Tony L. T. Zhan

Aging and compressibility of municipal solid wastes

The expansion of a municipal solid waste (MSW) landfill requires the ability to predict settlement behavior of the existing landfill. The practice of using a single compressibility value when performing a settlement analysis may lead to inaccurate predictions. This paper gives consideration to changes in the mechanical compressibility of MSW as a function of the fill age of MSW as well as the embedding depth of MSW. Borehole samples representative of various fill ages were obtained from five boreholes drilled to the bottom of the Qizhishan landfill in Suzhou, China. Thirty-one borehole samples were used to perform confined compression tests. Waste composition and volume-mass properties (i.e., unit weight, void ratio, and water content) were measured on all the samples. The test results showed that the compressible components of the MSW (i.e., organics, plastics, paper, wood and textiles) decreased with an increase in the fill age. The in situ void ratio of the MSW was shown to decrease with depth into the landfill. The compression index, Cc, was observed to decrease from 1.0 to 0.3 with depth into the landfill. Settlement analyses were performed on the existing landfill, demonstrating that the variation of MSW compressibility with fill age or depth should be taken into account in the settlement prediction.

An efficient approach for locating the critical slip surface in slope stability analyses using a real-coded genetic algorithm

A real-coded genetic algorithm is employed to develop a search approach for locating the noncircular critical slip surface in slope stability analyses. Limit equilibrium methods and the finite-element-based method are incorporated with the proposed search approach to calculate the factor of safety. Geometrical and kinematical compatibility constraints are established based on the features of slope problems to prevent slip surfaces from being unreasonable. A dynamic bound technique is presented to improve the search performance with more effective exploration within the solution domain. A number of examples are investigated that demonstrate the proposed search approach to be efficient in yielding accurate solutions to practical slope stability problems. The proposed search approach is stable and highly correlated with the results of independent analyses. Furthermore, this paper demonstrates the successful application of a real-coded genetic algorithm to noncircular critical slip surface search problems.

Vertical migration of leachate pollutants in clayey soils beneath an uncontrolled landfill at Huainan, China: A field and theoretical investigation

To assess the extent of leachate migration, continuous samples of clayey soils (about 9m) were obtained beneath a 17-year old uncontrolled landfill in southeastern China. The soil samples were sub sectioned and analyzed to determine the concentrations of chloride, sodium and COD in the pore water. Total nitrogen and soil organic matter content of the soil samples were also determined. Leachate-derived chloride was detected in the clayey soil to a maximum depth of 9m. Sodium and COD were found to migrate into the soils to depths of 3-4m due to the attenuation of solutes by the soil organic matter and clay minerals at the shallow soils. The estimated migration depths for the chloride are 3m in the case of pure diffusion. Advection and mechanical dispersion were found to be more important than molecular diffusion for this site with an 8m high leachate mound. By comparing the results obtained by the mathematical modeling for layered advection-dispersion problem with the measured concentration profiles, the ranges of the effective diffusion coefficient, retardation factor and dispersivity of the soils were estimated. Better fits are obtained by employing an artificial effective interface about 1m above the observed interface. The clayey soils showed a relatively high attenuation capacity for COD with the estimated retardation factor of 5.

Dependence of Gas Collection Efficiency on Leachate Level at Wet Municipal Solid Waste Landfills and Its Improvement Methods in China

AbstractLandfill gas (LFG) collection efficiency is low at many municipal solid waste (MSW) landfills in China. The relevant mechanism and potential solution for this problem are studied through laboratory gas permeability tests on MSW, field LFG extraction tests at a landfill subjected to leachate drawdown, numerical assessment of the LFG collection efficiency at the landfill with different leachate levels, and engineering application of the measures to improve LFG collection. The research work outcomes are as follows: High water content in the food-rich MSW is one of the major reasons causing the high leachate level at many Chinese landfills. High leachate mounds tend to result in a high degree of saturation in waste, and hence a low gas permeability of the waste. Therefore, the LFG collection efficiency at Chinese landfills with high leachate level is lower than expected. A drawdown of leachate level at a Chinese landfill by pumping resulted in a significant increase in the LFG collection rate and the ...

Instrumentation of an Unsaturated Expansive Soil Slope

To understand the complex soil-water interaction in an unsaturated expansive soil slope subjected to rainfall infiltration, a comprehensive instrumentation and monitoring program was carried out on an 11-m high cut slope in Hubei of China. The instrumentation included jet-filled tensiometers, thermal conductivity suction sensors, moisture probes, earth pressure cells, inclinometers, vertical movement points, artificial rainfall simulator, a tipping bucket rain gage, a vee-notch flow meter, and an evaporimeter. The technique and experience associated with each of the instrumentations are presented in this paper with an emphasis on the difference from the instrumentation in nonexpansive soils. In particular, a deliberate sealing scheme was adopted to prevent the potential bypass water flow through the shallow cracks into the installation holes for the suction sensors and the moisture probes. All the instruments worked well throughout the two month monitoring period, during which two artificial rainfall events were created. The recorded responses in pore-water pressure, water content, horizontal stress, and soil deformation were reasonably consistent with one another.

Wetting-Induced Softening Behavior of an Unsaturated Expansive Clay

Studying the wetting-induced softening behavior of unsaturated soils is important for understanding rain-induced slope failures in unsaturated soils. However, experimental studies on expansive soils are limited. This study investigates the wetting-induced softening characteristics of an unsaturated expansive clay via suction-controlled triaxial tests on recompacted and natural specimens. It was found that the wetting-induced swelling of the soil included a significant component of plastic strain and its magnitude decreased with the net confining stress. When a soil specimen was subjected to wetting at a constant deviator stress, a threshold value of soil suction was identified after which the axial strain increased greatly. This value tended to increase with the applied net stress ratio. After the threshold suction, the wetting process for recompacted specimens could be divided into a yielding process associated with volumetric strain hardening/softening and a failure process in which the volumetric behavior depended on the applied net stress ratio. The natural specimens exhibited a brittle failure like a heavily overconsolidated soil during wetting at a constant deviator stress and continuous volumetric dilation occurred after the threshold suction. The wetting-induced softening soil behavior revealed from the present study could interpret the rain-induced progressive slope failure in unsaturated expansive soils.

Parameter determination of a compression model for landfilled municipal solid waste: An experimental study:

The methods to determine the parameters of a one-dimensional compression model for landfilled municipal solid waste were investigated. In order to test the methods for parameter determination, long-term laboratory compression experiments were carried out under different surcharge loads (i.e. 100, 200 and 400 kPa). Based on the measured compression strain and the reported creep index, the modified primary compression indexes, pre-consolidation pressure and ultimate biodegradation-induced secondary compression strain were determined using the proposed methods. It was found that the simulated compression could not capture the measured secondary compression behavior when using a constant value of biodegradation-induced compression rate coefficient. The variation of the rate coefficient with the change of decomposition rate should be considered during modeling. Accordingly, the biodegradation-induced secondary compression strain in the compression model should be expressed in an incremental form in order to consider the variation of the rate coefficient.

Electrokinetic Dewatering of Sewage Sludge with Fixed and Moving Electrodes: Attenuation Mechanism and Improvement Approach

Abstract This paper presents a laboratory study on electrokinetic dewatering (EKD) of sewage sludge with fixed and moving electrodes. The experiments with fixed electrodes were conducted in two purposely designed Perspex cylinders to investigate the mechanism of attenuation in electroosmotic flow and dewatering efficiency. The experiments with moving electrodes were performed in an innovatively designed apparatus, in which the anode made of an array of stainless rods can be moved toward the fixed cathode plate. The fixed-electrode experiments showed that the electrical resistance of the dewatered section near the anode increased with the treating time, and hence the voltage gradient left for the undewatered section decreased. The increased electrical resistance resulted in an increase in the invalid energy consumption, and the reduced voltage gradient at the undewatered section resulted in an attenuation in electroosmotic flow and dewatering effect. Comparison tests conducted with the moving-anode apparat...

Influence Of Rainfall Pattern On The Infiltration Into Landfill Earthen Final Cover

Rainfall pattern is an important factor that affects the infiltration process into earthen final covers of landfills. In this paper, six typical rainfall patterns present in nature were sorted out. Then the rainfall patterns were used as the infiltration boundaries of an earthen final cover model, and numerical simulations of the infiltration process were carried out by the use of the commercial software SVFlux. The numerical simulation model was firstly verified by experimental data. A parametric study was performed to investigate the influence of rainfall patterns on the infiltration process in the silty soil cover. The numerical simulation results indicate that the rainfall pattern can significantly affect the infiltration process and cumulative infiltration under the same total amount of precipitation. The peak rainfall intensity occurs earlier, the cumulative infiltration will be larger. The advanced pattern (A1) will generate the largest amount of infiltration compared to other patterns. The numerical analyses also suggest that rainfall pattern should be taken into account in the performance assessment of earthen final covers.

A degradation model for high kitchen waste content municipal solid waste

Municipal solid waste (MSW) in developing countries has a high content of kitchen waste (KW), and therefore contains large quantities of water and non-hollocellulose degradable organics. The degradation of high KW content MSW cannot be well simulated by the existing degradation models, which are mostly established for low KW content MSW in developed countries. This paper presents a two-stage anaerobic degradation model for high KW content MSW with degradations of hollocellulose, sugars, proteins and lipids considered. The ranges of the proportions of chemical compounds in MSW components are summarized with the recommended values given. Waste components are grouped into rapidly or slowly degradable categories in terms of the degradation rates under optimal water conditions for degradation. In the proposed model, the unionized VFA inhibitions of hydrolysis/acidogenesis and methanogenesis are considered as well as the pH inhibition of methanogenesis. Both modest and serious VFA inhibitions can be modeled by the proposed model. Default values for the parameters in the proposed method can be used for predictions of degradations of both low and high KW content MSW. The proposed model was verified by simulating two laboratory experiments, in which low and high KW content MSW were used, respectively. The simulated results are in good agreement with the measured data of the experiments. The results show that under low VFA concentrations, the pH inhibition of methanogenesis is the main inhibition to be considered, while the inhibitions of both hydrolysis/acidogenesis and methanogenesis caused by unionized VFA are significant under high VFA concentrations. The model is also used to compare the degradation behaviors of low and high KW content MSW under a favorable environmental condition, and it shows that the gas potential of high KW content MSW releases more quickly.