Bio-hydro-mechanical Coupled Analyses Of Solid-liquid-gas Interactions In Landfilled Municipal Solid Wastes And Its Application

Municipal solid waste (MSW) landfill is a new form of infrastructure resulted from urbanization. A municipal solid waste landfill in China is concerned with four main geoenvironmental problems including landfill gas emission or explosion, leachate emission, settlement and instability. Engineered landfill differs from the conventional earth structures in that its waste body, environment and design are more complex. Furthermore, fluid flow, settlement and stability are highly coupled with each other in a landfill. An improved understanding of the solid-liquid-gas interactions in MSW landfill will help to obtain a better prediction and controlling measures for the four main geoenvironmental problems.The research works in this paper are funded by the major international joint research project "Generation, transport and systems engineering control of leachate and gas in municipal solid waste landfill", the general project "'Biochemical phase transition in solid waste mechanics and its application" and the key program "Solid-liquid-gas interactions in municipal solid waste landfill" from the National Science Foundation of China. Based on laboratory experiments, field tests and numerical simulations, bio-hydro-mechanical coupled analysis of solid-liquid-gas interactions in landfilled MSW were carried out. The main research works and conclusions are as follows:(1) Linear relationship between (C+H)/L and landfill gas production was found based on available biodegradation testing results. The degree of biodegradable solid matter loss was brought up as an indicator representing the degree of biodegradation. A biodegradation model which can take the effect of water content was proposed based on the first-order kinetics. This model is able to analyze solid mass loss and landfill gas generation during biodegradation process, and can be applied to represent the degree of biodegradation directly. Parametric sensitivity analyses of this model showed that landfill gas production per wet ton for Chinese MSW is about 1.3 times the value for the MSW in western developed countries. Landfill gas generation and utilization can be enhanced efficiently for Chinese MSW as it contains a very high content of easily biodegradable matter.(2)The water retention characteristic of MSW was found to be dependent on waste composition, biodegradation and compression processes based on available testing results. As Chinese MSW has a very high content of organics which contain much water in intraparticle voids, initial water content of Chinese MSW is much greater than that of western developed countries. Simulation results showed that van-Genuchten model (van Genuchten,1980) could satisfactorily model the water retention curve of Chinese MSW. For Chinese MSW, the value of parameter nvc ranges between 1.35 and 1.68. There is no clear relationship between nvG and void ratio for fresh Chinese MSW. However, a decrease of nvG with depth was found for borehole waste samples. The value of ovG has a wide range from 1.3 to 25.6, and doesn't show any correlation to void ratio for fresh Chinese MSW. However, it tends to increase with depth for borehole waste samples.(3) Based on Kozeny-Carmen model (Kozeny,1927; Carmen,1938,1956) and available hydraulic conductivity testing results, hydraulic conductivity of Chinese MSW was found to be mainly affected by particle structural properties (i.e. specific area and particle size distribution) and pore structural properties (void ratio or porosity). Simulation results show that the value of initial intrinsic permeability k0 tends to decrease with depth in Chinese landfills, and it is influenced by both biodegradation and compression processes. For Chinese MSW, the value of ko obtained from gas permeability tests was found to be an order larger than that analyzed from hydraulic tests. It was suggested to assess the intrinsic permeability of Chinese MSW by means of gas permeability tests.(4) Based on simulation results of the water retention curve and intrinsic permeability for Chinese MSW from van-Genuchten model and Kozeny-Carmen model, van-Genuchten-Mualem model (van Genuchten,1980) was found to give a good simulation of unsaturated liquid and gas permeabilities of Chinese MSW. yvG can reflect the effect of tortuosity of the MSW porous media,?e represents the effect of liquid saturation on unsaturated permeability, and the effect of porosity is considered through saturated permeability ks. Finally, an unsaturated-saturated fluid flow model was established for landfilled MSW based on above studies.(5) Based on analyses of available compression testing results, it was found that fresh Chinese MSW, which has a very large content of organics, has a much higher potential of both primary and secondary compression than that of MSW in western developed countries. Correlativity analysis was carried out between secondary compression and gas production through available testing results. Results show that power function can describe the relationship between the degrees of secondary compression and gas generation. The degree of secondary compression can be simplified as the degree of biodegradable solid matter loss when considering the direct effect of biodegradation on secondary compression. Based on above correlativity analysis, the determination method of secondary compression rate constant c in the stress-age coupled compression model (Chen et al.2010a) was proposed. Determination of the values of other parameters in the compression model was also discussed. The stress-age coupled model was tested using published compression data from two laboratory long-term compression tests. Results show that this model can give a satisfactory simulation of MSW under staged long-term compression.(6) Based on the proposed biodegradation model, unsaturated-saturated fluid flow model and compression model, a bio-hydro-mechanical coupled model was established for landfilled MSW. The frame of solid-liquid-gas interactions in landfilled MSW was proposed based on the coupled model. Key coupled model parameters include solid mass loss, liquid/gas source terms, pore pressure, saturation and porosity. Finite difference method and Gauss-Newton method were used to numerically solve the coupled model. The bio-hydro-mechanical coupled model was tested through simulations of a laboratory biodegradation test under compression and a field-scale pilot project. Monitoring and simulation results show that part of landfill gas generation potential might be lost through leachate drainage. Building up of the optimum biodegradation conditions can postpone the onset of peak landfill gas generation period. Landfill gas pressure will increase during leachate elevation process.(7) It was found that waste composition and water content are the most important factors affecting biodegradation, fluid flow and compression behaviors of landfilled MSW. Increase of organic content and water content will enhance both landfill gas generation and settlement. It was found that an increase of ww to 37.5% can result in the most significant enhancement. Inrease of liquid saturation will lower gas permeability so that landfill gas pressure builds up within waste body, especially near fully saturated. Landfill gas pressure can grow up to more than 2.2×104kPa if landfill gas is trapped. Besides, secondary compression behavior of MSW is affected by both biodegradation process and above stress. An increase of above stress will increase secondary compression rate. Furthermore, porosity change behavior is affected by both biodegradation and compression processes. For Chinese MSW, a gradual increase of porosity was found after a certain period of porosity decrease, which indicates that waste body tends to be softened continuously and a sudden collapse might happen. Finally, the findings of solid-liquid-gas interactions through above researches were summarized for practical applications in Chinese MSW landfill. It includes three main topics invoving the application of bioreactor landfill, control of liquid/gas, control of differential settlement and increase of landfill capacity. Corresponding controlling measures were proposed.