A modern landfill is really a giant, slow-cooking microbial reactor
Rubbish dumped in a sanitary landfill does not just sit there. Engineers design these sites as bioreactors where microbes spend decades dismantling food scraps, paper and garden waste, first with oxygen and then without it. The process runs through five phases, turns the trapped liquid acidic and back again, and exhales roughly equal parts methane and carbon dioxide.
Piling or pitting waste is the oldest way to get rid of it, and archaeologists call such heaps middens. Burying it systematically, with a fresh cover every day, only began in the 1940s, and engineered sanitary landfills spread in the 1960s and 1970s to replace open dumps. On a working site, trucks are weighed on arrival, unload at a tipping face, and bulldozers or compactors squash the load. Each evening it is buried under soil, wood chips, sprayed foam or even a removable blanket. Tight compaction stretches the life of the site.
Underground, the chemistry unfolds in stages. Fresh waste holds plenty of air, so oxygen-loving microbes feed first. Once the oxygen is spent, other bacteria take over, breaking material into volatile fatty acids that drag the pH of the liquid from about 7.5 down to 5.6, which can loosen metals into it. Then methane-producing microbes consume those acids, the liquid returns to neutral, and gas pours out. That methane stage lasts longest. Finally, as nutrients such as phosphorus run short, activity fades and air creeps back down from above.
The hazards come from both the liquid and the gas. Rain seeping through the waste becomes leachate loaded with heavy metals and organic pollutants, and ammonium can reach several thousand milligrams per litre because the airless conditions block the usual soil route for removing nitrogen. Modern sites sit on thick impermeable liners with collection pipes, but liners last perhaps several hundred years, so surrounding ground must be tested.
Landfills rank as the third largest source of methane worldwide, a gas that over 20 years traps around 82 times as much heat as carbon dioxide. Well-run sites capture it, either burning it off in flares or generating electricity; more than 850 American landfills recover their gas. Daily cover also keeps rats and flies away, and a full site can eventually be sealed and reclaimed for other uses.
Source: Landfill
