Now that the lid is off the proscriptive MSW landfill, the race is on to make improvements. Prudence suggests we make haste slowly.

By Richard T. Sprague and Gregory N. Richardson

Pile It Higher and Deeper?
The Bioreactive Landfill
Sustainable Development and Final Closure
An Alternative
Issues for Resolution

Roughly a decade has passed since the United States Environmental Protection Agency (USEPA) finalized regulations controlling the disposal of MSW under Subtitle D of the Resource Conservation and Recovery Act. As an industry, we spent the next five or six years permitting, designing, and constructing new landfill facilities that complied with the requirements of the Subtitle D regulations. Subsequently, we have spent the last four or five years wrestling with some of the implications of the "dry tomb" landfill envisioned in the regulations. The entire 10-year period has been quite rushed, and it might be time to reflect on whether the overall direction of our hubris makes sense in the longer term.

Pile It Higher and Deeper?

Many aspects of the modern Subtitle D MSW landfill derive from the Subtitle C hazardous waste (HW) landfill, for which serious regulatory upgrades began in the early to mid-1980s. Because the double-liner systems were so expensive, HW landfills soon realized they were selling "airspace" in a marketplace with decreasing competition (i.e., fewer permitted HW landfills). An early manifestation of this changed philosophy was the development of geonet drainage materials: airspace was too valuable to "waste" using 30 cm of sand when 0.3 cm of geonet/geotextile composite could be substituted as a leachate collection system. This same philosophy resulted in repeatedly amended permits for HW landfills, allowing ever-increasing elevations for the top deck of the closed HW landfill.

Whether intended or not, one of the aspects carried over from the HW landfill into the MSW landfill industry following promulgation of Subtitle D regulations was the "selling airspace" mentality. MSW managers became convinced that they, similar to their brethren in HW, needed to preserve the taxpayer or stockholder investment in expensive liner systems and closure systems by maximizing use of the airspace in new landfill facilities. This has led to MSW managers seeking to maximize in-place density of the MSW and minimize the volume of daily cover (hence, alternative daily cover [ADC] materials in place of soil). It is now leading to efforts–some extraordinary–to increase the final elevation of the top deck of MSW landfills.

The authors question whether piling it higher and deeper is precisely where we, as an MSW industry, should be heading. Several issues arise, the most prominent of which involve bioreactive landfills and postclosure land use. Each of these issues is discussed in more detail below, and an alternative approach is advanced that responds to these issues.

The Bioreactive Landfill

Over the past several years, considerable space in both popular and technical publications has focused on the bioreactive landfill, or bioreactor (Reinhart and Townsend, 2000). This literature suggests that the dry-tomb landfills mandated by Subtitle D will require several hundred years of postclosure care before stabilization occurs, if at all, and that bioreactors can shorten the period of stabilization to as little as 12-30 years. While the authors concur with this assertion, we believe data exist that indicate that piling it higher and deeper and current waste acceptance/screening practices might not be totally compatible with operation of a bioreactor.

Piling it higher and deeper might at some point lead to permeability problems that prevent the circulation of the very water required for the degradation process. The variation of density with depth can have a significant influence on the density, and therefore permeability, of the waste. The blue line in Figure 1 shows this density/depth relationship developed for one southern California landfill (Puente Hills) on the basis of field measurements of density and laboratory measurements of waste compressibility (Earth Technology, 1988). Based on this density/depth profile and representative compressibility values for MSW reported by Fassett et al. (1994), Kavazanjian et al. (1995) developed a Puente Hills MSW unit-weight profile shown by the green line in Figure 1. This is commonly used in stability analyses of MSW landfills in the absence of landfill-specific data.

Since the unit weight predictions in Figure 1 are based on landfills in southern California, it is expected that the moisture contents for these landfills is significantly less than those in this study. Discussions with Teresa Dodge with Puente Hills indicate that the average moisture content of their waste is estimated to be approximately 22%. It is recommended that designers adjust the density obtained from Figure 1 to agree with the typical moisture content of the waste in question. This adjustment in unit weight is performed using the following relationship:

where w is the moisture content at the landfill in question, g _T is the unit weight of the waste at a water content of w, and g _Puente is the unit weight of waste obtained from Figure 1. Moisture contents in Eastern landfills are commonly in the range of 30-35%, and in a bioreactor landfill the moisture content can exceed 60%. As waste depths approach 150 ft., one of the authors has measured MSW dry-unit weights as high as 2,500 pcy in landfills using soil as daily cover and 2,200 pcy in landfills using an ADC.

The permeability of the waste is also reduced by biofouling and filtering of the leachate as it is recirculated through the waste. Since researchers have shown that leachate can clog even the most permeable geotextile and sand (Koerner and Koerner, 1990), it is intuitive that highly compressed MSW will suffer the same fate. Additionally, the increased disposal of low-permeability biosolids in MSW landfills can only exacerbate this trend. Therefore, in landfills that pile it higher and deeper, owners and operators might have to introduce expensive vertical drainage systems within the refuse to ensure circulation of leachate without development of a perched zone of saturation. Ironically, such saturation halts the very degradation that motivates the recirculation effort to begin with.

The permeability effects discussed above would be totally independent of and additive with permeability effects attributable to daily cover, including ADC.

Sustainable Development and Final Closure

Some interest groups would maintain that the phrase sustainable landfill development is an oxymoron. The National Recycling Coalition (NRC) has, for example, developed a policy statement and position paper that defines landfills unacceptable developments in today’s society, at least in the US (Case, 2000). To eliminate this conundrum, NRC would have our society ban landfills and recycle everything, (perhaps) facetiously ignoring the readiness of our citizens to comply and the potential public health implications (e.g., a foodwaste slop bucket in every inner-city kitchen).

As solid waste management professionals, we have brought some of this position upon ourselves by adopting approaches from a very different HW-stream mentality, as well as by failing to advance our own strategy. To examine the situation in which we find ourselves, let’s look at where we are in terms of current postclosure-use planning, evaluate several definitions of sustainability, and plot a direction that extracts us from our current dilemma.

First, let’s look at where we are today. Subtitle D requires that the owner define the closure cap for a landfill cell during the permitting process for the cell. The cap must be compatible with the liner and leachate collection systems. However, these regulations do not require that the owner define the postclosure use of the landfill cell, which defines the nature of the cap from a redevelopment standpoint. All too often the postclosure-use decision is left until just before or just after the cell has closed. Since the landfill was constructed with "selling airspace" as the dominant driving force, the steepest sideslopes and shallowest top deck that the regulatory officials will permit largely define the configuration of the landfill cell. This configuration severely limits the future land-use scenarios available for a closed landfill cell, as does the failure to define the cap to meet the redevelopment land use.

On top of these constraints, essentially every closed landfill is surrounded by housing and/or commercial development. This surrounding land use occurs in spite of our best efforts to site new landfills far from existing development; no matter the care we place in siting the new facility, development has reached it prior to closure. Since this appears to be a predictable trend, it seems apparent that we should involve urban planners early in the siting and permitting process so that we can define a postclosure land use that will be compatible with the long-term built environment surrounding the landfill.

Second, let’s look at some definitions of sustainability, together with some solutions suggested by urban planners. Several definitions have been proposed. Recently, the National Council for Science and the Environment (National Academy of Science) proposed integration of economic security, ecological integrity, and social equity (National Council for Science and the Environment, 2001). Urban planners approach sustainability somewhat differently, while capturing some of the same attributes. To a visionary urban planner, the world we inhabit comprises four systemic elements: the natural environment, the human environment, the built environment, and the financial environment. According to this theory of urban planning, any proposed development must consider, incorporate, and optimize each of these elements into a systemic whole in order to be successful over any reasonably long period of time. In using this approach, the proposed development must substantively enhance all four of these elements so as to be seen as sustainable.

Urban planners would advocate that, during the planning process, the overall form, size, and composition of the landfill should be determined only through consideration of long-term uses. The short-term needs to dispose of materials currently seen as undesirable should not undermine the long-term goals of creating communities that integrate nature and society into a sustainable matrix. The welfare of future generations should never be compromised by our mistakes or shortcomings; every effort should be made to ensure that future generations are not unduly burdened by our current activities. In the case of landfills sited in locations that we know, with certainty, will eventually be subsumed by urban development, we bear responsibility not simply for lowering the initial costs of disposal but for determining the precedent use that all other uses will have to acknowledge for centuries to come.

Ultimately, MSW managers must consider the long-term equity of the landfill site to the taxpayers or stockholders and balance this equity with the shorter-term airspace value of the landfill.

An Alternative

With the permitting of a new site becoming increasingly impossible, the authors have begun to consider an alternative MSW landfill development model that challenges the concept that an expensive liner system should only be used once and that piling it higher and deeper is better. The concept is not new but models the landfill as an anaerobic digester vessel not unlike those constructed by municipal and industrial waste treatment facilities. Under this model, the most cost-effective operation is that which allows the fastest passive degradation of the waste and least expensive "recharge" of the vessel.

Increasing the speed of degradation implies that the moisture content of the waste will exceed 60%. For most landfills, this will require a significant amount of liquid addition to the waste. Typical nonarid regions have waste at 30% moisture. To raise this water content to 60% would require the addition of 61.4 gal. of water per ton of waste; for example, 2,000 tpd of waste would require 122,800 gal. of supplemental water per day. In semiarid regions, with the moisture content of waste as low as 15%, this number increases to as much as 105 gal. per ton of waste. Where would this water come from? Rainfall can provide this supplemental water only over a limited ratio of waste surface area to volume. As the waste is piled higher, the volume of the waste increases to the third power while the surface area increases only to the second power. Thus, the piling-it-higher-and-deeper landfills will require significant supplemental water other than precipitation.

From this perspective of water balance and problems with water circulation, the authors estimate that the optimum bioreactor cell will be of moderate size with heights less than 150 ft. and an area sized to hold five to eight years of waste. The alternative model is based on the anticipation that such cells could be constructed, filled, allowed to stabilize, and then mined to reclaim the airspace at a cost less than that of the one-time use mandated by piling it higher and deeper. Under this model, the site would contain a minimum of five cells, with one active, three aging, and one "inert" final cell. The aging cells would be mined for their landfill gas but would stabilize over a 15- to 30-year time period. Mining would separate the stabilized or composted materials from the inert using simple power-screen separators. The inert material retained by the screens would be disposed of in the long-term inert cell.

Beyond saving the cost of new cells, this model would reduce the need for expensive final covers placed over increasingly challenging slopes and reduce uncertain long-term monitoring and maintenance costs. The time frames for such a model are compatible with human life spans and the attention span of government. Models that have 200-plus-year events are not compatible with human life spans and the attention span of government.

Issues for Resolution

What challenges face the reusable-landfill operation? First, it is obvious that the reclamation operation must not produce significant objectionable odors, since society is ever encroaching on our buffers or health hazards to the field staff. This might lead to an increase in buffer area, but it might also fit into a well—master planned site development. Next, the reclamation operation must be able to mine and process the stabilized waste at a price less than the cost of a new liner. This might mean that we, as solid waste professionals, must actively support developing markets for the digested and stabilized waste, which will resemble compost. Finally, regulations would have to be modified to accommodate several changed conditions: the interim nature of the "final" closure cap on the digesting cells, the nature of the bottom liner for the inert-waste final cell, and the nature of the closure cap for the inert-waste final cell. These changes will require that the mentality of the current regulatory community be significantly challenged. Obviously we need a politically correct name for such a change–such an in-cell recycling–to broaden support for the concept.

References

Case, Clifford P. III. "Re: Docket No. F-2000-ALPA-FFFFF." www.nrc-recycle.org/P_Wforum/actions/landfill_letter.htm. Carter, Ledyard & Milburn for National Recycling Coalition. 2000.

Earth Technology. "In-Place Stability of Landfill Slopes, Puente Hills Landfill, Los Angeles, California." Report No. 88-614-1. The Earth Technology Corporation, Long Beach, CA. 1988.

Fassett, J.B., G.A. Leonards, and P.C. Repetto. "Geotechnical Properties of Municipal Solid Wastes and Their Use in Landfill Design." Proceedings, WasteTech 94 — Landfill Technology Conference. National Solid Waste Management Association, Charleston, SC. 1994.

Kavazanjian Jr., E., N. Matasovic, R. Bonaparte, and G.R. Schmertmann. "Evaluation of MSW Properties for Seismic Analysis." Proceedings of the Geoenvironment 2000 Specialty Conference. Vol. 2, pp. 1126-1141. ASCE, New Orleans, LA. February 1995.

Koerner, R.M. and G.R. Koerner. "Landfill Leachate Clogging of Geotextile (and Soil) Filters." EPA/600/2-91/025. Municipal Solid Waste and Residuals Management Branch, Environmental Protection Agency, Cincinnati, OH. 1990.

National Council for Science and the Environment. "Recommendations for Improving the Scientific Basis for Environmental Decisionmaking." www.cnie.org/2000conference/01.cfm. National Academy of Sciences. 2001.

Reinhart, D.R. and T.G. Townsend. Landfill Bioreactor Design & Operation. Lewis Publishers, New York, NY. 1997.

Richard T. Sprague is vice president and national director of landfill services with HDR Engineering Inc. in Denver, CO. Gregory N. Richardson is principal with GN Richardson & Associates Inc. in Raleigh, NC.