Geotechnical properties of municipal solid waste at different phases of biodegradation

Abstract

This paper presents the results of laboratory investigation conducted to determine the variation of geotechnical properties of synthetic municipal solid waste (MSW) at different phases of degradation. Synthetic MSW samples were prepared based on the composition of MSW generated in the United States and were degraded in bioreactors with leachate recirculation. Degradation of the synthetic MSW was quantified based on the gas composition and organic content, and the samples exhumed from the bioreactor cells at different phases of degradation were tested for the geotechnical properties. Hydraulic conductivity, compressibility and shear strength of initial and degraded synthetic MSW were all determined at constant initial moisture content of 50% on wet weight basis. Hydraulic conductivity of synthetic MSW was reduced by two orders of magnitude due to degradation. Compression ratio was reduced from 0.34 for initial fresh waste to 0.15 for the mostly degraded waste. Direct shear tests showed that the fresh and degraded synthetic MSW exhibited continuous strength gain with increase in horizontal deformation, with the cohesion increased from 1 kPa for fresh MSW to 16–40 kPa for degraded MSW and the friction angle decreased from 35° for fresh MSW to 28° for degraded MSW. During the triaxial tests under CU condition, the total strength parameters, cohesion and friction angle, were found to vary from 21 to 57 kPa and 1° to 9°, respectively, while the effective strength parameters, cohesion and friction angle varied from 18 to 56 kPa and from 1° to 11°, respectively. Similar to direct shear test results, as the waste degrades an increase in cohesion and slight decrease in friction angle was observed. Decreased friction angle and increased cohesion with increased degradation is believed to be due to the highly cohesive nature of the synthetic MSW. Variation of synthetic MSW properties from this study also suggests that significant changes in geotechnical properties of MSW can occur due to enhanced degradation induced by leachate recirculation.

Introduction

Bioreactor landfill technology involves injecting leachate and other supplemental liquids into the waste to accelerate or enhance the anaerobic biodegradation of MSW. Bioreactor landfills offer a sustainable way to achieve higher rates of MSW degradation, faster reduction of leachate and landfill gas pollution potential, and an increase in landfill volumetric capacity. They also offer significant reductions in post-closure management as a result of the reduced period for leachate and gas generation (Sharma and Reddy, 2004, Interstate Technology & Regulatory Council, 2006).

Under the U.S. Environmental Protection Agency (USEPA) Research Development & Demonstration Rule (40 CFR 258.4), several field demonstration projects have been initiated in the United States to assess the performance of bioreactor landfills as compared to the conventional landfills. Some of the well-designed demonstration projects showed accelerated stabilization of MSW based on the quantity and quality of landfill gas produced and the amount of landfill settlement (Reddy and Bogner, 2003, Interstate Technology & Regulatory Council, 2006, Benson et al., 2007). Several landfill failures have been attributed to accumulation of leachate within the landfills or uncontrolled leachate injection (Koerner and Soong, 2000, Blight, 2008). Therefore, controlled leachate recirculation operations are essential to prevent built-up of pore water pressures in the landfill and prevent any failures.

It is also believed that the degradation of organic matter in the waste changes the composition of solids matrix of the MSW; most degraded MSW is sometimes described as “muck-like” material. With the drastic changes in solids composition and increased moisture content, the mechanical behavior of MSW is expected to be quite different than that of fresh MSW (undegraded condition). It is critical to perform geotechnical stability analyses based on the properties that accurately describe the MSW under different phases of degradation. Several studies have been reported on geotechnical properties of MSW (Landva and Clark, 1990, Gabr and Valero, 1995, Kavazanjian, 2001, Zekkos, 2005, Grisolia and Napoleoni, 1996, Jones et al., 1997, Vilar and Carvalho, 2004, Reddy et al., 2009a, Reddy et al., 2009b); however, very few studies focused on determining the change in properties of MSW due to degradation under bioreactor landfill conditions (Wall and Zeiss, 1995, Van Impe and Bouazza, 1998, Pelkey et al., 2001, Hossain, 2002, Machado et al., 2006, Reddy et al., 2009a). In general, these studies did not systematically quantify the effects of leachate recirculation and enhanced degradation on the engineering properties of MSW.

Hossain (2002) reported controlled laboratory study on shredded MSW from a transfer station in which the MSW was anaerobically degraded in reactors with leachate recirculation. Based on the methane (CH₄) production, reactors were destructively sampled to obtain MSW samples degraded to different levels. Cellulose (C), hemicellulose (H) and lignin (L) concentrations of the samples were measured to better characterize the extent of degradation. A general trend of increased compressibility (compression index) and decreased shear strength (friction angle with zero cohesion) with level of degradation (represented by (C + H)/L) was reported. Additional studies performed following a similar approach, but using MSW from a landfill site, showed that both shear strength properties (cohesion and friction angle) and hydraulic conductivity decreased with increased decomposition of MSW (Hossain et al., 2009, Hossain and Haque, 2009).

Reddy et al. (2009a) determined the changes in geotechnical properties of landfilled MSW which was subjected to leachate recirculation for about 1.5 years. Because of the low amount of leachate recirculation, the extent of MSW degradation was minimal, leading to only minor differences between the properties of fresh and landfilled MSW. Because of heterogeneous composition of field MSW, quantifying the changes in properties solely due to biodegradation is often complicated. To overcome this problem, synthetic MSW can be used in laboratory investigations to determine the parameters affecting the behavior of MSW under controlled conditions. Synthetic MSW allows simulation of MSW with the desired composition and it can be prepared by mixing specific proportions of selected fresh products (paper, grass, etc.).

This paper presents the results of a comprehensive laboratory study to systematically quantify changes in geotechnical properties at various phases of biodegradation under highly controlled conditions in customized test reactors. The degradation process was monitored by measuring gas production rates and gas composition. Based on the testing of synthetic MSW exhumed at different phases of degradation, the changes in geotechnical properties are quantified as a function of degree of degradation.