Bioreactor Landfills: An Idea Whose Time Has Come

Sept. 1, 2002
Bioreactor landfill technology is not a new idea. Its genesis springs from the systematic treatment of wastewater that began in the late 1800s and early 1900s (Tchobanoglous and Burton, 1991). Bioreactor landfills can be thought of as an extension of anaerobic and aerobic digestion at wastewater treatment plants. They accelerate the biodegradation rate of MSW by adding leachate/water and possibly air and some nutrients. What could be simpler? Plenty. For starters, just coming up with a definition of a bioreactor landfill that everyone can agree on has been nearly impossible. For the purpose of this article the most recent definition proposed by SWANA (2001) will be used:A bioreactor landfill is any permitted Subtitle D landfill or landfill cell, subject to New Source Performance Standards/Emissions Guidelines, where liquid or air, in addition to leachate and landfill gas condensate, is injected in a controlled fashion into the waste mass in order to accelerate or enhance biostabilization of the waste.The key word in this definition is “controlled,” for the success of a bioreactor landfill lies as much in the operation as in the layout and design.Regulatory PoliciesThe major environmental concerns regarding MSW landfills are related to gas migration and leachate discharges. The current federal regulations governing MSW landfills under Subtitle D of the Resource Conservation and Recovery Act emphasize minimizing infiltration, collecting leachate generated by the landfill, and mandate maintenance of the landfill integrity for a minimum period of 30 years. These regulations create conditions that delay, rather than eliminate, the eventual degradation of MSW, leading to the creation of a “dry tomb.” The United States Environmental Protection Agency has repeatedly acknowledged–most recently in its proposed rule-making for the composite liner landfills currently in use–that the containment systems prescribed in the landfill rules will degrade and ultimately fail over time. This led to the promulgation of the rules in 1991:[E]ven the best liner and leachate collection systems will ultimately fail due to natural deterioration, and recent improvements in MSWLF containment technologies will be delayed by many decades at some landfills.Landfills continue to generate leachate and gas for decades following waste placement, possibly beyond the current 30-year postclosure monitoring period. Research has shown that a significant portion of the biodegradable fraction of waste placed in conventional MSW landfills remains relatively unstabilized following decades of landfilling (Rathje, 1999). Environmental impacts related to gas generation, leachate contamination of groundwater, and the long-term structural integrity of the containment system require long-term monitoring.Bioreactor landfills represent a fundamentally safer and better method of land disposal than the currently defined conventional landfills since waste is stabilized more rapidly. Bioreactor landfills can reduce the long-term pollution potential of the landfill by increasing the rate of waste settlement and stabilization, improving compaction densities, reducing the strength of leachate, and minimizing the long-term generation of landfill gas (LFG). Bioreactor landfills can be categorized broadly as aerobic or anaerobic. However, there are also ongoing studies of aerobic/anaerobic and semiaerobic bioreactors, which combine elements of both aerobic and anaerobic systems.Anaerobic Bioreactors
Aerobic landfill project, Williamson County, TNAnaerobic bioreactor landfills seek to stabilize landfilled waste rapidly by the addition of moisture to uniformly wet the waste mass. Landfill degradation of MSW frequently is rate-limited by insufficient moisture. The average landfilled MSW has a moisture content of only 25% wet weight (Emcon Associates, 1980). However, Farquhar and Rovers (1973) found that the maximum methane production in landfills occurred at moisture content of 60-80% wet weight. This suggests that most landfills are well below the optimum moisture content for methane production. Maier (1998) found average liquid absorptive capacities of waste reported in the literature to be between 16% and 29%, which equates to approximately 30-60 gal./yd.3 of waste. This represents a large potential capacity for leachate storage. It is recommended that liquid injection be suspended prior to reaching the liquid field capacity of the waste, due to watering in of gas extraction trenches and an increased potential for stability problems. Liquid can be injected into the waste via horizontal trenches, vertical wells, surface infiltration ponds, spraying, and prewetting of waste. Reinhart and Townsend (1997) describe these methods of liquid injection in some detail. Anaerobic bioreactor landfills initially should be carefully monitored. If the waste is wetted too rapidly, a buildup of volatile organic acids might lower the leachate pH, inhibiting the methane-producing bacteria population and reducing the rate of biodegradation. Leachate parameters (such as pH, volatile organic acids, and alkalinity) and LFG parameters (such as methane content) are direct indicators of an established methane-producing bacteria population. Optimal conditions for methane-producing bacteria are a pH of greater than 6.5. A high volatile organic acids-to-alkalinity ratio (>0.25) indicates that the leachate might have a low buffering capacity and conditions could soon inhibit methane generation.The gas content of anaerobic bioreactors is similar to that of conventional landfills, with methane and carbon dioxide each making up approximately 50% of the total LFG volume. When the methane content of the LFG exceeds approximately 40%, the methane-producing bacteria population can be considered established. A decrease in the methane gas content below 40% is a possible indication that the waste is becoming too wet or dry. Once the methane-producing bacteria population has become established, the rate of leachate recirculation may be increased.The cost of the basic piping, pumps, electricity, and increased LFG generation for an anaerobic bioreactor is expected to be offset by the avoided or delayed cost of leachate treatment, recovering additional airspace through increased landfill settlement, and LFG-to-energy royalties from electricity generation and direct gas usage by manufacturers or utilities.Benefits of anaerobic bioreactor landfills include:leachate storage within the waste mass,landfill airspace savings (increased rate of landfill settlement),more rapid waste stabilization than conventional landfills,increased methane generation rates (200-250% increase typical),potential for limited landfill mining, andlower postclosure costs.Aerobic BioreactorsAerobic bioreactors operate by the controlled injection of moisture and air into the waste mass through a network of horizontal and/or vertical pipes. Aerobic landfill processes are analogous to wet composting operations in which biodegradable materials are rapidly biodegraded using air, moisture, and increased temperatures created by biodegradation. Prior to air injection, liquid is pumped under pressure into the waste mass through injection wells in order to wet the waste mass to a moisture content between 50% and 70% by weight. Once optimal moisture conditions have been reached, air injection commences. Blowers typically are used to force air into the waste mass through a network of perforated wells that have been installed in the landfill. The rates of injection of air and leachate into the landfill are similar to the air and moisture application rates used in many composting systems. The aerobic process continues until most of the easily and moderately degradable compounds have been degraded and the compost temperature gradually decreases during the final phase of “curing” or maturation of the remaining organic matter.Optimum temperatures for waste degradation within an aerobic bioreactor landfill are between 140º and 160ºF. Due to the substantial amounts of heat generated, large quantities of leachate can be evaporated. Hudgins and Green (1999) reported leachate volume reductions of 86% and 50% at two aerobic bioreactor landfills. Waste temperatures are controlled by changing the rate of air and liquid injection. The potential for waste combustion typically is managed by ensuring that the waste mass is wetted adequately and air injection is uniform throughout the waste mass to minimize methane generation. Waste temperatures are maintained in the optimal range, and only enough air is injected into waste to support aerobic biodegradation.Aerobic bioreactor landfills are much more operationally intense than anaerobic bioreactor landfills. Weathers et al. (2001) determined that the additional power required to inject air into an aerobic bioreactor was 12 times higher than the power required to extract LFG in an anaerobic bioreactor. However, postclosure costs should be reduced substantially due to reductions in LFG generation and cover settlement.Because of higher reaction rates, aerobic biodegradation is a more rapid process than anaerobic biodegradation. Consequently, aerobic landfills offer the potential to achieve the same waste stabilization in two or four years that conventional landfills require decades or longer to reach. The rapid rate of waste stabilization in aerobic landfills offers the potential for mining of the landfill waste. It might be possible to divert a significant fraction of this stabilized waste from aerobic landfills, as will be discussed later.The following benefits have been observed at aerobic bioreactor landfills (Hudgins and Green, 1999):More rapid waste and leachate stabilizationLandfill airspace savings (increased rate of landfill settlement)Reduction of methane generation by 50-90%Capability of reducing leachate volumes by up to 100% due to evaporationPotential for landfill mining and sustainabilityReduction of environmental liabilitiesTable 1 compares conventional landfills with anaerobic and aerobic bioreactor landfills.Table 1. Comparison of Bioreactor LandfillsConventional LandfillAnaerobic BioreactorAerobic BioreactorTypical Settlement After:2 years
10 years
2-5%
15%
10-15%
20-25%
20-25%
20-25%
Anticipated Waste-Stabilization Time Frame30-100 years10-15 years2-4 yearsMethane Generation RateBase caseTwo times base case10-50% base caseLiquid Storage Capacity Utilized in Waste MassNone30-60 gal./yd.330-60 gal./yd.3Liquid EvaporationNegligibleNegligible50-80%*Average Capital CostLowMediumHighAverage O&M CostLowMediumHighAverage Closure/
Postclosure Cost
HighMediumLow* Liquid evaporation rate is highly dependent on site-specific characteristics.Bioreactor Landfill BenefitsGreenhouse Gas AbatementGreenhouse gas (GHG) abatement represents an area where benefits can be realized through the accelerated waste degradation rates offered by bioreactor landfills. Methane has been identified as a GHG, effective at trapping infrared radiation within Earth’s atmosphere, and so a potential contributor to the process of global warming. Aerobic bioreactor landfills can reduce GHG levels over conventional landfills. Conventional landfills and anaerobic bioreactor landfills convert organic matter and water into approximately equal parts methane and carbon dioxide. Aerobic bioreactor landfills convert organic matter and oxygen into predominantly carbon dioxide and water. Since methane is significantly more effective as a GHG than is carbon dioxide, aerobic bioreactor landfills reduce GHG emissions by minimizing methane emissions.Cost savings and benefits to the environment also might come from methane gas recovery if the waste in anaerobic bioreactor landfills is degraded more quickly. The capital costs required to recover and/or refine LFG are high. Timing is therefore critical, and methods that increase the methane production rate are often economically advantageous. Faster MSW degradation generates methane gas more quickly. This gas can be sold or used for energy on-site if it is produced in high-enough concentrations. Lessening the amount of gas generated from the landfill after capping also can decrease long-term postclosure care costs. This also decreases the risk of LFG migration and allows greater amounts of LFG to be captured or flared, reducing the amount of GHGs that escape into the atmosphere.Recycling MandatesRecycling rates of 25% or greater currently are mandated by many regulatory agencies. Other organizations, such as the Grassroots Recycling Network, would propose a recycling goal of zero waste generation (2002). Approximately 57% of the 229.9 million tons of MSW generated in the US in 1999 was landfilled (USEPA, 1999a). Despite increases in composting and recycling, this number is expected to rise to 240 million tons by 2005 (USEPA, 1999b). These statistics reflect the continuing reliance on modern Subtitle D landfills for the management of solid waste in the foreseeable future. Approximately 50-70% of MSW is composed of biodegradable waste. Short of a dramatic shift in thinking, Americans will continue to dispose of a large fraction of municipal food, yard, paper, and other types of putrescible waste by landfilling. Higher recycling rates might be achievable on the back end by mining stabilized waste from bioreactor landfills.Environmental ProtectionConfiguration of a landfill bioreactor. Click here for larger viewBioreactor landfills reduce the biodegradable fraction of the waste mass within a relatively short period of time. This stabilization of the waste mass will occur during the active period of landfilling and well within the postclosure care period. In the alternative dry-tomb scenario, stabilization of the waste mass may not occur until such time as the landfill cap and/or liner system fails. Therefore, bioreactor landfills create a stabilized waste mass and lessen the risk of leachate polluting the groundwater in the future. Leachate Strength ReductionBioreactor landfills decrease the strength of landfill leachate more rapidly than conventional landfills. Chemical oxygen demand (COD) is one often-used indicator of leachate strength. Reinhart and Townsend summarized measurements of COD half-lives for conventional and bioreactor landfills. Although the data are limited and only include limited full-scale application data, some rough conclusions can be made. The time it takes COD to be reduced by 50% (half-life) is about 10 times faster in a bioreactor landfill than in a conventional landfill. In the authors’ experience, liquid addition beyond landfill leachate and LFG condensate is frequently not required for many landfills in areas with higher precipitation (e.g., Northeast and Southeast US). Besides higher precipitation, many existing landfill facilities comprise a number of older closed landfills that still generate limited quantities of leachate. This additional leachate is ideal for supplementing the leachate flows from active landfills. Indeed, in the case of aerobic bioreactors, there might be surplus leachate generation beyond the capabilities of bioreactor landfills to absorb or evaporate. If additional liquid addition is required, bioreactor landfills can utilize stormwater or even offsite industrial/commercial/agricultural liquid wastes. The clogging potential of high-strength offsite liquid wastes on the leachate collection system should be evaluated prior to injection into the waste mass.Mined and screend landfill wasteMined waste being loaded into screenerFinger screenerStabilityThe stability of bioreactor landfills can be modeled using the methods typically used for conventional landfills (Schafer et al., 2002). Intermediate as well as final static conditions should be analyzed since wetted waste conditions prior to degradation may control stability. The minimum recommended factors of safety are 1.5 for final static conditions, 1.3 for intermediate static conditions, and 1.0 for seismic conditions, according to Schafer et al. The higher waste mass density, lower waste shear strength, and potential for buildup of pore pressures must be addressed in the design and operation of bioreactor landfills.The waste mass density in a bioreactor landfill is typically higher than in a conventional landfill due to the higher moisture content and increased consolidation of the waste mass. The average unit weight of MSW in a bioreactor landfill has been observed to be approximately 80 per cubic foot compared to 65-70 per cubic foot for a conventional landfill (Schafer et al., 2002). Pacey et al. (1999) surmised that the waste mass might be up to 30% heavier in a bioreactor landfill. Injecting leachate, LFG condensate, and other liquid sources might increase the potential for leachate seeps and stability issues. The following design and operational procedures can minimize this potential:Landfill operations should be graded away from outer slopes to minimize the potential of horizontal leachate migration toward outer slopes.Monitor sideslopes for leachate seeps and monitor pore pressures and liquid levels within the waste using pressure transducers and piezometers, respectively.Carefully select and use of daily cover materials (i.e., tarps, foam, construction and demolition material, and/or stripping of cover prior to waste placement).Record liquid injection dosing through flow meters and calculations of waste mass volume affected by each injection application.Liquid injection valving permits each injection point to be separately adjusted or shutoff.Uniform wetting of the waste mass through appropriately spaced injection points will minimize the potential for isolated saturated pockets of waste.Discontinue injection of liquid into waste mass within 100 ft. of outer slopes one year prior to final cover placement or if leachate seeps or significant pore pressures are observed near the sideslopes.Settlement Accelerating MSW degradation can reduce the need for new landfills by recapturing landfill airspace. Figure 1 provides typical landfill settlement for conventional, anaerobic, and aerobic bioreactor landfills. Conventional landfill settlement is typically around 10% of landfill height and generally occurs over a number of years as the waste decomposes (Koerner and Daniel, 1997). Pilot-scale landfill cells in Sonoma County, CA, and Mountain View, CA, exhibited settlement by as much as 20% and 14%, respectively, in leachate recirculation cells and approximately 8-10%, respectively, in the conventional dry cells (Reinhart and Townsend, 1997). Waste settlement varies greatly and is dependent on type of waste, amount of cover, and compaction. Typical anaerobic and aerobic bioreactors can be expected to generate between 20% and 25% settlement. However, aerobic bioreactors might achieve this settlement within two to four years, while anaerobic bioreactors might require five to 10 years. Increased rates of settlement before closure will permit additional MSW to be placed in the landfill before a cap is put in place. The placement of additional MSW in existing facilities can therefore reduce the need for new landfills. These benefits can be realized only when waste decomposes prior to closure, since once a landfill cap is put in place it might not be cost-effective to go back at a later date and reopen the facility. Landfill operators might opt to delay cap placement in order to take advantage of increased airspace created by additional waste settlement.Extending Bioreactor TechnologySustainabilityClick here for larger viewThe goal of bioreactor landfills is to stabilize the waste mass in a relatively short time frame of two to 10 years. The stabilized waste would have a minimized potential to generate LFG and further degrade if exposed to air and water. If the waste mass can be stabilized sufficiently in a bioreactor landfill, then it can be more safely excavated and recycled.After the waste has been stabilized, it then can be excavated and trommeled. Approximately half of the stabilized waste is a soil/compost mixture that may be utilized as landfill cover material. A fraction of the stabilized waste (approximately 10%) consists of metals, which could be recovered from the stabilized waste and recycled, thereby diverting it from the wastestream. The remaining fraction of stabilized waste consists of dirty plastic material and other miscellaneous inert materials. The dirty plastic material could be used as feedstock for low-grade plastic wood products or as a fuel source. The remaining miscellaneous inert material then would be landfilled.Landfill mining and reclamation can be used as a measure to remediate poorly designed or improperly operated landfills and to upgrade landfills that do not meet environmental and public health specifications. Excavators, screens, and conveyors are some of the typical equipment used in most operations. Complex operations recover additional materials and improve the purity of recovered materials and therefore have equipment in addition to that of simple operations.Environmental Control Systems Inc. (ECS) has formulated a patented process for developing sustainable landfills. The sustainable landfill developed by ECS consists of placing and treating waste in a number of smaller aerobic bioreactor cells that are constructed within the current or planned permitted boundaries of the landfill. Although smaller than the entire landfill, they are designed and constructed to Subtitle D standards, occupying only the footprint needed to mange the annual incoming wastestream. Following aerobic treatment, the stabilized waste mass is mined and sorted. A significant fraction of the stabilized waste is diverted from the landfill as previously described. The remaining miscellaneous inert material would be landfilled in a conventional landfill cell. Built closely together, each cell (five or six, depending on the waste characteristics, waste receipt, and waste degradation time) will experience waste filling, treatment, mining, and liner rehabilitation, if necessary, in a rotating sequence.

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