Mechanics of Compressibility and Strength of Solid Waste in Bioreactor Landfills

HOSSAIN, MD. SAHADAT. Mechanics of Compressibility and Strength of Solid Waste in Bioreactor Landfills (Under the direction of Prof. Mohammed A. Gabr) Bioreactor landfills are operated to enhance refuse decomposition, gas production, and waste stabilization. A major aspect of bioreactor landfill operation is the recirculation of collected leachate back through the refuse mass. While there are significant economic advantages to the operation of landfills as a bioreactor, our understanding of the mechanics governing accelerated waste degradation and its impact on waste geotechnical properties is limited. As such, there is a need to explain and quantify such impact on compressibility and shear strength parameters; these parameters are needed for the three design phases of landfill construction, operation, and post-closure. The overall objective of the research was to develop an understanding of change in refuse compressibility and strength during accelerated waste decomposition in landfills operated as bioreactors. An experimental program was performed to provide data on parameters describing MSW compressibility and strength properties as a function of the state of decomposition, gas generation, and physical characteristics of waste particles. The research links the measured parameters to the physical and biological changes that take place as waste decomposition is accelerated. The research provides data on the significance of using relatively small equipment and shredded waste relative to fieldestimated properties. The research develops a model of waste settlement that considers the effect of high moisture content and time-dependent property changes on waste compressibility. This research also develops a model for shear stress-displacement behavior of MSW in bioreactor landfills. Refuse samples representing various stages of decomposition, from fresh refuse to well decomposed refuse, were generated in laboratory-scale reactors that were operated under conditions designed to simulate decomposition in both traditional and bioreactor landfills. The reactors were destructively sampled to obtain refuse at various states of decomposition, based on the reactor’s methane production rate curve. In addition, the state of decomposition was quantified by measurement of the concentrations of cellulose (C), hemicellulose (H), and lignin (L), and (C+H)/L ratio. Reactors were sampled at each of four time points to obtain refuse in the anaerobic acid phase, the accelerated methane production phase, and early and late in the decelerated methane production phase. The experimental program was performed using oedometer and direct shear tests to determine the compressibility parameters and shear strength parameters and illustrate the effect of shredding and equipment size on compressibility and strength parameters for refuse (at different degrees of degradation). The extent of degradation was documented by gas production rates as well as (C+H)/L ratios. Oedometer test conducted on 63.5 mm, 100mm, 200mm diameter equipment with constant R, specimen to equipment size ratio, indicate that compressibility parameters are dependent on R. Compressibility parameters are similar with constant R even though the equipment size varies. Shredding of MSW affects mainly initial compression as observed from the test results on same equipment with variable R. For example, initial compression for MSW in 200mm equipment is 31% and 35% for R =0.34 and 0.17, respectively. Creep and biological strain rate of MSW is not affected by shredding. The variation of the magnitude of biological indices with varied R is minimal. The shear strength is affected by shredding as the light-weight reinforcing materials are shredded into smaller pieces during specimen preparation. The measured shearing angles are 31 o and 27 o for R=0.50 and 0.25, respectively. The larger components in the specimen act as better reinforcing element than shredded smaller components during the shear test. Compressibility increased with increasing gas production as solid-to-gas conversion took place. Testing results indicated a correlation between the coefficient of primary compression (Cc) and (C+H)/L ratio. The coefficient of primary compression (Cc) for all samples showed an increasing trend with decreasing (C+H)/L. Results indicated the creep index (Cα) to be independent of the state of waste decomposition. The creep index range was 0.02 to 0.03 for traditional and bioreactor samples in various states of decomposition. The magnitude of the biological indices varied with the state of decomposition and yielded the highest values (Cβ=0.19) when samples were actively decomposing and had substantial methane potential remaining. The amount of plastic, a non-degradable waste component, within test sample remains the same where as the paper content decreases with degradation as their structure matrix breaks down. The experimental results found that with degradation of degradable material, percentages of plastic content increases and contributes to decrease in friction angle. Accordingly, testing results indicated a correlation between strength parameters and (C+H)/L ratio. For example, measured shearing angle for bioreactor samples decreased from 32 o to 24 o as (C+H)/L ratio decreased from 1.29 to 0.25. The predicted shear behavior by the developed constitutive model for shear stress displacement was matched with the experimental results. Settlement prediction using a developed model considered all the aspects of biological decomposition, creep and matrix stiffness change as decomposition takes place with time. Seven field case studies show that the model predicts landfill settlement quite well including the biological decomposition component.