Waste material is an unavoidable by-product of human activities. Economic development, urbanization and improved living standards in cities increase the quantity and complexity of generated solid waste. If not managed properly, it leads to degradation of urban environment, puts strain on natural resources and leads to health problems. Landfills represent an environmentally acceptable disposal method of municipal solid waste on ground. They are engineered structures consisting of bottom liners, leachate collection and removal systems, and final covers (Figure 1).
Fig. 1 A typical modern landfill (Vesilind et al. 2002)
Estimation of long-term settlement in landfills is important for the design, construction, and maintenance as well as an integral part toward final closure of a site and its ultimate usage of MSW landfills (EI-Fadel and AI-Rashed 1998). Settlement prediction in landfills is complex and less understood compared to settlement in coarse- or fine-grained soils primarily because of the complex biodegradation process, heterogeneous nature of MSW, variable size, variable density, and different compression characteristics of waste (Wall and Zeiss 1995).The operationof an engineered landfill requires extensive knowledge of the different processes which occurs simultaneously in MSW during settlement.
Municipal solid waste (MSW) is defined to include wastes from residential, commercial, and institutional (e.g., schools, government offices) sources. This definition excludes many materials that are frequently disposed with MSW in landfills including combustion ash, water and wastewater treatment residuals, construction and demolition (C&D waste), and nonhazardous industrial process wastes (U.S. EPA 2007).
The composition of municipal solid waste varies greatly from municipality to municipality (country to country) and changes significantly with time. Information on waste composition is needed to estimate the amount of biodegradable organic carbon and to estimate the amount of recyclable material. In 2013, the EPA estimated MSW generation of 254 million tons in the United States. The composition of MSW is shown in Figure 2. The manner in which this waste is managed is illustrated in Figure 3.
Fig. 2 Total MSW generation (by material) (U.S. EPA, 2013)
More than 50% of the landfilled waste consists of paper, food and yard waste, which are biodegradable under anaerobic conditions (Barlaz et al., 2010).
Fig. 3 Management of MSW (U.S. EPA, 2013)
53 % of the waste is discarded and disposed in landfills, which makes landfilling as the primary method of disposing waste in the United States.
As over 50% of MSW and many other solid wastes are disposed of in landfills, a basic understanding of the design of a landfill is helpful. In the United States, the design and operation of landfills is regulated by Subtitle D of the Resource Conservation and Recovery Act, the New Source Performance Standards of the Clean Air Act, and related state regulations. Landfills have evolved from open dumps to highly engineered facilities designed to contain waste and separate it from the environment, capture contaminated water that contacts the waste (leachate), and control gas migration. A landfill site is typically excavated and lined with a system that includes layers to (1) minimize the migration of leachate to the groundwater, and (2) collect leachate for treatment. A typical cross section of a landfill is illustrated in Figure 3.
Fig 4. Typical cross-section of a landfill receiving MSW (Barlaz et al., 2010)
A common system used to restrict leachate migration consists of a 0.67- to 1-m-thick clay layer with a hydraulic conductivity of no more than 10−7 cm/s overlain with a geomembrane (GM). The GM is typically 1.5-mm-thick polyethylene. A drainage layer that contains a high-permeability material such as sand or gravel is placed above the liner to promote leachate collection. This layer has perforated pipes embedded to remove leachate from the landfill. A protective barrier is then installed above the leachate collection system to shield it from the equipment used to place and compact refuse. Waste may then be placed above the protective barrier, and it is covered daily to minimize wind blown refuse, odors, and the attraction of disease vectors. Daily cover alternatives include a 15-cm soil layer, spray-on foams, and synthetic materials that are rolled over the waste at the end of the working day. Once refuse has reached the design elevation, a final cover is applied. The final cover will include, at a minimum, a layer of low-permeability soil designed to minimize stormwater infiltration overlain by a layer of soil that will support vegetative growth. The final cover frequently includes a drainage layer and a GM beneath the vegetative layer. Vegetation serves to minimize erosion of the soil cover and to promote evapotranspiration (Barlaz et al., 2010).
MSW consists of multiphase media (gas, liquid, and solid) with each phase exhibiting spatial and temporal variations (El-Fadel and Khoury, 2000). Therefore, MSW settlement should depend on the contribution from all three phases.
Fig. 5 Phase Diagram for MSW (Hettiarachchi et al., 2008)
Settlement in landfilled waste is commonly described in terms of the five mechanisms identified by Sowers (1973):
Yen and Scanlon (1975) also identified five mechanisms of long-term settlement, which correspond closely to Sowers:
The likely incidence of settlement mechanisms during the life of a landfill site is shown in Figure 6.
Fig 6. Occurrence of settlement mechanisms and temporal classifications adopted by selected publications (McDougall, 2011)
Hence a combination of primary and secondary settlement conveniently classifies the mechanisms contributing to landfill settlement. Primary settlement occurs very quickly by comparison to the life of the landfill and can be adequately described by load-induced settlement models. Secondary settlement, driven by biodegradation effects, creep and physicochemical corrosion is a long-term phenomenon, which has for some time relied on time-dependent methods.
Influencing factors can be viewed as :
(i) factors defining the condition of the waste within a landfill cell, i.e. internal to the landfill (the boxed factors), and (ii) factors or site operations and controls or initial conditions which influence the internal conditions.
Figure 7 shows a range of factors influencing landfill settlement feeding into a simplified suite of primary and secondary settlement mechanisms.
Fig. 7 Mechanisms and factors influencing landfill settlement (McDougall, 2011)
Immediate compression is an abiotic mechanical process that occurs rapidly in response to an increase in stress. Mechanical creep involves the slow time-dependent abiotic yielding and reorientation of MSW constituents under constant stress, whereas biocompression involves abiotic mechanical creep coupled with biotic decomposition of the MSW organic fraction. Biocompression is primarily associated with anaerobic decomposition of the biodegradable organic fraction of the waste, which depends on the organic content, pH, moisture content, and temperature (Table 1).
Table 1. Environmental factors which most significantly impact upon MSW degradation in landfills (Yuen et al., 1994)
Optimum : 60 % and above
Pohland (1986), Rees (1980)
Optimum for methanogenesis:
6 to 8
6.4 to 7.2
Farquhar & Rovers (1973)
Optimum for methanogenesis:
45 (34 – 38°C)
Hartz et al (1982)
Mata-Alvarez et al (1986)
An idealized long-term settlement curve observed from laboratory large-scale simulators is illustrated in Figure 8 and can be divided into the following phases (Fei et al. 2013):
Phase 1 - Transitional phase (P1): It consists of primarily physical mechanisms such as particle reorientation and movement, raveling, creep, as well as compression or softening of waste constituents as moisture is introduced. The introduction of liquids gradually supports microorganism growth, but the microbial populations are still low and their activities do not contribute significantly to settlement.
Phase 2 - Active biodegradation phase (P2): Microorganisms actively consume waste and a peak in their activity is observed. The intensive and wide-spread microbial activities lead to biodegradation of MSW. As a result, rapid and significant settlement occurs. During this phase, and as the waste state changes,mechanisms such as creep and raveling may also contribute to settlement, but probably not significantly.
Phase 3 - Residual phase (P3): Availability and accessibility of organic MSW decrease steadily during the active biodegradation phase. When the majority of the substrate for microorganisms is depleted, biodegradation of MSW become stagnant and a reduction in amount and rate of settlement is observed, indicating stabilization of MSW. Residual biodegradation and the mechanical creep become the main contributions to the observed settlement.
Fig. 8 Idealized long-term settlement curve with three phases: transitional phase (P1), active biodegradation phase (P2) and residual phase (P3). (Modified from Fei et. al, 2013)
Active biodegradation phase, P2, has the largest contribution to the long-term settlement and hence, is studied in detail in the following sections.
To understand the biodegradation of waste in landfills, it is important to understand the general pathway of anaerobic biodegradation which is a complex process that requires the coordinated activity of several trophic groups of microorganisms (Madigan et al., 2003).
The primary biodegradable constituents in MSW are cellulose (C) and hemicellulose (H), which comprise approximately 40–60% of MSW by dry weight and account for greater than 90% of its methane potential (Barlaz et al. 1990), while the other major organic component, lignin, is at best only slowly degradable under methanogenic conditions (Colberg, 1988). Reported cellulose, hemicellulose, and lignin concentrations in residential refuse range from 28.8 to 54.3%, 6.6 to 11.9%, and 12.1 to 28% of dry weight, respectively (Barlaz, 2006). Data on the cellulose, hemicellulose, and lignin concentrations in municipal waste components are summarized in Table 2.
Table 2. Chemical Composition and Ultimate Methane Yield of Solid Waste Components (Eleazer et al., 1997).
aData are based on measurements in 2-L reactors that were operated to maximize methane production.
Additional values represent samples tested in subsequent studies.
MSW decomposition is a microbially mediated process that occurs in sequential phases referred to as hydrolysis, fermentation/acidogenesis, acetogenesis, and methanogenesis (Farquhar and Rovers 1973; Zehnder 1978; Barlaz et al. 1989; Pohland and Kim 1999; Levén et al. 2007).
The first phase is the hydrolysis of polymers (carbohydrates, fats, and proteins), which yields soluble sugars, amino acids, long-chain carboxylic acids, and glycerol. Equation 1. shows an example of a hydrolysis reaction where organic waste is broken down into a simple sugar, in this case, glucose (Ostrem, 2004).
Equation 1 : C6H10O4 + 2H2O → C6H12O6 + 2H2
In the second phase, acidogenic bacteria transform the products of the first reaction into short chain volatile acids, ketones, alcohols, hydrogen and carbon dioxide. The principal acidogenesis stage products are propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), acetic acid (CH3COOH), formic acid (HCOOH), lactic acid (C3H6O3), ethanol (C2H5OH) and methanol (CH3OH), among other. From these products, the hydrogen, carbon dioxide and acetic acid will skip the third stage, acetogenesis, and be utilized directly by the methanogenic bacteria in the final stage (Figure 9). Equations 2, 3 (Ostrem, 2004) and 4 (Bilitewski et al., 1997) represent three typical acidogenesis reactions where glucose is converted to ethanol, propionate and acetic acid, respectively.
Equation 2 : C6H12O6 ↔ 2CH3CH2OH + 2CO2
Equation 3 : C6H12O6 + 2H2 ↔ 2CH3CH2COOH + 2H2O
Equation 4 : C6H12O6 → 3CH3COOH
In the third phase, known as acetogenesis, the rest of the acidogenesis products, i.e. the propionic acid, butyric acid and alcohols are transformed by acetogenic or fatty acid oxidizing bacteria into hydrogen, carbon dioxide and acetic acid (Figure 9). Hydrogen plays an important intermediary role in this process, as the reaction will only occur if the hydrogen partial pressure is low enough to thermodynamically allow the conversion of all the acids. Such lowering of the partial pressure is carried out by hydrogen scavenging bacteria. Equation 5 represents the conversion of propionate to acetate, only achievable at low hydrogen pressure. Glucose (Equation 6) and ethanol (Equation 7) among others are also converted to acetate during the third stage of anaerobic biodegradation (Ostrem, 2004).
Equation 5 : CH3CH2COO-+ 3H2O ↔ CH3COO-+ H++ HCO3-+ 3H2
Equation 6 : C6H12O6+ 2H2O ↔ 2CH3COOH + 2CO2+ 4H2
Equation 7 : CH3CH2OH + 2H2O ↔ CH3COO-+ 2H2+H+
The fourth and final phase is called methanogenesis. During this stage, microorganisms convert the hydrogen and acetic acid formed by the acid formers to methane gas and carbon dioxide (Verma, 2002).The most common methanogenic substrates are acetate and CO2 plus H2. The bacteria responsible for this conversion are called methanogens and are strict anaerobes. Most methanogens have a pH optimum around 7 (Zinder, 1993). Should the activity of the fermentative organisms exceed that of the carboxylic acid degraders and methanogens, there will be an imbalance in the ecosystem. Carboxylic acids and H2 will accumulate and the pH of the system will fall, thus inhibiting methanogenesis. Waste stabilization is accomplished when methane gas and carbon dioxide are produced.
Equation 8 : CO2+ 4H2→ CH4+ 2H2O
Equation 9 : 2C2H5OH + CO2→ CH4+ 2CH3COOH
Equation 10 : CH3COOH → CH4+ CO2
The general scheme of anaerobic substrate biodegradation and microbial community relationships is illustrated in Figure 9.
Fig. 9 Overall process of anaerobic decomposition (Madigan et al., 2003)
Fig 10. Schematic representation of the course of anaerobic methane generation from complex organic substances showing scanning electron micrographs of individual microorganisms involved (Source: Waste to Energy Research and Technolgy Council)
In the overall anaerobic decomposition process, hydrolysis is the rate-limiting step when the substrate is complex solid organic material (e.g., C and H), whereas methanogenesis is rate-limiting when the substrate is solubilized (Noike et al. 1985; Pavlostathis and Giraldo-Gomez 1991; Vavilin et al. 1996).
The decomposition phases of MSW have unique leachate chemistry and biogas characteristics that have been linked to time-dependent compression phases of mechanical creep and biocompression (Hossain et al. 2003; Olivier and Gourc 2007; Ivanova et al. 2008; Gourc et al. 2010). During initial hydrolysis, fermentation, and acetogenesis (i.e., acid formation phase), carboxylic acids accumulate in the leachate and the leachate hydrogen ion concentration (pH) decreases (Barlaz et al. 1989; Pohland and Kim 1999). Mechanical creep is dominant during the acid formation phase (Wall and Zeiss 1995; Ivanova et al. 2008). Biocompression coincides with methanogenesis, which is characterized by methane generation, acid consumption, and increasing leachate pH. The transition from dominant mechanical creep to dominant biocompression has been linked to the onset of methane generation and acid consumption (Olivier and Gourc 2007; Ivanova et al. 2008; Bareither et al. 2010; Gourc et al. 2010).
The biodegradation of MSW takes place through the metabolic activity of microorganisms and results in changes in the mechanical and hydraulic properties of the waste as shown in Figure 11.
Fig 11. Schematic of the processes taking place during MSW biodegradation and examples of the parameters measured (Fei et al., 2014)
Landfills represent a complex and unique anaerobic ecosystem and are involved in the global cycling of organic carbon. Landfills serve as a repository for biogenic carbon that is biodegraded to CH4 and potentially recovered for energy. Methane may be recovered from landfills for use as an energy source, as occurs at an estimated 445 U.S. landfills (LMOP, 2007). When CH4 is recovered for beneficial reuse, it results in avoided emissions from power plants that rely on fossil fuels for energy production. However, as a result of CH4 that is not collected, landfills are estimated to be the second-largest source of anthropogenic CH4 emissions in the United States (US EPA, 2008).
Although cellulose and hemicellulose are intrinsically biodegradable under anaerobic conditions, they do not degrade completely, as some are protected by lignin and are not bioavailable. In addition, waste decomposition in a landfill is by no means complete, as environmental conditions are often suboptimal. Estimates of the fraction of MSW components that do biodegrade under even the most favorable optimal conditions range from 5% carbon sequestration of office paper to 42% sequestration for newspaper (Barlaz, 1998).
The burial of solid waste in a landfill initiates a complex series of chemical and biological reactions that has been described in a series of phases (Barlaz et al., 1989a). The rate and characteristics of leachate produced and biogas generated from a landfill vary from one phase to another, and reflect the microbially mediated processes taking place inside the landfill (Figure 12).
Figure 12. Phases of MSW degradation in a typical landfill (Pohland and Harper, 1986)
Environmental conditions in the landfill will have a significant impact on the rate of MSW decomposition. The factors that have most consistently been shown to affect the rate of refuse decomposition are the moisture content and pH, and it is generally accepted that refuse buried in arid climates decomposes more slowly than refuse buried in regions that receive greater than 50 to 100 cm of annual precipitation.
Landfill operation has evolved over the past several decades. Initially, landfills were operated to minimize water infiltration and therefore decomposition. With the advent of leachate collection and treatment, there has been increased interest in the operation of landfills to maximize waste decomposition and CH4 production. This is done by the recirculation of leachate and sometimes other liquids through the waste (Figure 13). Landfills operated to enhance decomposition are referred to as bioreactor landfills. The Solid Waste Association of North America (SWANA) has defined a bioreactor landfill as "any permitted Subtitle D landfill or landfill cell where liquid or air is injected in a controlled fashion into the waste mass in order to accelerate or enhance biostabilization of the waste." (U.S. EPA).
Bioreactor technology is a process based technology which involves physical, chemical and biological process with proper leachate management to recover bioenergy in the form of landfill gas and residue as manure.
Physical process - Phydical process involves, shredding of the waste to an uniform size, proper mixing of the waste etc.
Chemical process - Chemical process for enhancement of microbial growth involves leachate recirculation, pH adjustment, addition of buffers and nutrients etc.
Biological process - Bioreactor landfill operates under optimal anaerobic environmental conditions for enhancement of bio-degradation process.
Decomposition and biological stabilization of the waste in a bioreactor landfill can occur in a much shorter time frame than occurs in a traditional “dry tomb” landfill providing a potential decrease in long-term environmental risks and landfill operating and post-closure costs. As shown in Figure 14, the landfill settlement occurs at a much faster rate and as a result will stabilize earlier as compared to the one without recirculation. Potential advantages of bioreactors include:
Fig 13. An anaerobic bioreactor landfill Fig 14. Effect of leachate recirculation on settlement magnitude [Cell A]; [Cell F = control] (El Fadel , 1999)
The most common strategy to accelerate decomposition is to stimulate microbial activity by adding moisture to the waste via recirculation of leachate and addition of supplemental liquids, a practice that is becoming more common in North America (Pohland 1975; Barlaz et al. 1990; Pacey et al. 1999; Reinhart et al. 2002; Mehta et al. 2002; Warith 2002; Benson et al. 2007; Bareither et al. 2008a.) An assessment of state-of-the-practice at five full-scale landfills operating as bioreactors is studied (Table 3). This study focuses on effectiveness of liners and leachate collection systems, leachate generation rates, leachate recirculation practices and rates, effectiveness in moistening the waste, and settlement of the waste over time. Residential and commercial nonhazardous solid wastes are the dominant waste streams received at each landfill.
Table 3. General characteristics of landfills selected for study
The reason for implementing a bioreactor varied between landfills. At the two northeastern landfills Landfills L and G, state regulations require the operator to have the capacity to treat all leachate on-site. At these landfills, recirculation is the least expensive leachate management alternative and reducing treatment costs was a significant factor in the decision to recirculate leachate. Landfill G was also equally interested in airspace recovery and enhanced gas generation. Landfills M and D were interested in maximizing waste decomposition to reduce the long-term risk and associated costs for managing and treating leachate and gas. However, leachate disposal was an advantage at Landfills M and D too. At both landfills, off-site treatment required trucking leachate 40 to 60 km. Thus, significant cost and emissions savings were achieved at Landfills M and D by supplanting off-site treatment with recirculation. The motivation at Landfill Y was to monitor and document bioreactor performance in a full-scale landfill operation. Two modest-size cells were filled and covered rapidly so that bioreactor performance could be monitored and documented. Landfill Y also valued the cost savings accrued via leachate recirculation in lieu of leachate treatment.
Settlement data amenable to analysis were available for Landfills L, M, and Y. A baseline survey of the cover elevation at Landfill Y was conducted several months following waste placement. At each landfill, settlement was not monitored during the first month following waste placement, and initial compression was essentially complete prior to monitoring. Thus, the analysis presented herein corresponds to longer-term secondary compression due to mechanical and biological mechanisms.
Fig 15. Settlement curves for: a) NE, W, and pilot project conventional and recirculation areas at Landfill Y; b) conventional, recirculation, and Trench 5 areas at Landfill M
Settlement curves strain versus log time for the full-scale and pilot cells at Landfill Y are shown in Fig. 15(a). A change in the rate of compression occurs around 500 days for the pilot and full-scale bioreactor cells full-scale NE and W cells, bioreactor pilot cell. This change appears to correspond to the onset of active biodegradation, which increases the rate of settlement . This transition is associated with the onset of appreciable methane production at Landfill Y, which began 1–2 years following waste placement in both the pilot cell and the full-scale cells. A less distinct break is evident around 500 days in the settlement curve for the conventional pilot cell. This more subtle break is consistent with the modest methane production from the conventional pilot cell, which was around 20% of methane production from the bioreactor pilot cell and nearly ceased three years after waste filling.
Settlement curves for Landfill M are shown in Fig. 15(b). No distinct break was evident in the curves for Landfill M, and the settlement data were for conditions greater than 1,000 days after filling was complete (both conventional and bioreactor cells).Thus, active biodegradation was assumed to be occurring when the settlement data were collected at Landfill M. Both cells were actively producing methane during the period when the settlement data were collected.
The rate of settlement at landfills L, M, and Y was quantified using the secondary compression ratio Cα, which is commonly used in practice and describes the rate of change in strain relative to the logarithm of time (Sowers 1973; Bjarngard and Edgers 1990).Linear regression was used to determine Cα. For Landfill Y, Cα was computed for mechanical and biodegradation-induced compression, with the two mechanisms distinguished by the slopes before and after 500 days elapsed time. On average, the Cα are larger for both mechanical and biodegradation-induced compression in the bioreactor landfills compared to the conventional landfills 1.3 times for mechanical, 1.6 times for biodegradation.
Fig. 16 Relationship between biodegradation-induced secondary compression ratio and dose for recirculation in horizontal trenches
Figure 16 shows Cα for biodegradation-induced settlement as a function of the average dose for Landfills L, M, and Y in this study. the trend indicates that Cα increases as the recirculation dosage increases. Thus, greater settlement rates may be expected at landfills where recirculation is conducted more aggressively.
The objective of this study was to provide an overview of solid waste generation and the biological reactions involved in its decomposition. The emphasis was on anaerobic processes that reflect decomposition in landfills. Solid waste contains high concentrations of cellulose and hemicellulose that biodegrades under anaerobic conditions. The anaerobic biodegradation of solid waste requires the coordinated activity of several groups of microorganisms.
Increasing attention is being given to leachate recirculation in landfill bioreactors as an effective way to enhance the microbial decomposition of organic fraction of municipal solid waste. Such systems are operationally influenced to promote synergy between the inherent microbial community and controlled to accelerate the sequential phase of waste stabilization, primarily reflected by characteristics changes in quantity and quality of leachate and gas production.
Landfills are an essential part of an integrated waste management strategy. At this time, the majority of solid waste generated in the United States is disposed in landfills, where it decomposes to CH4 and CO2. The resulting landfill gas represents a source of energy. The development of a truly sustainable landfill will be important to the safe and effective management and control of waste in the future.