One-Dimensional Gas Flow Model for Horizontal Gas Collection Systems at Municipal Solid Waste Landfills

Extraction of biogas from horizontal layers above, below, and within municipal solid waste landfills is becoming more commonplace. A steady-state one-dimensional analytical landfill gas model was developed to assist in the assessment and design of such collection systems. The model simulates the distribution of gas pressure within a layer of landfill waste under a variety of operating conditions that include upper and lower boundaries specified at given fluxes or pressures. The model can be used to predict where maximum pressures will build up within the landfill and what vacuum pressures must be applied to achieve specific gas collection efficiency in a horizontal collection layer. The utility of the model was illustrated for several scenarios of interest. In the absence of gas collection from a landfill’s leachate collection system, considerable gas pressures can build up at the bottom of the landfill. The design of leachate collection systems for landfill gas removal should be considered from the outset. An evaluation of the parameters that impact vacuum requirements—waste depth, gas generation rate, and waste permeability—suggests that it may not be feasible to rely solely upon the leachate collection system for the removal of landfill gas. The model was thus used to illustrate cases where a horizontal collection layer underneath the landfill cap is used in conjunction with gas extraction from the bottom of the landfill. Several recommendations are proposed to improve the gas collection efficiencies for landfills utilizing horizontal gas collection layers.     

Biogeochemical Evaluation of Mechanisms Controlling CaCO3(s) Precipitation in Landfill Leachate-Collection Systems

A common failure mode for landfills is clogging of the leachate-collection system. The reduction in hydraulic conductivity associated with clogging causes a buildup of leachate head on the underlying liner, potentially increasing advective contaminant transport from the landfill and contaminating adjacent groundwater. In this paper, the biogeochemical model CCBATCH is used to link a primary cause of leachate collection system failure—CaCO3(s) precipitation?to anaerobic degradation of volatile fatty acids (VFAs) in column reactors used to study the clogging phenomena. One key to applying CCBATCH correctly was dividing the VFA conversion into two steps: conversion of propionate to acetate, carbonic acid, and methane; and acetate conversion to methane and carbonic acid. The primary driver for CaCO3(s) precipitation in the columns was acetate fermentation to CH4 and H2CO3, which increased the total carbonate concentration in the leachate and shifted the acid/base control to a weaker acid system, which caused an increase in solution pH. A second key to proper modeling was adding CO2(g) gas transfer to CCBATCH. The modeling results indicate that the kinetics of CO2(g) gas transfer was a key control over leachate chemistry once acetate fermentation was nearly complete. These results suggest that the best approach for the long-term control of CaCO3(s) clogging may be to enhance CO2(g) gas transfer from the leachate while buffering the leachate pH to near neutral. Taken together, these actions should decrease the yield of CaCO3(s) precipitated per mass of acetate removed.     

Contaminant Leaching Model for Dredged Material Disposal Facilities

This paper describes the hydrologic evaluation of leachate production and quality model, a screening-level tool to simulate contaminant leaching from a confined disposal facility (CDF) for dredged material. The model combines hydraulics, hydrology, and equilibrium partitioning, using site-specific design specifications, weather data, and equilibrium partitioning coefficients from the literature or from sequential batch or column leach tests of dredged material. The hydraulics and hydrology are modeled using Version 3 of the hydrologic evaluation of landfill performance model. The equilibrium partitioning model includes provisions for estuarine sediments that have variable distribution coefficients resulting from saltwater washout. Model output includes contaminant concentrations in the CDF profile, contaminant concentration and mass releases through the bottom of the CDF, and contaminant concentrations and masses captured by leachate collection systems. The purpose of the model is to provide sound information for evaluating the potential leachate impacts on ground water at dredged material CDFs and the effectiveness of leachate control measures.     

Clogging of Gravel Drainage Layers Permeated with Landfill Leachate

Ten flow cells, called mesocosms, are used to investigate the effect of different gravel sizes (38 and 19?mm) and operating conditions on clogging of leachate collection systems. These mesocosms simulated in real time and real scale the two-dimensional leachate flow conditions representative of a section of a continuous 300-mm-thick gravel drainage blanket adjacent to a leachate collection pipe in a primary leachate collection system. The tests were terminated after 6–12?years of operation. In some mesocosms the full 300?mm of gravel was saturated. In others, the leachate level was initially set at 100?mm and the upper 200?mm were unsaturated. Although the flow through all mesocosms was similar, the clogging in the fully saturated gravel was substantially more than in the partially saturated gravel. After 6?years of operation, typically, less than 10% of the initial pore space was filled with clog material in the unsaturated gravel. For the saturated zone, 45% of the initial pore space was filled with clog material in the fully saturated design as compared to only 31% in the partially saturated design. The 38?mm gravel performed much better than the 19?mm gravel. For example, it maintained a hydraulic conductivity that was higher than the 19?mm gravel even after operating for twice as long. Up to four mesocosms were placed in series, with the effluent from one mesocosm being the influent for another. The reduction in mass loading within the first mesocosm reduced the amount of clogging within the mesocosm later in series. There was a clear progression of decreasing amounts of initial pore space filled with clog material in the last mesocosm in series, and most of the clogging was due to the vertically percolating leachate.     

Particle Size and Clogging of Granular Media Permeated with Leachate

The effect of particle size (4-, 6-, and 15-mm nominal sizes) on the rate of clogging of columns of porous media permeated with municipal solid-waste leachate is examined. Clogging is shown to be more localized over a small volume of the porous media near the influent end of the column for smaller particles than for larger particles, where clogging was more uniformly distributed along the column. This is attributed to the greater surface area per unit volume of smaller particles allowing greater biofilm growth per unit volume. This increased the reduction in chemical oxygen demand (COD) and caused greater deposition of inorganic clog material per unit length of column than for larger particles. The distribution of methanogenic bacteria was found to closely correspond to the zones of most severe clogging. The bulk density of clog material is shown to be between 1.6 and 1.8 Mg∕m3. The chemical composition of the clog material is essentially independent of particle size, with calcium representing 26% of the dry mass of the clog material and CaCO3 being the main component of the clog. An examination of the yield of CaCO3 relative to COD indicates that the carbon in the CaCO3 represents <4% of the organic carbon represented by the drop in COD. Finally, the data from the column test is used to predict the expected time to clog for an actual landfill and were found to give results consistent with what was observed in the field.     

Performance of North American Bioreactor Landfills. I: Leachate Hydrology and Waste Settlement

An assessment of state-of-the-practice at five full-scale North American landfills operating as bioreactors is presented in this two-paper set. This paper focuses on effectiveness of liners and leachate collection systems, leachate generation rates, leachate recirculation practices and rates, effectiveness in moistening the waste, and settlement of the waste over time. Except in one case, the liner and leachate collection systems at the bioreactor landfills were similar to those used for landfills operated conventionally. Leachate generation rates increased approximately linearly with recirculation rate, but in all cases, the leachate generation rate was <300?L/m2?year. Leachate depths generally were maintained within regulatory requirements, even with the highest recirculation rates. Leakage rates from liners at bioreactor landfills, including alternative liner designs employing geosynthetic clay liners, are comparable to leakage rates from conventional landfills. Thus, based on the information gathered in this study, additional requirements or features for liners or leachate collection systems are not warranted for bioreactor landfills. Diminishing capacity of horizontal recirculation trenches is common. Experience at one landfill suggests that small doses at high frequency under substantial injection pressure can deter loss of trench capacity. Only those landfills that were aggressive in recirculation had achieved water contents near the field capacity. Increasing the amount of liquid that is added may be required to achieve field capacity at some landfills, particularly if a final cover is placed soon after waste grades are reached. The rate of time-dependent waste settlement attributed to biodegradation is about 1.6 times larger in bioreactor landfills than in conventional landfills, and increases as the recirculation dosage increases.     

Effectiveness of Scrap Tire Chips as Sorptive Drainage Material

Scrap tire disposal is a problem of growing concern. One solution to this problem is innovative methods for the reuse and recycling of scrap tires. Based on batch isotherm tests, scrap tire chips have been identified to be good sorbents of volatile organic compounds (VOCs) and could be used as leachate drainage layer material in solid waste landfills and in other similar applications. To demonstrate the effects of tire chips on the leachate they come in contact with in a drainage layer over a liner, large-scale tank tests simulating the drainage layer and the clay liner and also field tests were performed. Two cells were constructed in a landfill: one with scrap tire chips and the other with gravel leachate collection layer. According to the results of the large-scale tank tests and field tests, shredded tire chips have a significantly positive impact on the quality of the leachate with which they come in contact. The use of scrap tires in landfills would reduce the magnitude of the current tire disposal problem (a 1 ha landfill requires approximately 300,000 tires to fill 0.3 m of a leachate collection layer) and convert one waste into a beneficial construction material and simultaneously mitigate the problem of VOC transport from through landfill liners.     

Leachate Recirculation in Bioreactor Landfills Using Geocomposite Drainage Material

The key purpose of this study was to test the use of a permeable blanket made up of a geocomposite drainage layer (GDL) for leachate recirculation in municipal solid waste (MSW) landfills and to predict the observed leachate travel in the blanket using a numerical model. A 34?m long by 12?m wide permeable blanket made up of GDL was constructed at an active MSW landfill located in Michigan. Leachate was injected in the GDL using a perforated pipe placed centrally above the GDL along its length. Moisture content sensors, pressure transducers, thermistors, thermocouple sensors, and a vertical load sensor were embedded immediately below the GDL blanket to monitor the flow of injected leachate. After the blanket was covered with waste, leachate was injected into the blanket at rates ranging from 0.9 to 2.6?m3/h per meter length of the blanket. Data collected from the embedded sensors indicated that the injected leachate traveled at rates ranging from 5 to 18?m/h through the blanket depending upon the leachate injection rate. Only pressure transducers and thermistors were consistently able to detect migration of injected leachate once the blanket got saturated. Moisture content sensors could not register any change in readings once the blanket became saturated. Leachate injection pressure monitored over a period of about 12 months indicated no signs of clogging of the blanket. The leachate pressures measured immediately below the blanket were less than the net leachate injection pressure indicting that there was a head loss in the GDL blanket. Numerical modeling of liquid flow in the blanket indicated that predicted leachate travel in the blanket was consistent with the field data for assumed values of the waste hydraulic conductivity. In the absence of measured representative hydraulic properties of the waste, absolute verification of the field data was not possible.     

Numerical Evaluation of Landfill Stabilization by Leachate Circulation

This study compares various leachate management scenarios using a biologically reactive transport model, which is proposed in this study. The proposed model can be used to predict the contribution of biodegradation to contaminant attenuation and contaminant concentration in leachate over time. It can also be used to assess the extent of landfill stabilization in terms of local mass per bulk volume of remaining refuse available for transfer. A sensitivity analysis shows that landfill stabilization has significant sensitivity to most biokinetic parameters, the fluid-phase saturation constant, and the dissolution rate, in addition to the half-saturation constant and the retardation factor. The proposed model is applied to assess landfill stabilization under two control scenarios: leachate recycling versus continued input of clean water with no recirculation. The simulation results indicate that leachate recirculation provides more favorable conditions for development of an active anaerobic bacterial population and, hence, accelerates landfill stabilization.     

Treatment of Landfill Leachate by a Combined Anaerobic Fluidized Bed and Zeolite Column System

The use of a combined anaerobic fluidized bed and zeolite fixed bed system in sanitary landfill leachate treatment was investigated. Anaerobic treatability studies were successfully performed in the anaerobic fluidized bed reactor. The chemical oxygen demand (COD) removal was attained up to 90% with increasing organic loading rates as high as 18?g?COD/L?day after 80?days of operation. Good biogas production yield (Ygas) of 0.53?L biogas per gram removed COD with methane (CH4) content of 75% was obtained. The attached biomass concentration increased along the column height from bottom to top, and its mean value was found 6,065?mg/L after 100?days of operation. The anaerobically treated landfill leachate was further treated by a zeolite fixed bed reactor. While excellent ammonia removal (>90%) was obtained with the untreated zeolite, the regenerated zeolites showed higher performance. Consequently, this combined anaerobic and adsorption system is an effective tool to remove high COD and high ammonia in landfill leachate.     

Operation and Performance of Horizontal Wells for Leachate Control in a Waste Landfill

This paper presents data on gas and leachate flow rates and leachate levels obtained during a 600?day pumping trial on three retrofitted horizontal wells in a domestic waste landfill at Rainham, United Kingdom. The changes in gas and leachate flow rate with time and atmospheric pressure, and the interaction between the two flows, are discussed. The spatial variability of the response of the leachate levels within the landfill is explored with reference to the anisotropy and heterogeneity of the permeability of the waste. It is shown that horizontal wells can be an effective means of controlling leachate heads near the base of a landfill, and that leachate levels must be measured using piezometers with a discrete response zone rather than fully screened observation wells if meaningful results are to be obtained. It is argued that the large amounts of gas pumped from the nominally saturated zone of the landfill must have come from the ongoing degradation of the waste within the zone of influence of the well.     

垃圾渗滤液处理技术研究进展

介绍了渗滤液产生来源和特点，指出垃圾渗滤液是一种非常复杂的、难以生物处理的污水，并对垃圾渗滤液物理化学处理、生物处理及土地处理法进行了探讨，重点介绍了渗滤液的土地处理，包括渗滤液回灌、人工湿地法和渗滤液矿化垃圾生物反应床处理技术，指出了土地法处理垃圾渗滤液无论在技术上还是经济上比较是可行的。     

垃圾渗滤液处理技术研究进展

介绍了渗滤液产生来源和特点,指出垃圾渗滤液是一种非常复杂的、难以生物处理的污水,并对垃圾渗滤液物理化学处理、生物处理及土地处理法进行了探讨,重点介绍了渗滤液的土地处理,包括渗滤液回灌、人工湿地法和渗滤液矿化垃圾生物反应床处理技术,指出了土地法处理垃圾渗滤液无论在技术上还是经济上比较是可行的。     

Antioxidant Depletion from a High Density Polyethylene Geomembrane under Simulated Landfill Conditions

Accelerated aging tests to evaluate the depletion of antioxidants from a high density polyethylene geomembrane are described. The effects of temperature, high pressure, and continuous leachate circulation on the aging of geomembranes in composite liner systems are examined. The antioxidant depletion rates (0.05, 0.19, and 0.41?month?1 at 55, 70, and 85°C, respectively) obtained for the simulated landfill liner at 250 kPa vertical pressure are consistently lower than that obtained from traditional leachate immersion tests on the same geomembrane (0.12, 0.39, and 1.1?month?1 at 55, 70, and 85°C). This difference leads to a substantial increase in antioxidant depletion times at a typical landfill liner temperature (35°C) with 40 years predicted based on the data from the landfill liner simulators tests, compared to 15 years predicted for the same geomembrane based on leachate immersion tests. In these tests, the crystallinity and tensile yield strain of the geomembrane increased in the early stages of aging and then remained relatively constant over the testing period. There was no significant change in other geomembrane properties within the testing period.     

Estimation of Maximum Liquid Head over Landfill Barriers

To properly design a drainage layer for either landfill leachate collection systems or final covers, the designer must be able to estimate the maximum liquid head over the barrier for any proposed configuration. This paper presents four explicit formulas for estimating the maximum liquid head over an impervious sloping barrier. By means of numeric comparisons, McEnroe’s 1993 method is recommended for design of drainage layers for both bottom liners and final covers. Pipe slope is an important parameter that influences the maximum leachate head on the liner. Different combinations of base grade and pipe slope can directly affect the actual drainage behavior. If the pipe slope is steeper than the base grade, it will make a longer drainage distance and cause a high leachate head on the liner. A method for calculating the maximum liquid head in multilayered drainage media (e.g., geosynthetic and soil) is presented in the paper. The key consideration for this case is to determine the equivalent hydraulic conductivity of the combined drainage media under the phreatic surface under unconfined seepage conditions.     

Secondary Compression of Municipal Solid Wastes and a Compression Model for Predicting Settlement of Municipal Solid Waste Landfills

The estimation of the capacity and settlement of landfills is critical to successful site operation and future development of a landfill. This paper reports the results of a study on biodegradation behavior and the compression of municipal solid wastes. An experimental apparatus was developed which had a temperature-control system, a leachate recycling system, a loading system, and a gas and liquid collection system. Experiments were performed both with and without optimal biodegradation for comparative purposes. Test results indicated that settlement resulting from creep was relatively insignificant when the biodegradation process was inhibited. Compression due to decomposition under optimal biodegradation conditions was found to be much larger than compression associated with creep. The biodegradation process was significantly influenced by the operational temperature. A one-dimensional model is proposed for calculating settlement and estimating the capacity of the landfill under relatively optimal biodegradation conditions. The model was developed to accommodate the calculation of settlement in landfills when a multistep filling procedure was used. The calculation method is relatively simple and convenient for design purposes. Simulations of the physical processes showed that enhancing solid waste biodegradation during the filling stage can considerably increase the capacity of the landfill and reduce postclosure settlements.     

Modeling of Biodegradation in an Old Unregulated European Landfill

In this note an example of modeling of biodegradation processes of an old abandoned municipal solid waste landfill for its simulation is illustrated using the landfill dynamic simulation tool MODUELO. In this program the waste biodegradation model is based on the quantification of organic matter, its chemical composition, biodegradability, accessibility to microorganisms, and the ratio nonbiodegradable leachable organic matter to gasifiable matter. Data from a characterization campaign, presented elsewhere, were used to determine these parameters. The experimental information was completed with safety factors (to compensate for sampling uncertainty) and literature values. The degradation rates were determined, after having calibrated the hydrological model, by fitting the temporal series of pollutants’ concentration measured in the leachate and the biogas composition. The achieved fit of the simulated series compared to the measured data is reported as the result of this work. Given the limited information available, the obtained simulation model is considered an acceptable tool to study the future evolution of the landfill in different circumstances.     

Quality of Runoff Leachate Collected from Biosolids and Dairy-Manure Compost-Amended Topsoils

Compost amendment of pavement shoulder subgrades was attempted to mitigate shrinkage cracking, which otherwise would result in severe distress to adjacent pavement infrastructure systems. Two types of composts, biosolids and dairy-manure composts, were evaluated. As a part of this application, the runoff leachate emanating from the compost soils needs to be environmentally assessed as this leachate potentially ends up in storm sewer collection systems. As a part of the research, runoff collection systems were placed in both dairy-manure and biosolids compost-amended soil sections and a control untreated soil section. The collected water samples at various time intervals were then subjected to various chemical and environmental tests, including total suspended and dissolved solids, biochemical and chemical oxygen demand, total Kjeldahl nitrogen (TKN), and phosphorous measurements. This paper presents a summary of these test results from both sections and their comparisons with similar results from another leachate collected from a control or untreated section. These test results are also compared with a few local studies conducted on highway runoff samples and EPA benchmark values. Possible causes for chemical contaminant concentration differences in the leachate samples from both composts are also explained.     

Numerical Simulation of Transport of Four Heavy Metals in Kaolinite Clay

A multicomponent reactive solute-transport model was used to study the migration of dissolved heavy metals (Cd2+, Pb2+, Cu2+, and Zn2+) in a clay barrier subject to two leachates having different pH values. This solute-transport model is capable of simulating simultaneous processes of water flow, advective-dispersive-solute transport, and chemical reactions. The migration of these metals was simulated in a kaolinite landfill liner, which was assigned realistic physical and chemical properties and boundary conditions to model one-dimensional contaminant transport. The leachate input properties to the model were those of an actual leachate containing the four heavy metals. The numerical simulations were focused on the concentration profiles of these metals in the simulated clay barrier and leachate pH affects their mobilities. The numerical results indicate that with a nearly neutral leachate, the heavy metals mobility follows: Cd2+ < Pb2+ < Cu2+ < Zn2+. With an acidic leachate, the order changes to Pb2+ < Cu2+ < Zn2+ < Cd2+. Leachate pH has a significant effect on Cd2+ and Pb2+ mobility and a small effect on Cu2+ and Zn2+.     

Field Evaluation of a Leachate Collection System Constructed with Scrap Tires

Landfilling costs and the potential uses of scrap tires have prompted researchers to investigate beneficial reuses. One important application is the use of tire chips as a leachate collection material in municipal solid waste landfills. Laboratory and field studies were conducted to investigate the performance of tire chips as a drainage medium in landfills. The laboratory portion of the program included a series of hydraulic conductivity and compressibility tests. Two field test cells, one with tire chips and another with gravel as the control, were constructed. The tire-chip cell was instrumented with flowmeters, thermistors, and gas collection devices to evaluate the hydraulic performance as well as the potential for spontaneous combustion. Leachate collected from the two cells was analyzed to determine if tire chips would potentially contaminate the groundwater. The results indicated that adequate drainage conditions were present within the tire-chip layer. The presence of insignificant quantities of carbon monoxide, and the lack of oxygen, and recorded low temperatures suggested that a combustion hazard was not present. The field leachate data indicated that tire chips can be safely used as part of a landfill leachate collection layer, even though it may not be suitable to place them near drinking water sources.