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The purpose of this study is to compare different approaches to modeling the reductive dechlorination of chlorinated ethenes in the anaerobic region of an upward flow constructed wetland by microbial consortia. A controlled simulation experiment that compares three different approaches to modeling the degradation of chlorinated ethenes in wetland environments is conducted and investigates how each of the modeling approaches affect simulation results. Concepts like microbial growth in the form of a biofilm and spatially varying contaminant concentrations bring the validity of the CSTR assumption into question. These concepts are incorporated into the different modeling approaches to evaluate the CSTR assumption. Model simulations show that spatially varying contaminant concentrations have a significant effect on contaminant effluent concentrations. Additionally, the significance of the incorporation of a biofilm concept depends on the time characteristics of both diffusive mass transport and reaction kinetics.
The purpose of this study is to gain understanding of the dynamics of the processes that degrade Perchloroethene (PCE) to ethylene, or carbon dioxide (Co2), within the confines of a constructed wetland. A system dynamics approach will be used. This model is focused on identifying and optimizing the naturally occurring processes in stratified wetland sediment that reduce mass, toxicity, mobility, volume or concentration of contaminants in groundwater. Contaminant fate and transport within a wetland system is extremely complex and the mechanisms that drive wetland behavior are dynamic. Confidence in the model was built through verification and testing. Reasonable behavior resulted from a reasonable range of parameter values. The structure of the model represents the mechanisms and their interactions of an actual wetland system. This study provides a fundamental understanding of contaminant fate and transport in a constructed wetland and gives some insight for implementation. Testing identified specific parameters of typical wetland plant species, which could be optimized to have the most effect on contaminant fate. These parameters were the radius of aerobic influence and the number of roots per square meter. A remediation manager can use this model to explore system behavior by controlling or optimizing specific parameters to better manage contaminant fate and transport in a constructed wetland, saving time and resources.
The purpose of this study is to gain understanding of the dynamics of the processes that degrade Perchloroethene (PCE) to ethene, within the confines of the methanogenic zone of a constructed wetland. A system dynamics modeling approach is used. This model is focused on determining conditions that will enhance contaminant degradation. The chemical and biological processes within the methanogenic zone of a wetland system are extremely complex and dynamic processes. The model is broken up into three simultaneous processes: dechlorination, methanogenesis, and fermentation. The system behavior of the methanogenic zone can be adequately described by the classical formulations of representative microbial reactions acting simultaneously within each process in response to substrate limitation. The zone is assumed to be homogeneous and well mixed. This study provides a fundamental understanding of the complex interactions within the methanogenic zone of a constructed wetland and gives some insight for implementation. Testing identified flow rate, hydrogen concentration, and initial PCE biomass as specific parameters, which could be optimized to have the most effect on contaminant fate.
The purpose of this study is to gain understanding of the dynamics of the processes that degrade Perchloroethene (PCE) to ethene, within the confines of the methanogenic zone of a constructed wetland. A system dynamics modeling approach is used. This model is focused on determining conditions that will enhance contaminant degradation. The chemical and biological processes within the methanogenic zone of a wetland system are extremely complex and dynamic processes. The model is broken up into three simultaneous processes: dechlorination, methanogenesis, and fermentation. The system behavior of the methanogenic zone can be adequately described by the classical formulations of representative microbial reactions acting simultaneously within each process in response to substrate limitation. The zone is assumed to be homogeneous and well mixed. This study provides a fundamental understanding of the complex interactions within the methanogenic zone of a constructed wetland and gives some insight for implementation. Testing identified flow rate, hydrogen concentration, and initial PCE biomass as specific parameters, which could be optimized to have the most effect on contaminant fate.
With a sharp focus on environmental pollution and its impact on life and nature, scientists and engineers have studied the water treatment effect of natural wetlands for many years, resulting in the development of constructed wetlands (CWs) for treating wastewater. This informative new book provides current information and guidance on the construction, performance, operation, and maintenance of subsurface flow constructed wetlands of domestic and municipal wastewater. The focus of the volume is to evaluate the performance of horizontal subsurface flow constructed wetlands in treating domestic wastewater to establish the limit that can be safely discharged to agricultural drains. Two-step procedures were used for the preparation of this book. Using modeling and statistical analyses of treated water samples showed a significant difference between different media for the treatment of most pollutants. The authors went on to design artificial neural network models (ANNs) using Matlab software to simulate some of the experimental data and to anticipate the parameters of output concentration. The wetland systems have the ability to deal with various pollutants with different concentrations and to decrease the treated water to the standard limits. This volume presents the main role of emergent plants for treatment performance in the constructed wetlands and will be a very important resource for engineers in this field as well as for both undergraduate and graduate students.
Chlorinated ethenes are among the most prevalent groundwater contaminants at hazardous waste sites nationwide. In an attempt to manage the risks posed by these contaminants, while controlling costs, monitored natural attenuation (MNA) is being considered as a remediation strategy at many sites. MNA relies on naturally occurring physical, chemical, and biological processes in the subsurface to reduce the risk posed by the contamination. The implementation of MNA, however, requires a detailed understanding of these processes, and how they impact contamination at a particular site. One way to gain this understanding is through contaminant fate and transport modeling. In this study, a deterministic model that includes relevant fate and transport processes was applied to a chlorinated ethene-contaminated field site, at which spatial and temporal data had been collected. Parameters used for model input were obtained from the literature, experimental data, and by calibrating the model using concentration data from 1993. The model was then run in a predictive mode, and simulation results were compared to field data from 1999. Model performance was measured by comparison of observed and simulated concentration contour plots and evaluation of goodness-of-fit statistics. Over the six years the model was run in a predictive mode, the model was found to predict contaminant concentrations reasonably well for the three contaminants that were monitored.
Chlorinated ethene's physical properties as well as its ubiquitous state at DOD installations makes it a priority for innovative remediation efforts. Current techniques are expensive and time consuming to maintain. Constructed wetlands suggest an inexpensive and operational alternative to conventional technologies. Sub-surface flow wetlands provide the anaerobic zones necessary to reduce the recalcitrant chlorinated solvents prior to anaerobic or aerobic mineralization of its daughter products. A vertical flow cell to include sequential sedimentary layers of two hydric soil lifts and a mix of hydric soil and woody compost was the subject of this investigation. This study focused on the statistical significance among the three constructed strata. Concentrations of mono-carboxylic acids and other anions are indicators of the reductive conditions necessary for remediation. Acid anion concentrations were expected to be higher in the assumed anaerobic strata of the constructed cell as a result of the fermentation of humic substances. Decreases in sulfate and nitrate were also expected over the upward flowing, wetland profile due to the reductive, anoxic conditions. Evidence in this study validate these assumptions and suggest that constructed wetlands are a viable alternative to current remediation methods. Findings also suggest manipulation of the physical parameters such as strata depth, soil type, flow rate, etc of a wetland could increase the cell's remediation effectiveness.
​This volume provides a review of the past 10 to 15 years of intensive research, development and demonstrations that have been on the forefront of developing bioaugmentation into a viable remedial technology. This volume provides both a primer on the basic microbial processes involved in bioaugmentation, as well as a thorough summary of the methodology for implementing the technology. This reference volume will serve as a valuable resource for environmental remediation professionals who seek to understand, evaluate, and implement bioaugmentation.