The issue of how long will we need to carry on with landfill gas aftercare, is a serious one when you consider the cost of flaring and monitoring a landfill site for landfill gas production and migration, as the years go by.
M. Huber-Humer states in his paper “Dwindling Landfill Gas – Relevance and Aftercare Approaches, (Instiute of Waste Management BOKU, Univesity of Applied Life Scienceshile, Venna, Austria, 2007) 90% of the carbon broken down in a bioreactor landfill is found in landfill gas, only 10% remains in the organic load of leachate, so it is clear that it is the gas that is the major emittor and not the leachate.
At normal temperatures and water content in a landfill the decomposition of the organic matter in the landfill proceeds at a fairly standard rate in all MSW landfills. Utilizable gas formation and environmentally relevant gas emissions in such circumstances are thought to occur for a period of two to three decades.
That is not to say that gas production will stop even then, and a small amount of gas formation is generally estimated to continue for up to 100 years. Polluting leachate emissions can emerge considerably longer, and the parameters such as COD (chemical oxygen demand) and nitrogen compounds are particularly persistent. Salinity in leachate will not diminsh any more rapidly than will be achieved by simple dilutionn, so that will also remain high. This assertion has bee confirmed by investigations performed by Kruse (1994), Horing and Ehrig (1997) and Kriimpel-beck (1999) which all show that these parameters decrease very slowly over time. Accordingly, leachate from MSW-landfills must be treated for 100 to 200 years in order to comply with proper legal enforcement designed to protect water resources for future generations.
Therefore, although leachate quality is the determining factor in establishing the time span of landfill aftercare, it is collected quite easily at modern landfills with engineered techniques and attendance need not be very frequent, as in the event of pumps not operation the leachate will simply sit in the base of the lined landfills, awaiting action to remove it.
Landfill gas is different. Landfill gas is far harder to capture, even using modern equipment. In fact, because of the high methane content the resulting gas emissions are of paramount importance on a global scale in climate change.
A further conern must also be that when the top barrier fails at some later date, which it surely will water will infiltrate again and microbial decomposition rates in the waste will increase, leading to related effects upon gas formation, most probably at a time when gas recovery systems are no longer operational. Consequently, this leads to unforeseeable and largely unpredictable time-spans for landfill aftercare.
The EU landfill directive (Council Directive 1999/31/EC) stipulates in article 10 that landfill operators must ensure financial security for landfill closure and aftercare measures for a period of at least 30 years.
This time-span is also often interpreted as the “active aftercare phase” for a landfill site, including gas extraction and treatment, leachate collection and disposal. However, from a scientific and technical point of view, the real aftercare period will most probably last longer in order to achieve environmentally harmless emissions and this is particularly the case regarding leachate quality.
With respect to landfill gas, under optimal decomposition conditions, the greater part of methane formation in the landfill may already have taken place within 30 years after landfill closure. At first sight, compared to the amount of methane produced in the operational or landfill closure phase, it may seem to many people that the methane released in or even after the aftercare phase will to be negligible.
Nevertheless, in the latter phase gas extraction systems are almost certainly no longer in continuous operation, flares will be run sporadically due to automatic turn-offs when air in the gas rises, since by this stage gas volumes and methane contents are excessively low, making reliable flare equipment operation very difficult to achieve.
In a study performed to investigate conditions present in many older landfills in Germany, Kriirnpelbeck (1999) discovered that considerable methane emissions occur particularly in the transition period between active and passive aftercare measures, when active gas extraction, gas-utilization and/or flaring are no longer operating smoothly. Moreover, the period of “unusable” methane formation below the capability of standard flare equipment to burn-off, may last from several decades up to 100 years, this extended period will lead to the total volume of emitted landfill gas accumulating to provide an amount of gas which is consequently no longer negligible, particularly with respect to global warming.
Unfortunately, the rate of landfill gas production is reduced by the reduction of infiltration of rainfall which occurs after capping. For the best ability for the landfill gas to be produced and captured before the end of the active and passive aftercare stages, it would be better not to cap the landfills as is required by normal practise and regulations almost everywhere. Not capping would best serve the vital requirement for overall minimisation of methane gas emission in order to limit climate change from the landfill gas lost to the atmosphere.