Experimental Assessment of Coupled Physical-Biochemical-Mechanical-Hydraulic Processes of Municipal Solid Waste Undergoing Biodegradation.

Abstract

Proper management and disposal of municipal solid waste (MSW) remains an unresolved global problem. One solution to handle existing and future MSW is to move away from modern landfills that focus on containment and move towards bioreactor landfills that promote MSW biodegradation and enhance methane (CH4) generation and its collection as an alternative energy source. Solid, liquid and gas phases of MSW coexist in different proportions within a landfill, and evolve with time due to concurring and coupled physical-biochemical-mechanical-hydraulic processes during MSW biodegradation. A fundamental understanding of the concurring processes is needed to design, monitor, and operate bioreactor landfills effectively and efficiently. Seven large-size (d=300 mm; h=600 mm) laboratory landfill simulators were developed to degrade unprocessed MSW of variable waste composition that is representative of the MSW in a mega-scale landfill. The simulators were operated and monitored for up to four years to assess the evolution of the physical, mechanical, and hydraulic properties of MSW, the evolution of the biochemical characteristics of generated leachate and biogas, and population dynamics of MSW-degrading microorganisms. The coupled processes were found to be systematic, correlated to each other, and dependent on initial waste composition. Testing of MSW in fresh and fully-degraded (retrieved from laboratory simulators) states was performed to assess the physical and mechanical properties of MSW using a unique 300-mm diameter simple shear apparatus. The shear strength and compressibility of MSW changed due to biodegradation and was a function of the initial waste composition and the biodegradation state. A relationship between the shear strength and shear-wave velocity of MSW was established for fresh and degraded MSW. Laboratory results on CH4 generation and settlement of MSW during biodegradation generated as part of this study were supplemented by an extensive database synthesized from the literature that includes laboratory results and field measurements from numerous landfills. The database was analyzed to quantify the influence of moisture content of waste, overburden pressure, landfill monitoring and control, and temperature on MSW degradation. Based on the findings of this study, recommendations to promote MSW biodegradation include enhancing biodegradation conditions, optimizing initial waste composition, and increasing biogas collection efficiency.