Alternative cover systems for landfills

Alternative Landfill Covers: Time for Revolution in Waste Reclamation

Even a cursory examination of the existing, recent literature on the subject of landfill covers reveals that conventional techniques are significantly flawed. High implementation costs combined with a questionable long-term track record means that conventional landfill covers are less than ideal. This author's review of the literature unearthed significant studies that demonstrate potential solutions to this issue in the form of alternative landfill covers. One of the most promising new landfill covers being developed is called the evapotranspiration, which harnesses rather than fights natural hydrologic cycles. It can be concluded that this technique, as well as some other alterative methods, has the real potential to become a viable alternative to the conventional landfill covers that are showing their age.

Keywords alternative, landfill, covers, evapotranspiration, pollution, reclamation

Introduction

Clearly, the issue of waste management in the United States and the world is a significant one. Waste continues to pile higher every year, and landfills are filling up faster than ever (Albright et al. 71). When a landfill reaches its designated capacity, it is sealed and covered. In the past, this has meant the use of rather conventional techniques. Usually, a landfill is lined with compacted clay before even one piece of garbage is dumped there. Then, when the landfill has been filled to capacity, the entire site is covered over. The purpose of all of this engineering is simple: reduce the environmental damage and impact of the site by lessening the degree to which pollutants and contaminants can leach from the site through the introduction of either groundwater or rain. Landfill covers can be simple or complex, but are designed with individual sites in mind to control moisture, percolation, promote runoff, minimize erosion, and meet aesthetic demands (EPA 1). In other words, conventional landfill covers are designed to act like enormous box lids over and already-sealed system to isolate the site from the environment.

Unfortunately, this contained ideal is rarely realized in practice. Conventional containment techniques, such as compacted clay layers or geotextile membranes, have the tendency to be breached. Clay will crack and membranes can tear. Over time, this means that conventional landfill techniques are incapable of providing anywhere close to 100% isolation of the landfill from the environment. The end-result is contamination of the surrounding environmental system, especially via soil conditions and the local water table. Cleanup costs for contaminated landfill sites in the United States are already amount to several billion dollars (Dwyer, "Finding" 58). These costs can only be expected to rise unless better cover methods can be devised for landfills.

Alternative systems have been developed in an effort to provide better long-term functionality often at a fraction of the cost. These alternative landfill cover methods generally eschew efforts to provide absolute containment, recognizing the absurdity of such a goal. Instead, these alternative systems are designed to work on a site-specific basis with the local environmental conditions. Contamination of the surrounding environment is reduced or eliminated by harnessing natural processes that will operate indefinitely. The following literature review on the subject of alternative landfill covers reveals that the new systems being developed and implemented are more than ready to replace existing, conventional techniques.

Literature Review

The problem with conventional landfill covers

The conventional approach to landfill mediation and covers has been "entombment and preservation" (Hauser 1). By this, I mean that conventional landfill methods, especially when it comes to covers, has been primarily interested in isolating the landfill refuse from the surrounding soil and hydrologic cycle. Conventionally, this is accomplished through the use of barriers that limit the amount of water that can enter the landfill and, consequently, limit the amount of polluted water that can later leach back out (Hauser 1). One of the most commonly used water barriers employed in landfills as covers -- as well as a substrate material -- is clay. As a water impermeable barrier, clay has an impressive track record. Thus, it is little wonder that clay has been enlisted to serve as the water barrier that is present in nearly all landfills to prevent water infiltration (Koerner and Daniel 56).

Unfortunately, there are significant limitations to the continued use of clay in landfills either as a substrate or as a cover material. Compacted clay is expensive to implement at a landfill site, and compaction can be difficult at best in the variable site conditions of the typical landfill (Koerner and Daniel 53). Historically, clay has been used extensively, despite these shortcomings, largely because it fits well with the "entombment" approach to refuse mediation and because is a common, naturally occurring material. But as research methods have become more sophisticated and more scrutiny has been leveled at landfills, more and more problems have emerged regarding the use of compacted clay as a cover material. The primary strikes against the use of clay include the fact that it can be difficult to compact, it breaks down without adequate protection, it is susceptible to freeze-thaw cycles, differential settlement of refuse can cause cracking, and compacted clay is exceedingly hard to repair in the case of system failure (Koerner and Daniel 56).

Perhaps this is why a study of 544 landfills in California found that of those that used compacted clay as a cover material -- nearly all of them -- between 72% and 86% were suffering systemic failures (Dwyer, "Alternative" 50). Quite simply, it all of those cases, the compacted clay that was meant to act as an absolute barrier for water infiltration had failed and the "entombment" of the refuse was no longer a possibility. Worse, because of the difficulty associated with repairing landfill covers that use compacted clay, the possibility of repairing these numerous landfills is unlikely at best and exceptionally costly at worst. For these reasons, significant interest exists in the research community, not to mention among the public and in political circles, for the development of new landfill cover systems that are better able to protect health and the environment, though with a reduced overhead. This is where the discussion of alternative landfill covers begins.

Alternative systems simply defined

Fortunately, new and alternative landfill cover systems are being developed and experimented with. Many show significant promise at providing better environmental protection than conventional compacted clay methods. These new methods -- such as evapotranspiration, geosynthetic barriers, and anisotropic barriers -- are able to provide better protection for the environment and human health in most instances (Hauser 1). Alternative systems, by their nature, are relatively new and untested to varying degrees. But the key identifying characteristic that makes a landfill cover system "alternative" is its lack of inclusion in current regulations and laws on the subject of refuse reclamation (Dwyer, "Finding" 59). Evapotranspiration may be one of the most promising new alternative landfill cover systems in development, but because it is not clearly defined in all of the regulations on refuse disposal in the United States, it is still classified as an alternative system there.

Additionally, alternative systems generally consist of technologies that are able to accomplish waste reclamation and remediation at a lower cost than conventional landfill cover technologies (Dwyer, "Finding" 59). Compacted clay may be the tried and true technique for providing landfill covers in the United States, but paradoxically this does not mean that it will necessarily be the cheapest method. Because conventional landfill covers are designed with the intent of acting as an absolute barrier for water, the total system must be highly engineered in order to come close to reaching this goal. Combined with the fact that clay must often be shipped to the site of the landfill and the result is high implementation costs. Alternative landfill cover techniques, in contrast, are almost always less costly than the conventional techniques and, as we shall see, are often capable of out-performing conventional covers over long periods of time.

The proven value of alternative systems

When it comes to alternative landfill covers, there are a variety of techniques and technologies at the forefront of development. These include geosynthetic clay, capillary barriers, anisotropic barriers, and evapotranspiration. Of these listed methods, both anisotropic barriers and evapotranspiration have been shown to be good performers in landfill tests. They cost as little as half the cost of conventional compacted clay covers, and are expected to perform better in the long run (Dwyer, "Alternative" 51-52). Evapotranspiration, in particular, has become the prize of landfill engineering, especially in arid and semi-arid environments. It is much less costly than conventional techniques, but is perceived to be more natural than the "entombment" approach (Albright et al. 71). Three years of regimented testing at the Sandia National Laboratories in New Mexico put this method ahead of all other alternative cover techniques, at least in that environment. In those tests, the evapotranspiration cover even outperformed conventional clay compacted covers (Dwyer, "Finding" 63).

Evapotranspiration employs one or more layers of vegetation over a layer of fine grained soil, all of which is placed over the refuse and waste that is part of the filled landfill. Whereas conventional, compacted clay barriers are designed to prevent the infiltration of water into the waste below the cover, evapotranspiration takes a decidedly different approach. The cover technique actually uses to its advantage the high water storage capacity of fine grained soils to retain water in the soil above the waste and refuse. The water is "stored" in that layer until it is released back into the atmosphere either through evaporation from the soil or transpiration from the native vegetation that is planted on the surface (EPA 2). One of the only limitations of the evapotranspiration cover is its inability to function adequately outside of arid and semi-arid environments. Only there can the refuse be covered in a reasonable amount of soil with a storage capacity great enough to manage and store the water that will enter the soil at the landfill site. In humid climates, or ones with higher annual rainfalls, evapotranspiration is not as functional as a landfill cover (Albright et al. 75).

Whereas evapotranspiration covers have not been as effective in wetter climates, geomembranes are nearly as effective as evapotranspiration covers, but can function in essentially any climate (Albright et al. 75). This is an important feature in a landfill cover, because obviously many landfills in the United States will be built in the humid Southeast or the rainy Northwest. Landfill cover systems must be developed for those regions as well in order to protect the environment from contamination and human health from pollutants. In those places that evapotranspiration covers would be ineffective, geomembranes offer a useful alternative. They consist, basically, of a layer of synthetic material -- generally a plastic of some kind -- that diverts water around and away from the refuse site. In combination with compacted clay and, sometimes, surface vegetation, geomembranes are functional alternatives to conventional landfill covers.

But, of course, these are not the only alternatives available. Research is being conducted into more exotic and unexpected landfill cover systems, whose effectiveness is less certain though nonetheless promising. Research into the use of anaerobically digested lime-stabilized wastewater sludge and soil as a cover for landfills produced surprisingly positive results. The researchers found that, in fact, the cover method had no negative effect on the landfill and produced no leached contaminants more significant than those produced from a clay compacted, conventional cover. The true potential of this cover type is in its extreme cost effectiveness, the fact that it could serve as an outlet for lime waste, and the sludge actually enhances and hastens natural decomposition of the landfill waste (Rhew and Barlaz 499). While it might seem a bit unconventional, perhaps too alternative, to simply dump sludge on top of a landfill as a cover method, there is every indication that it could be a successful remediation strategy as an alternative cover.