May-June 2000

Landfill Bioreactors: A New York State Regulatory Perspective

With increased confidence in the ability of modern landfill containment systems to prevent pollution, now is the time to look ahead to landfill operations that will limit the potential for long-term pollution.

The modern landfill, constructed and operated in accordance with New York State’s solid waste management regulations, 6 NYCRR Part 360, is proving to be an environmentally sound method of MSW disposal. Research has shown that when used in a properly operated landfill, the double composite liner system adequate protects groundwater quality by acting as an effective leachate barrier. From a review of the operation plans that are being submitted for these modern landfills, however, it is obvious that we are still operating these highly efficient containment systems the same way as the unlined landfills of the past. Now that we have more confidence in the modern landfill’s ability to prevent pollution, it appears to be an appropriate time to look ahead to modern landfill operations that will limit the risk of long-term pollution.

The idea of using the modern lined landfill as a bioreactor is an operational concept that New York State encourages in an effort to address the problems associated with the "dry-tomb" method of MSW disposal. The negative perception of the dry-tomb landfill has long been utilized by organized interest groups to oppose the siting of new or expanded landfills. This perception typically characterizes the nondecomposed MSW mass in the landfill as a time bomb ticking away toward detonation at some future date, likely years after landfill closure, when the landfill’s environmental containment system begins to break down. The bioreactor concept, if implemented within a modern lined landfill, will promote enhanced waste-mass decomposition and reduce the potential for long-term pollution. Secondary benefits include optimized landfill operations via conservation of valuable airspace and thus the preservation of land resources for uses other than landfilling. Additionally, use of the bioreactor increases or enhances the potential for landfill-derived energy (biogas) production through improved landfill gas (LFG) production. This benefit is more evident since modern landfills tend to be large regional disposal facilities with significant waste masses able to sustain long-term gas production. The magnitude of waste in these modern facilities makes gas collection and recovery systems more economically viable. Collectively, these benefits should encourage today’s landfill operators to modernize operations in order to minimize the potential for long-term pollution.

Early in the spring of 1992, the New York State Department of Environmental Conservation’s Division of Solid & Hazardous Materials (then the Division of Solid Waste) initiated a rule-making revision to Part 360 to comply with Federal Municipal Solid Waste Landfill Regulations, 40 CFR Part 258, under Subtitle D of the Resource Conservation and Recovery Act. As part of this rule-making process, the state’s solid waste management regulations (Part 360) were revised in such a way to avoid disrupting the modernization of future landfill operations. The October 9, 1993, version of Part 360 was revised to include a passive regulatory requirement for new lined landfill operations to suggest operating the landfill as a MSW bioreactor, as well as incorporating thresholds for primary liner-system leakage. One of the goals of this rule-making effort was to develop a regulatory framework that is not only flexible but also encourages environmentally sound and resource-conscious landfill management.

Regulatory/Policy Overview

During the rule-making initiative, the Division of Solid & Hazardous Materials determined that the best way to promote what then was termed "active landfill management techniques" (including utilization of the landfill as a bioreactor) was to require that the landfill’s operation and maintenance manual address these modernized practices. Specifically, subdivision 360-2.9(a) of the state’s solid waste management regulations states:

It is the intent of the division to passively promote active landfill management practices, allowing for modernized operational methods to be voluntarily pursued without placing a regulatory hurdle for facility owners to overcome. The division realizes that a double composite liner system, as required by the state’s solid waste management regulations, is necessary for the protection of groundwater resources. Figure 1 depicts the cross-section of the double composite liner system required for New York State landfills. There are currently 38 active double-lined landfills operating throughout the state, some of which have been operational since 1987. Based on environmental monitoring data and facility reporting by these facilities, the division has not seen, nor is aware of, any groundwater-related impacts attributable to these double-lined landfill operations.

Figure 2. Representive Configuration of a Landfill Bioreactor (Click to View)

Landfill bioreactor operations typically require recirculation of leachate through the waste mass. The recirculated leachate in turn acts as a catalyst to promote active waste decomposition. In the past, there was a strong regulatory prohibition of any concept that would create additional hydraulic loading within an unlined waste mass. This position was taken because the release of leachate from an unlined landfill is a direct function of the magnitude of hydraulic loading to that landfill.

The advent of the double-lined landfill, with its ability to assess primary (upper) liner performance and to establish that a particular landfill has an acceptable level of leachate accountability, allowed the division to add the option of leachate recirculation in the operational requirements of section 360-2.17. More specifically, the following provisions of subdivision 360-2.17(j) must be complied with in order for the division to consider leachate recirculation:

The latter reference to a landfill’s operation and maintenance manual is to ensure that the division’s approval of leachate recirculation is contingent upon assurance that the landfill’s primary and secondary leachate collection and removal systems (LCRS) are designed with adequate accessibility for routine maintenance. This assurance is required since recirculation of leachate has the potential to actively promote increased biological clogging in the LCRS. Another issue is compliance with the regulatory performance standard that allows no more than 1 ft. of leachate head on the landfill’s primary (upper) liner. It is not the division’s intent to allow a landfill that will utilize leachate recirculation to effectively flood its waste mass with leachate. To do so would inevitably result in a violation of the landfill’s primary-liner leakage-rate threshold, heighten the potential for groundwater impacts, and possibly increase waste-mass instability.

Today, many regulators still have the misconception that leachate recirculation will allow or require a landfill’s waste mass to become fully saturated. As stated above, this is not a goal of an effective landfill bioreactor operation. Literature states that optimum waste-mass moisture content in the range of 30-40% is ideal for efficient waste-mass decomposition. Typical waste-mass moisture contents are well below this amount. In fact, active waste-mass decomposition, even in a damp Northeast climate, will tend to utilize the waste-mass moisture to a point where leachate production is reduced even though recirculation has taken place. One should not think, however, that landfills that recirculate leachate and have active waste-mass decomposition would no longer have to deal with leachate generation and treatment from these facilities. Ultimately, leachate will still be generated and require treatment. During 1998, New York State’s lined landfills collectively generated and collected more then 450 million gal. of leachate.

Another beneficial byproduct of leachate recirculation is enhanced generation of methane gas. If the LFG is treated or converted to other useful forms of energy, the landfill bioreactor will help to conserve natural-energy resources (see sidebar for status of New York State’s LFG recovery operations). This environmentally sound method of capturing and effectively utilizing landfill-derived methane gas also controls the emission of other offensive waste-decomposition gases. Horizontal gas collection systems, which can also recirculate leachate into the landfill, are being installed in some of the state’s landfills to help control landfill odors and emissions.

As stated above, the regulatory concerns for leachate recirculation tend to focus on the landfill’s ability to provide a high degree of leachate accountability; however, there are other concerns. A landfill bioreactor proposal should also be supported by an engineer’s evaluation for the structural integrity of the existing or proposed landfill liner and leachate collection system. The basis for this is that bioreactor operations have the potential to significantly increase waste-mass density over time. The recirculation of leachate in and of itself can increase the existing waste-mass density as much as 30% and thus needs to be evaluated to ensure the structural integrity of the landfill’s containment and leachate-collection system.

If the above demonstrations for leachate accountability and structural integrity are met, then a landfill operator may request approval to begin a landfill bioreactor operation. In doing this, he will need to modify the facility’s operation and maintenance manual. Typically, this plan will need to describe comprehensively how leachate will be recirculated and how operational compliance will be maintained. Part of this plan should include explicit instruction on the leachate recirculation rate. This plan should include a discussion regarding monitoring of the primary liner’s performance and the hydraulic loading of the landfill’s LCRS. It should address the proposed methods of leachate introduction into the landfill, as well as means for enhanced LFG-collection methods for effective odor and emissions control. Further, the plan should address the potential for internal ponding of recirculated leachate on layers of impervious daily or intermediate cover within the landfill. The plan should also cover how the bioreactor operation will be monitored analytically and physically with respect to waste-mass stabilization. In addition, the plan should have a separate contingency plan that would address—but not be limited to—such items as the potential for leachate surface seeps, odor issues, remedial cleaning/flushing of the LCRS to ensure a continuously free-flowing condition, and leachate leakage through the primary liner beyond the approved rate.

The Next Steps to Be Taken

Although the operation of a solid waste landfill as a bioreactor has significant merit, including economic benefits, reduced risk of long-term pollution, and enhanced methane production for energy generation, the concept also has several barriers preventing its widespread use. Among these barriers are the following:

Regulatory. Although New York State’s Part 360 Regulations and the federal Part 258 Regulations allow the use of leachate recirculation, the dominant regulatory approach is to operate landfills as dry tombs.

No Perceived Problem With Modern Dry-Tomb Landfills. Although those familiar with landfill operations may perceive the bioreactor concept as superior to the dry-tomb landfill operations, those outside of this group do not recognize the need for a change in policy.

Public Perception. The public might be more opposed to the bioreactor concept because the enhanced methane production and accelerated MSW decomposition may increase the perception that odor and air emissions problems will be more prevalent.

Less Reliance on Landfilling. With current solid waste management policy placing a high priority on the management of MSW within an integrated system, the reliance on the landfilling of raw MSW has been diminished. The trend in the management of solid wastes will move further toward the land disposal of only the nonrecoverable residual portion of the solid wastestream.

Capital Development Costs. Although increased capital costs would potentially be offset by decreased operational costs, a leachate recirculation system can still increase the capital start-up costs.

Operational Difficulties. Leachate recirculation operations are perceived as more complex, having increased monitoring requirements and higher potential for problems.

The barriers outlined above may prevent a fundamental change in the current practice of the dry-tomb operational concept to one more parallel to a bioreactor. Even if a fundamental change in the current practices is not advanced, there are mechanisms that can further promote the bioreactor concept within the realm of current landfill operations. Since advocates of the bioreactor concept claim the process will stabilize refuse in 10-15 years, the reduction of the 30-year postclosure care and monitoring period is perhaps the greatest incentive. The provisions of 40 CFR Part 258.61(b)(1) allow the director of an approved state to reduce the 30-year postclosure care period, provided the reduced period is sufficient to protect human health and the environment.

To enable states to consider a reduced postclosure care period, the landfill owner must justify that there is no potential for pollution. In most instances, a state will not commit to a reduced postclosure care period directly after closure. A more realistic approach would be to set forth pollution indicators whose installation would trigger a reduction or relaxation of the postclosure care requirements. Gathering data from the indicators might require the landfill owner to conduct monitoring beyond that which is typically required. This approach to achieving a reduction in the postclosure care requirements would be initiated on a case-by-case basis and would be heavily driven by performance-monitoring data.

The research and experimentation associated with the bioreactor concept must continue, but at the same time it is critical that current data be consolidated into a single reference source to guide further development of this concept. The consolidation of existing research will enable the bioreactor concept to move closer to the point where accepted standards of practice are available to ensure successful operation and monitoring of bioreactor landfills. The US Environmental Protection Agency (EPA), during the rule-making for 40 CFR Part 258, identified potential leachate-recirculation operational problems. These problems included increased leachate production, clogging of the leachate collection system, buildup of hydraulic head within the unit, an increase in air emissions and odor problems, and an increase in potential of leachate pollution from drift and/or runoff. Further, the Council of Great Lakes Governors Regional Biomass Program has undertaken a study that concludes that a number of current landfill regulations are potentially in conflict with bioreactor proposals. In a general sense, such regulations seek to control nuisance problems (odors, wind-blown debris, dust, and scavenging); health-hazard exposure (disease vectors, gases, and aerosols); leachate production; groundwater contamination; interference with equipment and access; onsite congestion; and impacts on long-term site integrity. In order to allay opposition to the bioreactor concept, it is imperative that acceptable operational practices be developed to work within the current regulatory framework for landfills and mitigate the concerns outlined by EPA. In addition, the consolidation of research would help to better define the best indicators for assessing the various performance aspects of the bioreactor. The development of performance indicators and standards of practice is critical to ensuring consistent performance and initiating any potential reduction in the postclosure care period.

The current research has set the stage for further analysis of the successes and failures associated with the existing studies. A consolidated document that analyzes existing research and concludes the most effective means to operate and monitor a bioreactor landfill may increase the chances that more bioreactor landfills will move forward. A document based on current research would afford landfill owners, regulators, and the public a more positive perception of bioreactor benefits and operation. Such a document also provides a reference source that will assist in justifying the potential benefits of landfill bioreactors.

Conclusion

In summary, New York is gaining experience with the operation of bioreactor landfill operations across the state. These experiences can be described as promising. The issues that we have experienced with bioreactor operations tend to center around odor- and gas-related problems at facilities that did not have adequate odor control and gas collection systems in place. New bioreactor concepts, such as those using an aerobic decomposition process to jump-start waste stabilization, are being pursued at landfills in the state. The use of properly designed gas-collection and recovery systems appears to be a standard necessary component if odors are to be controlled.

The Northeast continues to experience high solid waste disposal fees and shortfalls from time to time for available disposal capacity. Realizing this, landfill designers and operators are advancing optimized operation plans to conserve remaining airspace. The concept of leachate recirculation in the spirit of optimized landfill airspace and long-term biostabilization of the waste mass has merit. In addition, this article also notes that there are secondary benefits associated with landfill bioreactor operations. The regulatory perspective on this modern landfill concept in New York State, however, is one of moderation and caution. The state’s perspective hinges on the double-lined landfill’s ability to demonstrate acceptable leachate accountability and acceptable overall operational performance. This caution is founded on the fact that the operational performance of new landfills is being closely scrutinized by many. A single failure, even on an isolated account, could further impair the ability to gain public trust and support for these vital disposal facilities. These concerns should not present a barrier to the implementation of such new operational concepts, but rather should reinforce the need for comprehensive and well-thought-out operations and contingency plans associated with the modern landfill’s overall performance