558667
research-article2014
WMR0010.1177/0734242X14558667Waste Management & ResearchSivakumar Babu and Lakshmikanthan
Original Article
Estimation of the components of municipal solid waste settlement
Waste Management & Research 2015, Vol. 33(1) 30–38 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14558667 wmr.sagepub.com
GL Sivakumar Babu and P Lakshmikanthan
Abstract Estimation of the municipal solid waste settlements and the contribution of each of the components are essential in the estimation of the volume of the waste that can be accommodated in a landfill and increase the post-usage of the landfill. This article describes an experimental methodology for estimating and separating primary settlement, settlement owing to creep and biodegradation-induced settlement. The primary settlement and secondary settlement have been estimated and separated based on 100% pore pressure dissipation time and the coefficient of consolidation. Mechanical creep and biodegradation settlements were estimated and separated based on the observed time required for landfill gas production. The results of a series of laboratory triaxial tests, creep tests and anaerobic reactor cell setups were conducted to describe the components of settlement. All the tests were conducted on municipal solid waste (compost reject) samples. It was observed that biodegradation accounted to more than 40% of the total settlement, whereas mechanical creep contributed more than 20% towards the total settlement. The essential model parameters, such as the compression ratio (Cc’), rate of mechanical creep (c), coefficient of mechanical creep (b), rate of biodegradation (d) and the total strain owing to biodegradation (EDG), are useful parameters in the estimation of total settlements as well as components of settlement in landfill. Keywords Municipal solid waste, primary settlement, mechanical creep, biodegradation induced settlement
Introduction Prediction of municipal solid waste (MSW) settlement is one of the most important issues that must be considered for landfill design and operations. Over the last several decades, many researchers have performed field testing, laboratory testing, theoretical and numerical analyses in this field. A few of these researchers include: Sowers (1973), Edil et al. (1990), Yen and Scanlon (1975), Park and Lee (1997), Gourc et al. (2010) and Babu Sivakumar et al. (2010a, 2010b, 2011). Landfill settlement is the product of a combination of processes, including load and biodegradation-related phenomena. The monitoring of settlements is crucial to calculate the final volume of waste that might be accommodated, to determine when the final cover should be placed, and to avoid damage to biogas recovery and leachate recirculation systems (Hettiarachchi et al., 2009). Settlements create significant problems during the post-closure period of the landfill (Sowers, 1973; Durmusoglu et al., 2005). Deformations of high magnitude are observed after the primary compression of MSW in the landfill. Hence, landfill settlement mechanisms can be classified as a combination of primary and secondary settlements. It is commonly accepted that the secondary settlement of MSW occurs through mechanical and biological processes (Hettiarachchi et al., 2007a; Machado et al., 2008). Total settlement in MSW landfills can be considerable, from 25% to 50% of original fill thickness (Bjarngard and Edgers, 1990). Landfill settlement continues over an extended period of time, with a final
settlement that can approach 20%−30% of the initial fill height (Qian et al., 2002). Huitric (1981) recorded maximum observed settlements at Mission Canyon to be 80% of their estimated ultimate settlement (30%−34%). According to Edil et al. (1990) and Simoes and Campos (2002), the identification of settlement mechanisms in MSW landfills is important for the interpretation of geomechanical behaviour, proposition of long-term settlement models and carrying out long-term simulations that help in landfill design. Several approaches and models for estimating the total landfill settlement have been proposed. Though many researchers measured/predicted the total MSW settlement, the separation of components of settlement and the role played by each component of settlement needs to be established. While the measurements of total settlement are carried out in the field, the evaluation of components of settlements is useful in landfill engineering practice. The trends in landfill technology are changing; for example in conventional landfills, mechanical-related settlements, such as compression under loading and settlements owing to creep, dominate Indian Institute of Science, Bangalore, India Corresponding author: GL Sivakumar Babu, Indian Institute of Sciences, Civil, Bangalore 560012, India. Email: [email protected]
Downloaded from wmr.sagepub.com at Library - Periodicals Dept on February 15, 2015
31
Sivakumar Babu and Lakshmikanthan in total settlements, whereas biodegradation-related settlements are expected to be dominating in bioreactor landfills. The leachate circulation systems make the landfill saturated and enhance biodegradation. Traditionally, the settlement theory from the soils is used in MSW settlement analysis. Several factors like composition of MSW, moisture content, density, waste placement and age of waste have considerable effect on the settlement of MSW. MSW settlement is mainly attributed to: (1) physical and mechanical processes that include the reorientation of particles, movement of the fine materials into larger voids, and collapse of void spaces; (2) chemical processes that include corrosion, combustion and oxidation; (3) dissolution processes that consist of dissolving soluble substances by percolating liquids and then forming leachate; and (4) biological decomposition of organics with time depending on humidity and the amount of organics present in the waste. Several models are reported in the published literature to predict MSW landfill settlement. These models may be grouped as: (1) soil mechanics-based models, (2) empirical models, (3) rheological models, and (4) models incorporating biodegradation. The total settlement is essentially attributed to three components, viz, mechanical, creep and biodegradation and it is desirable to evaluate the three components of settlements. Models that integrate mechanical-, biodegradation- and physico-chemical-related phenomena were developed by Hettiarachchi et al. (2007a, 2007b, 2009), Gourc et al. (2010), Babu et al. (2010a, 2010b), McDougall (2011) and Liu et al. (2011). Of the models reviewed, models of Marques (2001) and Marques et al. (2003) are shown to consider all the mechanisms, as well as being simple for implementation in the landfill sites. Marques (2001) developed a composite rheological model to account for primary and secondary compression mechanisms, governed by rheological parameters that also accounts for waste degradation. The primary compression formulation is introduced as an ‘immediate compression’, which is independent of time, based on the observation that the respective process is linear for curves of void ratio as a function of the logarithm of the applied stress. The secondary compression is governed by the timedependent phenomenon in exponential function similar to the assumption in Gibson and Lo’s (1961) model, which is given by:
ε C = b∆σ (1 − e− ct′ ) (1a)
where ΔH is the settlement, H is the initial height of waste, Cc’ is the primary compression ratio, b is the coefficient of secondary mechanical compression, c is the secondary mechanical compression rate, Edg is the total compression owing to waste degradation, d is the secondary biological compression rate, t’ is the time elapsed since loading application and t” is the time elapsed since waste disposal. Marques et al. (2003) further developed the above model to consider lifts and lift placement intervals as MSW is usually placed in lifts in landfills over a period of years. In implementing the composite model, waste placement is idealised as progressing in a series of lifts. The thickness of the lifts may be set equal to the compacted thickness of the daily cells. After all lifts have been placed, the settlement ΔH of the landfill surface at time any time (t) is determined. The total strain is given below and the three ‘ε’ are defined by: ε = ε p + εc + εb (2a)
The three terms, εp, εc and εb, represent strain resulting from instantaneous response to applied load, time-dependent strain owing to mechanical creep and time-dependent strain owing to biological decomposition. The total settlement is given by: N
∆Η =
∑ H ε i
pi
i =1
+ ε Ci (t ) + ε Bi (t ) (2b)
where N is the number of lifts in the landfill, Hi is the initial thickness of compacted lift i, εPi is the strain in lift i resulting from instantaneous response to loading from overlying lifts, εCi is the strain at time t in lift i owing to mechanical creep associated with the stresses from self-weight and the weight of overlying lifts and εBi is the strain at time t in lift i owing to biological decomposition of lift i. The strains are given by:
N 1 ∆σ ij γiΗi + 2 j =i +1 ’ εΡi = Cc log 1 γiΗi 2
∑
1 εCi (t ) = b γ i Η i (1 − e− c (t −t ) ) + 2 i
(2c)
N
∑ ∆σ
ij (1 − e
j =i +1
− c ( t −t j )
) (2d)
The time-dependent biological degradation is proposed by Park and Lee (1997) and is given by:
ε b = Edg (1 − e
− dt "
σ + ∆σ ∆H − ct ′ = Cc′ log 0 + + ∆σ .b. 1 − e H σ 0 (1c) Edg . 1 − e− dt "
(
(
i
) (1b)
The settlement model is represented by:
ε Bi (t ) = EDG (1 − e − d (t −t ) ) (2e)
)
)
where γi is the unit weight of lift i is a composite value representing the weighted average of the compacted waste and daily cover in lift i. Because, t is a time after all lifts of the landfill have been placed, t – ti > 0 and t – tj > 0 for all values of i and j. Δσ is the change in vertical stresses imposed by lift j(j = i + 1) on lift i for j > i, and ti and tj are the times at which lifts i and j, respectively, were placed. Cc’ is the
Downloaded from wmr.sagepub.com at Library - Periodicals Dept on February 15, 2015
32
Waste Management & Research 33(1)
compression ratio, EDG is the total amount of strain that can occur owing to biological decomposition, d is the rate constant for biological decomposition (day-1), b is the coefficient of mechanical creep (m2 kN-1) and c is the rate constant for mechanical creep (day-1). The parameters, such as compression ratio, coefficient of mechanical creep, rate of biodegradation of MSW and the total biodegradable strain, are crucial in modelling settlements and need to be obtained from the experiments conducted on the MSW samples. To estimate the biodegradation induced settlements, it is assumed that degradation is a function of observed biogas production and the settlements begin with the onset of gas production. Settlement calculations are done based on the initial quantity of biodegradable content, the inerts, volume change and the time taken for biogas production. The values of the model parameters like b, c, d and EDG given in equation (1c) that are used in modelling MSW settlements are calculated from the experiments. Equation (1a) shows that the settlement is a function of overburden pressure as reflected in the second term. The contribution of primary settlements, effect of overburden and mechanically induced creep, as well as settlements owing to biodegradation, is important in landfill design and construction. Experiments have been conducted and results are obtained and analysed to examine this aspect. Depending on the amount of information available for a particular landfill under consideration, the parameter values need to be evaluated to obtain each component of settlement and ultimate settlement. In this study, the evaluation of the components of settlement is restricted to laboratory scale studies and further field scale studies have to be conducted for further analysis of settlements.
Objectives The objective of the study is to present an approach for classifying and estimating three components of settlements. The results of a series of laboratory triaxial tests, oedometer tests, creep tests and anaerobic reactor cell setups have been used to delineate the components of settlement. Triaxial tests have been conducted to separate primary and secondary settlement based on the time required for 100% pore pressure dissipation and coefficient of consolidation. The components of secondary settlement are separated based on the time required for the onset of biogas production in the gas generation cells. The laboratory triaxial tests, oedometer tests and creep tests are normally available in Civil Engineering and anaerobic reactor cell setups have been fabricated to understand the role of biodegradation in settlement.
Site description and composition analysis MSW rules in India specify that biodegradable wastes should be processed by composting, vermi composting, etc., and landfilling shall be restricted to non-biodegradable inert waste and compost rejects (Municipal Solid Waste Rules, 2000). Several pre-treatment methods have been developed in recent times in order to recover the materials and to minimise the organic content reaching the landfills. Composting has been adopted as a potential pre-treatment method in Mavallipura landfill located in the outskirts of Bangalore. The MSW
used in this study is the compost reject collected from the Mavallipura landfill site, Bangalore and is referred as MSW in this article. The MSW entering the landfill undergoes a number of processes before being landfilled. Hand sorting of the waste is done to recover the waste that can be reused or recycled. Aerobic windrow composting is done for a period of 2 months. Screening of the compost is done using large screens of size 35 mm and 16 mm to further segregate the recyclable materials. The waste retained by the large screens mostly contained of large plastics, rubber shoes, leather bags and other inert materials that were hand sorted or removed by other mechanical procedures. The waste of size
research-article2014
WMR0010.1177/0734242X14558667Waste Management & ResearchSivakumar Babu and Lakshmikanthan
Original Article
Estimation of the components of municipal solid waste settlement
Waste Management & Research 2015, Vol. 33(1) 30–38 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14558667 wmr.sagepub.com
GL Sivakumar Babu and P Lakshmikanthan
Abstract Estimation of the municipal solid waste settlements and the contribution of each of the components are essential in the estimation of the volume of the waste that can be accommodated in a landfill and increase the post-usage of the landfill. This article describes an experimental methodology for estimating and separating primary settlement, settlement owing to creep and biodegradation-induced settlement. The primary settlement and secondary settlement have been estimated and separated based on 100% pore pressure dissipation time and the coefficient of consolidation. Mechanical creep and biodegradation settlements were estimated and separated based on the observed time required for landfill gas production. The results of a series of laboratory triaxial tests, creep tests and anaerobic reactor cell setups were conducted to describe the components of settlement. All the tests were conducted on municipal solid waste (compost reject) samples. It was observed that biodegradation accounted to more than 40% of the total settlement, whereas mechanical creep contributed more than 20% towards the total settlement. The essential model parameters, such as the compression ratio (Cc’), rate of mechanical creep (c), coefficient of mechanical creep (b), rate of biodegradation (d) and the total strain owing to biodegradation (EDG), are useful parameters in the estimation of total settlements as well as components of settlement in landfill. Keywords Municipal solid waste, primary settlement, mechanical creep, biodegradation induced settlement
Introduction Prediction of municipal solid waste (MSW) settlement is one of the most important issues that must be considered for landfill design and operations. Over the last several decades, many researchers have performed field testing, laboratory testing, theoretical and numerical analyses in this field. A few of these researchers include: Sowers (1973), Edil et al. (1990), Yen and Scanlon (1975), Park and Lee (1997), Gourc et al. (2010) and Babu Sivakumar et al. (2010a, 2010b, 2011). Landfill settlement is the product of a combination of processes, including load and biodegradation-related phenomena. The monitoring of settlements is crucial to calculate the final volume of waste that might be accommodated, to determine when the final cover should be placed, and to avoid damage to biogas recovery and leachate recirculation systems (Hettiarachchi et al., 2009). Settlements create significant problems during the post-closure period of the landfill (Sowers, 1973; Durmusoglu et al., 2005). Deformations of high magnitude are observed after the primary compression of MSW in the landfill. Hence, landfill settlement mechanisms can be classified as a combination of primary and secondary settlements. It is commonly accepted that the secondary settlement of MSW occurs through mechanical and biological processes (Hettiarachchi et al., 2007a; Machado et al., 2008). Total settlement in MSW landfills can be considerable, from 25% to 50% of original fill thickness (Bjarngard and Edgers, 1990). Landfill settlement continues over an extended period of time, with a final
settlement that can approach 20%−30% of the initial fill height (Qian et al., 2002). Huitric (1981) recorded maximum observed settlements at Mission Canyon to be 80% of their estimated ultimate settlement (30%−34%). According to Edil et al. (1990) and Simoes and Campos (2002), the identification of settlement mechanisms in MSW landfills is important for the interpretation of geomechanical behaviour, proposition of long-term settlement models and carrying out long-term simulations that help in landfill design. Several approaches and models for estimating the total landfill settlement have been proposed. Though many researchers measured/predicted the total MSW settlement, the separation of components of settlement and the role played by each component of settlement needs to be established. While the measurements of total settlement are carried out in the field, the evaluation of components of settlements is useful in landfill engineering practice. The trends in landfill technology are changing; for example in conventional landfills, mechanical-related settlements, such as compression under loading and settlements owing to creep, dominate Indian Institute of Science, Bangalore, India Corresponding author: GL Sivakumar Babu, Indian Institute of Sciences, Civil, Bangalore 560012, India. Email: [email protected]
Downloaded from wmr.sagepub.com at Library - Periodicals Dept on February 15, 2015
31
Sivakumar Babu and Lakshmikanthan in total settlements, whereas biodegradation-related settlements are expected to be dominating in bioreactor landfills. The leachate circulation systems make the landfill saturated and enhance biodegradation. Traditionally, the settlement theory from the soils is used in MSW settlement analysis. Several factors like composition of MSW, moisture content, density, waste placement and age of waste have considerable effect on the settlement of MSW. MSW settlement is mainly attributed to: (1) physical and mechanical processes that include the reorientation of particles, movement of the fine materials into larger voids, and collapse of void spaces; (2) chemical processes that include corrosion, combustion and oxidation; (3) dissolution processes that consist of dissolving soluble substances by percolating liquids and then forming leachate; and (4) biological decomposition of organics with time depending on humidity and the amount of organics present in the waste. Several models are reported in the published literature to predict MSW landfill settlement. These models may be grouped as: (1) soil mechanics-based models, (2) empirical models, (3) rheological models, and (4) models incorporating biodegradation. The total settlement is essentially attributed to three components, viz, mechanical, creep and biodegradation and it is desirable to evaluate the three components of settlements. Models that integrate mechanical-, biodegradation- and physico-chemical-related phenomena were developed by Hettiarachchi et al. (2007a, 2007b, 2009), Gourc et al. (2010), Babu et al. (2010a, 2010b), McDougall (2011) and Liu et al. (2011). Of the models reviewed, models of Marques (2001) and Marques et al. (2003) are shown to consider all the mechanisms, as well as being simple for implementation in the landfill sites. Marques (2001) developed a composite rheological model to account for primary and secondary compression mechanisms, governed by rheological parameters that also accounts for waste degradation. The primary compression formulation is introduced as an ‘immediate compression’, which is independent of time, based on the observation that the respective process is linear for curves of void ratio as a function of the logarithm of the applied stress. The secondary compression is governed by the timedependent phenomenon in exponential function similar to the assumption in Gibson and Lo’s (1961) model, which is given by:
ε C = b∆σ (1 − e− ct′ ) (1a)
where ΔH is the settlement, H is the initial height of waste, Cc’ is the primary compression ratio, b is the coefficient of secondary mechanical compression, c is the secondary mechanical compression rate, Edg is the total compression owing to waste degradation, d is the secondary biological compression rate, t’ is the time elapsed since loading application and t” is the time elapsed since waste disposal. Marques et al. (2003) further developed the above model to consider lifts and lift placement intervals as MSW is usually placed in lifts in landfills over a period of years. In implementing the composite model, waste placement is idealised as progressing in a series of lifts. The thickness of the lifts may be set equal to the compacted thickness of the daily cells. After all lifts have been placed, the settlement ΔH of the landfill surface at time any time (t) is determined. The total strain is given below and the three ‘ε’ are defined by: ε = ε p + εc + εb (2a)
The three terms, εp, εc and εb, represent strain resulting from instantaneous response to applied load, time-dependent strain owing to mechanical creep and time-dependent strain owing to biological decomposition. The total settlement is given by: N
∆Η =
∑ H ε i
pi
i =1
+ ε Ci (t ) + ε Bi (t ) (2b)
where N is the number of lifts in the landfill, Hi is the initial thickness of compacted lift i, εPi is the strain in lift i resulting from instantaneous response to loading from overlying lifts, εCi is the strain at time t in lift i owing to mechanical creep associated with the stresses from self-weight and the weight of overlying lifts and εBi is the strain at time t in lift i owing to biological decomposition of lift i. The strains are given by:
N 1 ∆σ ij γiΗi + 2 j =i +1 ’ εΡi = Cc log 1 γiΗi 2
∑
1 εCi (t ) = b γ i Η i (1 − e− c (t −t ) ) + 2 i
(2c)
N
∑ ∆σ
ij (1 − e
j =i +1
− c ( t −t j )
) (2d)
The time-dependent biological degradation is proposed by Park and Lee (1997) and is given by:
ε b = Edg (1 − e
− dt "
σ + ∆σ ∆H − ct ′ = Cc′ log 0 + + ∆σ .b. 1 − e H σ 0 (1c) Edg . 1 − e− dt "
(
(
i
) (1b)
The settlement model is represented by:
ε Bi (t ) = EDG (1 − e − d (t −t ) ) (2e)
)
)
where γi is the unit weight of lift i is a composite value representing the weighted average of the compacted waste and daily cover in lift i. Because, t is a time after all lifts of the landfill have been placed, t – ti > 0 and t – tj > 0 for all values of i and j. Δσ is the change in vertical stresses imposed by lift j(j = i + 1) on lift i for j > i, and ti and tj are the times at which lifts i and j, respectively, were placed. Cc’ is the
Downloaded from wmr.sagepub.com at Library - Periodicals Dept on February 15, 2015
32
Waste Management & Research 33(1)
compression ratio, EDG is the total amount of strain that can occur owing to biological decomposition, d is the rate constant for biological decomposition (day-1), b is the coefficient of mechanical creep (m2 kN-1) and c is the rate constant for mechanical creep (day-1). The parameters, such as compression ratio, coefficient of mechanical creep, rate of biodegradation of MSW and the total biodegradable strain, are crucial in modelling settlements and need to be obtained from the experiments conducted on the MSW samples. To estimate the biodegradation induced settlements, it is assumed that degradation is a function of observed biogas production and the settlements begin with the onset of gas production. Settlement calculations are done based on the initial quantity of biodegradable content, the inerts, volume change and the time taken for biogas production. The values of the model parameters like b, c, d and EDG given in equation (1c) that are used in modelling MSW settlements are calculated from the experiments. Equation (1a) shows that the settlement is a function of overburden pressure as reflected in the second term. The contribution of primary settlements, effect of overburden and mechanically induced creep, as well as settlements owing to biodegradation, is important in landfill design and construction. Experiments have been conducted and results are obtained and analysed to examine this aspect. Depending on the amount of information available for a particular landfill under consideration, the parameter values need to be evaluated to obtain each component of settlement and ultimate settlement. In this study, the evaluation of the components of settlement is restricted to laboratory scale studies and further field scale studies have to be conducted for further analysis of settlements.
Objectives The objective of the study is to present an approach for classifying and estimating three components of settlements. The results of a series of laboratory triaxial tests, oedometer tests, creep tests and anaerobic reactor cell setups have been used to delineate the components of settlement. Triaxial tests have been conducted to separate primary and secondary settlement based on the time required for 100% pore pressure dissipation and coefficient of consolidation. The components of secondary settlement are separated based on the time required for the onset of biogas production in the gas generation cells. The laboratory triaxial tests, oedometer tests and creep tests are normally available in Civil Engineering and anaerobic reactor cell setups have been fabricated to understand the role of biodegradation in settlement.
Site description and composition analysis MSW rules in India specify that biodegradable wastes should be processed by composting, vermi composting, etc., and landfilling shall be restricted to non-biodegradable inert waste and compost rejects (Municipal Solid Waste Rules, 2000). Several pre-treatment methods have been developed in recent times in order to recover the materials and to minimise the organic content reaching the landfills. Composting has been adopted as a potential pre-treatment method in Mavallipura landfill located in the outskirts of Bangalore. The MSW
used in this study is the compost reject collected from the Mavallipura landfill site, Bangalore and is referred as MSW in this article. The MSW entering the landfill undergoes a number of processes before being landfilled. Hand sorting of the waste is done to recover the waste that can be reused or recycled. Aerobic windrow composting is done for a period of 2 months. Screening of the compost is done using large screens of size 35 mm and 16 mm to further segregate the recyclable materials. The waste retained by the large screens mostly contained of large plastics, rubber shoes, leather bags and other inert materials that were hand sorted or removed by other mechanical procedures. The waste of size
