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Feature
Article September/October 2000
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A comparison study between a bioreactor landfill and a conventional dry-type landfill gives bioreactor technology a boost. By Pat Sullivan and G. Alexander Stege Bioreactor
Landfills A study of the expected benefits and impacts of the bioreactor technology on MSW landfills was conducted in order to estimate potential changes in air and greenhouse gas (GHG) emissions and methane-recovery potential. The study was designed to compare bioreactor landfills to the conventional Resource Conservation and Recovery Act (RCRA) Subtitle D ("dry-type") landfills, which are commonly used in the landfill industry. The objective of this study was to demonstrate that although there will likely be an increase in landfill gas (LFG) generation at a bioreactor landfill, this will not necessarily result in an increase in emissions of pollutants present in LFG. The study was also designed to demonstrate that with the increase in LFG generation, ancillary benefits of GHG reductions and increased methane-recovery potential will be realized. Bioreactor landfills are gaining increasing prominence in the solid waste industry as an alternative to the typical MSW or "sanitary" landfill, which is designed and operated in accordance with RCRA Subtitle D or state-equivalent regulations. Subtitle D was promulgated on the premise that refuse in a landfill must be kept dry in order to prevent the formation and migration of leachate and LFG, which could impact groundwater underlying the landfill, cause air-quality impacts, or create subsurface combustible-gas problems. Subtitle D has generally been successful in minimizing the formation of leachate and controlling the migration of leachate out of the refuse mass. However, the regulation has met with limited success in the area of LFG. The dry entombment of a landfill cannot eliminate LFG generation; rather, it just slows the rate of microbial degradation so that LFG is produced over a longer period of time. This phenomenon has substantially lengthened the postclosure maintenance period for the operation of LFG collection and control systems. In addition, it has restricted the potential for recovery of LFG for energy production. With the increased technical knowledge that has been gained over the last decade since Subtitle D was promulgated, it is clear that alternative landfill design and operational scenarios should be considered to better manage the generation of LFG and decrease the postclosure maintenance period for landfills. Landfills only generate revenue while waste is being accepted. As such, long-term (more than 30 years) postclosure obligations place an undue financial burden on landfills, forcing landfill owners and operators to increase disposal costs to fund the postclosure activities. The bioreactor landfill considered in this study is an MSW landfill that utilizes enhanced microbial processes under anaerobic conditions to accelerate the degradation of refuse. This enhanced degradation can serve to more rapidly stabilize the refuse mass while producing LFG more quickly and at higher rates. In terms of LFG, this creates a twofold benefit. Significant LFG generation (and subsequent recovery) is anticipated to be limited to a 15- to 20-year life after landfill closure, thereby significantly limiting the postclosure period for LFG control. Also, the methane-recovery potential at a bioreactor landfill creates a more financially viable situation because LFG generation occurs at higher levels over a shorter time period, thus allowing for more methane recovery with less operational cost (i.e., fewer years of operation). With the enhanced microbial activity in a bioreactor landfill, LFG generation and recovery rates are expected to increase substantially as a result of accelerated and more complete degradation. This increased rate of LFG generation must be managed properly so that impacts to air quality do not occur. This bioreactor landfill emissions study included an evaluation of projected LFG generation rates at a typical dry-type landfill as compared to LFG production at a similarly sized bioreactor landfill. LFG generation is known to have a direct impact on LFG emissions, which include methane, nonmethane organic compounds (NMOCs), volatile organic compounds (VOCs), and hazardous air pollutants (HAPs). In addition, the LFG recovery rate (i.e., the amount of LFG collected) will affect the amount of secondary pollutant emissions (e.g., nitrogen oxides, or NO; carbon monoxide, or CO; combustion particulate; and sulfur dioxide) that will be created by LFG control devices. After evaluating LFG production, the study related the LFG generation and recovery rates to the emission rates for the pollutants identified above. The study used methane, NMOCs, NOx, and CO as representative pollutants for landfills. Emission estimates for these pollutants were completed for a typical dry-type landfill and then compared to an equivalently sized bioreactor landfill. This comparison included an assessment of peak annual emissions, which is a commonly used value for air-quality permitting (i.e., potential to emit), and emissions over the operational and postclosure life of the landfill (a 75-year life was assumed). Secondary pollutant emissions were determined by predicting LFG recovery rates under a controlled scenario and assuming that the recovered LFG would be combusted in an enclosed ground flare, a standard control device at MSW landfills. Hypothetical Landfill Scenario To complete a comparative analysis of bioreactor versus dry-type landfills, the study delineated an evaluation of a typical landfill scenario. This scenario represents a medium-size MSW landfill, which is similar in size to many in operation today. The medium-size landfill was assumed to have a capacity of 20 million tons and an active life of 40 years. The annual refuse disposal rate for the typical landfill was assumed to be an average of 500,000 tpy or 1,603 tpd. LFG generation-rate estimates for the bioreactor and dry-type landfills were developed using the US Environmental Protection Agency’s (EPA) first-order decay-rate LFG generation model. Two separate model runs were completed for the landfill: one for the bioreactor scenario and one for the dry-type scenario of the same size. The projected differences in LFG generation between the bioreactor landfill and the dry-type landfill were accounted for by altering the refuse decay-rate constant (k value) of the refuse. The selection of k values for the various model runs was determined by considering default values prescribed by EPA within the New Source Performance Standards (NSPS) and other regulations, values derived from LFG recovery-rate calibration studies completed by SCS Engineers, and predicted values for bioreactor landfills from ongoing pilot tests. The study did not alter the ultimate methane generation-rate (Lo value) constant since this parameter was not assumed to be affected by the increased moisture and microbial activity in a bioreactor landfill. Because a bioreactor landfill is specifically designed to accelerate refuse degradation and LFG production, a k value of 0.1 was selected for use in the bioreactor model run. For comparison purposes, a k value of 0.05 is prescribed by EPA in the NSPS regulations for wet climates (greater than 25 in. of rain per year); the NSPS prescribes a k value of 0.02 for dry climates. EPA’s Compilation Air Pollutant Emission Factors (AP-42) document (Section 2.4 on landfills) sets forth a k value of 0.04 for wet climates and 0.02 for dry climates. A k value of 0.02 was selected for use in the dry-type model run (i.e., typical MSW landfill in a dry climate). As such, the 0.1 k value represents a fivefold increase in refuse degradation over the dry-type landfill. An Lo value of 100 m3/Mg was selected for use in the model runs for both landfill types. An Lo value of 100 m3/Mg represents the value prescribed by AP-42 for use with MSW landfills. LFG Collection and Control Systems In the study, landfill gas collection and control systems (GCCSs) were assumed to be installed at both the bioreactor and dry-type landfills. The schedule for installation of this system at the dry-type landfill was based on NSPS requirements. It was assumed that a typical dry-type landfill would not be operated with a GCCS unless required by the regulations. Therefore, an NSPS Tier 2 emission-rate calculation was assumed to be conducted. NSPS require the installation of a GCCS when an emission threshold of 50 Mg per year of NMOCs is exceeded. A default NMOC concentration of 595 ppm by volume was used for the Tier 2 calculations; this value represents the AP-42 default for NMOCs at MSW landfills. Based on the Tier 2 results, GCCSs were assumed to be installed at the dry-type landfills within 30 months after first exceeding the 50-Mg threshold (year 12 of the study), as required by NSPS. The GCCSs were assumed to have a collection efficiency of 75%, which is the average value prescribed by AP-42 and is typical for NSPS landfills. As allowed by NSPS, GCCSs at the dry-type landfills were assumed to be taken out of service after 20 years of postclosure operation (year 61 of the study). GCCSs at bioreactor landfills were assumed to be installed in the third year of landfill operation, regardless of the NMOC emission rate. This was done since bioreactor landfills will begin to generate LFG very quickly and LFG collection and control will be necessary to limit LFG migration and emissions. In fact, bioreactor landfills are typically designed to be operated with LFG control systems in place during the early life of the landfill. GCCSs at the bioreactor landfills were assumed to be taken out of service within three years after the landfill’s NMOC emission rate dropped below 50 Mg after closure, in accordance with NSPS. GCCSs at bioreactor landfills were assumed to have a collection efficiency of 90%. This increase in collection efficiency versus dry-type landfills was justified based on the fact that bioreactor landfills are typically designed with extremely comprehensive GCCSs in order to effectively control the high levels of LFG generation. Furthermore, bioreactor landfills are maintained in highly controlled environments that are more conducive to increased LFG collection and control, including substantial liner and cover systems. Methane and GHG Emissions Methane emissions from the landfills were estimated by assuming that methane comprises 50% of the LFG at the bioreactor and dry-type landfills, which is typical for MSW landfills. There is some evidence to suggest that methane concentrations in LFG at bioreactor landfills could vary from the 50% value; however, these data are not conclusive enough at this time and therefore were excluded from the study. Methane emissions were calculated over a 75-year landfill operational and postclosure life. Annual methane emissions were summarized for each year of the study. For years when GCCSs were not in place, the total methane emissions were estimated to be equal to the total amount of methane generation. For years when GCCSs were in operation, methane emissions were estimated to be equal to the amount of uncollected methane (25% or 10%) plus the amount of methane not destroyed in the LFG flare (2% of the amount of methane processed through the flare = 98% destruction efficiency). Methane is a very potent GHG. In recognition of this fact, EPA has established the Landfill Methane Outreach Program to promote the control of methane emissions from landfills. Methane is 21 times more potent than carbon dioxide as a GHG. This calculation also was used to estimate the methane-recovery potential for each of the landfill types. This was done by estimating the recovered methane potential and converting it to a corresponding energy potential, assuming a content of 500 Btu per standard cubic foot (i.e., approximately 50% methane). Nonmethane Organic Compounds NMOC emissions from landfills were estimated in similar fashion as the methane emissions. NMOCs were assumed to be present in a concentration of 595 ppm by volume, as prescribed by AP-42 for MSW landfills. This value was used for both types of landfills, although there is a possibility that NMOC concentrations could vary for a bioreactor landfill. However, data are not available that could justify the use of an alternative NMOC value for bioreactor landfills. NMOC emissions were calculated over a 75-year landfill life. For years when GCCSs were not in place, the total NMOC emissions were estimated to be equal to the total amount of NMOC generation. For years when GCCSs were in operation, the emissions were estimated to be equal to the amount of uncollected NMOCs (25% or 10%) plus the amount of NMOC not destroyed in the LFG flare (2% of the amount processed through the flare = 98% destruction efficiency). Volatile Organic Compounds EPA has determined that VOCs comprise 39% of the NMOCs at MSW landfills, per AP-42; therefore, the emissions described above were used to gauge the VOC emissions at these landfills. VOCs are an ozone precursor and, as such, can have regulatory impacts for MSW landfill in ozone nonattainment areas. Hazardous Air Pollutants The majority of HAPs emitted from an MSW landfill are NMOCs; therefore, the NMOC emissions described above can also be used as a gauging point for HAP emissions from landfills. Any increases or decreases in NMOC emissions at bioreactor versus dry-type landfills would result in a requisite increase or decrease in HAP emissions. HAP emissions from landfills will soon be regulated under the upcoming Maximum Achievable Control Technology (MACT) standard for MSW landfills and the Urban Air Toxics Strategy (UATS). As such, it is important to consider the impacts that bioreactor landfills may have on HAP emissions from landfills. Secondary Pollutants From LFG Combustion Devices Secondary pollutant emissions from the LFG combustion devices include undestroyed methane, NMOCs, VOCs, and HAPs in addition to NOx, sulfur oxides, CO, and particulate matter less than 10 microns generated from combustion. Enclosed ground flares can achieve an organic compound destruction efficiency of greater than 99% in most cases. For this study, however, the destruction efficiency was assumed to be 98% to be consistent with NSPS requirements. The controlled emission rates for NOx and CO were calculated using emission factors derived from the Best Available Control Technology (BACT) standard, which has been established for enclosed ground flares. The BACT for NOx is 0.06 lb./MM Btu; the BACT for CO is 0.2 lb./MM Btu. LFG for the bioreactor and dry-type landfills was assumed to have a heating value of 500 Btu/ft.3 Emission factors for NOx and CO for other types of LFG combustion devices, such as IC engines, gas turbines, steam boilers, and open flares, are available and do vary from the values for enclosed flares. An enclosed flare was used in this study, however, since it has a well-established BACT standard. Projected emissions and methane-recovery potential from this typical landfill scenario are summarized in Table 1.
Methane and GHG Emissions Based on the above, a properly designed and operated bioreactor landfill is expected to have lower methane, NMOC, VOC, and HAP emissions as compared to a similarly sized dry-type landfill. The air-quality benefits of these emission reductions are substantial when evaluating the peak year but are especially significant when assessing the entire operational and postclosure life of a landfill. The projected reductions in methane emissions will have a significant benefit on the reduction of GHGs. Methane accounts for over 18% of the worldwide GHG emissions (as carbon-dioxide equivalents). In the United States, landfills emit more than 36% of the anthropogenic methane emissions nationwide. A relatively large landfill gas-to-energy (LFGTE) project (approximately 10 MW) is estimated to avoid 65,700 tpy of carbon-dioxide emissions through displacement of utility-produced electricity. The operation of a medium-size landfill as a bioreactor landfill could potentially result in an additional reduction of more than 1.7 million tpy of carbon-dioxide emissions, based on our study. Therefore, the potential benefits of bioreactor landfills on the reduction of GHG emissions are significant and worthy of further evaluation. In addition, any GHG emission reductions, above that which is required by air-quality regulations, could result in a viable GHG credit that has monetary value. As such, the increased reductions in GHG emissions, which occur with a bioreactor landfill, could create viable GHG credits that would generate income for a landfill owner or operator. Energy-Recovery Potential Based on the results of the study, a bioreactor landfill is expected to result in an increase of approximately 76% in methane-recovery potential as compared to a dry-type landfill. This will result in a corresponding up to 76% increase in the recoverable Btu value from the same landfill, which could provide for a substantial rise in the energy-recovery potential from MSW landfills. In addition, bioreactor landfills produce this additional methane in larger quantities over a shorter period of time and during the operational life of the landfill. Because of this, energy-recovery scenarios are more financially attractive since an equivalent amount of energy would be recovered in less time, resulting in lower annual operating costs over the life of the project. Further, the methane-generation scenarios for a bioreactor landfill correspond better with the projected lives for the various energy-recovery equipment used at landfills, such as IC engines, gas turbines, and boilers. As such, an LFGTE facility at a bioreactor can generate more energy from a single piece of equipment before such equipment must be replaced or overhauled. NMOC Emissions Under the NSPS and Emission Guidelines (EG), MSW landfills are regulated for NMOC emissions. Compliance with the NSPS and EG is achieved by limiting NMOC emissions to below regulatory standards. The operation of a landfill as a bioreactor landfill will result in a substantial decrease in actual NMOC emissions throughout the life of a landfill. EPA predicted that the implementation of the NSPS and EG would result in a reduction of approximately 82,000 tpy of NMOC emissions at 355 landfills nationwide. That is an average of approximately 231 tpy per landfill. The operation of a landfill as a bioreactor landfill could potentially result in an additional decrease of approximately 40 tpy (when adjusted for the NMOC concentration used by EPA in promulgating the regulations). This represents a possible increase of approximately 17% in reductions, which could occur at an NSPS/EG site operated as a bioreactor landfill. Again, this amount of reduction is clearly significant. In addition to its use in NSPS/EG compliance, NMOCs have also been established as a regulated pollutant for MSW landfills under the federal Prevention of Significant Deterioration (PSD) program. PSD is a preconstruction air permitting program for major sources of emissions in attainment areas. PSD must be evaluated for applicability for any new landfills or sites proposed for expansion where NMOC emissions may increase. PSD is triggered by new landfills or modifications to existing minor source landfills that emit at least 250 tpy of NMOCs. PSD is triggered for modifications to existing major source landfills that produce 50 tpy of NMOCs. As such, reductions in NMOC emissions for a bioreactor landfill could help to limit emissions so that PSD, which is a burdensome program, is not triggered. VOC Emissions Since VOCs comprise 39% of the NMOCs at MSW landfills, NMOC emission reductions will also result in substantial VOC reductions. As stated above, VOCs are an ozone precursor, and as such they can have regulatory impacts for MSW landfills in ozone nonattainment areas. New Source Review (NSR) is a preconstruction permitting program for major emission sources in nonattainment areas. It contains some very stringent air-quality requirements, including the requirement to purchase emissions offsets, which can be extremely expensive (e.g., $5,000-$25,000 per tpy in California). NSR can be triggered for emission increases as low as 10 tpy in extreme nonattainment areas for ozone. As such, even small increases in VOC emissions could be detrimental to a landfill project. The reduction in VOC emissions occurring at a bioreactor landfill could be used to help avoid NSR applicability and the burdensome costs associated with emission offsets. In addition, any VOC emission reductions, above that required by air-quality regulations, could result in credible emission reduction credits (ERCs) that have monetary value. As such, the increased reductions in VOC emissions, which occur with a bioreactor landfill, could create viable ERCs that would generate income for a landfill owner or operator. Hazardous Air Pollutants As previously mentioned, the majority of HAPs emitted from an MSW landfill are NMOCs. Therefore, NMOC emission reductions that occur at a bioreactor landfill will also result in reductions of HAP emissions. HAPs in LFG can present potential human health and ecological risks when emitted into ambient air. Many of the HAPs in LFG emissions are carcinogenic or otherwise toxic. As such, reductions in HAP emissions with a bioreactor landfill will result in decreased health risks to human and ecological receptors. Also, HAP emissions from landfills will soon be regulated under the upcoming MACT standard for MSW landfills (due in November 2000) and the UATS (regulations are expected in 2004). HAP emissions reductions at bioreactor landfills may assist a landfill in compliance with MACT and/or UATS regulations. Secondary Pollutants From LFG Combustion Devices Secondary pollutant emissions from the LFG combustion devices were shown to increase for a bioreactor versus a dry-type landfill. While this long-term increase in secondary pollutant emissions is not expected to be detrimental to air quality, these emissions will have an adverse impact on air-quality permitting for these devices, particularly in nonattainment areas. Because current federal, state, and local air-quality regulations require the permitting of potential-to-emit levels for pollutants, the high peak-year secondary pollutant emissions for a bioreactor landfill will be burdensome during permitting. Therefore, this ancillary adverse impact of bioreactor landfills must be considered when permitting LFG control devices under current air-quality regulations. The potential issues with secondary pollutant emissions should be considered when planning a bioreactor landfill project. This bioreactor landfill emissions study was developed based on available information on bioreactor landfills. It is clear from the conclusions of the study that bioreactor landfills are expected to have a generally positive impact on air quality and GHG emissions from landfills. Therefore, further study into these positive impacts is warranted, but this can only be accomplished by evaluating actual conditions at full-scale bioreactor landfills. Data from pilot studies, while valuable, are limited, and extrapolation of these data to a full-scale bioreactor landfill operation is questionable. Also, many of the aforementioned technical concepts for bioreactor landfills might be consistent with the manner in which landfills are currently regulated as emission sources. As such, it is clear that the concept of a bioreactor landfill will necessitate a reevaluation of current regulatory programs that govern emissions and possible regulatory or policy changes to facilitate bioreactor development. Pat Sullivan is the air-quality technical manager for SCS Engineers in Dublin, CA. G. Alexander Stege is a project scientist for SCS Engineers in Phoenix, AZ.
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