How Landfills become "Landfill Gas Producing" (Methanogenic)
THE BASIC PROCESSES OF WASTE DECOMPOSITION
First it is necessary to explain briefly the microbiological and chemical processes which give rise to gas and leachate within a domestic waste landfill, during and beyond the active life of the site.
Probably the most widely-reproduced graph in the field of landfill science is that proposed as long ago as 1973 by Farquhar and Rovers, showing the generalised changes in composition of landfill gas with time. (To be added.)
There are three major phases in the decomposition of domestic wastes, summarised below:
Phase 1: Aerobic decomposition rapidly (typically in less than a month) uses up oxygen which is present within the wastes.
Phase 2: Anaerobic and facultative organisms (acetogenic bacteria) hydrolyse and ferment cellulose and other putrescible materials, producing simpler, soluble compounds such as volatile fatty acids (with a high biochemical oxygen demand (BOD) value) and ammonia.
Phase 3: More sensitive and slower growing methanogenic bacteria gradually become established and start to consume simple organic compounds, producing the mixture of carbon dioxide and methane (plus various trace constituents) which is released as landfill gas.
Phase 1 is short, perhaps lasting only a few days or weeks. However, it may persist for long periods, producing significant quantities of cartoon dioxide, in shallow (<3 m) deposits of waste where air can readily enter the waste, or if air is drawn into waste by pumping. Significant quantities of hydrogen (up to about 20% by volume) can be produced, particularly if the site is dry.
Phase 2 can last for as little as 6 months in most current high organic content and high input rate non-hazardous landfills, but for lower input rate, lower organic content wastes, this stage can take years, especially if cold conditions prevail and waste input rate is very low.
The landfill gas expert recognises how advanced this stage is by the leachates produced during this stage.
Leachate is the contaminated water which percolates through and from waste and is mostly derived from rainwater.
Phase 2 type leachates are characterised by high BOD values (commonly >10 000 mg/l); and high ratios of BOD:COD (commonly 0.7 or greater) indicating that a high proportion of the soluble organic materials are readily degradable. Other typical characteristics of the leachate are acidic pH values (typically 5 or 6), strong unpleasant smells and high concentrations of ammonia (often 500-1 000 mg/l) are to be expected. The aggressive chemical nature of such leachate assists in dissolution of other components of wastes, so leachates can contain high levels of iron, manganese, zinc, calcium and magnesium. Gas production consists mainly of carbon dioxide with lesser quantities of methane and hydrogen.
Phase 3, the methanogenic stage commences as the available chemically bound oxygen molecules in the waste mass are fully consumed.
The landfill gas expert becomes familiar in the odour of the methane gas and can usually tell when methanogenicity is present, due to changes in smell, and with the onset of the characteristic methanogenic "landfill odour".
Click for more landfill gas facts here.
|
