What Happens in a Landfill? Slow Decay, Methane and a Very Long Aftercare

A sanitary landfill is an engineered containment structure rather than a hole, and the components are specified by regulation in most countries:

• •A liner system at the base, typically compacted clay overlaid with a thick plastic geomembrane, designed to stop liquid reaching groundwater
• •A leachate collection layer above the liner, with perforated pipes draining the contaminated liquid that percolates through the waste to a collection point for treatment
• •Waste placed in thin layers, compacted by heavy machines and covered daily with soil or an approved alternative to control odour, vermin and windblown litter
• •Gas collection wells drilled into the completed cells, connected to a vacuum system that draws off the methane
• •A capping system on completion, with a low-permeability layer, drainage and topsoil, which keeps rainwater out and gas in
• •Monitoring boreholes around the perimeter checking groundwater, and aftercare obligations typically running thirty years or more after closure

Decomposition proceeds in phases as the chemistry changes. In the first days to weeks, oxygen trapped with the waste is consumed by aerobic bacteria, warming the mass. Once it is gone, an acid-forming phase begins, in which bacteria break complex material into organic acids, lowering the pH and mobilising metals into the leachate. Methanogenic archaea then establish and convert those acids into methane and carbon dioxide, which is the dominant phase and can continue for decades, producing landfill gas that is roughly half methane. The process is slow because a compacted, capped landfill is dry and sealed, and moisture is the limiting factor: organic material needs water to decompose and a well-capped site keeps it out. That is the paradox at the centre of landfill design, since the measures that protect groundwater also preserve the waste, which is why some operators now run bioreactor landfills that deliberately recirculate leachate to accelerate decomposition and finish the gas production sooner.

Landfill methane is a significant share of human methane emissions, and capture is therefore a high-return climate measure. Collection systems recover a proportion that depends heavily on how well the site is capped and how early the wells are installed, with estimates ranging from around half to over eighty percent in the best-run facilities. Captured gas is either flared, which converts methane to the far less potent carbon dioxide, or used, either burned in engines to generate electricity or cleaned to pipeline quality and injected into the gas network. Uncaptured gas is a hazard as well as an emission, since methane is explosive in air between roughly five and fifteen percent, and there have been fatal explosions where gas migrated laterally into buildings adjacent to old sites. Satellite monitoring has recently changed the picture considerably, with instruments detecting large plumes from individual landfills worldwide and showing that reported emissions from many sites have been substantially understated.

Leachate is the other output and it is nasty. Rainwater and moisture from the waste percolate down, dissolving whatever they pass through, and the result contains organic compounds, ammonia, heavy metals, chloride and, increasingly a concern, per- and polyfluoroalkyl substances from consumer products, which are extremely persistent and poorly removed by conventional treatment. Old unlined sites leached directly into groundwater, and the legacy of that is substantial: a great many closed landfills predate liner requirements and are monitored, remediated or simply left. Modern sites collect and treat leachate, either on site or by tankering it to a wastewater works, and the obligation continues for decades after closure, which raises the awkward question of who pays when the operator no longer exists, addressed in most regulatory systems by requiring financial provision up front.

The direction of travel in most developed countries is away from landfill entirely, driven by a hierarchy that ranks prevention first, then reuse, recycling, energy recovery and disposal last. European rules have progressively restricted the landfilling of untreated biodegradable waste, and several countries now send almost nothing to landfill, sending it instead to incineration with energy recovery, which removes the methane problem and creates arguments about emissions and about whether guaranteed feedstock contracts undermine recycling. Landfill taxes have been effective at changing behaviour, since the cost is directly avoidable by diverting material. Completed sites are capped and reused as parks, golf courses and, increasingly, solar installations, which suit them because the land cannot be built on and has grid connections from the gas engines. Landfill mining, extracting buried material for recycling or fuel, is periodically proposed and rarely economic. The largest remaining problem is elsewhere, since open dumping without liners, gas capture or leachate control is still the normal method of disposal for a substantial share of the world's population.

A modern landfill is a sealed engineered container with a clay and plastic liner, leachate drainage, daily cover and gas wells, and its own protective design slows decomposition by keeping moisture out, which is why excavated waste is found largely intact after decades. Buried organic material decays anaerobically and produces landfill gas that is roughly half methane, a potent greenhouse gas that is captured, flared or burned for electricity with recovery rates that satellite monitoring suggests are often overstated. Policy across most developed countries now diverts waste away from landfill entirely.