The Hazard Defines the Liner System
Landfills that hold construction rubble like stones, wood, drywall, shingles, bricks, etc. offer substantially lower hazard potential than municipal solid waste sites which typically accept household waste, commercial solid waste, sludge, some generator waste and non-hazardous industrial waste.
Different types of landfills and their associated hazard levels require different types of liner systems. The most basic is a single-liner system, which are generally employed for low-risk construction landfills where harmful liquid wastes are not accepted for disposal.
Municipal solid waste landfills typically employ double-liner systems, where the topmost layer is designed to collect leachate either to be reapplied to the waste heap in the case of bioreactor models, or to be removed and treated separately in the case of dry landfills. In either case, no leachate should pass the topmost layer, so the lower layer is designed to be both a leak-detection system and backup barrier in case the primary layer fails.
Hazardous waste sites are an entirely different kettle of fish and may require substantially more protection, but they will always start, at minimum, with a 2-layer liner system. Depending on the type of waste, additional protections and monitoring systems may be required.
Protecting Soil and Groundwater
All landfills have some level of moisture, whether it’s generated through natural decomposition processes or is introduced through some type of precipitation. As this moisture percolates through the waste mass, it picks up a variety of contaminants. The specific contaminants depend on the nature of the waste itself, but nearly always include hazardous chemicals such as volatile organic compounds, chloride, nitrogen, solvents, phenols, and heavy metals. This kind of contaminated liquid is a hazardous byproduct of landfills, and exposure to the local environment represents a significant risk. When exposed to the soil, those contaminants infiltrate soil layers and may migrate to nearby ecosystems, or to nearby aquifers and other freshwater sources such as wells for drinking water, lakes and streams.
Even hazardous gases formed through biological processes (landfill gas) are known to move through soil and collect in nearby buildings or even in low lying land areas. Some typical but dangerous landfill gases are ammonia, sulfide, methane and carbon dioxide. Human exposure to soil, air, and water contaminated by dumps and landfills have long been associated with respiratory disease, cancers, and birth defects.
Regulations enacted by the Environmental Protection Agency, as well as state and local governments, define the minimum requirements for operating a municipal solid waste landfill. For example, the EPA requires the installation of a layer of two feet or more of compacted clay, with a maximum hydraulic conductivity along the bottom and sides of the landfill, overlaid by a flexible, durable geomembrane, such as high-density polyethylene. The purpose of this type of liner system is to protect groundwater and soil from leachate releases.
In practice, there are many types of liners which are used for the control of leachate. These options include a variety of geomembranes, geosynthetic clay liners, geotextiles, geogrids, geonets, and geocomposites. Each type of liner has specific uses and abilities.
Controlling Leachate
Leachate within a landfill must not only be isolated from the local soil and groundwater sources, but must be collected for redirection according to the landfill model in use. While both natural and synthetic liners may serve in these roles, the chief concern is the ability of any given liner to maintain both integrity and impermeability over the life of the landfill. Since landfills are typically expected to produce leachate and landfill gases for up to 50 years, the durability and longevity of the liner system and its materials must be carefully considered.
The ideal liner for containing leachate within a landfill must have the following properties:
- Highly flexible
- High tensile strength
- Durable
- Resistant to abrasion and punctures
- Highly chemically resistant
- Able to withstand temperature variations
- Resistant to UV degradation
Ultimately, a landfill liner is intended to be an impermeable barrier which is installed under modern engineered landfill sites. The liner prevents migration of leachate and its toxins into underlying aquifers and nearby surface waters.
