Landfill

As the waste is placed in the landfill, the void spaces contain high volumes of molecular oxygen (O₂). 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.

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.

Hydrolysis of the biodegradable fraction of the solid waste begins in the acid formation phase, which leads to rapid accumulation of volatile fatty acids (VFAs) 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 (COD). Long-chain volatile organic acids (VOAs) are converted to acetic acid (C₂H₄O₂), CO₂, and hydrogen gas (H₂). High concentrations of VFAs increase both the biochemical oxygen demand (BOD) 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.

The acid formation phase intermediary products (e.g., acetic, propionic, and butyric acids) 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, experessed as oxygen demand, decreases at a rapid rate with increases in CH₄ and CO₂ gas production. This is the longest decomposition phase.

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 (ORP) 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.[6]

When precipitation falls on open landfills, water percolates through the garbage and becomes contaminated with suspended and dissolved material, forming leachate. 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 ingress and new leachate formation. However, liners must have a lifespan, be it several hundred years or more. Eventually, any landfill liner could leak,[7] so the ground around landfills must be tested for leachate to prevent pollutants from contaminating groundwater.

Rotting food and other decaying organic waste creates decomposition gases, especially CO₂ and CH₄ from aerobic and anaerobic decomposition, respectively. Both processes occur simultaneously in different parts of a landfill. In addition to available O₂, the fraction of gas constituents will vary, depending on the age of landfill, type of waste, moisture content and other factors. For example, the maximum amount of landfill gas produced can be illustrated a simplified net reaction of diethyl oxalate that accounts for these simultaneous reactions:[8]

4 C₆H₁₀O₄ + 6 H₂O → 13 CH₄ + 11 CO₂

On average, about half of the volumetric concentration of landfill gas is CH₄ and slightly less than half is CO₂. The gas also contains about 5% molecular nitrogen (N₂), less than 1% hydrogen sulfide (H₂S), and a low concentration of non-methane organic compounds (NMOC), about 2700 ppmv[8]

Waste disposal in Athens Greece

Landfill gases can seep out of the landfill and into the surrounding air and soil. Methane is a greenhouse gas, and is flammable and potentially explosive at certain concentrations, which makes it perfect for burning to generate electricity cleanly. Since decomposing plant matter and food waste only release carbon that has been captured from the atmosphere through photosynthesis, no new carbon enters the carbon cycle and the atmospheric concentration of CO₂ is not affected. Carbon dioxide traps heat in the atmosphere, contributing to climate change.[9] In properly managed landfills, gas is collected and flared or recovered for landfill gas utilization.

Poorly run landfills may become nuisances because of vectors such as rats and flies which can spread infectious diseases. The occurrence of such vectors can be mitigated through the use of daily cover.

A group of wild elephants interacting with a trash dump in Sri Lanka

Other potential issues include wildlife disruption due to occupation of habitat[10] and animal health disruption caused by consuming waste from landfills,[11] dust, odor, noise pollution, and reduced local property values.

Landfills in Canada are regulated by provincial environmental agencies and environmental protection legislation.[13] Older facilities tend to fall under current standards and are monitored for leaching.[14] Some former locations have been converted to parkland.

Landfill bans in Europe

In the European Union, individual states are obliged to enact legislation to comply with the requirements and obligations of the European Landfill Directive.

Finland, Sweden, Denmark, Poland, Germany, the Netherlands, Belgium, Austria and Slovenia have laws banning or severely restricting the disposal of household trash via landfills.[15]

Landfilling is currently the major method of municipal waste disposal in India. India also has Asia's largest dumping ground in Deonar, Mumbai.[16] However issues frequently arise due alarming growth rate of landfills and poor management by authorities.[17] On and under surface fires have been commonly seen in the Indian landfills over the last few years.[18]

Landfilling practices in the UK have had to change in recent years to meet the challenges of the European Landfill Directive. The UK now imposes landfill tax upon biodegradable waste which is put into landfills. In addition to this the Landfill Allowance Trading Scheme has been established for local authorities to trade landfill quotas in England. A different system operates in Wales where authorities cannot 'trade' amongst themselves, but have allowances known as the Landfill Allowance Scheme.

U.S. landfills are regulated by each state's environmental agency, which establishes minimum guidelines; however, none of these standards may fall below those set by the United States Environmental Protection Agency (EPA).[19]

Permitting a landfill generally takes between five and seven years, costs millions of dollars and requires rigorous siting, engineering and environmental studies and demonstrations to ensure local environmental and safety concerns are satisfied.[20]