Geotechnical and Geoenvironmental

Coupled Flow-Mechanical Modeling of Bioreactor Landfills

Hanumanth Kulkarni, PhD Candidate, Advisor: Dr. Krishna Reddy

Currently, there are more than three thousand landfills in the United States in which approximately 55% of the municipal solid waste (MSW) generated is being disposed. The majority of these landfills are designed on the premise that liquid generation should be minimized through the design of liner, leachate collection and removal, and final cover systems. Recently, “bioreactor”landfills are being designed on the contrasting premise that leachate recycling, water addition, and other operating strategies should induce an enhanced environment for biochemical degradation of MSW. Compared to conventional “dry tomb” landfills, a bioreactor landfill rapidly stabilizes MSW within a short period into a form where potential contaminant leaching is minimized and environmental effects are reduced. The material that remains in the landfill after stabilization consists of non-biodegradable waste (metal, plastic, glass) as well as residual biodegradable materials. During the process of landfill stabilization, waste mass is lost, and the landfill mass will settle, decreasing volume. Bioreactor landfills are receiving a great deal of attention from environmental professionals because they offer a sustainable way to achieve increased waste degradation along with benefits such as reduction in leachate pollution potential and rapid increase in landfill volumetric capacity. They also offer significant reductions in post-closure management as a result of the reduced period for landfill leachate and gas generation and improved potential for more rapid land reuse.

The changing moisture contents and associated enhanced degradation during leachate recirculation influence the geotechnical engineering properties and hence the viability of various bioreactor designs. Inadequate designs have led to failure of several bioreactor landfills in the past. Therefore, a comprehensive research program has been undertaken at the University of Illinois at Chicago to measure, model, and evaluate the geotechnical stability of anaerobic bioreactor landfills through controlled laboratory experiments, cooperative investigations with ongoing field scale projects, and the development of field-validated coupled flow-mechanical model with accelerated decomposition rates under anaerobic conditions. The specific objectives of the research program are: (1) to characterize the composition and engineering properties of waste at various stages of anaerobic decomposition; (2) to investigate various geophysical methods to monitor spatial and temporal variation in moisture content and/or changes in engineering properties; and (3) to investigate the interrelated effects of leachate recirculation and the geotechnical stability through coupled fluid flow-mechanical modeling. Ultimately, the laboratory and modeling results from this project are to be validated with field monitoring and performance data obtained at several bioreactor landfills, and then develop practical guidelines for the design, construction and monitoring of bioreactor landfills.

To date, we have characterized the changes in composition and geotechnical properties of MSW due to enhanced degradation under leachate recirculation operations. Both field and laboratory research is conducted to accomplish the following: (1) investigate moisture distribution and changes in composition and biochemical characteristics of MSW subjected to leachate recirculation at Orchard Hills landfill located in Davis Junction (Illinois, USA); (2) investigate changes in geotechnical properties of MSW due to leachate recirculation at Orchard Hills landfill; (3) investigate degradation behavior and changes in geotechnical properties of MSW at different levels of degradation during leachate recirculation in controlled laboratory bioreactors; and (4) investigate degradation behavior and changes in geotechnical properties of synthetic MSW with specific controlled composition at different levels of degradation in controlled laboratory bioreactors. We have also evaluated different geophysical methods to determine spatial and/or temporal variation of moisture content and/or engineering properties or indicators that provide indirect information on engineering properties. The geophysical methods evaluated included electrical resistivity tomography, electromagnetic conductivity surveys, ground penetrating radar, well logging (using electromagnetic conductivity and natural gamma probes), and seismic surveys. All of these tests were performed at the Orchard Hills landfill.

Currently, we are developing a coupled flow-mechanical model to investigate the implications of leachate recirculation in bioreactor landfill operations on slope stability and settlement of MSW. As a part of this effort, a constitutive model describing the stress-strain behavior of MSW is developed. We are in the process of validating the model based on the field monitoring data at different bioreactor landfill sites. Ultimately, this research will provide general design guidance to the practicing engineers to develop safe and effective bioreactor landfills.

This project is funded by the National Science Foundation through a grant (#xxxxx), Environmental Research and Education Foundation and CREED.

Remediation of Contaminated Subsurface using Nanoscale Iron Particles

Kenneth Darko-Kagya, PhD Candidate, Advisor: Professor Krishna Reddy

Through the accidental or improper release of toxic chemicals (such as chlorinated organic pollutants), numerous sites have become contaminated. Lately, success has been achieved by using iron filings (zerovalent iron, Fe0) as a reactive material in permeable reactive barriers to remediate such contaminants in groundwater. Nanoscale zerovalent iron (nZVI) particles have the potential to be superior to iron filings, both in terms of initial rates of reduction and total moles of contaminants reduced per mole of iron. Instead of waiting for the contaminants to pass through the permeable reactive barriers, the nZVI particles can be injected into the contaminated source zones for rapid and effective detoxification of the contaminants. However, the heterogeneous subsurface can seriously diminish the effective distribution/delivery of nZVI particles. Recently held USEPA-NSF workshop concluded that the reactivity of the nZVI particles has been well documented, but there is only sparse data on the fate and transport of nZVI particles in different subsurface conditions. The delivery of nZVI particles into the contaminated zones uniformly and in required amounts in a controlled manner is essential for effective remediation.

Our research objective is to investigate transport and reactivity of nZVI particles in different geologic media and then identify promising strategies for the enhancement of transport of nZVI particles under different subsurface conditions. Our hypotheses are that: (1) as a result of aggregation, nZVI particles can be transported only to limited distances in subsurface; and (2) enhancement strategies such as use of dispersants, pressurized and/or electrokinetic systems have potential to enhance transport of nZVI particles in subsurface.

To date, we have investigated the reactivity of nZVI particles with a variety of organic pollutants such as PCP and DNT and found out that nZVI particles possess unique characteristics to effectively transform the toxic organic compounds into nontoxic form. However, nZVI particles agglomerate and settle quickly, thus limiting their application for in-situ remediation of any polluted site. We have investigated different dispersants to overcome this problem and found that green compounds such as aluminum lactate can effectively retard agglomeration and settlement of nZVI and allow them to be transported in porous media such as soils. We have quantified the real-time transport of nZVI particles in different soils using a magnetic susceptibility sensor system. The unique aspect of our research is quantifying the changes in reactivity of nZVI particles as they are being transported in soils. The nZVI particles undergo complicated geochemical interactions when they are transported in complex geologic settings, resulting in reduced reactivity. With the help of SEM and TEM images, we are able to understand the changes in morphology of the nZVI particles.

Currently, methods to enhance transport of nZVI particles while maintaining their reactivity are being investigated. Ultimately, this research will lead to a remediation technology that can help cleanup polluted soils and groundwater at so many sites across the country and worldwide in an effective and sustainable manner.

This research is funded by the National Science Foundation through a grant (# xxxx) with Professor Krishna Reddy as PI and Professor Amid Khodadoust as Co-PI.

Topic revision: r3 - 2010-08-30 - 16:50:16 - Main.tmatthes
 
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