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.     

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.     

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 hybrid method for quasi-three-dimensional slope stability analysis in a municipal solid waste landfill

Limited space for accommodating the ever increasing mounds of municipal solid waste (MSW) demands the capacity of MSW landfill be maximized by building landfills to greater heights with steeper slopes. This situation has raised concerns regarding the stability of high MSW landfills. A hybrid method for quasi-three-dimensional slope stability analysis based on the finite element stress analysis was applied in a case study at a MSW landfill in north-east Spain. Potential slides can be assumed to be located within the waste mass due to the lack of weak foundation soils and geosynthetic membranes at the landfill base. The only triggering factor of deep-seated slope failure is the higher leachate level and the relatively high and steep slope in the front. The valley-shaped geometry and layered construction procedure at the site make three-dimensional slope stability analyses necessary for this landfill. In the finite element stress analysis, variations of leachate level during construction and continuous settlement of the landfill were taken into account. The "equivalent" three-dimensional factor of safety (FoS) was computed from the individual result of the two-dimensional analysis for a series of evenly spaced cross sections within the potential sliding body. Results indicate that the hybrid method for quasi-three-dimensional slope stability analysis adopted in this paper is capable of locating roughly the spatial position of the potential sliding mass. This easy to manipulate method can serve as an engineering tool in the preliminary estimate of the FoS as well as the approximate position and extent of the potential sliding mass. The result that FoS obtained from three-dimensional analysis increases as much as 50% compared to that from two-dimensional analysis implies the significance of the three-dimensional effect for this study-case. Influences of shear parameters, time elapse after landfill closure, leachate level as well as unit weight of waste on FoS were also investigated in this paper. These sensitivity analyses serve as the guidelines of construction practices and operating procedures for the MSW landfill under study.     

Determination of specific gravity of municipal solid waste

This investigation was conducted to evaluate experimental determination of specific gravity (G_s) of municipal solid waste (MSW). Water pycnometry, typically used for testing soils was adapted for testing MSW using a large flask with 2000 mL capacity and specimens with 100–350 g masses. Tests were conducted on manufactured waste samples prepared using US waste constituent components; fresh wastes obtained prior and subsequent to compaction at an MSW landfill; and wastes obtained from various depths at the same landfill. Factors that influence specific gravity were investigated including waste particle size, compaction, and combined decomposition and stress history. The measured average specific gravities were 1.377 and 1.530 for as-prepared/uncompacted and compacted manufactured wastes, respectively; 1.072 and 1.258 for uncompacted and compacted fresh wastes, respectively; and 2.201 for old wastes. The average organic content and degree of decomposition were 77.2% and 0%, respectively for fresh wastes and 22.8% and 88.3%, respectively for old wastes. The G_s increased with decreasing particle size, compaction, and increasing waste age. For fresh wastes, reductions in particle size and compaction caused occluded intraparticle pores to be exposed and waste particles to be deformed resulting in increases in specific gravity. For old wastes, the high G_s resulted from loss of biodegradable components that have low G_s as well as potential access to previously occluded pores and deformation of particles due to both degradation processes and applied mechanical stresses. The G_s was correlated to the degree of decomposition with a linear relationship. Unlike soils, the G_s for MSW was not unique, but varied in a landfill environment due both to physical/mechanical processes and biochemical processes. Specific gravity testing is recommended to be conducted not only using representative waste composition, but also using representative compaction, stress, and degradation states.     

The effects of daily cover soils on shear strength of municipal solid waste in bioreactor landfills

Bioreactor landfills are operated to enhance refuse decomposition, gas production, and waste stabilization. The major aspect of bioreactor landfill operation is the recirculation of collected leachate back through the refuse mass. Due to the accelerated decomposition and settlement of solid waste, bioreactor landfills are gaining popularity as an alternative to the conventional landfill. The addition or recirculation of leachate to accelerate the waste decomposition changes the geotechnical characteristics of waste mass. The daily cover soils, usually up to 20–30% of total MSW volumes in the landfill, may also influence the decomposition and shear strength behavior of MSW. The objective of this paper is to study the effects of daily covers soils on the shear strength properties of municipal solid waste (MSW) in bioreactor landfills with time and decomposition. Two sets of laboratory-scale bioreactor landfills were simulated in a laboratory, and samples were prepared to represent different phases of decomposition. The state of decomposition was quantified by methane yield, pH, and volatile organic content (VOC). Due to decomposition, the matrix structure of the degradable solid waste component was broken down and contributed to a significant decrease in the reinforcing effect of MSW. However, the daily cover soil, a non-degradable constituent of MSW, remains constant. Therefore, the interaction between daily cover soil particles and MSW particles will affect shear strength behavior. A number of triaxial tests were performed to evaluate the shear strength of MSW. The test results indicated that the shear strength of MSW was affected by the presence of cover soils. The friction angle of MSW with the presence of cover soil is higher than the friction angle of MSW without any cover soils. The friction angle of MSW increased from 27° to 30° due to the presence of cover soils for Phase 1 samples. The increased strength was attributed to the friction nature of sandy soil that was used as daily covers soils. Therefore, the effects of cover soils on the shear strength properties of MSW should be evaluated and taken into consideration during stability analyses and design.     

Bioreactor landfills: experimental and field results

Bioreactor landfills allow a more active landfill management that recognizes the biological, chemical and physical processes involved in a landfill environment. This paper presents the results of an experimental study carried out to determine the effect of solid waste size, leachate recirculation and nutrient balance on the rate of municipal solid waste (MSW) biodegradation. Higher rates of MSW biodegradation eventually cause a reduction of the contaminant life span of the landfill and decrease in the cost of long term monitoring. The study indicated that the smaller the size of the MSW the faster the biodegradation rate of the waste. In addition, the paper presents the results of leachate recirculation on solid waste biodegradation in a full-scale landfill site, which is located in Nepean, Ontario, Canada. The leachate was recirculated into the landfilled solid waste for 8 years through infiltration lagoons. Similar results to those obtained in the laboratory scale experiments were noted. The average pH of the leachate in the early stages of recirculation was on the acidic range of the pH scale, however, the pH value was in the range of 7-8 after 2 years of leachate recirculation. The concentration of chloride remained fairly constant at about 1000 mg/l during the leachate recirculation period. A decreasing trend of the organic load, measured as biological oxygen demand and chemical oxygen demand, was observed. Recovery of landfill air space was also noted because of the enhanced subsidence and decomposition of the solid waste.     

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.     

A critique of risk modeling and risk assessment of municipal landfills based on U.S. Environmental Protection Agency techniques

Environmental risks are a growing public concern in America. The threat of contaminated drinking water from municipal solid waste (MSW) landfills is receiving increased attention from citizens and from the United States Environmental Protection Agency (USEPA). Examination of USEPA data collected from MSW landfills reveals approximate risks posed by these landfills. USEPA research suggests that 60% of MSW landfills present less than a 1 in 10 billion risk of cancer incidence. Another 6% pose risks less than 1 in a billion, while 17% present risks less than 1 in a million. This study addresses the USEPA risk assessment techniques and models for estimating MSW landfill risks. USEPA data for toxic constituents of landfill leachate are also analysed in order to better understand the difficulties encountered in estimating landfill risks. The study also presents a brief discussion of public perceptions of risk as they relate to communicating the USEPA landfill model results to those who might be affected.     

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.     

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.     

The current situation of solid waste management in China

With economic development, the quantity of solid waste is increasing rapidly in China; the total quantities of municipal solid waste (MSW), industrial solid waste (ISW), and hazardous waste (HW) in 2002 were 136.5 million tons, 945 million tons, and 10 million tons, respectively. In 2002, the quantity of MSW disposed of was 74.04 million tons, 89.30% of which was landfilled, 3.72% was incinerated, and 6.98% was composted. There are currently 651 disposal facilities for MSW in China. Mining gangue is the largest component of ISW, making up 27.5% of the total. In the Chinese industrial sector, the coal mining and processing industry contributed most to the total quantity of ISW, with 16.0% of the total quantity of ISW generated by this sector. In total, 44% of HW was recycled, 27% was stored, 13.5% was disposed of, and 15.4% was discharged. Of the total HW generated, 40% was produced by the chemical materials and chemical products industry. Five categories of HW, i.e., waste alkali, waste acid, inorganic fluoride waste, copper waste, and inorganic cyanide waste, made up 57.8% of the total HW generated. Solid waste pollution has become a huge challenge faced by those involved in Chinese environmental management, but this can be seen as an opportunity to improve environmental quality. This article introduces the strategies taken to improve solid waste management in China.     

An Overview of Solid Waste Management and Plastic Recycling in Qatar

Municipal solid waste management (MSWM) constitutes one of the most crucial health and environmental problems facing authorities in the Arabian Gulf. Recent literature on current solid waste management (SWM) in Qatar has been reviewed in this paper, and a focused study has been carried out to provide a review on the total amount of municipal solid waste generated, stored, collected, disposed as well as the constituents of the waste. The analysis showed that Qatar produced around 2,000,000 tons of solid municipal waste annually, corresponding to a daily generation rate per capita of about 2.5 kg. About 60% of MSW is organic material and about 300 kg is composed daily. Landfill and composting is considered the most appropriate waste disposal techniques in Qatar. Um-Al-Afai landfill has nearly 80% of MSW. Because of the increased migration in Qatar, there is a sharp rise in the volume and also in the variety of solid waste. It is important to alleviate societal concerns over the increased rate of resource consumption and waste production; thus, policy makers have encouraged recycling and reuse strategies to reduce the demand for raw materials and to decrease the quantity of waste going to landfill. An example of the benefit of mechanical recycling of plastics compared to land filling and composting was conducted by GaBi 4 life cycle analysis tool which showed the benefits to the global warming and human toxicity. Recycling is the favored solution for plastic waste management, because it has a lower environmental impact on the defined impact categories, from Global Warming Potential (GWP) and Human Toxicity Potentials (HTP) indicators.     

Bio-drying and size sorting of municipal solid waste with high water content for improving energy recovery

Bio-drying can enhance the sortability and heating value of municipal solid waste (MSW), consequently improving energy recovery. Bio-drying followed by size sorting was adopted for MSW with high water content to improve its combustibility and reduce potential environmental pollution during the follow-up incineration. The effects of bio-drying and waste particle size on heating values, acid gas and heavy metal emission potential were investigated. The results show that, the water content of MSW decreased from 73.0% to 48.3% after bio-drying, whereas its lower heating value (LHV) increased by 157%. The heavy metal concentrations increased by around 60% due to the loss of dry materials mainly resulting from biodegradation of food residues. The bio-dried waste fractions with particle size higher than 45 mm were mainly composed of plastics and papers, and were preferable for the production of refuse derived fuel (RDF) in view of higher LHV as well as lower heavy metal concentration and emission. However, due to the higher chlorine content and HCl emission potential, attention should be paid to acid gas and dioxin pollution control. Although LHVs of the waste fractions with size <45 mm increased by around 2× after bio-drying, they were still below the quality standards for RDF and much higher heavy metal pollution potential was observed. Different incineration strategies could be adopted for different particle size fractions of MSW, regarding to their combustibility and pollution property.     

Nanosilver impact on methanogenesis and biogas production from municipal solid waste

Silver nanoparticles (AgNPs, nanosilver) released from industrial activities and consumer products may be disposed directly or indirectly in sanitary landfills. To determine the impact of AgNPs on anaerobic digestion of landfill waste, municipal solid waste (MSW) was loaded in identical landfill bioreactors (9L volume each) and exposed to AgNPs (average particle size=21nm) at the final concentrations of 0, 1, and 10mgAg/kg solids. The landfill anaerobic digestion was carried out for more than 250 days, during which time the cumulative biogas production was recorded automatically and the chemical property changes of leachates were analyzed. There were no significant differences in the cumulative biogas volume or gas production rate between the groups of control and 1mgAg/kg. However, landfill solids exposed to AgNPs at 10mg/kg resulted in the reduced biogas production, the accumulation of volatile fatty acids (including acetic acid), and the prolonged period of low leachate pH (between 5 and 6). Quantitative PCR results after day 100 indicated that the total copy numbers of 16S rRNA gene of methanogens in the groups of control and 1mgAgNPs/kg were 1.97±0.21×10(7) and 0.90±0.03×10(7), respectively. These numbers were significantly reduced to 5.79±2.83×10(5)(copies/mL) in the bioreactor treated with 10mgAgNPs/kg. The results suggest that AgNPs at the concentration of 1mg/kg solids have minimal impact on landfill anaerobic digestion, but a concentration at 10mg/kg or higher inhibit methanogenesis and biogas production from MSW.     

An attempt to perform water balance in a Brazilian municipal solid waste landfill

This paper presents an attempt to model the water balance in the metropolitan center landfill (MCL) in Salvador, Brazil. Aspects such as the municipal solid waste (MSW) initial water content, mass loss due to decomposition, MSW liquid expelling due to compression and those related to weather conditions, such as the amount of rainfall and evaporation are considered. Superficial flow and infiltration were modeled considering the waste and the hydraulic characteristics (permeability and soil-water retention curves) of the cover layer and simplified uni-dimensional empirical models. In order to validate the modeling procedure, data from one cell at the landfill were used. Monthly waste entry, volume of collected leachate and leachate level inside the cell were monitored. Water balance equations and the compressibility of the MSW were used to calculate the amount of leachate stored in the cell and the corresponding leachate level. Measured and calculated values of the leachate level inside the cell were similar and the model was able to capture the main trends of the water balance behavior during the cell operational period.     

Investigation of the application of an enzyme-based biodegradability test method to a municipal solid waste biodrying process

This paper presents a study to evaluate the recently developed enzymatic hydrolysis test (EHT) through its repeated application to a waste treatment process. A single waste treatment facility, involving a biodrying process, has been monitored using three different methods to assess the biodegradable content of the organic waste fractions. These test methods were the anaerobic BMc, aerobic DR4 and the EHT, which is a method based on the enzymatic hydrolysis of the cellulosic content of waste materials. The input municipal solid waste (MSW) and the output solid recovered fuel (SRF) and organic fines streams were sampled over a period of nine months from a single mechanical biological treatment (MBT) facility. The EHT was applied to each stream following grinding to <10 mm and <2 mm, in order to investigate the effect of particle size on the release of dissolved organic carbon (DOC) from enzyme hydrolysis. The output organic fines were found to more biodegradable than the MSW input and SRF output samples in each of the test methods, significantly (p < 0.05) for the EHT and DR4 methods, on the basis of DOC released and oxygen consumed, respectively. The variation between sample replicates for the EHT was higher where sample sizes of <2 mm were analysed compared to sizes of <10 mm, and the DOC release at each phase of the EHT was observed to be higher when using particle sizes of <2 mm. Despite this, additional sample grinding from the <10 mm to a smaller particle size of <2 mm is not sufficiently beneficial to the analysis of organic waste fractions in the EHT method. Finally, it was concluded that as similar trends were observed for each test method, this trial confirms that EHT has the potential to be deployed as a practical operational biodegradability monitoring tool.     

Management of municipal solid waste in the Three Gorges region

As the fourth phase of the Three Gorges reservoir project commenced in 2008, the rate of water flow in the Yangtze River has obviously decelerated further downstream and water clarity within the storage facility has decreased. Meanwhile, the rate of urbanization in the region is adding to the amount of municipal solid waste (MSW) being generated by every day life. The composition of the waste is becoming more diversified and complicated, thereby presenting an increasing threat to the ecological environment and water resources of the Three Gorges region. This paper is a probe into MSW in terms of its characteristics as well as methods of storage, collection, transportation, recycling, treatment and disposal, the protection of environmental ecosystems. Municipal solid waste management (MSWM) is one of the major environmental problems in the Three Gorges region, and indeed the whole of China. Based on the analysis of the present situation of MSWM and its treatment/disposal, some methods of sorting, recycling, decomposing, incineration and reuse are described, sanitary landfill as the main disposal method in Chongqing city, incineration being the second. Sanitary landfill or dump was also used for MSW treatment in the Three Gorges region, and this paper also provides some suggestions for improving MSWM in the Three Gorges region.     

Implementing separate waste collection and mechanical biological waste treatment in South Africa: A comparison with Austria and England

The degradation of organic compounds found in municipal solid waste (MSW) under the anaerobic landfill conditions produces gas and liquid emissions that can protract well into the landfill after-care period. The European Landfill Directives regulate the amount and nature of the organic compounds disposed into landfills. In South Africa and other developing countries, MSW is still landfilled without any kind of pre-treatment. This paper presents a pilot project of mechanical biological waste treatment (MBWT) in South Africa implemented at municipal level in the city of Durban using passively aerated open windrows. Based on case studies from Austria, England and South Africa, a waste minimisation model which can facilitate full-scale implementation of MBWT in developing countries is presented. MSW was treated in open windrows for 8 weeks. Composting temperature reached a maximum of 65 °C in less than 10 days. The results of eluate tests on waste samples from the windrows at the end of composting show a reduction of BOD₅ and BOD₅/COD ratios equal to 35.7% and 16.7%, respectively. The percent waste composition of the treated MSW was 28.3% putrescibles, 17.4% garden refuse, 13.3% plastic, 12.4% fabrics, 12% paper and other elements. The waste composition shows that more than 40% of un-treated organic material and also more than 40% non-biodegradable and recyclable materials are still landfilled without any form of biological treatment or resource recovery. A simple wet and dry waste collection model can promote recycling, treatment of biological waste before landfilling, resource recovery, labour intensive jobs and hence sustainable landfilling in the South African scenario as well as in similar developing countries.