Leachate Recirculation or Recycling
Leachate recirculation can be applied to all types of landfills from the current “EU Waste Regulations Compliant” MSW landfills to the most basic (with little engineering and management) seen in the developing nations.
Recirculation is also central to the philosophy of so called “flushing landfills” where the intention is to accelerate the decomposition, and then the washing (or flushing) out of all the remaining contaminants to eventually render the landfill safe from groundwater emissions, and available for high grade after-use.
In the discussion which follows we have considered all three landfill types.
Recirculation in Landfills to EU (Waste Management Regulations/Landfill Directive) Requirements
Recycling leachate in municipal solid waste (MSW) landfills can provide:
- a means of disposal (not only short-term as it percolates through the waste but also by allowing the waste to absorb (soak-up) the leachate);
- enhancement of the rate of landfill stabilisation (encouraging both the onset of fermentation leading to gas) and reduced long term settlement;
- increased gas yields).
It can also save money by:
- allowing postponement of the date when a planned on-site leachate treatment plant will be needed;
- allowing more time before leachate treatment starts such that initial composting/fermentation has removed the initial very high BOD/COD concentrations seen in new landfills and new landfill cells, which would otherwise imply the provision of much larger, much more aeration intensive leachate plants than would be necessary after a few months of suitably applied recirculation;
- possibly providing a means of storm flow balancing
Leachate recycling would also seem to be a positive measure when the alternative (that is absence of moisture within a landfill) is considered. Modern lined and capped containment landfill practice is often referred to as “dry-tomb landfilling”, which will postpone the onset of emissions, rather than prevent them. It the waste is too dry it will never decompose, if decomposition does not take place before a geological (or other) event disrupts the lining, groundwater pollution will take place from landfill leachate. (In reality groundwater pollution will still occur unless decomposition and “flushing” has taken place when the containment ruptures, but at least encouraging decomposition is a start.)
Although leachate recycling has been gaining recognition worldwide, the merits of recycling MSW leachate are controversial. Since the introduction of close to the current levels of Waste Regulation in the mid-1980s, some regulatory agencies have encouraged leachate recycling in MSW landfills, while others have prohibited it.
Critics point to the risk that it may increase the hydraulic loading on the landfill containment system, and clearly if leachate was allowed to build-up within the landfill to a level above the elevation assumed for the purpose of the design it would either increase the risk of groundwater pollution, or give rise to a greater rate of this. This depends on whether the lining is a “perfect” HDPE (or equivalent) FML membrane (with no holes, tears, or areas of deterioration), or simply just a clay layer.
Leachate recycling should in any event not be allowed to continue to the point when excessive leachate retention periods in contact with the waste then raise the non-organic pollutant loads which are usually diffusion rate limited. Clearly monitoring should be undertaken to ensure that excessively high metals etc do not to build-up in the recalculated leachate, otherwise subsequent treatment and disposal will become disproportionately more complex and expensive.
Leachate recycling is normally today only allowed (in the UK), in composite-lined landfills where adequate leachate drainage is present to ensure that permitted maximum leachate levels are not exceeded, and a high level of monitoring is undertaken to demonstrate this. (However, permitted practice does vary in different regions, and we would welcome comments on this statement placed on our forum.)
Recirculation in Flushing Landfill Designs
Flushing landfills are fully lined containment landfills, generally experimental, where the intention is to accelerate the decomposition, and then the washing (or flushing) out of all the remaining contaminants.
Leachate recycling is best used during the early stage in this fermentation/ leaching, wet-cell approach to MSW management. In this approach, the leachate recycling-accelerated stabilisation period is followed by a period of washing the decomposed MSW residues with clean water.
During this time, residues that would leak out of the landfill and pollute groundwaters are deliberately leached out of the wastes in a controlled manner. Without the clean-water leaching step, leachate recycling will not protect groundwater resources. At this time, however, Subtitle D regulations prohibit adding clean-water to leach removable components of waste, which is an integral part of the wet-cell approach.
Plastic bags can prevent recycled leachate from interacting with the MSW and are a particular hindrance to “flushing”. Landfill gas production rates also depend on the waste's moisture content, waste that is not exposed to the recycled leachate will also produce landfill gas at a slower rate than non-bagged wastes. Shredding waste before burial can help shorten the duration of landfill gas production and decrease the time for landfill stabilisation.
Recirculation in Non Engineered Landfills/Developing Nations
From the foregoing discussion of lined landfills a set of guidelines for recirculation of leachate in non-engineered landfills can also be proposed:
- recirculation can provide balancing during wet weather when leachate which would otherwise escape can be soaked back into the waste;
- by improving the wetting of the waste, stabilisation will be improved and if landfill gas can be collected the amount of gas and the early payback potential to recoup the investment will be maximised. Utilisation of gas is of benefit to the local community as it generates biofuel and if generation is added can also supply the local power grid. See the Landfill gas Web Site.);
- after initial fermentation/ acetogenesis the recirculated leachate will be more easily treated aerobically, after the initial very high BOD and COD drops;
But, great care is needed for non-engineered landfills to ensure that unseen and unknown the recirculated leachate does not enter groundwater and then pollute wells and appear downstream in water courses. (This is what nearly happened at Foxhall Landfill, in the UK.) Once a plume of contamination has been created the remedial measures can be very expensive, or the pollution take many years to dissipate.
Other severe risks from recirculation arise if the level of leachate in the landfill is not carefully monitored and controlled. If leachate levels rise in the waste, breakouts may rapidly develop uncontrollably and cause surface water pollution, however, worse can occur. A number of landfills have suffered collapse of sloping faces and the presence of high leachate levels has been a major if not the primary contributor.
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