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.     

Leachate generation and biogas energy recovery in the Jebel Chakir municipal solid waste landfill,Tunisia

A survey was conducted between 2006 and 2008 in order to identify municipal solid waste (MSW) composition and its influence on leachate generation and to assess the amount of biogas yield from the Jebel Chakir landfill in Tunis City. The organic fraction was the predominant compound in the MSW, followed by paper, fine, plastic, leather, rubber, metal, textile, glass and ceramic. The average MSW moisture content varies from 60 % in the wet season to 80 % in the dry one. The recognised MSW composition is well representative if compared to that of cities in developing countries. A large leachate quantity is produced in the landfill of Jebel Chakir, despite the negative water balance of the site. Based on the annual MSW landfilled quantities and using the LandGEM model, the expected peak landfill gas (LFG) production is estimated to occur 1 year after the landfill closure with a rate of 3.53 × 10⁷ m³/year. The analysis of the potential conversion of LFG to electric energy shows it at a total LFG-to-electricity energy of around 257 GWh with a heating value of 4,475 kcal/m³ based on an LFG collection efficiency of 33 % and energy efficiency of 33 % giving an economic feasibility for a 10 MW power plant.     

Modeling of perched leachate zone formation in municipal solid waste landfills

The paper presents a 1D mathematical model for the simulation of the percolation fluxes throughout a landfill for municipal solid waste (MSW). Specifically, the model was based on mass balance equations, that enable simulation of the formation of perched leachate zones in a landfill for MSW. The model considers the landfill divided in several layers evaluating the inflow to and outflow from each layer as well as the continuous moisture distribution. The infiltration flow was evaluated by means of the Darcy’s law for an unsaturated porous medium, while the moisture distribution evaluation has been carried out on the basis of the theory of the vertically distributed unsaturated flow. The solution of the model has been obtained by means of the finite difference method. The model has been applied to a semi-idealized landfill located in Palermo landfill (Bellolampo). Specifically, field measurements were conducted to determine the relationship between waste density and applied vertical strain. This relationship was then used to relate vertical strain to waste porosity. The inflow rate to the system was simulated via a synthetic hyetograph whose characteristics have been identified in a previous hydrologic study.Three simulations, each with a different initial moisture content, were conducted. The model results showed a different response of the landfill in terms both of flow rates throughout the landfill and moisture profile. Indeed, the initial moisture content drastically influenced not only the formation of perched leachate zones but also their extension. The model can be a useful tool in predicting potential for the formation of perched leachate zones.     

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.     

Future landfill emissions and the effect of final cover installation--a case study

Municipal solid waste (MSW) landfills are potential long-term sources of emissions. Hence, they need to be managed after closure until they do not pose a threat to humans or the environment. The case study on the Breitenau MSW landfill was performed to evaluate future emission levels for this site and to illustrate the effect of final cover installation with respect to long-term environmental risks. The methodology was based on a comprehensive assessment of the state of the landfill and included analysis of monitoring data, investigations of landfilled waste, and an evaluation of containment systems. A model to estimate future emission levels was established and site-specific predictions of leachate emissions were presented based on scenario analysis. The results are used to evaluate the future pollution potential of the landfill and to compare different aftercare concepts in view of long-term emissions. As some leachable substances became available for water flow during cover construction due to a change in the water flow pattern of the waste, a substantial increase in leachate concentrations could be observed at the site (e.g. concentrations of chloride increased from 200 to 800 mg/l and of ammonia-nitrogen from 140 to about 500 mg/l). A period of intensive flushing before the final cover installation could have reduced the amount of leachable substances within the landfill body and rapidly decreased the leachate concentrations to 11 mg Cl/l and 79 mg NH₄-N/l within 50 years. Contrarily, the minimization of water infiltration is associated with leachate concentrations in a high range for centuries (above 400 mg Cl/l and 200 mg NH₄-N/l) with low concomitant annual emission loads (below 12 kg/year of Cl or 9 kg/year of NH₄-N, respectively). However, an expected gradual decrease of barrier efficiency over time would be associated with higher emission loads of 50 kg of chloride and 30 kg of ammonia-nitrogen at the maximum, but a faster decrease of leachate concentration levels.     

Hydrological performance of MSW incineration residues and MSW co-disposed with sludge in full-scale cells

Water flows were analysed for the filling phase and the first 4 years after closure of two types of full-scale landfill cells: 'special cells' containing mostly fly ash from municipal solid waste (MSW) incineration disposed with other special/hazardous waste, and 'biocells' (biological cells) containing co-disposed MSW and food industry sludge. The landfill cells were constructed about -1.5 m above sea level (masl) at Lomma Bay, southern Sweden. The hydrological effects of water intrusion into the special cells from surroundings and sludge moisture within the biocells were studied. HELP modelling of hydrological processes predicted delay in peaks of leachate generation from uncovered special cells following rain, which was not confirmed. Faster leachate production as a response to rainfall from special cells than from biocells was observed. It was inferred that special waste has more intensive channelling, lower water absorption and higher hydraulic conductivity than mixtures of sludge/MSW. To avoid convergence problems in modelling uncovered special cells, the use of a 5 cm deep top layer with saturated hydraulic conductivity 1.7 x 10(-3) cm s(-1), porosity 0.437, and field capacity 0.105, is suggested.     

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.     

Effect of leachate recirculation on the migration of copper and zinc in municipal solid waste and municipal solid waste incineration bottom ash co-disposed landfill

Municipal solid waste incinerator (MSWI) bottom ash was allowed to be disposed of with municipal solid waste (MSW) in landfill sites in the recently enacted standard of China. In this study, three sets of simulated landfill reactors, namely, conventional MSW landfill (CL), conventional MSWI bottom ash and MSW co-disposed landfill (CCL), and leachate recirculated MSWI bottom ash and MSW co-disposed landfill (RCL), were operated to investigate the environmental impact of the co-disposal. The effect of leachate recirculation on the migration of Cu and Zn in the co-disposed landfill was also presented. The results showed that the co-disposal of MSWI bottom ash with MSW would not enhance the leaching of Cu and Zn from landfill. However, the co-disposal increased the Cu and Zn contents of the refuse in the bottom layer of the landfill from 56.7 to 65.3 mg/kg and from 210 to 236 mg/kg, respectively. The recirculation of the leachate could further increase the Cu and Zn contents of the refuse in the bottom layer of the landfill to 72.9 and 441 mg/kg, respectively. Besides these observations, the results also showed that the co-disposed landfill with leachate recirculation could facilitate the stabilization of the landfill.     

Sustainable disposal of municipal solid waste: Post bioreactor landfill polishing

Sustainable disposal of municipal solid waste (MSW) requires assurance that contaminant release will be minimized or prevented within a reasonable time frame before the landfill is abandoned so that the risk of contamination release is not passed to future generations. This could be accomplished through waste acceptance criteria such as those established by the European Union (EU) that prohibit land disposal of untreated organic matter. In the EU, mechanical, biological and/or thermal pretreatment of MSW is therefore necessary prior to landfilling which is complicated and costly. In other parts of the world, treatment within highly engineered landfills is under development, known as bioreactor landfills. However, the completed bioreactor landfill still contains material, largely nonbiodegradable carbon and ammonia that may be released to the environment over the long-term. This paper provides a conceptual analysis of an approach to ensure landfill sustainability by the rapid removal of these remaining materials, leachate treatment and recirculation combined with aeration. The analysis in this paper includes a preliminary experimental evaluation using real mature leachate and waste samples, a modeling effort using a simplified mass balance approach and input parameters from real typical bioreactor cases, and a cost estimate for the suggested treatment method.     

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.     

Assessment of internal condition of waste in a roofed landfill

Recently, roofed landfills have been gaining popularity in Japan. Roofed landfills have several advantages over non-roofed landfills such as eliminating the visibility of waste and reducing the spread of offensive odours. This study examined the moisture balance and aeration conditions, which promote waste stabilisation, in a roofed landfill that included organic waste such as food waste. Moisture balance was estimated using waste characterization and the total amount of landfilled waste. Internal conditions were estimated based on the composition, flux, and temperature of the landfill gas. Finally, in situ aeration was performed to determine the integrity of the semi-aerobic structure of the landfill.With the effects of rainfall excluded, only 15% of the moisture held by the waste was discharged as leachate. The majority of the moisture remained in the waste layer, but was less than the optimal moisture level for biodegradation, indicating that an appropriate water spray should be administered. To assess waste degradation in this semi-aerobic landfill, the concentration and flow rate of landfill gas were measured and an in situ aeration test was performed. The results revealed that aerobic biodegradation had not occurred because of the unsatisfactory design and operation of the landfill.     

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.     

Monitoring operational and leachate characteristics of an aerobic simulated landfill bioreactor

Long-term biodegradation of MSW in an aerobic landfill bioreactor was monitored as a function of time during 510 days of operation. Operational characteristics such as air importation, temperature and leachate recirculation were monitored. The oxygen utilization rates and biodegradation of organic matter rates showed that aerobic biodegradation was feasible and appropriate to proceed in aerobic landfill bioreactor. Leachate analyses showed that the aerobic bioreactor could remove above 90% of chemical oxygen demand (COD) and close to 100% of biochemical oxygen demand (BOD5) from leachate. Ammonium (NH4+), nitrate (NO3-) and sulphate (SO4(2-)) concentrations of leachate samples were regularly measured. Results suggest that nitrification and denitrification occurred simultaneously, and the increase in nitrate did not reach the levels predicted stoichiometrically, suggesting that other processes were occurring. Leachate recirculation reduced the concentrations of heavy metals because of the effect of the high pH of the leachate, causing heavy metals to be retained by processes such as sorption on MSW, carbonate precipitation, and hydroxide precipitation. Furthermore, the compost derived from the aerobic biodegradation of the organic matter of MSW may be considered as soil improvement in the agricultural plant production. Bio-essays indicated that the ecotoxicity of leachate from the aerobic bioreactor was not toxic at the end of the experiment. Finally, after 510 days of degradation, waste settlement reached 26% mainly due to the compost of the organic matter.     

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.     

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.     

Modelling of environmental impacts of solid waste landfilling within the life-cycle analysis program EASEWASTE

A new computer-based life-cycle assessment model (EASEWASTE) has been developed to evaluate resource and environmental consequences of solid waste management systems. This paper describes the landfilling sub-model used in the life-cycle assessment program EASEWASTE, and examines some of the implications of this sub-model. All quantities and concentrations of leachate and landfill gas can be modified by the user in order to bring them in agreement with the actual landfill that is assessed by the model. All emissions, except the generation of landfill gas, are process specific. The landfill gas generation is calculated on the basis of organic matter in the landfilled waste. A landfill assessment example is provided. For this example, the normalised environmental effects of landfill gas on global warming and photochemical smog are much greater than the environmental effects for landfill leachate or for landfill construction. A sensitivity analysis for this example indicates that the overall environmental impact is sensitive to the gas collection efficiency and the use of the gas, but not to the amount of leachate generated, or the amount of soil or liner material used in construction. The landfill model can be used for evaluating different technologies with different liners, gas and leachate collection efficiencies, and to compare the environmental consequences of landfilling with alternative waste treatment options such as incineration or anaerobic digestion.     

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.     

Longitudinal data analysis in support of functional stability concepts for leachate management at closed municipal landfills

Landfill functional stability provides a target that supports no environmental threat at the relevant point of exposure in the absence of active control systems. With respect to leachate management, this study investigates “gateway” indicators for functional stability in terms of the predictability of leachate characteristics, and thus potential threat to water quality posed by leachate emissions. Historical studies conducted on changes in municipal solid waste (MSW) leachate concentrations over time (longitudinal analysis) have concentrated on indicator compounds, primarily chemical oxygen demand (COD) and biochemical oxygen demand (BOD). However, validation of these studies using an expanded database and larger constituent sets has not been performed. This study evaluated leachate data using a mixed-effects regression model to determine the extent to which leachate constituent degradation can be predicted based on waste age or operational practices. The final dataset analyzed consisted of a total of 1402 samples from 101 MSW landfills. Results from the study indicated that all leachate constituents exhibit a decreasing trend with time in the post-closure period, with 16 of the 25 target analytes and aggregate classes exhibiting a statistically significant trend consistent with well-studied indicators such as BOD. Decreasing trends in BOD concentration after landfill closure can thus be considered representative of trends for many leachate constituents of concern.     

Use of zeolitised coal fly ash for landfill leachate treatment: a pilot plant study

Treatment of municipal solid waste (MSW) landfill leachate generally results in low percentages of nutrient removal due to the high concentration and accumulation of refractory compounds. For this reason, individual physical, chemical and biological processes have been used for the treatment of raw landfill leachate and sometimes for the mixture of domestic wastewater and landfill leachate. In this work, the possibility of treating landfill leachate was tested in a bench-scale pilot plant by a two-step method combining adsorption and coagulation-flocculation. Zeolite synthesized from coal fly ash, a by-product of coal-fired power stations, was used in this study both as a decantation aid reagent and as an adsorbent of COD and NH4-N. The coagulation-flocculation step was performed by the use of aluminium sulphate and a polyelectrolyte (ACTIPOL A-401). The leachate was collected directly from a storage unit of the organic fraction of MSW, before it was composted. For this reason the raw leachate was diluted before treatment. The sludge was recirculated to enhance the removal efficiency of nutrients as well as to optimize flocculant saving and to decrease sludge production. The results showed that it is possible to remove 43%, 53% and 82% of COD, NH4-N, and suspended solids, respectively. Therefore, this method may be an alternative for ammonium removal, as well as a suitable pre- or post-treatment step, in combination with other processes in order to meet regulatory limits.     

Municipal solid waste management in China: a comparative analysis

This paper illustrates an overview of the past and present MSWM strategies in China. A comparison is made with MSWM in China, and other developed and developing countries to identify and analyze the problems of existing MSWM, and evaluate some effective suggestion to overcome the limitations. Rapid urbanization and economic growth are the main factors of increasing MSW generation in China. The generating MSW has 55.86 % food waste with high moisture contain due to unavailable source separation. Chinese MSWM is dominated by 60.16 % landfilling, whereas incineration, untreated discharge, and other treatments are 29.84, 8.21, and 1.79 %, respectively. In 2014, a total of 604 sanitary landfills, 188 incineration plants, and 26 other units were used for MSWM. With the magnitude of timing, the increasing rate of incineration unit and disposal capacity is higher than the landfill. In 2004–2014, the disposal capacity of landfill and incineration is increased from 68.89 to 107.44 and 4.49 to 53.3 million tons, respectively. However, the heating value in the majority of Chinese incineration plants is 3000–6700 kJ/kg and the inappropriate leachate treatment can be found in 47 % landfill sites. A proper taxation system for MSW disposal is not fully implemented in China, which has a negative impact on overall MSW recycling. From the comparative study of MSWM, it is revealed that the source separation MSW collection, high energy recovery from incineration plants, appropriate leachate treatment, effective landfill location and management, increase waste recycling and proper taxation system for MSW disposal are essential to improve MSWM in China.