at two storm-water outfalls to receive storm-water runoff and allow infiltration beneath the beach dunes. A laboratory column experiment was performed to supplement this installation and determine the potential hydraulic and bacterial removal efficiency of the sand comprising the Kure Beach dunes. Columns constructed using sand collected at different depths of the dune were used to analyze the affect of bacteria application on infiltration and to examine the changes in bacteria removal that occur as infiltration rates are affected by bacteria-laden water application. Sand columns were loaded over a 60-day period with either bacteria-free storm water or storm water spiked with Escherichia coli. The seepage rate for the bacteria-spiked storm-water treatment was significantly lower (p<0.05) than the seepage rate of the bacteria-free treatment, particularly toward the end of the study. Bacteria application likely compounds the impact of sediment clogging at the sand/storm-water interface. This study indicates the need for maintenance when implementing a DIS or similar sand filter to maintain design infiltration rates, especially if reduced infiltration rates are not planned for in the design. However, a decrease in seepage rate was correlated with a decrease in effluent bacteria concentration in the bacteria-spiked storm-water sand columns. Thus, optimization is required to provide maximum infiltration of storm-water while maintaining bacteria removal efficiency.     

Effect of Hexavalent Chromium on Performance of Membrane Bioreactor in Wastewater Treatment

The presence of toxic hexavalent chromium poses a great challenge in biological wastewater treatment. In this study, the performance of a membrane bioreactor (MBR) for the treatment of synthetic domestic wastewater in the presence of chromium was investigated. The carbonaceous pollutant removal is not affected by Cr(VI) with concentration ranging from 0.4 to 10 mg/L; it becomes slightly lower when the Cr(VI) is 50 mg/L. The nitrification efficiency of above 99% can be achieved when the waste stream is free of the metal or contains 0.4 mg/L chromium. When its concentration is 10 mg/L, nitrification efficiency above 50% is found; however, it becomes deteriorated in the presence of 50 mg/L chromium. The positive biomass growth, though lower than conventional activated sludge process, can be achieved at Cr(VI) concentration less than 10 mg/L; a decline in the cell growth occurs when the metal concentration is increased to 50 mg/L. Significant accumulation for the metal is observed when its concentration is 0.4 mg/L; however, almost no metal removal is observed when the concentration is above 10 mg/L. During eight-month continuous operation, the presence of Cr(VI) has an insignificant effect on the flux. The nitrifiers in the MBR are more sensitive to the presence of Cr(VI) than heterotrophs.     

Distribution of the Microbial Community Structure in Sulfur-Based Autotrophic Denitrification Columns

Substrate-dependent evolution of a bacterial community capable of transforming nitrate was examined in sulfur-based autotrophic denitrification columns. The 16S rRNA genes and denaturing gradient gel electrophoresis (DGGE) analysis revealed that the initial bacterial consortium was well adapted to column operation time and distribution of nitrate concentration. In the lower part of a 200-day operated column where nitrate was introduced, a bacterial strain designated OTU DE-1 was abundant, occupying 92% of the community. The species was identified as Thiobacillus denitrificans with a similarity of 97% by BLAST search. A heterotroph designated as OTU DE-2 showing a similarity of 94% to Cenibacterium arsenoxidans was then enriched in the middle part of the column occupying 82% of the community, indicating the presence of organic electron donors. Interestingly, OTU DE-5 with a similarity of 98% to Chlorobium limicola, which is commonly present in hydrogen sulfide-rich environments, was found in the upper part of the column where sulfate level was high with little nitrate. The microbial community was consistent with observed concentrations of nitrate, nitrite, and sulfate with column height and operation time.     

Transport and Deposition of Suspended Soil Colloids in Saturated Sand Columns

Understanding colloid mobilization, transport, and deposition in the subsurface is a prerequisite for predicting colloid-facilitated transport of strongly adsorbing contaminants and further developing remedial activities. This study investigated the transport behavior of soil-colloids extracted from a red-yellow soil from Okinawa, Japan. Different concentrations of suspended-soil colloids (with diameter <1??μm) were applied, at different flow velocities and pH conditions, to 10-cm long water-saturated columns repacked with either Narita (mean diameter D50 = 0.64??mm) or Toyoura (mean diameter D50 = 0.21??mm) sands. The transport and retention of colloids were studied by analyzing colloid effluent breakthrough curves (BTCs), particle size distribution in the effluent, and colloid deposition profiles within the column. The results showed a significant influence of flow velocity: Low flow velocity caused tailing of colloid BTCs with higher reversible entrapment and release of colloids than high flow velocity. The finer Toyoura sand retained more colloids than the coarser Narita sand at low pH conditions. The deposition profile and particle size distribution of colloids in the Toyoura sand clearly indicated a depth-dependent straining mechanism. By fitting colloid transport models (one-site and two-site models) to the colloid effluent breakthrough curves, transport and deposition of colloids in Narita sand at low pH were best described by a one-site attachment-detachment model, whereas colloid transport and deposition in Toyoura sand at low pH were better captured by a two-site attachment, detachment, and straining model. The coupled effects of solution chemistry, colloid sizes, and medium surface properties have a dominating role in particle-particle and particle-collector interactions in colloid transport and deposition.     

Bioclogging of Sand due to Biodegradation of Aircraft Deicing Fluid

The biodegradation of propylene glycol (PG) and PG-based aircraft deicing fluid (ADF) at initial concentrations of 400–100,000?mg/L was investigated in saturated sand columns operated under nitrogen-limited conditions that are expected occur in the environment. PG biodegradation resulted in the accumulation of 0.4–1.4?mg volatile solids/g sand, which decreased the hydraulic conductivity of the sand by 23–99.8%. At loading up to 0.27?mg ADF or PG/g sand/d, greater than 99% PG removal and 88% soluble chemical oxygen demand (COD) removal were achieved. At higher loading, removal efficiency decreased but the removal rate increased to 11.2?mg?PG/g sand/day and up to 10.7?mg COD/g sand/day. As ADF or PG loading increased causing more nitrogen-limited conditions and likely a greater amount of PG fermentation, cell yields decreased and a greater fraction of incomplete mineralization of the ADF and PG were noted as measured by higher residual soluble COD. The results indicate that natural attenuation of PG in groundwater is likely to occur in association with potentially significant bioclogging.     

Effect of Sand Columns on the Undrained Load Response of Soft Clays

When sand columns are used as vertical drains in soil improvement schemes, the possible reinforcing role that these columns can play in regards to improving the bearing capacity is usually neglected in design. The objective of this paper is to evaluate the degree of improvement in the mechanical properties of soft clays in practical applications involving the use of sand drains or sand columns in clayey soils. For this purpose, 32 isotropically consolidated undrained triaxial tests were performed on normally consolidated kaolin specimens. The parameters that were varied were the diameter of the sand columns, the height of the columns, the type of columns (geotextile encased versus nonencased), and the effective confining pressure. Test results indicated that sand columns improved the undrained strength significantly even for area replacement ratios that were less than 18%. The increase in undrained strength was accompanied by a decrease in pore pressure generation during shear and an increase in Young’s modulus. The drained shear strength parameters were found to be relatively unaffected by the sand column reinforcement, except for fully penetrating columns with high area replacement ratios.     

Impact of Salinity on MS-2 Sorption in Saturated Sand Columns—Fate and Transport Modeling

This research investigated the sorption and transport of MS-2 in saturated sand under a wide range of salinities using one-dimensional column experiments. The salinity varied from 0 ppt (fresh water) to 30 ppt. The MS-2 in the fresh water showed very weak adsorption due to having the same negative charge as the sand. Increasing the salinity concentrations dramatically enhanced MS-2 adsorption. The MS-2 breakthrough revealed the existence of reversible and irreversible sorption sites in the sand. Salinity increased MS-2 attachment by compressing the double layers of MS-2 and reversible sorption sites. The salinity also changed some reversible sorption sites into irreversible sorption sites by reversing to positive surface charges of silica powder. An advection-dispersion-sorption model with a two-site reversible-irreversible kinetic sorption was developed to describe MS-2 breakthrough under different salinity conditions. The sorption parameters were estimated and their independence was evaluated by minimizing the total squared error of the MS-2 data. The proposed model showed good agreement with the experimental data for a wide range of salinity levels from fresh water to near seawater. The strong sorption shown in the MS-2 breakthrough at high salinity levels above 8 ppt was able to distinguish the proposed model from other sorption models. This study promotes the understanding of the viral sorption with salinity and provides a useful model for coastal management of viral migration in saline coastal groundwater.     

Influence of the Grain-Size Distribution Curve of Quartz Sand on the Small Strain Shear Modulus Gmax

The paper presents a study of the influence of grain-size distribution curve on the small strain shear modulus Gmax of quartz sand with subangular grain shape. The results of 163 resonant column tests on 25 different grain-size distribution curves are presented. It is demonstrated for a constant void ratio that while Gmax is not influenced by variations in the mean grain-size d50 in the investigated range, it significantly decreases with increasing coefficient of uniformity Cu = d60/d10 of the grain-size distribution curve. Well-known empirical formulas (e.g., Hardin’s equation with its commonly used constants) may strongly overestimate the stiffness of well-graded soils. Based on the RC test results, correlations of the constants of Hardin’s equation with Cu have been developed. The predictions using Hardin’s equation and these correlations are in good accordance with the test data. Correlations of the frequently used shear modulus coefficient K2,max with Cu and empirical equations formulated in terms of relative density, are also given in the paper. A comparison of the predictions by the proposed empirical formulas with Gmax-data from the literature and a micromechanical explanation of the experimental results are provided. Correction factors for an application of the laboratory data to in situ conditions are also discussed.     

Visualizing Search and Solution Spaces in the Optimum Design of Biaxial Columns

Optimum detailing of reinforcement bars for biaxial columns is a nontrivial task and there is no guidance in the codes of practice on this issue. Conventional methods of producing a single design solution and the use of optimization methods to find a single global optimum solution is not what practicing engineers require. This paper introduces an innovative approach to design, which emphasizes the interactive use of evolutionary computing in design and a new approach to visualization of search and solution spaces. This replaces the “black box” nature of the optimization techniques with an interactive approach in which the designer is in full control of the process. Instead of locating a single solution, clusters of good designs are identified in color, allowing the designer to conduct a more focused search in the areas of high performance design solutions. This method enables the designer to view individual solutions in the vicinity of the local peaks in order to learn more about the interrelationship between design parameters. Interactive visualization tools enable designers to develop a deeper understanding of the search and solution spaces and assist them in evaluating the merits of alternative designs and in their decision making. A by-product of visualization is knowledge discovery, which allows the designer to discover novel solutions that would not have been possible by conventional means. This approach could be easily extended to any stage of multidisciplinary design, especially to the conceptual stage of the design process. For example, the position of bars within a column is similar to the position of columns in a floor layout optimization problem.     

Modeling of Corroded FRP-Wrapped Reinforced Concrete Columns in Axial Compression

This paper discusses the mechanical behavior of reinforced concrete columns wrapped with fiber-reinforced polymer (FRP) sheets. A numerical routine was developed to predict the behavior of the columns using a step-by-step technique. The routine is based on an existing model and was modified to account for confinement provided by the traditional steel as well as the external FRP wraps. Several empirical equations for the confined concrete were calibrated with results from experimental tests from different published papers. The most accurate equation was incorporated into the routine to predict the stress-strain relation of the column up to failure. A different confinement to the outer concrete cover and the inner core was used to account for the FRP wraps and the transverse steel. The model was calibrated with experimental results from different experiments on FRP-wrapped reinforced concrete columns.The model was taken one step further by using it to predict the behavior of reinforced concrete columns, with a combination of steel corrosion and CFRP wraps. The columns modeled were subjected to harsh corrosive environment over 44 months. The model successfully predicted the load deformation in both axial and circumferential directions in corroded and intact columns, both wrapped and unwrapped, with good accuracy. The analysis forms a solid foundation for accurate evaluation of the effect of corrosion and wrapping on reinforced concrete columns.     

金属及合金中铬的光度分析

论述了金属及合金中铬的光度分析方法新近发展状况。     

Biodegradation of Tetrachloroethene by Chitin Fermentation Products in a Continuous Flow Column System

The ability of chitin fermentation products to promote tetrachloroethene (PCE) reduction was evaluated in a continuous-flow column system to identify how different electron donors affect reductive dechlorination. Natural chitin fermentation products were initially used to support PCE reduction. Acetate (3.5?mM) was the dominant fermentation product, followed by propionate (0.1?mM), butyrate (0.1?mM), and hydrogen (100?nM). After chlorinated ethene concentration profiles reached pseudo steady state, the ability of individual fermentation products (acetate, acetate+propionate, propionate, or formate) to support PCE reduction was evaluated. None of the fermentation products tested stimulated dechlorination as well as the suite generated from chitin (kPCE = 6.9?day?1); however, acetate-stimulated PCE dechlorination the best (kPCE = 5.3?day?1), followed by formate (kPCE = 2.4?day?1), acetate+propionate (kPCE = 1.8?day?1), and propionate (kPCE = 1.2?day?1). Similar trends were observed for the PCE daughter products trichloroethene and dichloroethene. Free energies of individual fatty acid reactions were calculated and shown to be useful predictors of dechlorination performance, except for the case of acetate+propionate. Hence, acetate is the dominant fatty acid controlling dechlorination in the chitin-enhanced system, propionate appears to have an inhibitory effect when present with acetate alone, and other unidentified nutrients produced during chitin fermentation likely contribute to dechlorination activity as well.     

铬蓝黑R分光光度法测定钢中微量铬

研究了铬蓝黑Ｒ分光光度法测定钢中微量铬的方法。在ＨＡｃ－ＮａＡｃ弱酸性介质中,铬与铬蓝黑Ｒ形成蓝色络合物,最大吸收波长位于６００ｎｍ处,表现摩尔吸光系数ε＝２．１９×１０４,铬含量在０～６０μｇ／５０ｍｌ范围内符合比尔定律。方法用于钢中微量铬的测定,结果满意。     

邻苯二酚紫—溴化十六烷基三甲铵分光光度法测定钢中微量铬

本文研究了PV—CTMAB光度法测定铬.在HAc—NaAc缓冲介质中,显色配合物最大吸收位于650nm处,表观摩尔吸光系数为6.2×10~4.铬含量在0～14μg/25ml范围内符合比尔定律.本方法应用于钢中微量铬的测定,结果满意.     

Performance of Geosynthetic-Encased Stone Columns in Embankment Construction: Numerical Investigation

This paper presents the results of a numerical investigation into the performance of geosynthetic-encased stone columns (GESCs) installed in soft ground for embankment construction. A three-dimensional finite-element model was employed to carry out a parametric study on a number of governing factors such as the consistency of soft ground, the geosynthetic encasement length and stiffness, the embankment fill height, and the area replacement ratio. The results indicate among other things that additional confinement provided by the geosynthetic encasement increases the stiffness of the stone column and reduces the degree of embankment load transferred to the soft ground, thereby decreasing the overall settlement. It is also shown that the geosynthetic encasement has a greater impact for cases with larger stone column spacing and/or weaker soil. Also revealed is that unlike isolated column loading conditions, full encasement may be necessary to ensure maximum settlement reduction when implementing GESCs under an embankment loading condition. Practical implications of the findings are discussed in detail.     

Influence Zone for End Bearing of Piles in Sand

An attempt is made to establish an analytically based estimate of the influence zone surrounding the tip of a loaded pile in sand. In the framework of the cavity expansion theory and a confined local failure mechanism, explicit expressions are derived in which the sizes of the upward and downward influence zones are properly linked with the angle of shearing resistance, the stiffness, the volumetric strain, and the mean effective stress of the sand at the pile tip. Based on a series of parametric analyses, the mean range of the influence zone is suggested. For piles in clean sand, the influence zone above the pile tip is between 1.5 and 2.5D and the zone below the tip ranges from 3.5 to 5.5D, where D is pile diameter. For piles in more compressible silty sand, the influence zone extends between 0.5 and 1.5D above the pile tip and between 1.5 and 3D below the tip. Because of its analytical nature, the present study may provide a meaningful insight into the current empirical interpretations.     

Shaft Capacity of Open-Ended Piles in Sand

This paper presents the results from an experimental investigation designed to examine the effect of soil-core development and cyclic loading on the shaft resistance developed by open-ended piles in sand. An instrumented open-ended model pile was installed either by driving or jacking into an artificially-created loose sand deposit in Blessington, Ireland. The tests provided continuous measurements of the soil-core development and the radial effective stresses during installation and subsequent load tests. The equalized radial effective stresses developed at the pile-soil interface were seen to be dependent on the degree of soil displacement (plugging) experienced during installation, the distance from the pile toe, and the number of load cycles experienced by a soil element adjacent to the pile shaft. A new design method for estimating the shaft capacity of piles in sand is proposed and compared with measurements made on prototype field-scale piles.     

松辽盆地宋站地区沙河子组储层砂体特征与分布预测

为了有效预测宋站地区沙河子组有利储集层,依据岩心、测井、地震以及其他分析测试等资料,并结合前期沉积相研究的认识,对沙河子组储层岩石类型、物性、含气性以及地球物理响应特征进行了研究。研究区沙河子组储层岩石类型主要为砂砾岩,孔隙度一般小于10%,渗透率普遍小于1×10-3μm2,认为沙河子组砂砾岩为低孔—低渗型储层。研究发现:区内主要发育的辫状河三角洲和扇三角洲两种沉积相带均可作为有利储集相带,其发育的砂砾岩体普遍含气,且砂砾岩厚度越大,其产气能力越强,表明沉积环境和砂砾岩厚度是影响储层发育的关键因素。并通过地震反演等技术对研究区砂体厚度和平面展布进行预测,提出研究区有利储层分级划分方案。     

Instability Conditions of Loose Sand in Plane Strain

When a loose sand specimen is loaded under an undrained condition, it may become unstable. The instability conditions may be specified by an instability line determined using undrained tests. However, the instability behavior of sand has seldom been studied under plane-strain conditions. Experimental data obtained under both triaxial and plane-strain conditions are presented in this paper to define the instability conditions of loose sand under plane-strain conditions. Using the state parameter, a unified relationship can be established between the normalized slope of instability line and the state parameters for both axisymmetric and plane-strain conditions. Using this relationship, the instability conditions established under axisymmetric conditions can also be used for plane-strain conditions.