Landfilling is one of the traditional options considered to address “The Waste Problem,” dubbed for the continuous, largely anthropogenic, generation of waste (inclusive of industrial, residential, commercial, agricultural origins and more). Solid waste as defined by the EPA encompasses “any garbage, refuse, sludge from a waste treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semi-solid, or contained gaseous material resulting from industrial, commercial, mining and agricultural operations, and from community activities” (USEPA, 40 CFR 257.2). Tied to population growth and socioeconomic development, waste generation continues to grow and with it the footprint of traditional “dry tomb” landfills. Restricted by federal regulations, but supplemented by increasingly sophisticated engineering systems and technologies, modern landfills have evolved from traditionally open dumps. The philosophy of modern landfills centers strictly on containment and isolation of the material; out of sight and encased by heavily engineered systems to protect from leachate-induced environmental and health hazards. However, bioreactor landfills offer an alternative landfilling philosophy focused on controlled material biodegradation that carries potential as an innovative landfill approach offering a more sustainable solution to the waste problem.
Bioreactor landfills, unlike conventional dry tomb municipal solid waste (MSW) landfills, purposefully recirculate air and liquids through the waste structure to support the microbial degradation processes. Introduction of liquids facilitates microbial movement and nutrient transport, while the introduction of air accelerates biodegradation and bio-stabilization processes (Ritzkowski and Stegmann, 2013). Central to the principle of bioreactor landfills, recirculation of leachate through the system stimulates organic decomposition, producing combustible gas, which can be collected to generate electricity. Figure 1 and Figure 2 visualizes the skeleton of a bioreactor landfill and the recirculation process for electricity generation respectively, while Figure 3 depicts an example of a hybrid aerobic-anaerobic sequenced landfill and Figure 4 depicts a conventional landfill with leachate recirculation.
In the United States, MSW landfills are regulated by Subtitle D of the Resource Conservation and Recovery Act (RCRA). Federal code permits leachate recirculation in accordance with 40 CFR 258.28, which restricts liquid addition to the reintroduction of leachate and condensation into Subtitle D landfills. Bioreactors require tightly controlled liquid addition and management to function optimally, so at first glance, bioreactor landfills are not compatible with federal regulations (Pacey et al., n.d. and Reinhart et al., 2002).
Some states have interpreted 40 CFR 258.28 to mean that no liquid addition is permissible with the exception of reintroducing leachate and condensation, but Subtitle D doesn’t expressly prohibit the use of amendments in MSW landfills, so other states have a broader interpretation of the code that prohibits the introduction of bulk liquid wastes, but allow the use of amendments, like clean water, to promote bioreactor microbial processes (Pacey et al., n.d.).
A 1997 Solid Waste Association of North America survey found that six US states permit bioreactor landfills by supplementing federal regulations with additional state-specific requirements with another eight states viewing bioreactor landfills positively with plans to approve them or willing to consider a proposal to approve them. Some states have embraced bioreactor landfill technology. The New York Code of Federal Regulations 360-2.9 states: “…active landfill management techniques to encourage rapid waste mass stabilization and alternate energy resource production and enhanced landfill gas emission collection systems are encouraged…” (Pacey et al., n.d. and Reinhart et al., 2002).
Most states have no specific regulations regarding bioreactor landfills and allow the recirculation of leachate and condensation while three states prohibit bioreactor landfills and another eight states indicate they would not approve a bioreactor landfill with the primary reason cited being that most of the state’s landfills are unlined (Pacey et al., n.d.).
The US Environmental Protection Agency (EPA) describes three types of bioreactor landfills: aerobic, anaerobic, and hybrid aerobic-anaerobic (US Environmental Protection Agency, n.d.). Additionally, many traditional MSW landfills are operated with leachate and condensation recirculation and will be discussed in this paper as partial bioreactor landfills. Key details for each type of landfill are discussed below and Table 1 from Berge et al., 2009 provides an excellent comparison and can be viewed in Appendix A.
Figure 1. Anaerobic Bioreactor Landfill Schematic
Bioreactor landfills were commonly operated under anaerobic conditions. Leachate is continuously recirculated through the waste stratum, facilitating microbial anaerobic degradation and producing gas--primarily methane. The gas collection system then funnels the gas for combustion and energy generation. Circulating liquids through an enclosed system and high gas generation poses obvious geotechnical hazards of slope stability failure as additional pressures from leachate and gas, when not maintained properly, reduce the effective strength of the soil-waste stratum. Another primary challenge with purposefully accelerating the biostabilization of the waste is the potential of waste densification and subsequent settlement, which may induce additional stress to waste liners containing the whole system.
A persistent issue with anaerobic bioreactor landfills is also the danger of ammonia accumulation in leachate, which is toxic and carries high potential risk for the environment and community health and safety, should the leachate with high concentration of ammonia reach an aquifer or groundwater table. Buildup of ammonia is excreted when organic nitrogen comes in contact with leachate and hydrolyzes (Burton and Watson-Craik, 1998). Depending on pH levels (nitrogen tends to hydrolyze to ammonium in basic liquids), ammonium concentration increases in leachate and poses greater risk from discharge if left untreated (Price et al., 2003). For long term maintenance, leachate nitrification treatment is needed to nitrify the leachate ammonium to less toxic nitrate. Based on the literature, a common technique to curb the accumulation of ammonia is with the use of an ex-situ denitrification treatment facility whereby leachate collected is subjected to biological treatment with introduction of nitrifiers, which transforms the toxic ammonia to less hazardous forms of nitrates and nitrites that can then be reintroduced to the waste matrix. However, reintroduction of nitrate to the system may enable denitrification of nitrates to a gaseous state, arriving at the same potential slope stability risk with weakened effective stresses.
Despite structural stability concerns and potential risk from discharge, anaerobic decomposition facilitates high methane generation, improves leachate quality, and accelerates bio-stabilization. From an efficiency perspective, anaerobic bioreactors can also have time-cost savings; Berge et al., 2009 found that anaerobic bioreactor landfills can be 20% less costly (when leachate recirculation begins during active filling) to 9% more costly (when leachate recirculation begins post-landfill closure) when compared to a traditional MSW landfill.
Figure 2. Aerobic Bioreactor Landfill Schematic
Traditional bioreactor landfills operate by enhancing waste stabilization by capitalizing on anaerobic microorganisms, but more recently, there has been interest in aerobic bioreactor landfill systems. Aerobic bioreactor landfills introduce oxygen to the landfill to promote aerobic conditions, typically by utilizing horizontal and vertical wells, like those used for gas extraction and leachate injection (Reinhart et al., 2002).
Aerobic bioreactor landfills lead to increased concern of landfill fires: the aerobic microbial respiration process is exothermic, releasing energy in the form of heat that can create landfill fires and the introduction of oxygen adds fuel that can exacerbate fire risk. Additionally, nitrous oxide, which is a more potent greenhouse gas than methane, can be emitted. There are also increased power costs associated with the air injection process (Reinhart et al., 2002).
Despite the above concerns, there are multiple advantages of aerobic bioreactor landfills over the traditional anaerobic process. Research has shown that aerobic waste decomposition results in faster waste stabilization. Additionally, aerobic bioreactor landfills do not produce methane as a byproduct, nor do they produce other gases typically associated with foul odors from anaerobic processes. Another added benefit is that chemicals that aren’t degraded by anaerobic processes can be broken down under aerobic conditions, offering improved treatment for organic wastes and ammonia. Finally, the addition of air to the landfill helps to strip moisture from the system, which aids in drying out landfills to minimize leachate production (Reinhart et al., 2002).
Berge et al., 2009 found that aerobic bioreactor landfills are approximately 13% less costly than a traditional landfill due primarily to real-time landfill space savings that allow the placement of additional waste before another cell must be utilized.
Figure 3. Aerobic-Anaerobic Schematic of a Sequenced Hybrid Bioreactor Landfill
As the name implies, hybrid bioreactor landfills utilizes both aerobic and anaerobic phases to optimize the benefits from both decomposition processes. Biological decomposition, encompassing both anaerobic and aerobic, both occur simultaneously as the landfill stabilizes. Aerobic degradation is the major method of decomposition during the initial phase as the waste is filled in the landfill, with large surface area exposed to air. Microorganisms use the entrapped oxygen to break down organic matter to simpler polymers, resulting in high carbon dioxide and low methane emissions. Simpler monomers are anaerobically broken down to a mix of organic acids. Methanogenesis ensues using remaining carbon dioxide, hydrogen, and inorganic acids, releasing methane and lowering dissolved solid concentrations in the leachate.
One example of a hybrid model introduces forced aeration of older landfills that’s largely undergone anaerobic decomposition. Purposeful aeration of an older anaerobic bioreactor landfill essentially converts the site to a hybrid bioreactor landfill and helps address remediation of persistent pollution from unmaintained landfills left to long-term anaerobic decomposition (Ritzkowski and Stegmann, 2013). The same paper studied a landfill aeration project in Germany using Landfill Simulation Reactors (LSRs) to assess and extrapolate the oxygen requirements for aeration to accomodate for the decomposition of biodegradable organic carbon embedded in the waste matrix, arguing that forced aeration allows the landfill to reach an acceptable bio-stabilized state at a faster rate. A slew of research has been conducted previously to optimize the leachate quality from aerobic bioreactors and biogas generation from anaerobic decomposition. Xu et al., 2015 found that partial aeration of the surface layer accelerated the decomposition of volatile fatty acids (VFAs) and higher aeration frequency shortened the acidogenic phase, allowing methanogenesis to ensue faster. However, batch reactor experimentation by Brummeler and Koster, 1989 (electronically published in 2003) have previously suggested that over aeration (referenced as long composting time in the paper) leads to significant loss in potential methane production. In either case, length and volume of air (and leachate) circulation incur associated high operating and maintenance costs, but, if phased correctly, can be an economical approach due to higher rates of methane production (Berge et al. 2009). Ultimately, hybrid reactors aim to optimize the energy capture potential of the system in such a way that minimizes environmental and health risks by removal to nitrates and accelerating biostabilization.
Figure 4. Conventional Landfill with Leachate Recirculation Schematic
A true bioreactor landfill utilizes enhanced microbial processes to transform and stabilize waste through the careful and controlled introduction of liquids at optimized levels. Many traditional MSW landfills utilize some leachate and condensation recirculation, leading to some beneficial microbial processes to hasten waste stabilization, but in these landfills, the liquid injection process isn’t optimized. So, while these partial bioreactor landfills aren’t aligned with the “dry tomb” approach of traditional MSW landfills, they also don’t act in the same fashion as bioreactor landfills. Since these partial bioreactor landfills aren’t optimized, they don’t realize the same potential benefits as their true bioreactor counterparts, like increased output of methane for electrical generation or enhanced waste degradation leading to airspace savings, but they do result in leachate treatment savings.
Morris et al., 2003 studied a traditional MSW landfill that utilized leachate recirculation to enhance waste degradation in anaerobic conditions. The study found that, with the exception of ammonia, the leachate rapidly achieved typical chemical concentrations of a mature landfill, total landfill gas production increased and the duration of landfill gas production decreased. Of note, is that the study’s measured landfill gas volume didn’t agree with the landfill gas generation models; it is believed that the lower measured gas production is a result of unoptimized leachate recirculation being channeled along preferential flow paths. The study also found that the total volume decreased by 19% over the 13-year study. The study supports the idea that a traditional MSW landfill benefits from the positive effects of moisture recirculation to create a partial bioreactor case, leading to a reduction in post-closure monitoring and care.
Leakage control is a standard design consideration taken for typical “dry tomb” MSW landfills to avoid potential health and environmental hazards from accidental leachate leakages. Since bioreactor landfills purposefully circulate leachate material through the waste matrix as a way of economical biostabilization, design constraints must account for continuously wet conditions. Leachate distribution methods in bioreactor landfills include spray irrigation, infiltration ponds, subsurface trenches or wells, and direct application on the working surface (Reddy, 2006). High degree of anisotropy and large void spaces are some of the key characteristics of waste that can increase seepage risk. Anisotropic waste mass makes it difficult to predict leachate flow paths from the leachate distribution system. Instead preferential flow paths are created that often spreads laterally towards the slopes of the landfill instead of diffusing over a larger volumetric space. Along with waste characteristics, the pressure of the leachate distribution also increases the potential for leakages as leachate is effectively pushed further from the trenches/wells, diffusing over a larger volume of saturated material which may seep through the side face of the slope. Figure 5, below, summarizes the various causes of leachate seeps in landfills (Xu et al., 2013).
Xu et al. 2013, summarizes general seepage control strategies as the following: (1) assessing target volumes of leachate to be circulated, (2) managing soil cover permeability, (3) grade waste lifts, and (4) increase trench/well setback distance.
(1) Decreasing the total volume of leachate circulating in the matrix does lower the risk of leakage, considering all other variables remain constant, but doing so may also affect methane production, which then affects energy output, and the financials of making the bioreactor landfill an economically feasible system. Predictive cost models would arguably be difficult to use to assess the cost benefits of varying target volumes of leachate, especially due to site-specific variables such as landfill age, size, and primary materials present. However, one can come to a basic understanding that to vary the leachate flow rate (and maintain specific pressure) in the distribution system requires additional energy inputs that have direct correlation with costs. (2-3) Soil cover permeability and graded slopes both help prevent pooling and the inadvertent lateral migration of liquids towards the slopes. (4) The setback distance (distance between the trench and landfill surface) can work in tandem with decreasing the total leachate volume being recirculated to lower the potential for seepage. Lowering the volume out output can decrease the zone of saturation around the injection trench so that the zone does not propagate to the sides of the landfill.
Though operating a “wet” system, such as bioreactor landfills, pose potential leakage problems that may be detrimental to human health and the environment, design engineers have access to proven techniques that can mitigate and reduce the impact of the potential hazard while maintaining an economically feasible and sustainable landfilling system.
Traditional MSW landfills utilize a compaction philosophy that maximizes compaction to pack the maximum amount of waste into the minimal amount of volume due to the high cost of landfill construction. This philosophy doesn’t carry over well to the implementation of bioreactor landfills: hydraulic conductivity is inversely related to specific weight, so efficient packing of waste reduces the ability of moisture to move through the waste to enable enhanced degradation processes. Compaction also contributes to anisotropy in the compacted waste profile that can increase lateral movement of moisture; this could also lead to a channeling effect due to preferential flow paths and leachate seeps that decrease the rate of waste degradation, so uniformity of the waste profile in bioreactor landfills is preferred (Reinhart et al., 2002). The addition of liquids to the solid waste increases the in-situ density, potentially increasing weight by up to 30%, so proper design considerations need to be made for stability and for the leachate and landfill gas collection systems (Pacey et al., n.d.).
Early research efforts show that anaerobic bioreactor landfills settle approximately two to three times faster than “dry tomb” landfills without additional compaction effort while aerobic bioreactor landfills have slightly less volume recovery, so volume “lost” due to non-compaction of the waste when placed can be recovered through anaerobic and aerobic waste degradation and moisture addition. The volume reduction and stabilization tend to occur within five to ten years of bioreactor process implementation, so “as-built” bioreactor landfills can capitalize on volume recovery during active filling whereas “retrofit” bioreactor landfills realize airspace recovery post-closure. Further research is required to optimize compaction processes in bioreactor landfills to achieve the goal of enhanced waste degradation and volume recovery without hindering moisture movement (Reinhart et al., 2002; Pacey et al., n.d.).
Most bioreactor landfills utilize anaerobic decomposition processes to promote rapid waste stabilization, and as a byproduct, produce increased quantities of landfill gas when compared with traditional MSW landfills. Since landfill gas is typically the source of most odors at a landfill and methane gas is a potent greenhouse gas, the design and operation of effective landfill gas collection systems is critical at bioreactor landfills (Reinhart et al., 2002).
Techniques and systems for gas control are typically similar to those utilized at traditional MSW landfills, but bioreactor landfills produce gas at a greater rate earlier in their life cycle, as depicted in Figure 6 from Qiyong & Jiaoju, 2011. As a result of the early spike of landfill gas production, other techniques may need to be implemented at early stages to capture the large volume of gas. Some measures recommended from Reinhart et al., 2002 include: gas collection from the leachate collection system, horizontal wells installed within the waste, and additional surface collection systems. Well-designed gas capture systems can lead to excellent opportunities for use in a landfill gas energy project and maximized greenhouse gas offsets (Pacey et al. n.d.).
Pacey et al., n.d. also notes that enhanced gas production can have negative impacts to side slope and cover stability if the gas capture system isn’t adequate. Uplift pressure from the landfill gas during the installation phase of geomembranes can lead to ballooning, instability, and soil loss, so increased venting or aggressive extraction methods should be utilized and considerations should be made for gas pressures on slope stability when the gas collection system is shut down.
Bioreactor landfills typically have faster waste decomposition than traditional MSW landfills, leading to earlier stabilization of waste and leachate, reducing the time period where post-closure monitoring and maintenance must be accomplished. Aerobic waste decomposition leads to especially fast waste stabilization and can degrade chemicals, like ammonia and organics, that anaerobic processes cannot (Reinhart et al., 2002). Estimates of leachate and waste stabilization timelines for bioreactor landfills are three to ten years post-closure (Pacey et al., n.d.).
Leachate quality is typically measured in terms of chemical oxygen demand (COD) or total organic carbon (TOC) and laboratory, pilot-scale, and field studies of bioreactor landfills have shown low concentrations of hazardous organics and rapid reduction of COD or TOC when compared with traditional MSW landfills. The removal of hazardous organics is optimized in bioreactor landfills by: 1) stripping of volatile organics due to increased gas production, 2) optimized conditions for biodegradation, and 3) promotion of the immobilization of contaminants via humification. Humification is the process by which organic matter is oxidized to reach a mature and stable state (Reinhart et al., 2002). Figure 7 from Grossule et al., 2018 provides a general kinetic comparison of bioreactor landfill COD and ammonia removal with time.
The leachate quantity and quality post-closure at bioreactor landfills is lower and better when compared with traditional MSW landfills and more amenable to on-site treatment. Additionally, the improved quality of the leachate in bioreactor landfills leads to reduced risk of contaminant migration in the event of a containment system failure (Pacey et al. n.d.).
Whether bioreactor landfills are operated anaerobically, aerobically or some hybridized combination of both, consistent monitoring is necessary to ensure operations abide by federal and state regulations, but also to manage site safety and that the systems are in ideal working conditions conducive for biostabilization. One aspect monitored in bioreactor landfills includes nitrogen management of ex-situ treatment of leachate before being recirculated in the waste matrix. Nitrate concentration in leachate is monitored both for safety and productivity concerns. Ammonia concentrations can potentially increase in leachate and pose health hazards in the event of ground water table contamination, while increased nitrates in leachate facilitates denitrification, accelerates biostabilization, and reduces nitrogen levels (byproduct of denitrification). Another major concern for monitoring is correlated with the aerobic degradation phase as it is an exothermic process and increases landfill temperatures, which pose dangerous risk of internal fires. Many other monitoring efforts are observed, especially those federally regulated to maintain public safety, notably groundwater monitoring wells to ensure the potability of the water supply.
Since bioreactor landfills are actively operated to increase rates of degradation, these engineered systems can be biostablized at a faster rate. At closure, leachate quality and on-site treatment may remain with limited external inputs as the system’s production of landfill gas (methane and carbon dioxide) declines along with associated environmental risks (Warith, 2002). A smaller time period for post-closure care also carries smaller costs relative to post-closure needs of “dry tomb” landfills. Though full scale bioreactor landfills are largely viewed as research facilities, their potential to harness energy sustainably and faster treatment time to closure, opens more opportunities for land reuse, especially in space constrained areas.
Engineered systems do not operate in a vacuum. Heavily engineered systems such as a bioreactor landfills operate under strict accordance to rules of operations outlined by RCRA. Regardless of the benefits of such a landfill, non-engineering perspectives should be taken into account before proceeding and during operations. Ongoing research on bioreactor landfill test sites indicate a large economic potential for cost-savings and energy generation, but such benefits may arguably be hampered by negative societal perception brought by traditional “dry tomb” landfills. For example, landfills tend to be located closer to lower socioeconomic class neighborhoods as communities maintain a NIMBY (not-in-my-backyard) attitude towards waste management. What assessment factors need to be considered with determining locations of waste management facilities that’s both economically feasible and equitable for the local community? Waste generation is also tied directly to socioeconomic status with booming economies in more developed countries producing more waste. How do we hold higher class individuals with more disposable income accountable for greater waste generation? Bioreactor landfill operations are largely experimental due to perceived risks and the limitations of federal and local restrictions. How can engineers best educate the public and advocate for policies that ensure a high threshold for safety, but also does not impede constant innovation? Currently, the regulations are strictly defined for containment purposes, but as the body of knowledge regarding bioreactor landfills continue to grow (especially in the bio-chemo-hydro-mechanical interactions within the waste matrix), we may be able to take advantage of the full potential of this disposal method that balances human health and environmental safety, sustainable energy output, and micro- and macroeconomics.
Bioreactor landfills offer the potential to revolutionize anthropogenic waste management techniques, but federal regulations are restrictive and potentially slow the speed of discovery and growth of knowledge from bioreactor landfills. Bioreactor landfills come in several different forms, but the philosophy behind operations remains consistent. As an alternative to traditional, “dry tomb” landfills, bioreactor landfills actively recirculate leachate through the waste matrix to accelerate biostabilization while collecting usable energy output. The risks posed by the engineered system encompasses human safety and environmental health, which is why many engineering techniques are implemented to prevent disasters such as leachate leakages contaminating the groundwater table and waste mass destabilization from settlement, leachate, and gas pressures. Largely considered experimental systems, the body of knowledge governing bioreactor landfill operations continues to grow. With time and further research, bioreactor landfills may help lessen the social stigma associated with landfills and ignite support for the use of bioreactor landfills as an energy efficient and sustainable method for waste management.
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