Waste management is today one of the greatest headaches for burgeoning metropolitan areas. An economy and lifestyle that requires excessive material turnover and wastage has led to mountains of waste which is a potential threat to the environment. Where recycling is not practised, landfill is used. What are the techniques and problems related to dumping millions of tonnes of mixed waste into the ground?
Burying waste has been practised for millenia. The modern method involves a lined (plastic or clay) pit, where solid waste is compacted and eventually covered by earth.
When organic material decomposes, it can create two problems: leachate and methane gas. Leachate is a iquid which accumulates at the bottom of the landfill, and can be siphoned out for special treatment and separate disposal.
Landfill gas is mostly methane, and rises through the waste mound, or may enter escape paths in the ground system around the landfill site. In badly managed landfill sites this has led to explosions. Landfill gas can be allowed to escape into the air at non-dangerous concentrations, or, ideally, captured and used as a fuel.
Landfill mining, a sector of the more general "urban mining", refers to the practice of excavating waste deposits for materials which may be recycled.
Urban mining, or landfill mining, is a field which is dependent on the economic value of materials. It has traditionally been cheaper to import raw material from mines than to recover it from waste in a landfill. The first time a waste landfill was excavated in Europe was in 1990 (Spitzau Landfill, Vienna). Germany conducted a pilot demonstration project at Burghof/Horrheim Deponie, in 1993-4. Since then, several thousand tonnes of waste have been excavated from German landfills. The main reason has been for environmental safety reasons, such as water table protection, or for reassignment of land use. In only 13% of the cases, was the operation specifically to recover materials for recycling.1
From an environmental perspective, it would seem logical to reutilise materials as much as possible, preserving the environment from the impacts of mining, and the depletion of resources. Examples of recovery of heavy metals from hazardous waste landfills are: nickel and vanadium from heavy oil, selenium from mercury-selenium slurry, tellurium from concentrates from non-ferrous industries, and copper from PCB-containing transformers. Copper is less present in landfills than other metals, such as iron, but has a much higher market value. Overall, copper provides twice the revenue as iron in landfill mining operations.