How Are Landfills Constructed?
Municipal solid waste landfills are designed and constructed to meet specifications to ensure the protection of the environment including our surface waters, groundwater, and air quality. The two most important aspects of landfill design include liner installation and landfill leachate collection/management. Landfills are designed into cells. Cells are individual waste disposal units or mini-landfills that inter-connect by welded liner systems.
Liner Installation: The base of each landfill cell is composed of many layers of liner material. The liners are constructed of high density polyethylene (HDPE) plastic. The first layer is composed of an over-lapping geo-synthetic clay layer (GCL). The GCL is composed of the equivalent to bentonite clay and offers the last defense against leakage of leachate to the groundwater. If liquid comes into contact with the GCL it immediately softens and expands to close off any holes, therefore halting any leaks. The middle layer consists of the primary and secondary liners. These impermeable HDPE liners are the main defense against leachate leakage. A geo-textile composite drainage layer separates the primary and secondary liners. The geo-net allows any leakage from the primary liner to flow by gravity, along the top of the secondary liner, to a sump to be pumped out for treatment. If a leak is detected between the two liners it can be located and repaired. The final layer is comprised of another geo-textile composite drainage layer. This layer is located on top of the primary liner system and allows leachate to flow quickly into the leachate collection system and offers protection against objects which could possibly penetrate the liner systems.
The HDPE liners are welded together to cover the entire surface area of the landfill. The liner panels are welded together by a machine “mouse” that forms a double fusion weld. This type of weld forms an air channel between the weld of two liner panels. The air channel is then “air tested” and must hold specified air pressures for an extended period of time to ensure that no leakage can occur. Another weld typically performed during landfill construction is the extrusion weld. This weld is conducted by heating a string of HDPE to a specified temperature, thereby melting it to form a “bead”. The bead is then forced against the interface of two sections of liner. After the HDPE bead has cooled the weld is “vacuum” tested to ensure no leakage can occur. During construction further quality assurance testing is also conducted. These tests include sheer, peel, and conformance testing of the liner systems to meet strict specifications.
Leachate Management: All precipitation that falls onto an open landfill cell is absorbed into the waste. As the precipitation leaches through the waste it picks up contaminants such as metals, nitrogen, silt, salts, volatile organic compounds, and other oxygen demanding wastes. The liquid waste, known as leachate, is collected from the bottom of the lined landfill in the leachate collection system. The leachate collection system is comprised of a network of inter-connected perforated collection piping that flows by gravity to a sump area. From the sump, leachate is pumped to a stabilization basin for pretreatment. After pretreatment, the leachate undergoes further treatment through a conventional wastewater treatment plant or constructed wetland treatment system. Tertiary treatment through the use of a continuous backwashing sand filter polishes the treated leachate (effluent) exiting the wastewater treatment plant before discharge into the Northeast Cape Fear River. This discharge is permitted by the North Carolina Department of Environment and Natural Resources, Division of Water Quality. The permit allows for the discharge of 50,000 gallons/day. The effluent must meet permit limits for Biochemical Oxygen Demand, ammonia nitrogen, pH, Total Suspended Solids, zinc, phenol, alpha-terpineol, benzoic acid, and p-cresol.