In 1965, the U.S. government passed the Solid Waste Disposal Act, regulating the development and management of landfills throughout the country. While prior dumps were open to the air and soil, new legislation required the construction of sanitary landfills. These were designed to mitigate the spread of toxins and pollutants to the nearby environment around the landfill. A modern sanitary landfill includes a bottom impermeable liner, a system in order to gather and remove liquids (leachate), a stormwater management system, and a methane collection mechanism that removes built-up gas. The nature of a sanitary landfill means it must be constantly monitored while it collects and removes gas and liquids from the waste.
A sanitary landfill is separated into large cells, ranging from a few hundred square feet to acres in size. These cells are stacked up atop each other, compacted and often topped with a daily cover like soil or degradable films. Above the impermeable bottom layer are materials like gravel or another granular substance that prevents large pieces of waste from escaping the cell. This gravel serves as a filter in order to funnel liquids into the leachate collection system.
As each landfill cell is created, garbage is dumped within the site and compacted. Daily covers are used in order to discourage odor and pests, as well as minimize contact with the outside environment. This can come in the form of 6 inches or more of soil, poured atop the waste, or another kind of cover in the form of a geotextile fabric, ash or dust, compost materials, foam products, or numerous other alternatives.
A sanitary landfill needs to account for the byproducts of waste breakdown. Leachate and methane can be valuable when collected and reused, but hazardous if allowed to escape into the environment. Leachate can be funneled and collected via filtering materials and a series of pumps and piping connected into the leachate management system. Oftentimes, the next step is to collect it in a leachate pond, which is also lined with an impermeable geotextile to prevent seepage into the environment. Since leachate is so heavily concentrated with toxins and chemicals, a corrosion and chemically resistant liner is required in order to successfully contain the liquid and not degrade over time.
Methane is produced by decaying organic matter and is also the primary component of natural gas. If methane collected from the landfill is recaptured, it can be used or sold in order to generate electricity. Sanitary landfills utilize gas extraction wells to isolate the methane before transporting it to further treatment. The RCRA includes specific regulations in reference to methane management in its section on “Explosive Gasses Control.” 40 CFR 258.23:
(a) Owners or operators of all (sanitary landfills) must ensure that:
(1) The concentration of methane gas generated by the facility does not exceed 25 percent of the lower explosive limit for methane in facility structures (excluding gas control or recovery system components); and
(2) The concentration of methane gas does not exceed the lower explosive limit for methane at the facility property boundary.
(b) Owners or operators of all MSWLF units must implement a routine methane monitoring program to ensure that the standards of paragraph (a) of this section are met.
(1) The type and frequency of monitoring must be determined based on the following factors:
(i) Soil conditions;
(ii) The hydrogeologic conditions surrounding the facility;
(iii) The hydraulic conditions surrounding the facility; and
(iv) The location of facility structures and property boundaries.
(2) The minimum frequency of monitoring shall be quarterly.
(c) If methane gas levels exceeding the limits specified in paragraph (a) of this section are detected, the owner or operator must:
(1) Immediately take all necessary steps to ensure protection of human health and notify the State Director;
(2) Within seven days of detection, place in the operating record the methane gas levels detected and a description of the steps taken to protect human health; and
(3) Within 60 days of detection, implement a remediation plan for the methane gas releases, place a copy of the plan in the operating record, and notify the State Director that the plan has been implemented. The plan shall describe the nature and extent of the problem and the proposed remedy.
Here’s an example breakdown of what a closed and completed modern sanitary landfill may look like, from the topmost layer to the bottom:
- Vegetation Layer: Up to 18 inches of soil which allows for erosion-preventing vegetation to take root.
- Geotextile Mat: A non-woven geotextile fabric that protects underlayment from roots and erosion.
- LLDPE Liner: This is a Linear Low Density Polyethylene liner which prevents the garbage below from reaching the surface, or from water permeating into the waste.
- Closed Compacted Cells: These are the finished, compacted cells of waste that have been sealed in and covered.
- Protective Soil Layer: A protective layer of soil between 1 and 2 feet that separates the lower layers of filtering materials from the waste being dumped above.
- Granular Layer: This funnels the liquids and leachate into the leachate collection system which removes it from the landfill.
- Geotextile Mat: A permeable, protective layer between the lower plastic liner and granular layer above.
- HDPE Liner: High Density Polyethylene liners come in sections that are heat-welded in order to be completely impermeable. This is the primary layer which prevents liquids or leachate from seeping further down into the ground.
- Compacted Cell Layer: 1 to 2 feet of compacted clay which serves as a secondary liner separating the landfill from the groundwater
