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.     

Geotechnical properties of fresh municipal solid waste at Orchard Hills Landfill, USA

This paper presents the results of a laboratory investigation to determine the geotechnical properties of fresh municipal solid waste (MSW) collected from the working phase of Orchard Hills Landfill located in Davis Junction (Illinois, USA). Laboratory testing was conducted on shredded MSW to determine the compaction, hydraulic conductivity, compressibility, and shear strength properties at in-situ gravimetric moisture content of 44%. In addition, the effect of increased moisture content during leachate recirculation on compressibility and shear strength of MSW was also investigated by testing samples with variable gravimetric moisture contents ranging from 44% to 100%. Based on Standard Proctor tests, a maximum dry density of 420 kg/m(3) was observed at 70% optimum moisture content. The hydraulic conductivity varied in a wide range of 10(-8)-10(-4)m/s and decreased with increase in dry density. Compression ratio values varied in a close range of 0.24-0.33 with no specific trend with the increase in moisture content. Based on direct shear tests, drained cohesion varied from 31 to 64 kPa and the drained friction angle ranged from 26 to 30 degrees. Neither cohesion nor friction angle demonstrated any correlation with the moisture content, within the range of moisture contents tested. The consolidated undrained triaxial shear tests on saturated MSW showed the total strength parameters (c and phi) to be 32 kPa and 12 degrees, and the effective strength parameters (c' and phi') to be 38 kPa and 16 degrees. The angle of friction (phi) decreased and cohesion (c) value increased with the increase in strain. The effective cohesion (c') increased with increase in strain; however, the effective angle of friction (phi') decreased first and then increased with the increase in strain. Such strain-dependent shear strength properties should be properly accounted in the stability analysis of bioreactor landfills.     

Removal of unburned carbon from municipal solid waste fly ash by column flotation

Unburned carbon (UC) is the major source of organic contaminants in municipal solid waste (MSW) fly ash. So most organic contaminants can be removed by the removal of the UC from the MSW fly ash. In this paper, we first used a technique of column flotation to remove UC from MSW fly ash. The influences of column flotation parameters on the recovery efficiency of UC were systematically studied. It was found that the UC recovery efficiency was greatly influenced by the gas flow rate, pH value, collector kerosene's concentration and the types of fly ash. By optimizing the above parameters, we have successfully removed 61.2% of the UC from MSW fly ash having 5.24% UC content. The removal mechanism was well accounted for the kinetic theory of column flotation and surface-chemistry theory. The results indicate that the column flotation technique is effective in removing the UC from MSW fly ash, and show that there is a strong possibility for practical application of this technique in removing the organic contaminants from MSW fly ash.     

Predicting the heating value of MSW with a feed forward neural network

The influence of the heating value of municipal solid waste (MSW) is very important on the combustion efficiency of MSW incinerators. The heating value of MSW is determined by the elementary chemical composition of its various components. Commonly, calorimetric measurement and empirical methods are available for this determination. In this analysis, the relationship between the physical composition and the low heating value (LHV) was studied. A feed forward neural network (FFNN) can be very helpful in predicting the heating value of MSW from its physical composition. The results of this analysis show that the prediction of LHV of MSW with FFNN is much better than conventional models.     

Geotechnical properties of municipal solid waste at different phases of biodegradation

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.     

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.     

Chloride extraction for quality improvement of municipal solid waste incinerator ash for the concrete industry.

Coal ash from power stations has long been used successfully in the cement industry as binders in several Portland formulations. This is not the case for municipal solid waste (MSW) ash as chloride concentrations, ranging from 10 to 200 g kg(-1) dry weight in the bottom and fly ash, respectively, exceed the maximum allowable concentration in most cement mixtures. To reduce chloride content in MSW bottom ash, a laboratory investigation was carried out based on the exhaustive washing in tap water. The influence of operative parameters such as temperature, granulometric properties and solid/liquid ratio of extraction was evaluated. In addition to optimization of the mentioned operative parameters for full-scale application, the paper gives preliminary indications on mechanistic aspects of the washing operation.     

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.     

Combustible and incombustible speciation of Cl and S in various components of municipal solid waste

Chlorine (Cl) and sulfur (S) in municipal solid waste (MSW) are important reactive elements during combustion. They generate the acidic pollutants HCl and SOx, and, furthermore, produce and suppress organic chlorinated compounds. Nevertheless, few practical reports about Cl and S content in MSW have been published. In combustion and recycling processes, both combustible Cl and S, and incombustible Cl and S species are equally important. This paper presents the results of a comprehensive study about combustible and incombustible Cl and S in MSW components, including kitchen garbage, paper, textiles, wood and leaves, plastics and small chips. By integrating this collected data with data about MSW composition, not only the overall content of Cl and S in MSW, but also the origins of both combustible and incombustible Cl and S were estimated. The average Cl content in bulk MSW was 3.7 g/kg of raw MSW, of which 2.7 and 1.0 g/kg were combustible and incombustible, respectively. The Cl contribution from plastics was 76% and 27% with respect to combustible and incombustible states. The average S content in bulk MSW was 0.81 g/kg of raw MSW, of which 0.46 g/kg was combustible and 0.35 g/kg was incombustible. Combustible S was mainly due to synthetic textiles, while incombustible S was primarily from paper.     

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.     

Residential recycling programs: Environmental,economic and disposal factors

The type of recycling program initiated by state or local government is related to (a) the environmental awareness of the population and the subsequent reflection of that awareness in government, (b) the state's municipal solid waste (MSW) disposal situation, and (c) the market conditions for recycled materials. Two hypotheses are studied: (1) voluntary participation recycling programs are initiated when environmental awareness is the driving force and market conditions are good and (2) mandatory participation recycling programs result when the driving force is MSW disposal difficulties.New Jersey and Oregon, two of the states, with statewide mandatory and voluntary recycling programs respectively, are studied with respect to the events leading to the enactment of their recycling programs. The hypotheses are supported by the events in each state. Hypothesis 2 is further supported by data from 48 states comparing information on each state's MSW disposal situation with the local and statewide recycling programs reported in a survey. The percentages of mandatory recycling programs increase with increasing MSW disposal difficulty. A measure of environmental awareness must be developed before hypothesis one can be studied with cross-sectional data.     

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.     

Accelerated carbonation of steel slags in a landfill cover construction

Steel slags from high-alloyed tool steel production were used in a full scale cover construction of a municipal solid waste (MSW) landfill. In order to study the long-term stability of the steel slags within the final cover, a laboratory experiment was performed. The effect on the ageing process, due to i.e. carbonation, exerted by five different factors resembling both the material characteristics and the environmental conditions is investigated. Leaching behaviour, acid neutralization capacity and mineralogy (evaluated by means of X-ray diffraction, XRD, and thermogravimetry/differential thermal analysis, TG/DTA) are tested after different periods of ageing under different conditions.Samples aged for 3 and 10 months were evaluated in this paper. Multivariate data analysis was used for data evaluation. The results indicate that among the investigated factors, ageing time and carbon dioxide content of the atmosphere were able to exert the most relevant effect. However, further investigations are required in order to clarify the role of the temperature.     

Field evaluation of resistivity sensors for in situ moisture measurement in a bioreactor landfill

The ability of resistance-based sensors to measure in situ waste moisture content in a landfill was examined. One hundred and thirty-five resistance-based sensors were installed in a leachate recirculation well field at a bioreactor landfill in Florida, US. The performance of these sensors was studied for a period of over 6 years. The sensors were found to respond to an increase in moisture resulting from leachate recirculation. It was observed that 78% of sensors worked successfully in the field during the study period. The initial spatial average moisture content determined by the sensor readings (using a laboratory-derived calibration) was 42.8% compared to 23% from gravimetric readings. Eighteen sensors (13%) showed that they were saturated before liquid addition, and no change in moisture content was observed in these sensors during the study period. Laboratory-derived calibration methods resulted in an over-estimation of moisture content. An alternate field-calibration method, where wetted sensor output was assumed equal to the average of gravimetric measurements for wet samples, was evaluated. The final spatial average moisture contents were 64.2% and 44.4% for the laboratory-derived and field-derived calibration methodologies, respectively, compared to 45% measured gravimetrically from excavated waste samples. When moisture content was determined using a mass balance approach, the result was 34.6%. The results suggest that when appropriately calibrated, resistivity-based sensors can be used to obtain a reasonably accurate estimate of local moisture content. However, caution should be taken to extend the moisture content values that are representative of waste surrounding the sensors to estimate the overall moisture content on the landfill-wide scale.     

Moisture retention of municipal solid waste mixed with sewage sludge and ash in a semi-arid climate.

Mechanisms involved in moisture storage in refuse are explored using data from four sets of experiments in a semi-arid climate. Two laboratory series of experiments contained municipal solid waste (MSW) amended with sewage sludge, one with higher proportions of ash in the MSW than the other. Outdoor experiments contained waste streams with different proportions of ash. Field cells compared moisture retention of refuse and MSW co-disposed with sewage sludge. Sewage sludge at high loads was found to increase the moisture storage relative to unamended MSW. Belt-pressed sludge retained water as bound water that was released by decay and changing pH. Sun-dried sludge also retained more moisture than MSW alone. In gravimetric terms, ash reduced the storage potential of MSW, in laboratory and outdoor experiments. However, outdoor experiments released less leachate from ash-rich refuse than middle-income waste with no ash fraction.     

Evaluation of the geotechnical properties of MSW in two Brazilian landfills

The characteristics of municipal solid waste (MSW) play a key role in many aspects of waste disposal facilities and landfills. Because most of a landfill is made up of MSW, the overall stability of the landfill slopes are governed by the strength parameters and physical properties of the MSW. These parameters are also important in interactions involving the waste body and the landfill structures: cover liner, leachate and gas collection systems. On the other hand, the composition of the waste, which affects the geotechnical behavior of the MSW, is dependent on a variety of factors such as climate, disposal technology, the culture and habits of the local community. It is therefore essential that the design and stability evaluations of landfills in each region be performed based on the local conditions and the geotechnical characteristic of the MSW. The Bandeirantes Landfill, BL, in São Paulo and the Metropolitan Center Landfill, MCL, in Salvador, are among the biggest landfills in Brazil. These two disposal facilities have been used for the development of research involving waste mechanics in recent years. Considerable work has been made in the laboratory and in the field to evaluate parameters such as water and organic contents, composition, permeability, and shear strength. This paper shows and analyzes the results of tests performed on these two landfills. The authors believe that these results could be a good reference for certain aspects and geotechnical properties of MSW materials in countries with similar conditions.     

A design chart for estimation of horizontal displacement in municipal landfills

This paper describes the development of a design chart for the estimation of maximum horizontal displacement within a municipal landfill using the height and the side slope of the landfill. The design chart is based on the results of a finite element parametric study in which the behaviour of the municipal solid waste (MSW) was modeled using a non-linear elastic hyperbolic model. The model input parameters, i.e. non-linear stiffness, shear strength and unit weight of MSW, were obtained from laboratory testing data and an extensive stochastic numerical modelling exercise. Non-linear variations of unit weight as well as Young’s modulus of MSW with depth were incorporated in the finite element analyses. The validity of the design chart was assessed using field monitoring results from a large landfill located in Ontario, Canada.     

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.     

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.     

Interpretations of surface movements of a landfill built on steeply sloping ground. A cautionary case history

The surface of a municipal solid waste (MSW) landfill cell, built on steeply sloping ground, was discovered to be moving downhill. The movements were measured and considered by the owners to be alarming. In an attempt to stop the movement, the compacted soil stability berm at the toe of the cell was raised by 5 m in two stages. The rate of movement declined, but the waste surface continued to move. Eventually, the cell was not filled to its designed full level. The paper describes the design of the landfill cell and its underliner, as well as three independent investigations into the causes of the surface movement. The first two investigators assumed that the surface movements represented shear displacements at the interface of the geomembrane and its protective geo-fabric, and therefore that the cell was failing in shear. The third investigation, undertaken in preparation of evidence in a civil court case, 7 years later, concluded that the movements resulted from down-slope shear straining of the land-filled MSW. The later investigation concluded that the MSW cell had been, and is still shear-stable, and it points to differences in behaviour between MSW landfill slopes and comparable slopes in mine waste or natural soils.