Landfill gas (LFG): results from the biological decomposition of organic wastes when in its anaerobic (methanogenic) phase. LFG consists of two bulk gases namely methane (CH₄) and carbon dioxide (CO₂). Methane production during the methanogenic phase is initially as high as 60-65% but over time, say 15 years, it declines as the available biodegradable carbon is depleted. Complete depletion or stabilisation takes decades, perhaps 30 to 50 years. When methane has declined to <20% it is problematic as it cannot be reliably destroyed by burning in a conventional LFG flare and when diluted in air in the range 15% down to 5% CH_{4 ,} it is potentially explosive.

Methane and carbon dioxide are so-called Green House Gases (GHG’s). Although methane is the simplest hydrocarbon it is at least 21 times more harmful as a GHG than carbon dioxide. However, the UK Environment Agency has recently stated its belief that methane is in fact perhaps 100 times more destructive than carbon dioxide during the first 5 years as a GHG and is pressing for action on methane now. LFG has been identified as a major contributor to global warming and should therefore be destroyed to simpler less damaging forms, i.e. CO₂ and H₂O (water).

Landfill gas also contains so-called ‘trace’ gases including: -

• Nitrogen
• Oxygen
• Sulphides, e.g. Hydrogen Sulphide (H_2S)
• Disulphides
• Carbon Monoxide
• Ammonia
• Mercaptans
• Plus many others

Leachate: is highly contaminated water that forms from biodegradation of organic wastes + water soluble waste + rainwater = a chemical ‘soup’ that kills all aquatic life forms and so it must be treated on site or off-site before it can be safely released into the environment.

Leachate contains two contaminants which cause major pollution of rivers and groundwaters:

• Ammonia – even very low concentrations will kill fish and other aquatic life forms.
• COD - Dissolved organic compounds measured as Chemical Oxygen Demand – these reduce the dissolved oxygen levels and will also destroy all aquatic organisms.
• BOD - Biochemical oxygen demand is a chemical procedure for determining the uptake rate of dissolved oxygen by the biological organisms in a body of water.
• BOD₅ – is a test that measures the rate of oxygen uptake by micro-organisms in a sample of water at a temperature of 20°C a period of five days in the dark.

Other pollutants found in leachate are:

• Pesticides & herbicides such as Mecoprop.
• Metals such as cadmium, chromium, copper, lead etc.
• Suspended solids
• Dissolved methane

• The following table shows published values for ammonia concentrations that will kill fish and other aquatic life forms:

Note: NH₃ is the un-ionised form of ammonia; NH₄⁺ is the ionised form.

Typical NH₃ value for landfill leachate: 1000mg/l; (it can be considerable higher than this).