Biofilm Growth and Mineral Precipitation in Synthetic Leachate Columns

The results of a numerical model developed to describe the clogging of the leachate collection systems are compared with data from column experiments conducted with synthetic landfill leachate. A biofilm kinetic model is used to calculate the utilization of organic substrates (acetate and propionate) and growth of active clog material, and the substrate removal is then linked to mineral precipitation. Calculated changes in the chemical oxygen demand and Ca2+ of the effluent leachate and the porosity profiles along the length of the column show encouraging agreement with measured values. Means to obtain reasonable parameter values are presented. The favorable outcome represents completion of an essential first step toward being able to predict what controls clogging in landfill leachate collection systems.     

Particle Size and Clogging of Granular Media Permeated with Leachate

The effect of particle size (4-, 6-, and 15-mm nominal sizes) on the rate of clogging of columns of porous media permeated with municipal solid-waste leachate is examined. Clogging is shown to be more localized over a small volume of the porous media near the influent end of the column for smaller particles than for larger particles, where clogging was more uniformly distributed along the column. This is attributed to the greater surface area per unit volume of smaller particles allowing greater biofilm growth per unit volume. This increased the reduction in chemical oxygen demand (COD) and caused greater deposition of inorganic clog material per unit length of column than for larger particles. The distribution of methanogenic bacteria was found to closely correspond to the zones of most severe clogging. The bulk density of clog material is shown to be between 1.6 and 1.8 Mg∕m3. The chemical composition of the clog material is essentially independent of particle size, with calcium representing 26% of the dry mass of the clog material and CaCO3 being the main component of the clog. An examination of the yield of CaCO3 relative to COD indicates that the carbon in the CaCO3 represents <4% of the organic carbon represented by the drop in COD. Finally, the data from the column test is used to predict the expected time to clog for an actual landfill and were found to give results consistent with what was observed in the field.     

Leachate Recirculation in Bioreactor Landfills Using Geocomposite Drainage Material

The key purpose of this study was to test the use of a permeable blanket made up of a geocomposite drainage layer (GDL) for leachate recirculation in municipal solid waste (MSW) landfills and to predict the observed leachate travel in the blanket using a numerical model. A 34?m long by 12?m wide permeable blanket made up of GDL was constructed at an active MSW landfill located in Michigan. Leachate was injected in the GDL using a perforated pipe placed centrally above the GDL along its length. Moisture content sensors, pressure transducers, thermistors, thermocouple sensors, and a vertical load sensor were embedded immediately below the GDL blanket to monitor the flow of injected leachate. After the blanket was covered with waste, leachate was injected into the blanket at rates ranging from 0.9 to 2.6?m3/h per meter length of the blanket. Data collected from the embedded sensors indicated that the injected leachate traveled at rates ranging from 5 to 18?m/h through the blanket depending upon the leachate injection rate. Only pressure transducers and thermistors were consistently able to detect migration of injected leachate once the blanket got saturated. Moisture content sensors could not register any change in readings once the blanket became saturated. Leachate injection pressure monitored over a period of about 12 months indicated no signs of clogging of the blanket. The leachate pressures measured immediately below the blanket were less than the net leachate injection pressure indicting that there was a head loss in the GDL blanket. Numerical modeling of liquid flow in the blanket indicated that predicted leachate travel in the blanket was consistent with the field data for assumed values of the waste hydraulic conductivity. In the absence of measured representative hydraulic properties of the waste, absolute verification of the field data was not possible.     

Biogeochemical Evaluation of Mechanisms Controlling CaCO3(s) Precipitation in Landfill Leachate-Collection Systems

A common failure mode for landfills is clogging of the leachate-collection system. The reduction in hydraulic conductivity associated with clogging causes a buildup of leachate head on the underlying liner, potentially increasing advective contaminant transport from the landfill and contaminating adjacent groundwater. In this paper, the biogeochemical model CCBATCH is used to link a primary cause of leachate collection system failure—CaCO3(s) precipitation?to anaerobic degradation of volatile fatty acids (VFAs) in column reactors used to study the clogging phenomena. One key to applying CCBATCH correctly was dividing the VFA conversion into two steps: conversion of propionate to acetate, carbonic acid, and methane; and acetate conversion to methane and carbonic acid. The primary driver for CaCO3(s) precipitation in the columns was acetate fermentation to CH4 and H2CO3, which increased the total carbonate concentration in the leachate and shifted the acid/base control to a weaker acid system, which caused an increase in solution pH. A second key to proper modeling was adding CO2(g) gas transfer to CCBATCH. The modeling results indicate that the kinetics of CO2(g) gas transfer was a key control over leachate chemistry once acetate fermentation was nearly complete. These results suggest that the best approach for the long-term control of CaCO3(s) clogging may be to enhance CO2(g) gas transfer from the leachate while buffering the leachate pH to near neutral. Taken together, these actions should decrease the yield of CaCO3(s) precipitated per mass of acetate removed.     

Effect of Gas on Pore Pressures in Wet Landfills

The waste in a landfill may become saturated due to many reasons, including leachate recirculation or extreme precipitation. As high saturation levels in waste are achieved, the permeability of the waste to landfill gas decreases. This may result in pore pressures that are greater than what would be predicted by fluid statics. A theoretical model for estimating the excess pore pressure at the bottom of saturated waste is derived. A finite difference procedure is then presented as an approximate solution to the model. It was found that below the level of saturation, the steady-state excess pore pressure distribution increases linearly similar to a hydrostatic distribution. Combining its effect with the static water pressure, the excess pore pressure may be accounted for by using an equivalent unit weight of fluid that is artificially higher than water. A parametric study of the input parameters showed that the equivalent unit weight of the pore fluid was highly dependent on the hydraulic conductivity of the waste, particularly if the hydraulic conductivity of the waste is lower than about 2×10?6?m/s.     

Nutrient Retention in Vegetated and Nonvegetated Bioretention Mesocosms

Thirty well-established 240L bioretention mesocosms were used to investigate retention of dissolved nutrients by bioretention systems. Ten mesocosms were comprised of 80?cm sandy loam, ten of 80?cm loamy sand, and ten of pea gravel with 20?cm of loamy sand. Half were vegetated with shrubs/grasses, while the other half had no vegetation (barren). In the first part of our study, the loam and sand mesocosms were dosed with synthetic storm water comprising 0.8?mg?L?1 total phosphorus (TP) and 4.8?mg?L?1 total nitrogen (TN). TP retention in the vegetated loam was 91% compared to 73% in the barren, and TN retention was 81% compared to 41% in the barren loam. TP retention was 86–88% in the sand treatments, while TN retention in the vegetated sand was 64%, compared to 30% in the barren. In the second part of our study, all 30 mesocosms were loaded weekly with 45?cm of tertiary effluent with high nutrient loads (22.3?m?year?1 hydraulic load at a flow-weighted average of 4.5?mg?L?1 TP and 4.8?mg?L?1 TN, or 1,012?kg?ha?1?year?1 TP and 1,073?kg?ha?1?year?1 TN). After 50 weeks of loading, cumulative TP retention was 92% in the vegetated loam, 67% in the sand, and 44% in the vegetated gravel. However, TP retention by barren media was 56% in the loam, 39% in the sand, and 14% in the gravel. Cumulative TN retention was 76% in the vegetated loam, 51% in the sand, and 40% in the vegetated gravel. In contrast, maximum TN removal by barren media was 18% in the loam. The increase in TP retention by vegetated systems substantially exceeds phosphorus uptake rates for plants, suggesting that other processes are involved. The increase in TN retention by vegetated systems also exceeds nitrogen uptake rates for plants, suggesting that denitrification is involved.     

Sorbent Wicking Device for Sampling Hydrophobic Organic Compounds in Unsaturated Soil Pore Water. I: Design and Hydraulic Characteristics

The hydraulic characteristics of horizontally installed sorbent wick sampling devices were evaluated through wick tracer studies and laboratory soil column experiments to assess the influence of horizontal wick length and sampler interface design on sampling pore water in unsaturated soils. The nominal sampler design consisted of a cylindrical porous metal interface packed with granular-activated carbon encapsulating the end of a fiberglass wick that extended 100 cm horizontally from the interface before dropping 100 cm vertically to a collection vessel. The maximum sampling rate of horizontally installed wick systems declines exponentially with increasing horizontal wick length, while the vertical length influences the range of soil–water pressures that may be sampled. The nominal design sampled pore water from clay loam laboratory columns at 8 to 14 mL?h?1 under steady-state infiltration conditions and 2 to 5 mL?h?1 under draining conditions across a ?10 to ?45 cm H2O soil–water pressure range. Sampling rates in medium-grained sand under similar flow conditions were less than that of the clay loam due to reduced water content and reduced interface/soil contact area. An analysis of observed sampling velocities versus calculated soil water contents and hydraulic conductivities indicated that the design performs best when the soil water content is greater than 0.15 and unsaturated hydraulic conductivity is greater than 0.2 cm?h?1. A hydraulic model was developed that estimates the sampling velocity of the nominal design based on sampler interface pressure, which was linearly correlated with soil pressure.     

Vortex characteristics due to nozzle clogging in water caster mould: modelling and validation

In continuous slab casting, clogging in the submerged entry nozzle (SEN) ports leads to flow asymmetry and vortex formation in the mould. Knowledge of vortexing and its influence on product quality is fundamental for defect-free production. In this study, the interconnected effects of nozzle clogging and SEN submergence depth, variation on flow asymmetry and vortex characteristics in a 0.4 scale water caster have been characterised by CFD investigation and validated with experimental results from the authors’ previous work. Mean flow velocities at the sub-meniscus and near the port exit predicted by the computational model are compared with the time-averaged values of the impeller probe velocity measurements and found to be in reasonable agreement. Three different clogging conditions (0, 33 and 66% in the left port of the SEN) for SEN submergence depth of 60?mm are studied and the 66% clogging produced vortices having largest diameter, which is consistent with the experimental observations. The effects of SEN submergence depth on flow asymmetry and vortexing are investigated with three different conditions – 40, 60 and 80?mm. It is found that the shallow SEN submergence depth (40?mm) produces vortices of largest diameter and the flow is most stable for a SEN submergence depth of 60?mm among the three cases. Vortex bending towards the SEN as noticed in the experimental observations is also observed in the computational study. This work illustrates the possibility of capturing features of vortexing using validated CFD model.     

Field Evaluation of a Leachate Collection System Constructed with Scrap Tires

Landfilling costs and the potential uses of scrap tires have prompted researchers to investigate beneficial reuses. One important application is the use of tire chips as a leachate collection material in municipal solid waste landfills. Laboratory and field studies were conducted to investigate the performance of tire chips as a drainage medium in landfills. The laboratory portion of the program included a series of hydraulic conductivity and compressibility tests. Two field test cells, one with tire chips and another with gravel as the control, were constructed. The tire-chip cell was instrumented with flowmeters, thermistors, and gas collection devices to evaluate the hydraulic performance as well as the potential for spontaneous combustion. Leachate collected from the two cells was analyzed to determine if tire chips would potentially contaminate the groundwater. The results indicated that adequate drainage conditions were present within the tire-chip layer. The presence of insignificant quantities of carbon monoxide, and the lack of oxygen, and recorded low temperatures suggested that a combustion hazard was not present. The field leachate data indicated that tire chips can be safely used as part of a landfill leachate collection layer, even though it may not be suitable to place them near drinking water sources.     

Operation and Performance of Horizontal Wells for Leachate Control in a Waste Landfill

This paper presents data on gas and leachate flow rates and leachate levels obtained during a 600?day pumping trial on three retrofitted horizontal wells in a domestic waste landfill at Rainham, United Kingdom. The changes in gas and leachate flow rate with time and atmospheric pressure, and the interaction between the two flows, are discussed. The spatial variability of the response of the leachate levels within the landfill is explored with reference to the anisotropy and heterogeneity of the permeability of the waste. It is shown that horizontal wells can be an effective means of controlling leachate heads near the base of a landfill, and that leachate levels must be measured using piezometers with a discrete response zone rather than fully screened observation wells if meaningful results are to be obtained. It is argued that the large amounts of gas pumped from the nominally saturated zone of the landfill must have come from the ongoing degradation of the waste within the zone of influence of the well.     

Simulation of Construction and Demolition Waste Leachate

Solid waste produced from construction and demolition (C&D) activities is typically disposed of in unlined landfills. Knowledge of C&D debris landfill leachate is limited in comparison to other types of wastes. A laboratory study was performed to examine leachate resulting from simulated rainfall infiltrating a mixed C&D waste stream consisting of common construction materials (e.g., concrete, wood, drywall). Lysimeters (leaching columns) filled with the mixed C&D waste were operated under flooded and unsaturated conditions. Leachate constituent concentrations in the leachate from specific waste components were also examined. Leachate samples were collected and analyzed for a number of conventional water quality parameters including pH, alkalinity, total organic carbon, total dissolved solids, and sulfate. In experiments with the mixed C&D waste, high concentrations of total dissolved solids (TDS) and sulfate were detected in the leachate. C&D leachates produced as a result of unsaturated conditions exhibited TDS concentrations in the range of 570–2,200 mg∕L. The major contributor to the TDS was sulfate, which ranged in concentration between 280 and 930 mg∕L. The concentrations of sulfate in the leachate exceeded the sulfate secondary drinking water standard of 250 mg∕L. The leachate produced from lysimeters exposed to conditions of constant flooding possessed a greater concentration of dissolved constituents than leachate from the unsaturated lysimeters. The sulfate concentration ranged from 950 to 1100 mg∕L. In both scenarios, the primary ions contributing to the dissolved solids were sulfate and calcium, both a result of gypsum drywall. The flooded lysimeters remained at a constant pH of 11 throughout the experiment, whereas the pH dropped to neutral conditions (pH 6–7) in the unsaturated columns after 1 month of the leaching experiment. The high pH of leachate from the flooded columns was attributed to concrete. Based on leaching tests on individual waste components, wood and cardboard were the primary materials contributing to dissolved organic carbon. The organic carbon concentrations in the leachate were generally lower than typical municipal waste leachate. The biological conversion of sulfate to sulfide was evident in many columns, and was most pronounced in the unsaturated, mixed-waste columns.     

稳定流与非稳定流情况下非饱和—饱和区域重金属运移模拟

为定量研究稳定流与非稳定流情况下非饱和带—饱和区域内水流及溶质迁移过程,在非饱和带及饱和区域流动理论及溶质运移理论基础上,建立了非饱和带—饱和区域内水流方程与污染物运移模型,采用Van Genuchten-Mualem模型来描述相对渗透率函数,基于Toughreact软件对模型进行模拟计算并给出模拟结果。模拟结果得出:在稳定流情况下,非饱和带及饱和区域流速均逐渐减小,最后趋于稳定;而非稳定流情况下,流速初始阶段逐渐减小,后期随着各季度降水量的不同流速大体呈周期性变化。在总降水量相同及其他条件相同的情况下,非稳定流相对于稳定流对溶质运移存在促进作用,即其运移速率更快,但影响尺度不大;随着时间的推移,污染物将透过包气带进入地下水中,其中钠离子最快,六价铬次之,铀出现最晚。     

Fe-Si-B非晶合金制备过程中的水口结瘤分析

利用扫描电镜和能谱仪对采用不同坩埚制备Fe-Si-B非晶合金薄带过程中的水口结瘤物进行了物相分析,研究了水口结瘤物的来源及形成机制.通过热力学计算,验证了钢液中Al、Si氧化物夹杂产生的必然性.结果表明:水口结瘤中的物相主要有Al、Si、Mg的氧化物,以及少量Na、Fe、B等元素的氧化物.结瘤物中存在的MgO来源于坩埚材料,可通过选择适当的坩埚材质来避免.     

Probabilistic Unsaturated Flow along the Textural Interface in Three Capillary Barrier Models

The probabilistic flow within capillary barrier models is evaluated by coupling a first-order reliability (probabilistic) model to a variably saturated flow model. The objective is to determine the most significant uncertain variable to probabilistic flow and the effect of different textural combinations. The model inputs include the mean and standard deviation of each uncertain variable. The van Genuchten curve-fitting model for unsaturated flow is used. The uncertain flow variables are saturated water content, residual water content, saturated hydraulic conductivity, and model parameters alpha (α) and n. A clay loam soil sample overlying a loamy sand sample, a clay loam sample overlying a sand sample, and a loamy sand sample overlying a sand sample are used to represent finer soils overlying coarser soil. Evaluations near the textural interface show that model parameters alpha (α) and n=most significant uncertain variables. These are related to the water entry pressure and the width of pore size distribution of the soils. Generally, soils that have the least uniform pore size distribution are the most effective capillary barriers. In this study, the clay loam overlying loamy sand satisfied this condition and performed better than the other soil combinations.     

Urban Particle Capture in Bioretention Media. I: Laboratory and Field Studies

Bioretention is a novel stormwater best-management practice that uses a mixture of soil/sand/mulch as adsorptive filtration media that can capture both urban particulates and dissolved pollutants while promoting infiltration. This study conducted a series of laboratory column experiments and field observations, which showed that: (1) bioretention media stratification occurs with runoff percolation due to particulate deposition; (2) bioretention filter media are clogging limited, instead of breakthrough limited; and (3) both depth filtration and cake filtration significantly contribute to urban particle capture. Because of the fine size of bioretention media, incoming suspended solids cannot significantly penetrate below 5–10?cm of the media in the column tests and approximately 20?cm in the monitored field facility. Bioretention filters under intermittent flow conditions exhibited higher solids loading capacity (in kg/m2) before clogging than under continuous flow conditions. The clay components in incoming total suspended solids assume critical responsibility for bioretention media clogging. The media resistance due to solids deposition was estimated through Darcy’s law. The hydraulic conductivity of two media types decreased from 54±23 and 72±46?cm/h to less than 10?cm/h due to particle capture. Experimental results suggest that a 20-cm media depth is sufficient for bioretention design and maintenance procedures (media replacement) for runoff particle capture.     

Functional Relationship to Describe Drains with Entrance Resistance

Numerical flow models usually represent drains as a system dependent boundary condition. If soil is saturated, drains act as the Dirichlet boundary condition with pressure head set equal to zero, and if soil is unsaturated, drains act as the Neumann boundary condition with flow set equal to zero. The underlying assumption is that drains exhibit ideal behavior. In reality, however, this is generally not so, and the flow encounters additional resistances due to pipe slotting and clogging of the envelope material around the drains. To account for the resulting resistance, a Hooghoudt-type boundary condition was developed that prescribes drain flow in relation to the groundwater level at a reference position. The measured drain discharge in an old drainage system was compared with calculated discharge assuming an ideal drain. It was found that the ideal drain assumption led to large errors in simulated discharge. With a correctly formulated and calibrated Hooghoudt boundary condition, however, more accurate drain discharges were obtained.     

49MnVS非调质钢水口堵塞原因分析

 非调质钢中含有较多的铝和硫,常在连铸过程中因水口蓄积钙铝酸盐或硫化钙而发生堵塞,影响生产的顺利进行。为了探究非调质钢的水口堵塞原因并提出预防措施,采用X射线荧光分析、X射线衍射分析和热力学计算,分析了非调质钢49MnVS的水口堵塞结瘤物。结果表明,水口堵塞物主要由CaO·6Al2O3、CaO·Al2O3、MgO·Al2O3和金属铁相组成,生产中钙的收得率仅为预期的53.33%;热力学计算表明,水口堵塞的原因是钢液中的钙铝质量比较低,生成了大量高熔点的CaO·6Al2O3和CaO·Al2O3夹杂物,夹杂物在水口处黏附聚集;同时,由于MgO-C质水口耐火材料被侵蚀,MgO在絮流区与Al2O3碰撞生成MgO·Al2O3,铁液滴在夹杂物孔隙中凝固黏结,使堵塞物进一步长大,将水口完全堵塞。因此,可以通过优化喂线操作、控制钢中的钙铝比位于0.127～0.225、改进水口材质等措施来预防水口堵塞。     

Shear Strength and Shear-Induced Volume Change Behavior of Unsaturated Soils from a Triaxial Test Program

A series of unsaturated soil triaxial tests were performed on four soils including sand, silt, and a low plasticity clay. Attempts were made to correlate unsaturated soil properties from these tests and data from the literature with soil-water characteristics curve (SWCC), soil gradation, and saturated soil properties. The feasibility of estimating unsaturated soil property functions from saturated soil properties, SWCCs and gradation data, is demonstrated. A hyperbolic model for estimation of the unsaturated soil parameter, ?b, versus matric suction is presented. Shear induced volume change behavior was also studied, and results are included in this paper. Although not correlated with soil index properties, these shear-induced volume change data are important to complete stress-deformation analyses, and represent a significant addition to the existing data base of unsaturated soil properties.     

Performance of North American Bioreactor Landfills. II: Chemical and Biological Characteristics

The objective of this research was to examine the performance of five North American bioreactor landfills. This paper represents the second of a two-part series and addresses biological and chemical aspects of bioreactor performance including gas production and management, and leachate chemistry. The data support accelerated methane generation at several landfills (k = 0.08–0.21?1/year) relative to the AP-42 default decay rate (k = 0.04?1/year). While the data indicate that gas collection increases at bioreactor landfills, a general relationship between decay rate and moisture added or wet weight water content could not be identified. There was no indication that gas collection increases appreciably when the water content reaches 40%. Most of the leachates at the landfills in this study were commingled from cells operating as a bioreactor and conventionally. Nevertheless, trends in pH and BOD:COD in the bioreactor leachates were consistent with the impacts of enhanced biological activity. Ammonia concentrations also increased over time but remained below levels reported to be inhibitory. For both heavy metals and speciated organic chemicals, there was no indication that bioreactor landfill leachate is significantly different from leachate generated at conventional landfills.     

Electrokinetic Remediation Using Surfactant-Coated Ceramic Casings

Electrokinetic remediation is an emerging technique that can be used to remove metals from saturated or unsaturated soils. In unsaturated soils, control of the medium's water content is essential. Previously used electrode designs have caused detrimental soil wetting due to excess electroosmotic flow out of ceramic-encased anodes. We tested a method to reverse the electroosmotic flow at the anode by treating the ceramic casing with the cationic surfactant hexadecyltrimethylammonium (HDTMA). Laboratory tests showed the untreated ceramic had an electroosmotic permeability of 2.4 × 10?5 cm2 V?1 s?1. Ceramic treated with HDTMA had an electroosmotic permeability of ?1.3 × 10?5 cm2 V?1 s?1. Under an applied electric potential, electroosmotic flow was reversed in the HDTMA-treated ceramic, indicating a reversed zeta potential due to formation of an HDTMA bilayer on the ceramic surface. Field tests conducted over a 6-month period showed negligible water loss from HDTMA-treated ceramic (0.03 L h?1) compared to untreated ceramics (up to 6 L h?1). The results indicated that a surfactant treatment to the anode ceramic casing can greatly improve the application of electrokinetics in unsaturated environments.