Reliability assessment of bioreactor landfills using Monte Carlo simulation and coupled hydro-bio-mechanical model.

The performance of a bioreactor landfill is highly influenced by the simultaneous interactions of several coupled processes that occur within the landfill. In addition, the high uncertainty and spatial variability in the geotechnical properties of municipal solid waste (MSW) poses significant challenge in accurately predicting the performance of bioreactor landfills. In this study, a 2D coupled hydro-bio-mechanical (CHBM) model was employed to predict the behavior of MSW in bioreactor landfills. The numerical model integrated a two-phase flow hydraulic model, a plane-strain formulation of Mohr-Coulomb constitutive model, and a first order decay biodegradation model. The statistical ranges (mean and standard deviation) of some of the major influential MSW properties were derived from the published studies. Random fields of spatially variable MSW properties were generated following the log-normal distribution. Reliability-based analysis was carried out by performing several realizations of Monte-Carlo simulations and the statistical response of the output results including the moisture distribution, pore fluid pressures, landfill settlement, and interface shear response of the composite liner system were quantified. The results clearly indicate the importance of considering spatial variability of the geotechnical MSW properties and its influence on the performance of bioreactor landfills during leachate injection operations. A comparison of the results with the deterministic analysis was performed to evaluate the relative benefits and to emphasize the need for reliability-based analysis for effective design of bioreactor landfills.

Influence of dynamic coupled hydro-bio-mechanical processes on response of municipal solid waste and liner system in bioreactor landfills.

A two-dimensional (2-D) mathematical model is presented to predict the response of municipal solid waste (MSW) of conventional as well as bioreactor landfills undergoing coupled hydro-bio-mechanical processes. The newly developed and validated 2-D coupled mathematical modeling framework combines and simultaneously solves a two-phase flow model based on the unsaturated Richard's equation, a plain-strain formulation of Mohr-Coulomb mechanical model and first-order decay kinetics biodegradation model. The performance of both conventional and bioreactor landfill was investigated holistically, by evaluating the mechanical settlement, extent of waste degradation with subsequent changes in geotechnical properties, landfill slope stability, and in-plane shear behavior (shear stress-displacement) of composite liner system and final cover system. It is concluded that for the given specific conditions considered, bioreactor landfill attained an overall stabilization after a continuous leachate injection of 16years, whereas the stabilization was observed after around 50years of post-closure in conventional landfills, with a total vertical strain of 36% and 37% for bioreactor and conventional landfills, respectively. The significant changes in landfill settlement, the extent of MSW degradation, MSW geotechnical properties, along with their influence on the in-plane shear response of composite liner and final cover system, between the conventional and bioreactor landfills, observed using the mathematical model proposed in this study, corroborates the importance of considering coupled hydro-bio-mechanical processes while designing and predicting the performance of engineered bioreactor landfills. The study underscores the importance of considering the effect of coupled processes while examining the stability and integrity of the liner and cover systems, which form the integral components of a landfill. Moreover, the spatial and temporal variations in the landfill settlement, the stability of landfill slope under pressurized leachate injection conditions and the rapid changes in the MSW properties with degradation emphasizes the complexity of the bioreactor landfill system and the need for understanding the interrelated processes to design and operate stable and effective bioreactor landfills. A detailed discussion on the results obtained from the numerical simulations along with limitations and key challenges in this study are also presented.

Slope stability of bioreactor landfills during leachate injection: effects of heterogeneous and anisotropic municipal solid waste conditions.

In bioreactor landfills, leachate recirculation can significantly affect the stability of landfill slope due to generation and distribution of excessive pore fluid pressures near side slope. The current design and operation of leachate recirculation systems do not consider the effects of heterogeneous and anisotropic nature of municipal solid waste (MSW) and the increased pore gas pressures in landfilled waste caused due to leachate recirculation on the physical stability of landfill slope. In this study, a numerical two-phase flow model (landfill leachate and gas as immiscible phases) was used to investigate the effects of heterogeneous and anisotropic nature of MSW on moisture distribution and pore-water and capillary pressures and their resulting impacts on the stability of a simplified bioreactor landfill during leachate recirculation using horizontal trench system. The unsaturated hydraulic properties of MSW were considered based on the van Genuchten model. The strength reduction technique was used for slope stability analyses as it takes into account of the transient and spatially varying pore-water and gas pressures. It was concluded that heterogeneous and anisotropic MSW with varied unit weight and saturated hydraulic conductivity significantly influenced the moisture distribution and generation and distribution of pore fluid pressures in landfill and considerably reduced the stability of bioreactor landfill slope. It is recommended that heterogeneous and anisotropic MSW must be considered as it provides a more reliable approach for the design and leachate operations in bioreactor landfills.

Towards a highly-efficient fuel-cell catalyst: optimization of Pt particle size, supports and surface-oxygen group concentration.

In the present work, methanol oxidation reaction was investigated on Pt particles of various diameters on carbon-nanofibers and carbon-black supports with different surface-oxygen concentrations, aiming for a better understanding of the relationship between the catalyst properties and the electrochemical performance. The pre-synthesized Pt nanoparticles in ethylene glycol, prepared by the polyol method without using any capping agents, were deposited on different carbon supports. Removal of oxygen-groups from the carbon supports had profound positive effects on not only the Pt dispersion but also the specific activity. The edge structures on the stacked graphene sheets in the platelet carbon-nanofibers provided a strong interaction with the Pt particles, significantly reconstructing them in the process. Such reconstruction resulted in the formation of more plated Pt particles on the CNF than on the carbon-black and exposure of more Pt atoms with relatively high co-ordination numbers, and thereby higher specific activity. Owing to the combined advantages of optimum Pt particle diameter, an oxygen-free surface and the unique properties of CNFs, Pt supported on heat-treated CNFs exhibited a higher mass activity twice of that of its commercial counterpart.

Study of mechanism of enhanced antibacterial activity by green synthesis of silver nanoparticles.

The extensive use of silver nanoparticles needs a synthesis process that is greener without compromising their properties. The present study describes a novel green synthesis of silver nanoparticles using Guava (Psidium guajava) leaf extract. In order to compare with the conventionally synthesized ones, we also prepared Ag-NPs by chemical reduction. Their optical and morphological characteristics were thoroughly investigated and tested for their antibacterial properties on Escherichia coli. The green synthesized silver nanoparticles showed better antibacterial properties than their chemical counterparts even though there was not much difference between their morphologies. Fourier transform infrared (FTIR) spectroscopic analysis of the used extract and as-synthesized silver nanoparticles suggests the possible reduction of Ag(+) by the water-soluble ingredients of the guava leaf like tannins, eugenol and flavonoids. The possible reaction mechanism for the reduction of Ag(+) has been proposed and discussed. The time-dependent electron micrographs and the simulation studies indicated that a physical interaction between the silver nanoparticles and the bacterial cell membrane may be responsible for this effect. Based on the findings, it seems very reasonable to believe that this greener way of synthesizing silver nanoparticles is not just an environmentally viable technique but it also opens up scope to improve their antibacterial properties.

Improved critical current density of MgB2--carbon nanotubes composite.

In the present study, we report a systematic study of doping/admixing of carbon nanotubes (CNTs) in different concentrations in MgB2. The composite material corresponding to MgB2-x at.% CNTs (35 at.% > or = x > or = 0 at.%) have been prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural/microstructural characterization employing XRD, Scanning electron microscopic (SEM), and Transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by Physical property measurement system (PPMS) and electrical transport measurements have been done by the four-probe technique. The microstructural investigations reveal the formation of MgB2-carbon nanotube composites. A CNT connecting the MgB2 grains may enhance critical current density due to its size (approximately 5-20 nm diameter) compatible with coherence length of MgB2 (approximately 5-6 nm) and ballistic transport current carrying capability along the tube axis. The transport critical current density (Jct) of MgB2 samples with varying CNTs concentration have been found to vary significantly e.g., Jct of the MgB2 sample with 10 at.% CNT addition is approximately 2.3 x 10(3) A/cm2 and its value for MgB2 sample without CNT addition is approximately 7.2 x 102 A/cm2 at 20 K. In order to study the flux pinning effect of CNTs doping/ admixing in MgB2, the evaluation of intragrain critical current density (JJ) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on Jc is obtained for 10 at.% CNTs admixed MgB2 sample at 5 K, the Jc reaches approximately 5.2 x 10(6) A/cm2 in self field, -1.6 x 10(6) A/cm2 at 1 T, approximately 2.9 x 10(5) A/cm2 at 2.6 T, and approximately 3.9 x 10(4) A/cm2 at 4 T. The high value of intragrain Jc in 10 at.% CNTs admixed MgB2 superconductor has been attributed to the incorporation of CNTs into the crystal matrix of MgB2, which are capable of providing effective flux pinning centres. A feasible correlation between microstructural features and superconducting properties has been put forward.