A review of key landfill design and operational factors under current scrutiny and several strategies for resolving the conflicts among industry professionals.

By Gregory N. Richardson and Richard T. Sprague

Landfill Gas
Wet Versus Dry Landfills
Table 1. Leakage of Alternative Liner Systems
Alternative Final Covers
Rebuilding Industry Forum
References

Fifteen years ago under amendments to the Resource Conservation and Recovery Act (RCRA), Congress mandated the first multilayered liner and cap systems (including geomembranes) for hazardous waste landfills, thereby creating the basis for "dry tomb" storage of wastes. During the intervening years, the Environmental Protection Agency (EPA) and state agencies have carried these mandates forward, with some slight alterations, into multilayered liner and cap systems for solid waste landfills. Following separate statutory and regulatory paths under the Air Quality Analysis Workshop, EPA and state agencies have developed regulations controlling emissions of regulated gaseous materials from landfills. These regulations require passive or active systems associated with the cap system to control the release of greenhouse gases from large landfills and may unilaterally modify the cap barrier layer requirements described under RCRA.

Overlying these already-conflicting requirements is a movement among various waste management professionals toward bioreactors, or "wet tombs." These bioreactors are intended to accelerate landfills’ digestion of degradable organic materials and generation of greenhouse gases. The performance standards mandated by these requirements often conflict, creating a dilemma for design, regulatory, and contractor personnel alike.

Landfill Gas

In response to commitments made by the executive branch to reduce volatile organic compound emissions by the United States in support of world global-warming issues, Congress expanded the Clean Air Act (CAA) in 1988 to regulate municipal solid waste landfills (MSWLFs). This control was expanded this year with the enactment of New Source Performance Standards (NSPS) for nonmethane organic compound (NMOC) emissions from MSWLFs.¹ Training seminars covering these new regulations were conducted by EPA in most states in partnership with state solid waste regulatory groups. A significant emphasis was placed on active gas-recovery systems and potential "wet" landfill operation to accelerate waste decomposition and thus increase landfill gas (LFG) recovery rates.

The new NSPS regulations are a very good example of standards generated with little or no input from the owner/operator or design community and carry potentially significant costs with no benefit to the facility and little or no benefit to the environment. The levels of gas emissions assumed in the NSPS are based in part on the Compilation of Air Pollutant Emission Factors, or AP-42, first published in 1972. Actual measured NMOC gas emission rates in contemporary "dry" MSWLFs have consistently been shown to be dramatically less than EPA presumptive values. This is undoubtedly in part the result of more restrictive waste acceptance criteria at these landfills, with hazardous-waste exclusion programs and restrictions of sludge acceptance being the norm. Fortunately, the NSPS regulations allow for actual onsite gas-emission measurements using a Tier Two sampling strategy. currently both the Solid Waste Association of North America (SWANA) and the National Solid Waste Management Association are attempting to gather case histories of recently measured gas-emission rates to support a more reasonable default level. In theory, this effort should include the data from all facilities that have conducted Tier Two field tests over the past two years. Unfortunately, there is no simple way to identify all such facilities.

Concurrent with the CAA emphasis on active recovery systems and capture of LFG, the industry has lost all federal tax incentives for such expensive systems, and the deregulation of the electrical utility industry has lessened the market for power produced. It is increasingly difficult or impossible to financially justify active gas-recovery systems on the basis of energy or heat generated. Here again SWANA is leading the effort to reinstate tax credits, with active support by such owners as the Delaware Solid Waste Authority.² One must wonder why SWANA exspends considerable effort to lobbying for federal legislation that is clearly essential to achieving the goals of recently enacted CAA NSPS.

Wet Versus Dry Landfills

An increasing number of lined MSWLFs recirculate leachate back to the waste to accelerate decomposition. This may be done to reduce leachate treatment costs, to gain airspace due to a natural volume reduction, or out of a concern for the future integrity of dry landfills. While these positive benefits exist, the operation of a wet landfill also creates a greater opportunity for bioclogging of the leachate collection system and may increase the potential for odor generation. It also increases the potential for sideslope leachate seeps and instabilities and the rate of waste subsidence.

On December 16, 1998, EPA issued a memorandum that clarified its Office of Solid Waste’s policy on leachate recirculation in MSWLFs. Under current 40 CFR Part 258 requirements, leachate recirculation is only allowed in MSW landfills that have a 2-ft.-thick compacted clay liner with a permeability equal to or less than 1 x 10^-7 cm/sec. Alternative composite liners using geosynthetic clay liners (GCLs) are not currently allowed where leachate will be recirculated to the waste.

The great irony here is that the EPA study clearly shows that the use of GCLs reduces the rate of leachate leakage from a composite liner system. This reduction in leachate volume with time is clearly shown in the EPA data presented in Table 1 that identifies three typical stages in the life of an MSWLF: (1) construction and preoperation, (2) operation, and (3) postclosure. A technical report presenting these data has been a "pending publication" by EPA for the past year but remains unpublished.

Table 1. Leakage of Alternative Liner Systems
(gal./ac./day)⁴

I = as initially constructed

II = during active waste placement

III = with final cover in place

One month after EPA issued this memorandum, this article’s co-author, Gregory Richardson, contacted all major manufacturers of GCLs to determine what they intended to do about this situation. Not one was aware of it! Further, at this point in time the three major GCL manufacturers have contacted EPA regarding this regulatory shortfall, but no coordinated industry response is being formulated. EPA is reviewing this problem, but to date this review has been private and no potential forum for industry input has been established. The use of GCL as an alternative to frequently expensive and questionable 1 x 10^-7 cm/sec. clays has become increasingly common⁵ but now might limit those facilities from considering wet landfill operations.

Alternative Final Covers

As the first generation of Subtitle D landfills are reaching the end of their operational life, considerable technical questions have been raised regarding both the design of and technical justification for the expensive final covers proposed for these facilities. Concerns have included the following:

• The use of an infiltration barrier system on the 4:1 to 3:1 sideslopes common to the industry has created a slope stability problem that might lead to the inevitable failure of such covers.
• Landfills that recirculate leachate will undergo significant settlements that could damage the expensive barrier covers. And why install a barrier cover to limit surface-water infiltration if leachate is being pumped beneath the cover?
• Are barrier covers required in arid and semiarid regions of the US?
• Are barrier covers required to meet the new NSPS requirements?

Having spent millions of dollars to install a liner system, most owners naturally question placement of an essentially impermeable final cover over the waste. This need is based on EPA’s concern that lined landfills should not, over the long term, become "bathtubs" that eventually fill and release leachate to the environment. Subtitle D and all states require that an interim cover be placed on waste that will not receive additional waste for more than 30 days and that the final cover be placed within one year of final waste placement. Obviously, if such tactics as leachate recirculation allow the operator to essentially degrade the waste in a decade, there is a good argument for a more flexible time frame for closure or more creative and functional interim closures. A good example of such innovation is the use of an exposed geomembrane interim cover by the Delaware Solid Waste Authority over waste that will receive additional waste in the decade to come.⁶ Should recirculation facilities be allowed to close out during the waste degradation phase with a more permeable cover designed for erosion resistance? After the degradation is completed, will it still be necessary to place an impermeable cover over the stabilized waste? Obviously the use of a wet technology requires more thought than simply figuring out the physical process of recirculation.

Alternative final covers for arid and semiarid regions of the US are being informally studied by a small work group buried deep within EPA’s Remediation Technologies Development Forum (RTDF). One of the RTDF teams is focused on phytoremediation, and somehow the Alternative Cover Assessment Program (ACAP) is now part of this group’s agenda. The second meeting of ACAP was hosted by EPA Region 9 on March 22-23, 1999. Of the 71 attendees, only five were designers and three were owners, and no geosynthetic manufacturers were present. Regulators from western states made up the bulk of the group, with a small number of academics and researchers from similar Department of Energy (DOE) research programs invited. (Note: Some of the DOE research programs have existed since 1985 and focus on 1,000-year caps having minimal slope and estimated to cost in excess of $1.4 million/ac. These costs are more than 10 times the cost of the prescriptive RCRA cap.) The ACAP group has already finalized the design and installation manual of a large lysimeter that it thinks demonstration landfills should install beneath prospective final covers. The lysimeter is 10 x 20 m in area and is estimated to cost $50,000 to install. The existence of the ACAP program will no doubt surprise most landfill owners and designers in the West.

Unfortunately, the ACAP group intends to collect and analyze data during a five-year period before issuing its guidance or report. This schedule will be too late for the first generation of landfill cells implementing closure under Subtitle D requirements. Perhaps EPA and the states should consider delaying final-closure requirements until critical closure issues are addressed by the ACAP program. Allowing owners to implement an interim closure would save many owners from needless expenditures while not relieving them from monitoring, maintenance, and corrective-action requirements.

Rebuilding Industry Forum

It is obvious that significant design and operational considerations are currently evolving that will impact all MSWLFs. It is also apparent that owners, designers, and geosynthetic manufacturers are playing a minor role in the ongoing development. During the promulgation of RCRA Subtitle D, the industry enjoyed an open forum that was in whole responsible to one person, Robert Landreth of EPA’s Risk Reduction Engineering Laboratory. Under Landreth, designers, owners, and manufacturers were given a forum to provide their input, be it test data or operational experience. This open forum died with the retirement of Landreth.

How can designers, owners, and geosynthetic manufacturers reestablish their voice in the development of regulations that govern us? The authors suggest the following:

1. SWANA’s Landfill Division should issue position papers, perhaps for SWANA ratification, recommending a stay of some closure requirements pending the results of the ACAP program and wet-versus-dry field trials.
2. Designers should voluntarily support or participate in research activities related to solid waste landfill design issues. HDR Engineering, for example, has committed a number of engineering hours to support the ACAP program on a Midwestern landfill. As another example, G.N. Richardson & Associates consultants committed funding to support its employees in graduate-level research that will support a better understanding of wet landfill operation and planning.
3. Geosynthetic manufacturers should offer support to research in solid waste landfill design, such as the ACAP program. This support should be in the form of expertise, funding, materials, or material research. Such involvement is essential for manufacturers if their products are to remain acceptable and might lead to a new generation of materials better suited to revised design criteria.
4. EPA-sponsored programs, such as ACAP, should expand their membership to include representation from the standards development and regulated communities. This expansion could include an official representative from the following organizations: ASTM D35 on Geosynthetics, ASTM D18 on Soil and Rock, SWANA Landfill Division, and APWA, National Conference of Mayors, National Conference of Counties, or some similar group. As official representatives, these members would be charged with representing the interests of their organizations rather than those of their own personal organization.

Both the designers of modern landfills and the manufacturers of the geosynthetics used in these systems must be made aware of and be brought into the technical evolution of solid waste landfill design. Very important design considerations are currently being reviewed by EPA in a manner that minimizes input from the very community that is purported to be served. Additionally, it is very clear to the authors that delays in final closure requirements should be allowed for lined landfills. Such a postponement would allow time for important technical and philosophical developments to occur regarding final covers and would not compromise the safety of the environment. With typical RCRA Subtitle D final covers costing in excess of $120,000/ac., local governments cannot afford to build in advance of these important questions being properly answered.

References

¹ EPA, Standards of Performance for New Stationary Sources, 40 CFR Part 60, updated 1999.

² Raymond L. Huitric. "Landfill Gas Management Focus." SWANA Integrated Solutions. October 8, 1999.

³ EPA, Municipal Solid Waste Landfill Criteria, 40 CFR Part 258, amended through 1996.

⁴ Othman, M.A., R. Bonaparte, B.A. Gross, and D. Warren. "Evaluation of Liquids Management Data for Double-Lined Landfills." Prepared for EPA Office of Research and Development, Cincinnati, OH. 1998.

⁵ Richardson, Gregory N. "Geosynthetic Clay Liners: Alternative Subtitle D Liner Systems." Geotechnical Fabrics Report. May 1997.

⁶ Skelly, Dawn. "A Landfill Goes Green." Geotechnical Fabrics Report. April 1999.

MSW
May / June, 2000