Decoupling MSW settlement into mechanical and biochemical processes – Modelling and validation on large-scale setups

Forecasting settlements of non-hazardous waste is essential to ensure the integrity and durability of landfill covers over time. Over a short time span, the survey of settlements may also contribute to the investigation of the biodegradation processes. This paper addresses secondary settlements of Municipal Solid Waste (MSW), a heterogeneous and time-evolving material.An analysis of available experimental data from different pilots and the literature was conducted to quantify the influence of biodegradation on MSW secondary settlements. After making assumptions about the various features of the waste and their constitutive relationships, a one-dimensional biomechanical model to predict the secondary settlement has been developed. The determination of the total secondary settlement was obtained by the addition of two separate parts, the mechanical settlement, due to creep, and the biochemical settlement, due to the degradation of the organic matter. The latter has been evaluated based on the observed biogas production. Using the data from different recent large-scale experiments that provide a monitoring of biogas production, a method for predicting the biochemically-induced settlements is proposed and validated on these tests. The relative contributions of mechanical and biochemical settlements are also calculated and discussed as a function of waste pre-treatment and operation conditions (biological pre-treatment, shredding, leachate injection). Finally, settlement may be considered as a relevant indicator for the state of biodegradation.     

Constitutive model for municipal solid waste incorporating mechanical creep and biodegradation-induced compression

A constitutive model is proposed to describe the stress–strain behavior of municipal solid waste (MSW) under loading using the critical state soil mechanics framework. The modified cam clay model is extended to incorporate the effects of mechanical creep and time dependent biodegradation to calculate total compression under loading. Model parameters are evaluated based on one-dimensional compression and triaxial consolidated undrained test series conducted on three types of MSW: (a) fresh MSW obtained from working phase of a landfill, (b) landfilled waste retrieved from a landfill after 1.5 years of degradation, and (c) synthetic MSW with controlled composition. The model captures the stress–strain and pore water pressure response of these three types of MSW adequately. The model is useful for assessing the deformation and stability of landfills and any post-closure development structures located on landfills.     

Parametric study of MSW landfill settlement model

A newly developed and validated constitutive model that accounts for primary compression and time-dependent mechanical creep and biodegradation is used for parametric study to investigate the effects of model parameters on the predicted settlement of municipal solid waste (MSW) with time. The model enables the prediction of stress strain response and yield surfaces for three components of settlement: primary compression, mechanical creep, and biodegradation. The MSW parameters investigated include compression index, coefficient of earth pressure at-rest, overconsolidation ratio, and biodegradation parameters of MSW. A comparison of the predicted settlements for typical MSW landfill conditions showed significant differences in time-settlement response depending on the selected model input parameters. The effect of lift thickness of MSW on predicted settlement is also investigated. Overall, the study shows that the variation in the model parameters can lead to significantly different results; therefore, the model parameter values should be carefully selected to predict landfill settlements accurately. It is shown that the proposed model captures the time settlement response which is in general agreement with the results obtained from the other two reported models having similar features.     

Evolution of unsaturated hydraulic properties of municipal solid waste with landfill depth and age

Successful modeling of liquid and air flow and hence designing of liquid and air addition systems in the landfills are constrained by the lack of key parameters of unsaturated hydraulic properties of municipal solid waste (MSW), which are strongly dependent on the depth of burial and the degree of decomposition. In this study, water retention curves (WRC) of MSW are measured using pressure plate method on samples repacked according to the in situ unit weight measured during borehole sampling, representing the MSW in shallow, middle, and deep layers. The measured WRC of MSW is well-reproduced by the van Genuchten-Mualem model, and is used to predict the unsaturated hydraulic properties of MSW, including water retention characteristics and unsaturated hydraulic conductivity. The estimated model parameters are consistent with other studies, suggesting that the pressure plate method yields reproducible results. As the landfill depth and age increase, the overburden pressure, the highly decomposed organic matter and finer pore space increase, hence the capillary pressure increases, causing increases in air-entry values, field capacity and residual water content, and decreases in steepness of WRC and saturated water content. The unsaturated hydraulic properties of MSW undergo changes with landfill depth and age, showing more silt loam-like properties as the landfill age increases.     

Modeling microbiological and chemical processes in municipal solid waste bioreactor,part I: Development of a three-phase numerical model BIOKEMOD-3P

The numerical computer models that simulate municipal solid waste (MSW) bioreactor landfills have mainly two components – a biodegradation process module and a multi-phase flow module. The biodegradation model describes the chemical and microbiological processes. The models available to date include predefined solid waste biodegradation reactions and participating species. Some of these models allow changing the basic composition of solid waste. In a bioreactor landfill several processes like anaerobic and aerobic solids biodegradation, nitrogen and sulfate related processes, precipitation and dissolution of metals, and adsorption and gasification of various anthropogenic organic compounds occur simultaneously. These processes may involve reactions of several species and the available biochemical models for solid waste biodegradation do not provide users with the flexibility to simulate these processes by choice. This paper presents the development of a generalized biochemical process model BIOKEMOD-3P which can accommodate a large number of species and process reactions. This model is able to simulate bioreactor landfill operation in a completely mixed condition, when coupled with a multi-phase model it will be able to simulate a full-scale bioreactor landfill. This generalized biochemical model can simulate laboratory and pilot-scale operations in order to determine biochemical parameters important for simulation of full-scale operations.     

Approach for the use of MSW settlement predictions in the assessment of landfill capacity based on reliability analysis

In the analysis and design of municipal solid waste (MSW) landfills, there are many uncertainties associated with the properties of MSW during and after MSW placement. Several studies are performed involving different laboratory and field tests to understand the complex behavior and properties of MSW, and based on these studies, different models are proposed for the analysis of time dependent settlement response of MSW. For the analysis of MSW settlement, it is very important to account for the variability of model parameters that reflect different processes such as primary compression under loading, mechanical creep and biodegradation. In this paper, regression equations based on response surface method (RSM) are used to represent the complex behavior of MSW using a newly developed constitutive model. An approach to assess landfill capacities and develop landfill closure plans based on prediction of landfill settlements is proposed. The variability associated with model parameters relating to primary compression, mechanical creep and biodegradation are used to examine their influence on MSW settlement using reliability analysis framework and influence of various parameters on the settlement of MSW are estimated through sensitivity analysis.     

Waste settlement in bioreactor landfill models

Prediction of landfill settlement is one of the important parameters that affects the design and maintenance of bioreactor landfills. Due to the large number of variables involved in the settlement mechanism, accurate prediction of landfill settlement is a real challenge. The operational protocol of a landfill, the presence of municipal sludge from treatment plants, the addition of soybean peroxidase (SBP) enzymes, and the fraction of organic matter in the municipal solid waste (MSW) have to be reflected in the parameters of any model used to predict the settlement of MSW. In this work, a biodegradation-induced settlement model incorporating two parameters (A and B) was developed. The settlement data of two researchers were used to estimate the parameter values with two different approaches; the first considered the overall experiment and results, and the second separated the aerobic phase, if present, from the anaerobic phase. The rate of initial settlement occurring under aerobic conditions has been greater than that under anaerobic conditions. Parameters increased with the increase in the concentration of enzymes and with the presence of sludge in both aerobic and anaerobic stages. Increasing organic content of MSW has resulted in the enhancement of the biodegradation rate and settlement. This has been reflected on the higher values of the parameters compared to their values in the absence of organic waste.     

Perpetual landfilling through aeration of the waste mass; lessons from test cells in Georgia (USA).

Municipal solid waste (MSW) landfills worldwide are experiencing the consequences of conventional landfilling techniques, whereby anaerobic conditions are created within the landfilled waste. Under anaerobic conditions within a landfill site slow stabilization of the waste mass occurs, producing methane, (an explosive 'green house' gas) and leachate (which can pollute groundwater) over long periods of time. As a potential solution, it was demonstrated that the aerobic degradation of MSW within a landfill can significantly increase the rate of waste decomposition and settlement, decrease the methane production and leachate leaving the system, and potentially increase the operational life of the site. Readily integrated into the existing landfill infrastructure, this approach can safely and cost-effectively convert a MSW landfill from anaerobic to aerobic degradation processes, thereby effectively composting much of the organic portions (one of the potentially polluting elements in a conventional landfill site) of the waste. This paper summarizes the successful results of two separate aerobic landfill projects located in Georgia (USA) and discusses the potential economic and environmental impacts to worldwide solid waste management practices.     

Aging and compressibility of municipal solid wastes

The expansion of a municipal solid waste (MSW) landfill requires the ability to predict settlement behavior of the existing landfill. The practice of using a single compressibility value when performing a settlement analysis may lead to inaccurate predictions. This paper gives consideration to changes in the mechanical compressibility of MSW as a function of the fill age of MSW as well as the embedding depth of MSW. Borehole samples representative of various fill ages were obtained from five boreholes drilled to the bottom of the Qizhishan landfill in Suzhou, China. Thirty-one borehole samples were used to perform confined compression tests. Waste composition and volume-mass properties (i.e., unit weight, void ratio, and water content) were measured on all the samples. The test results showed that the compressible components of the MSW (i.e., organics, plastics, paper, wood and textiles) decreased with an increase in the fill age. The in situ void ratio of the MSW was shown to decrease with depth into the landfill. The compression index, Cc, was observed to decrease from 1.0 to 0.3 with depth into the landfill. Settlement analyses were performed on the existing landfill, demonstrating that the variation of MSW compressibility with fill age or depth should be taken into account in the settlement prediction.     

Development of a purpose built landfill system for the control of methane emissions from municipal solid waste

In the present paper, a new system of purpose built landfill (PBLF) has been proposed for the control of methane emissions from municipal solid waste (MSW), by considering all favourable conditions for improved methane generation in tropical climates. Based on certain theoretical considerations multivariate functional models (MFMs) are developed to estimate methane mitigation and energy generating potential of the proposed system. Comparison was made between the existing waste management system and proposed PBLF system. It has been found that the proposed methodology not only controlled methane emissions to the atmosphere but also could yield considerable energy in terms of landfill gas (LFG). Economic feasibility of the proposed system has been tested by comparing unit cost of waste disposal in conventional as well as PBLF systems. In a case study of MSW management in Mumbai (INDIA), it was found that the unit cost of waste disposal with PBLF system is seven times lesser than that of the conventional waste management system. The proposed system showed promising energy generation potential with production of methane worth of Rs. 244 millions/y ($5.2 million/y). Thus, the new waste management methodology could give an adaptable solution for the conflict between development, environmental degradation and natural resources depletion.     

Settlement analysis of fresh and partially stabilised municipal solid waste in simulated controlled dumps and bioreactor landfills

The patterns of settlement of fresh as well as partially stabilised municipal solid waste (MSW), undergoing degradation in five different landfill lysimeters, were studied elaborately. The first two lysimeters, R1 and R2, contained fresh MSW while the other three lysimeters, R3, R4 and R5, contained partially stabilised MSW. R1 and R3 simulated conventional controlled dumps with fortnightly disposal of drained leachate. R2 and R4 simulated bioreactor landfills with leachate recirculation. Fortnightly water flushing was done in R5. Settlement of MSW, monitored over a period of 58 weeks, was correlated with the organic carbon content of leachate and residual volatile matter in the MSW to establish the relationship between settlement and organic destruction. Compressibility parameters such as modulus of elasticity and compression indices were determined and empirical equations were applied for the settlement data. Overall settlements up to 49% were observed in the case of landfill lysimeters, filled with fresh MSW. Landfill lysimeters with liquid addition, in the form of leachate or water, experienced lower primary settlements and higher secondary settlements than conventional fills, where no liquid addition was practised. Modified secondary compression indices for MSW in lysimeters with leachate recirculation and flushing were 30%-44% higher than that for lysimeters where no liquid addition was done. Secondary settlements in bioreactor landfills were found to vary exponentially with time.     

Municipal solid waste effective stress analysis

The mechanical behavior of municipal solid waste (MSW) has attracted the attention of many researchers in the field of geo-environmental engineering in recent years and several aspects of waste mechanical response under loading have been elucidated. However, the mechanical response of MSW materials under undrained conditions has not been described in detail to date. The knowledge of this aspect of the MSW mechanical response is very important in cases involving MSW with high water contents, seismic ground motion and in regions where landfills are built with poor operation conditions. This paper presents the results obtained from 26 large triaxial tests performed both in drained and undrained conditions. The results were analyzed taking into account the waste particles compressibility and the deformation anisotropy of the waste samples. The waste particles compressibility was used to modify the Terzaghi effective stress equation, using the Skempton (1961) proposition. It is shown that the use of the modified effective stress equation led to much more compatible shear strength values when comparing Consolidated-Drained (CD) and Consolidated-Undrained (CU), results, explaining the high shear strength values obtained in CU triaxial tests, even when the pore pressure is almost equal to the confining stress.     

The effect of moisture regimes on the anaerobic degradation of municipal solid waste from Metepec (México)

The State of México, situated in central México, has a population of about 14 million, distributed in approximately 125 counties. Solid waste management represents a serious and ongoing pressure to local authorities. The final disposal site ("El Socavón") does not comply with minimum environmental requirements as no liners or leachate management infrastructure are available. Consequently, leachate composition or the effects of rain water input on municipal solid waste degradation are largely unknown. The aim of this work was to monitor the anaerobic degradation of municipal solid waste (MSW), simulating the water addition due to rainfall, under two different moisture content regimes (70% and 80% humidity). The study was carried out using bioreactors in both laboratory and pilot scales. The variation of organic matter and pH was followed in the solid matrix of the MSW. The leachate produced was used to estimate the field capacity of the MSW and to determine the pH, COD, BOD and heavy metals. Some leachate parameters were found to be within permitted limits, but further research is needed in order to analyze the leachate from lower layers of the disposal site ("El Socavón").     

A laboratory study on the MSW mechanical behavior in triaxial apparatus

Shear strength characterization of MSW materials is a mandatory task when performing analyses related to landfill design and landfill geometry improvements. Despite the considerable amount of research focusing on MSW mechanical behavior there remain certain aspects which are not completely understood and deserve attention in particular the case of the undrained behavior of MSW. This paper presents the results of a comprehensive laboratory testing program using a large-scale triaxial apparatus at the Federal University of Bahia, Salvador, Brazil. The effect of factors such as confining pressure, unit weight, fiber content, rate of loading and over-consolidation on the MSW mechanical response were investigated. Tested samples presented typical MSW shear/strain curves (concave upward) in all the tests, despite the pore water pressure reaching levels almost equal to the confining pressure. The obtained results show that increasing confining stress, unit weight, loading rate, fiber content and over-consolidation lead to an increase in the MSW shear strength. The importance of the fibrous components in the waste behavior is highlighted and graphs showing the variation of the MSW shear strength with fiber content in different drainage conditions are shown. The authors believe these results could be of interest to many companies, especially considering the new trend of plastic material recycling (prior landfilling) for energy recovery purposes.     

Comparison of aerobic and anaerobic stability indices through a MSW biological treatment process

A complex mechanical-biological waste treatment plant designed for the processing of mixed municipal solid wastes (MSW) and source-selected organic fraction of municipal solid wastes (OFMSW) has been studied by using stability indices related to aerobic (respiration index, RI) and anaerobic conditions (biochemical methane potential, BMP). Several selected stages of the plant have been characterized: waste inputs, mechanically treated wastes, anaerobically digested materials and composted wastes, according to the treatment sequence used in the plant. Results obtained showed that the main stages responsible for waste stabilization were the two first stages: mechanical separation and anaerobic digestion with a diminution of both RI and BMP around 40% and 60%, respectively, whereas the third stage, composting of digested materials, produced lesser biological degradation (20-30%). The results related to waste stabilization were similar in both lines (MSW and OFMSW), although the indices obtained for MSW were significantly lower than those obtained for OFMSW, which demonstrated a high biodegradability of OFMSW. The methodology proposed can be used for the characterization of organic wastes and the determination of the efficiency of operation units used in mechanical-biological waste treatment plants.     

Long-term behavior of municipal solid waste landfills

A method is presented to predict the long-term behavior of element concentrations (non-metals and metals) in the leachate of a municipal solid waste (MSW) landfill. It is based on water flux and concentration measurements in leachates over one year, analysis of drilled cores from MSW landfills and leaching experiments with these samples. A mathematical model is developed to predict the further evolution of annual flux-weighted mean element concentrations in leachates after the “intensive reactor phase”, i.e. after the gas production has dropped to a very low level. The results show that the organic components are the most important substances to control until the leachate is compatible with the environment. This state of low emissions, the so-called “final storage quality”, will take many centuries to be achieved in a moderate climate.     

Prediction of syngas quality for two-stage gasification of selected waste feedstocks

This paper compares the syngas produced from methane with the syngas obtained from the gasification, in a two-stage reactor, of various waste feedstocks. The syngas composition and the gasification conditions were simulated using a simple thermodynamic model. The waste feedstocks considered are: landfill gas, waste oil, municipal solid waste (MSW) typical of a low-income country, the same MSW blended with landfill gas, refuse derived fuel (RDF) made from the same MSW, the same RDF blended with waste oil and a MSW typical of a high-income country. Energy content, the sum of H2 and CO gas percentages, and the ratio of H2 to CO are considered as measures of syngas quality. The simulation shows that landfill gas gives the best results in terms of both H2+CO and H2/CO, and that the MSW of low-income countries can be expected to provide inferior syngas on all three quality measures. Co-gasification of the MSW from low-income countries with landfill gas, and the mixture of waste oil with RDF from low-income MSW are considered as options to improve gas quality.     

Unsaturated consolidation theory for the prediction of long-term municipal solid waste landfill settlement.

The understanding of long-term landfill settlement is important for landfill design and rehabilitation. However, suitable models that can consider both the mechanical and biodecomposition mechanisms in predicting the long-term landfill settlement are generally not available. In this paper, a model based on unsaturated consolidation theory and considering the biodegradation process is introduced to simulate the landfill settlement behaviour. The details of problem formulations and the derivation of the solution for the formulated differential equation of gas pressure are presented. A step-by-step analytical procedure employing this approach for estimating settlement is proposed. The proposed model can generally model the typical features of short-term and long-term behaviour. The proposed model also yields results that are comparable with the field measurements.     

Gasification characteristics of MSW and an ANN prediction model

Gasification characteristics make up the important parts of municipal solid waste (MSW) gasification and melting technology. These characteristics are closely related to the composition of MSW, which alters with climates and seasons. It is important to find a practical way to predict gasification characteristics. In this paper, five typical kinds of organic components (wood, paper, kitchen garbage, plastic, and textile) and three representative types of simulated MSW are gasified in a fluidized-bed at 400-800 degrees C with the equivalence ratio (ER) in the range of 0.2-0.6. The lower heating value (LHV) of gas, gasification products, and gas yield are reported. The results indicate that gasification characteristics are different from sample to sample. Based on the experimental data, an artificial neural networks (ANN) model is developed to predict gasification characteristics. The training and validating relative errors are within +/-15% and +/-20%, respectively, and predicting relative errors of an industrial sample are below +/-25%. This indicates that it is acceptable to predict gasification characteristics via ANN model.