Landfills store waste materials at an engineered and self-contained site to decompose in a controlled setting. The term “landfill” implies a simple earth excavation filled with garbage. While crude waste disposal practices occurred over many previous decades, modern landfills are carefully prepared structures with many common design features. Many landfills safely serve a secondary function such a recreational park or golf course when their maximum capacities are reached. Most landfills are solid waste landfills that receive municipal refuse originating from homes, businesses and institutions. There are some specialized landfills that receive specific industrial wastes including liquids. Even within solid waste landfills, liquid containment is essential because of leachate production. Leachate is an organic waste liquid, a by-product of solid waste moisture and decomposition, mixed with any precipitation or groundwater infiltrating through the landfill. Modern landfill designs will include a bottom layer, a leachate recovery system, a surface cap, and gas release capability, as methane is generated within landfills as organic wastes decompose.
Wastes are deposited and distributed on the prepared bottom layer of a landfill by garbage trucks and earthmoving equipment. The design of the bottom layer is very important to avoid leachate contamination of the groundwater adjacent to the landfill site. The bottom layer also serves in the recovery of leachate by providing a liquid filter to separate the solid waste from leachate and route it toward collection points placed within the landfill envelope. In a typical design, leachate is allowed to infiltrate through a permeable geo-textile membrane into sloped sand layer(s) beneath the stored waste, but is contained by synthetic liners and the clay base beneath the sand. Perforated leachate drain pipes placed within the sand (or other permeable course soil) then collect the waste liquid at engineered low-points. Groundwater monitoring wells are typically installed around the perimeter of the landfill and sampled frequently to detect any leakage from the system.
When waste placement in an area of the landfill is maximized, surface cover to protect the stored waste from water infiltration is constructed. It is important to eliminate water infiltration into the landfill as much as possible. Moisture adds to leachate production, which increases the leachate collection, handling, and disposal costs. Landfill covers are designed with an upper layer of top soil and surface vegetation, and a sand layer (or other permeable course soil) above a clay layer placed on top of the stored waste. The sand layer provides lateral drainage for any water that infiltrates through the topsoil. The clay cap protects the stored waste from water infiltration above. Vertical pipes are placed through the surface cover and into the stored waste to collect, vent, or burn the methane gas produced as the waste decomposes. In some landfill applications it is cost effective to recover, process, and store the methane gas as a fuel source.
In complex multiple-cell landfills, several iterations of these basic layers may be constructed and filled with additional waste levels in different horizontal and vertical segments.