The bioreactor landfill technology offers many potential benefits to the landfill industry, but there are many unanswered questions at this time, which means proceeding with cautious optimism. Going forward with full-scale bioreactor projects will better define and make use of the bioreactor technology.

By Pat Sullivan

Site-Specific Feasibility
Conceptual Design Issues
Conclusions

A bioreactor landfill is an MSW landfill that utilizes enhanced microbial processes under anaerobic (and possibly even aerobic) conditions to accelerate the degradation of refuse. This enhanced degradation can serve to more rapidly stabilize the refuse mass while producing landfill gas (LFG) and leachate more quickly and at higher rates. This is accomplished through the control of moisture, temperature, pH, nutrients, and/or other properties within the refuse mass.

A bioreactor landfill is specifically designed and operated to ensure that the enhanced microbial processes can occur in an unrestricted but controlled fashion. The LFG and leachate control systems for a bioreactor landfill are expanded to account for the increased gas and leachate production. When properly managed, a bioreactor landfill is anticipated to allow microbial degradation to occur to the maximum extent possible during the active life of the landfill.

With the increased technical knowledge that has been gained over the last decade since Resource Conservation and Recovery Act 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 (e.g., 30-year) postclosure obligations place an undue financial burden on landfills, forcing landfill owners and operators to increase disposal costs to fund the postclosure activities. For LFG, these postclosure maintenance periods could be more like 45-60 years when dealing with the dry entombed landfill regulated under Subtitle D. The concept of a bioreactor landfill has been developed to address these problems.

Bioreactor landfills are being considered in the solid waste industry as an alternative to this type of landfill. The typical MSW or "sanitary" landfill is designed and operated in accordance with Subtitle D or state-equivalent regulations. Subtitle D was promulgated on the premise that refuse in a landfill be kept dry to prevent the formation and migration of leachate and LFG, which could impact groundwater underlying the landfill, affect air quality, or create subsurface combustible gas problems.

The refuse within a bioreactor landfill must be kept extremely moist in order to achieve the desired result; this will cause increased leachate generation. This occurrence would be contrary to the intent of Subtitle D; however, since current leachate control systems as required by Subtitle D have been effective in minimizing leachate migration, a bioreactor landfill is not expected to result in any increased environmental impacts as a result of leachate migration. A bioreactor landfill is merely a variation on the accepted practice of leachate recirculation, as allowed under Subtitle D.

Subtitle D has generally been successful in minimizing the formation of leachate and controlling the migration of leachate out of the refuse mass. The regulation, however, 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 enhanced microbial activity in a bioreactor landfill, LFG generation and recovery rates are expected to increase substantially over the short term as a result of the accelerated and more complete degradation that will occur. This increased rate of LFG generation must be managed properly so that effects to air quality do not occur; the current LFG control technology should be sufficient to manage these potential impacts.

In terms of LFG, this creates a twofold benefit. LFG generation (and subsequent recovery) is anticipated to be limited to a 10- to 15-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) for an LFG-to-energy facility. In terms of leachate, the increased generation is fully manageable under the control systems that would be built into the bioreactor landfill and required by Subtitle D. Furthermore, the bioreactor landfill is anticipated to stabilize leachate quality in a more rapid fashion so that the costs to manage leachate throughout the postclosure period will also be minimized.

There is an additional, and maybe more important, benefit to a bioreactor landfill in that the refuse mass is more quickly stabilized, resulting in increased airspace capacity within the same landfill footprint and a potential for more rapid redevelopment of the landfill after closure because of better settlement scenarios. This would allow more waste to be placed into the same airspace, which should help alleviate any landfill-capacity shortage and allow additional revenue to be generated from the same landfill footprint. For the current dry-type landfills, the recovery of airspace occurs too slowly to be of any use to the landfill operator during the active life of the landfill. Also, landfill settlement will continue to occur at a dry-type landfill well into the postclosure period.

In summary, designing landfills for enhanced waste decomposition has a number of clear potential advantages for landfill operators, including:

• more predictable long-term landfill performance;
• rapid stabilization of the waste to a stage where the ongoing leachate and LFG generation is environmentally acceptable without continued collection and treatment activities;
• controllable gas yields during operation with a more economical production profile and reduced waste of gases, which could help reduce the greenhouse gas burden;
• leachate with a reduced treatment requirement during operation;
• longer landfill lifetimes resulting from a better use of landfill airspace and resulting in a reduced need to site new landfills and more revenue from existing landfills;
• reduced postclosure costs;
• a reduction in the financial uncertainty associated with potential long-term liabilities;
• increased land value after closure as a result of better waste stabilization and more options for land use, including construction on top of refuse;
• improved public perception in that any concerns over the long-term risks associated with landfills are reduced and useful products are being produced;
• the potential for mining of the degraded waste to recover humic material and other recyclables and allow inspection of any lining system.

Site-Specific Feasibility

The decision to implement a bioreactor landfill is clearly a site-specific one. There are many factors that can contribute to the potential viability and ultimate success of a bioreactor landfill, as discussed below.

Market Drivers

The driving force behind the use of reactive landfill technologies arises from market concerns and community expectations that conventional landfills are no longer practical and profitable as a means of disposal for MSW. This perception has placed landfill owners and operators in the position of encountering increasing resistance to the siting of new landfills and the expansion of existing landfills, as well as facing demands for higher operational standards at existing facilities.

The value of airspace in existing facilities, therefore, becomes a very important concept, creating opportunities for improved operation to lengthen landfill lives and/or reuse the airspace made available through enhanced degradation of the waste. Rapid stabilization of the waste and recycling of the airspace in a bioreactor landfill will also serve to reduce the long-term liability inherent in conventional landfill practices and substantially reduce postclosure costs. The combination of these two elements makes the bioreactor technology very attractive for the future of the landfill industry.

Comparison to Benchmarks

The performance of an enhanced landfill should be evaluated against certain benchmarks that will allow comparison in performance and cost-effectiveness to existing landfill technologies. These benchmarks should be used in any proposed feasibility study as guides to determining as to the feasibility and financial viability of a bioreactor project. Table 1 outlines a series of possible benchmarks for evaluating various aspects of an enhanced reactive landfill.

Table 1. Bioreactor Landfill Benchmarks

Need for a Feasibility Study

A feasibility study might be necessary to evaluate whether a candidate site could benefit from the enhanced bioreactor technology. The decision to implement a bioreactor landfill is clearly a site-specific endeavor, in which all pertinent site issues must be evaluated. Furthermore, the full-scale use of the bioreactor landfill technology has not been approved in many jurisdictions.

Bioreactor landfills currently in operation in the United States are either not full-scale operations or are merely landfills that recirculate leachate. As such, there are very little data on the long-term operations of true bioreactor landfills. This is truly cutting-edge technology that must be evaluated for feasibility on a case-by-case and site-specific basis. Hence, a thorough and comprehensive feasibility study may be a necessity.

Conceptual Design Issues

Many issues must be evaluated in the conceptual design phase of a proposed bioreactor project in order to make an informed decision as to the site-specific feasibility of the bioreactor technology. These issues include, but are not limited to, increased airspace for refuse disposal; reduced postclosure costs and long-term liabilities; regulatory approval for the design, construction, and operation of a bioreactor landfill; LFG control; leachate management; and life-cycle project economics.

Rapid Waste Decomposition and Stabilization

The more rapid decomposition and stabilization of waste expected to occur during the life of a bioreactor landfill would provide increased airspace for refuse disposal, a reduced postclosure period, and increased potential for postclosure development. While these outcomes would be positive for the project, the rapid settlement of the refuse mass could potentially cause problems with the LFG-recovery, liquids-recirculation, and landfill-monitoring systems if not properly addressed in the design process.

Landfill Gas Control

During the life of a bioreactor landfill, the overall quantity of LFG generated per unit of waste (LFG yield) is expected to be the same or slightly higher than that of a Subtitle D landfill. In a bioreactor landfill, however, this quantity of LFG is expected to be generated at an accelerated rate when compared to a Subtitle D cell. The peak LFG generation (and recovery rate) will be higher for a bioreactor landfill.

This means that an LFG recovery system must be sized appropriately and may include a combination of horizontal collectors and vertical wells designed to control the accelerated generation and peak flows. In addition, it might be more cost-effective to combine leachate recirculation systems with the LFG recovery system. It might also prove effective to use the LFG collection system to distribute air prior to the landfill reaching the anaerobic phase of its life cycle (i.e., aerobic degradation). The increased potential for energy recovery as a result of the accelerated LFG generation rate must also be evaluated.

Leachate Recirculation/Liquids Addition

A key feature of any bioreactor landfill is the recirculation and/or addition of liquids into the waste mass. For most landfills, it will be necessary to add liquids to the leachate redistribution system in order to sustain a flow high enough to maintain moisture levels in the waste and peak levels of waste decomposition and stabilization. The source(s) of these additional liquids must be evaluated, including the potential for the addition of sewage sludges and other liquid wastes, which could be added to the liquid stream during the life of the landfill to enhance waste degradation. The latter source carries additional benefits of adding needed microbes to the waste and potentially generating of additional tipping fees.

Any assessment of a bioreactor landfill must look at the effects of rapid waste settlement on the liquids distribution system. Regulatory concerns regarding the acceptance of wastes with high liquid content (e.g., sewage sludges) and the potential for biofouling of the liquids distribution system caused by the introduction of sewage sludge into the liquid stream must also be evaluated.

Containment Systems

It might be necessary to develop a landfill containment system for the bioreactor landfill that is more conservative than a normal Subtitle D liner and leachate control systems. The reasons for this are that (1) the bioreactor landfill concept has only recently been considered as an alternative to traditional landfilling techniques, and (2) the regulatory community has limited experience with enhanced landfills. Although engineering studies have shown that the Subtitle D single-liner systems, with leachate control, should be sufficient to handle even the heavy liquid loads from a bioreactor landfill, many regulatory jurisdictions are not comfortable with the concept.

Initial Aerobic Waste Degradation

Recently, the waste industry has been investigating the possibility of injecting air into the waste mass during the initial stages of placement to aerobically enhance waste degradation. Wastes could decompose significantly faster under aerobic conditions than under anaerobic conditions. In addition, the ultimate potential production of LFG could be reduced significantly in theory. More study is needed, however, to assess the potential for and the possible control of fugitive emissions during the aerobic decomposition period. One possible remedy includes a more thorough load-checking program for household hazardous wastes and substances that can produce nonmethane organic compounds as fugitive emissions. Another might include extensive presorting of waste prior to placement.

Regulatory and Permitting Issues

As part of any feasibility study, the regulatory framework under which a project such as this would be proposed and approved must be evaluated. The design, construction, and operations of an enhanced bioreactor landfill present several unique issues in terms of compliance with existing environmental regulations for landfills. A feasibility study must identify and assess how proposed design, construction, and operational aspects of a bioreactor landfill would have to be changed or enhanced to comply with environmental regulations and permits or where the regulations and permits would have to be modified to fit the bioreactor technology.

Landfill Design, Construction, and Operations

It is clear that bioreactor landfills must be designed, constructed, and operated in a different fashion than conventional landfills. The manner in which landfill construction and operation would be changed would be based on the results of a feasibility study as previously outlined. The design of a bioreactor landfill must be flexible to allow future modifications as site-specific operational issues are discovered. It is likely that the design, construction, and operations of a bioreactor landfill will evolve significantly as the technology becomes more well defined.

Financial Viability Analysis

A feasibility study can show that a bioreactor landfill is technically possible; however, such a landfill must also be cost-effective to construct and operate. Before a bioreactor project is implemented, it might be prudent to complete a cost-benefit analysis in order to evaluate whether a bioreactor project is appropriate at a given site or within a given regulatory jurisdiction. Even if the technology is shown to be effective, it might not work well at every site.

Conclusions

The bioreactor landfill technology offers many potential benefits to the landfill industry and could even result in wholesale changes to the way the industry conducts its business. There are many unanswered questions at this time, however, that must temper the enthusiasm and cause the industry to proceed with cautious optimism. Many of these questions will not be answered until full-scale implementation is put into action. The landfill industry would be best served by going forward with full-scale bioreactor projects even though the technology is not fully developed. Only then will enough information be gained in order to better define and make use of the bioreactor technology.

Part 2 of this series on bioreactor landfills will evaluate each of the above issues in greater detail and provide a summary of information on bioreactor landfills that are currently in the operational phase.

MSW
July/August 2000