Landfill

A landfill is a site for the disposal of waste materials, including municipal solid waste. It is the oldest and most common form of waste disposal, although the systematic burial of waste with daily, intermediate, and final covers only began in the 1940s. In the past, waste was simply left in piles or thrown into pits.
Landfills take up significant amounts of land and pose environmental risks. Some landfill sites are used for waste management purposes, such as temporary storage, consolidation, and transfer, or for various stages of processing waste material, such as sorting, treatment, or recycling. Unless they are stabilized, landfills may undergo severe shaking or soil liquefaction during an earthquake. Once full, the area over a landfill site may be reclaimed for other uses.
Both active and restored landfill sites can have significant environmental impacts which can persist for many years. These include the release of gases that contribute to climate change and the discharge of liquid leachates containing high concentrations of polluting materials.

Operations

Operators of well-run landfills for non-hazardous waste meet predefined specifications by applying techniques to:

confine waste to as small an area as possible
compact waste to reduce volume

They can also cover the waste daily with layers of soil or other materials, such as wood chips and fine particles.
During landfill operations, a scale or weighbridge may weigh waste collection vehicles on arrival and personnel may inspect loads for wastes that do not accord with the landfill's waste-acceptance criteria. Afterward, the waste collection vehicles use the existing road network on their way to the tipping face or working front, where they unload their contents. After loads are deposited, compactors or bulldozers can spread and compact the waste on the working face. Before leaving the landfill boundaries, the waste collection vehicles may pass through a wheel-cleaning facility. If necessary, they return to the weighbridge for re-weighing without their load. The weighing process can assemble statistics on the daily incoming waste tonnage, which databases can retain for record keeping. In addition to trucks, some landfills may have equipment to handle railroad containers. The use of "rail-haul" permits landfills to be located at more remote sites, without the problems associated with many truck trips.
Typically, in the working face, the compacted waste is covered with soil or alternative materials daily. Alternative waste-cover materials include chipped wood or other "green waste", several sprayed-on foam products, chemically "fixed" bio-solids, and temporary blankets. Blankets can be lifted into place at night and then removed the following day prior to waste placement. The space that is occupied daily by the compacted waste and the cover material is called a daily cell. Waste compaction is critical to extending the life of the landfill. Factors such as waste compressibility, waste-layer thickness and the number of passes of the compactor over the waste affect the waste densities.

Sanitary landfill life cycle

The term landfill is usually shorthand for a municipal landfill or sanitary landfill. These facilities were first introduced early in the 20th century, but gained wide use in the 1960s and 1970s, in an effort to eliminate open dumps and other "unsanitary" waste disposal practices. The sanitary landfill is an engineered facility that separates and confines waste. Sanitary landfills are intended as biological reactors in which microbes will break down complex organic waste into simpler, less toxic compounds over time. These reactors must be designed and operated according to regulatory standards and guidelines covered by the field of environmental engineering.
Aerobic decomposition is often the first stage by which wastes are broken down in a landfill. This process is followed by four stages of anaerobic degradation. Solid organic material typically decays rapidly as larger organic molecules degrade into smaller molecules. These smaller organic molecules begin to dissolve and move to the liquid phase, followed by hydrolysis of the organic molecules, and the hydrolyzed compounds then undergo transformation and volatilization as carbon dioxide and methane, with the rest of the waste remaining in solid and liquid phases.
During the early phases, little material volume reaches the leachate as the biodegradable organic matter of the waste undergoes a rapid decrease in volume. Meanwhile, the leachate's chemical oxygen demand rises with increasing concentrations of the more recalcitrant compounds compared to the more reactive compounds in the leachate. Successful conversion and stabilization of the waste depends on how well microbial populations function in syntrophy.
The life cycle of a municipal landfill undergoes five distinct phases, as follows:

Initial adjustment (Phase I)

As the waste is placed in the landfill, the void spaces contain high volumes of molecular oxygen. With added and compacted wastes, the O₂ content of the landfill bioreactor strata gradually decreases. Microbial populations grow, density increases. Aerobic biodegradation dominates, i.e. the primary electron acceptor is O₂.

Transition (Phase II)

The O₂ is rapidly degraded by the existing microbial populations. The decreasing O₂ leads to less aerobic and more anaerobic conditions in the layers. The primary electron acceptors during transition are nitrates and sulphates since O₂ is rapidly displaced by CO₂ in the effluent gas.

Acid formation (Phase III)

Hydrolysis of the biodegradable fraction of the solid waste begins in the acid formation phase, which leads to rapid accumulation of volatile fatty acids in the leachate. The increased organic acid content decreases the leachate pH from approximately 7.5 to 5.6. During this phase, the decomposition intermediate compounds like the VFAs contribute much chemical oxygen demand. Long-chain volatile organic acids are converted to acetic acid, CO₂, and hydrogen gas. High concentrations of VFAs increase both the biochemical oxygen demand and VOA concentrations, which initiates H₂ production by fermentative bacteria, which stimulates the growth of H₂-oxidizing bacteria. The H₂ generation phase is relatively short because it is complete by the end of the acid formation phase. The increase in the biomass of acidogenic bacteria increases the amount of degradation of the waste material and consuming nutrients. Metals, which are generally more water-soluble at lower pH, may become more mobile during this phase, leading to increasing metal concentrations in the leachate.

Methane fermentation (Phase IV)

The acid formation phase intermediary products are converted to CH₄ and CO₂ by methanogenic microorganisms. As VFAs are metabolized by the methanogens, the landfill water pH returns to neutrality. The leachate's organic strength, expressed as oxygen demand, decreases at a rapid rate with increases in CH₄ and CO₂ gas production. This is the longest decomposition phase.

Final maturation and stabilization (Phase V)

The rate of microbiological activity slows during the last phase of waste decomposition as the supply of nutrients limits the chemical reactions, e.g. as bioavailable phosphorus becomes increasingly scarce. CH₄ production almost completely disappears, with O₂ and oxidized species gradually reappearing in the gas wells as O₂ permeates downwardly from the troposphere. This transforms the oxidation–reduction potential in the leachate toward oxidative processes. The residual organic materials may incrementally be converted to the gas phase, and as organic matter is composted; i.e. the organic matter is converted to humic-like compounds.

Social and environmental impact

Landfills have the potential to cause a number of issues. Infrastructure disruption, such as damage to access roads by heavy vehicles, may occur. Pollution of local roads and watercourses from wheels on vehicles when they leave the landfill can be significant and can be mitigated by wheel washing systems. Pollution of the local environment, such as contamination of groundwater or aquifers or soil contamination may occur as well.

Leachate

When precipitation falls on open landfills, or when water is released from the breakdown of waste, water percolates through the waste and becomes contaminated with suspended and dissolved material, forming leachate enriched with organic matter, heavy metals, organic contaminants, and other contaminants present in the waste. If this is not contained it can contaminate groundwater. All modern landfill sites use a combination of impermeable liners several metres thick, geologically stable sites, and collection systems to contain and capture this leachate. It can then be treated and evaporated. Once a landfill site is full, it is sealed off to prevent precipitation entering the landfill and formation of new leachate. However, liners have a lifespan, often several hundred years or more, but eventually any landfill liner could leak, so the ground around landfills must be tested for leachate to prevent pollutants from contaminating groundwater.
The largest problem in sanitary landfills with regard to leachate quality is nitrogen, particularly in the form of ammonium nitrogen. Hydrolysis of waste results in the release of carbon species such as bicarbonate and acetic acid as well as the release of ammonium. The anaerobic environment present in landfills does not allow for coupled nitrification-denitrification, the typical nitrogen removal pathway in soils, which can lead to an accumulation of ammonium in the leachate and concentrations upwards of several thousand milligrams per liter.