A team from the Department of Engineering at the University of Cambridge, UK, describes a way to gain economic and environmental value from rubbish tips.
For the last half century landfill has been regarded as a cheap and easy way for the UK to dispose of unwanted materials. However, landfill sites for municipal solid waste (MSW) are reaching, or have already reached, capacity, and there are concerns about their safety and environmental desirability. Increased recycling and material re-use raises the possibility of gaining value from discarded materials.
Landfill mining, the practice of excavating the contents of sites consisting of MSW and waste from construction and industrial sources, is starting to be explored in the UK, but there is precedent in other parts of the world. The first recorded example is in Tel Aviv, Israel, in 1953. No further instances of landfill mining are recorded until the late 1980s when a number of pilot schemes began in the USA, and one in India. Projects in Germany, Sardinia and Sweden began in the early 1990s. Over recent years schemes have been initiated in many other parts of the world.
Standard open-cast mining techniques are commonly used for excavating landfill sites. The materials are then separated for processing, using equipment and techniques already used for MSW. Typically, the material will pass through a series of trommel screens to filter out coarse materials (which cannot be processed, such as appliances and fabrics) and fine material, like soil, leaving the remainder for sorting. A magnetic separator collects the ferrous scrap, and other waste is separated using tools such as an air classifier.
It’s a dirty job
There are several possible motivations for mining landfill sites, applying in varying proportions in different cases, and caution is needed in generalising from one situation, or country, to another.
• Safety concerns – old sites may have to be reclaimed because they are unsafe. The most common hazard is toxic materials that leach into groundwater. Before the 1970s, UK landfill sites used only a single liner between the pit and the ground and so are vulnerable to damage.
• Value of the recyclable material – with the rising costs of energy and many raw materials, old landfill sites may have the same economic advantages as other mining opportunities. It has been thought that the density of valuable materials in landfill sites is sometimes as high as the levels of some metal ores which are mined commercially.
The resource that might be economically recovered from MSW is variable. The composition of landfills varies not only with location and age, but can be heterogeneous even within a single cell. At one extreme all MSW is sent to landfill, typified by urban communities in developed Western countries such as the USA and UK. This leads to landfill with a high proportion of potentially valuable materials, such as metals, where a level of 17% was recorded in Edinburg, USA.
At the other end of the spectrum are countries where scavenging is a part of life. Not only has the waste sent to landfill already been stripped of anything with an immediate resale value, but sites are scavenged for combustible or recyclable resources. Organic matter can be used either for energy recovery (usually via pyrolysis or gasification), or to make compost by anaerobic decomposition. Plastics in older rubbish dumps are generally regarded as too contaminated to be easily separable, but in all cases the energy content is available for reclamation.
However, in the UK this is impeded by legislation.
• Landfills have become valuable real estate in other cases. This has been a strong incentive in The Netherlands, where waste sites are close to urban developments and the land can be valuable for housing, recreational purposes or industrial development. Rather than relocating the excavated waste, the volume is reduced by removing anything that can be recycled.
• The wider green agenda – a significant motivation in the UK is the lack of capacity and difficulty in identifying new sites. The Environment Agency estimates that landfills in Bedfordshire and Essex have only between one and three years of remaining capacity. These concerns are echoed across parts of Europe, leading, for example, to MSW from Italy being transported to Germany for processing. Landfill mining and re-processing alleviates the problem by reducing the volume of material. Another factor is that sites are generally unstable for 30 years after closure. This leads to costs associated with inspection, maintenance and, in some cases, leachate treatment, offset by revenue from methane generation. However, mining of unstable sites is hazardous because of gas and corrosive leachates, so only stable sites are thought to be suitable.
Whatever the particular local motivation, landfill mining is like any other mining process – finite. Once existing sites have been mined, the opportunity (or need) will have passed. Future sites are expected to contain few materials of value for recycling or energy generation.
The economics of landfill mining are improving, and despite inevitable local opposition, it is on the agenda. The number of sites suitable for mining, however, is quite small. Common reasons for exclusion are their position (proximity to housing), or if they have already been used for recreational purposes or development. Perhaps the most promising areas are those currently in use. Rather than closing them as they reach capacity, accelerated decomposition methods might be used to allow them to be mined and kept open.
Recycling is increasing, so the amount of waste going to landfill is falling, but desposals remain the obvious short-term option. Long-term, increasing prices for raw materials will force the development of safe re-processing.