With waste footwear an increasing problem in landfills around the world, E&T investigates how industry and academia are developing ways to recycle worn-out shoes instead.
Shoe shops around the world are doing a roaring trade. Sturdy leather boots, frivolous pink stilettos, ultra-trendy trainers and simple summer flip-flops are flying off the shelves, as people hop from fashion to fashion, from school to wedding breakfast to fitness regime to dinner party. So great is the demand for footwear that Selfridges has just opened the world's largest shoe department, offering 5,000 styles in an area of 35,000ft2. The major retailer clearly believes the current trend of global shoe sales doubling every 20 years will continue. We buy 20 billion pairs annually, and by 2013 the industry will be worth $230bn.
Great news for manufacturers and retailers; bad news for the planet. Around 95 per cent of the shoes we buy end up in landfill. Around the world, millions of tonnes of old and unwanted shoes are disposed of in this way every year, where the chemicals contained within them contaminate soils and groundwater. Meanwhile, rotting organic material, such as leather and wood, contributes to climate change by releasing methane, a potent greenhouse gas.
The Centre for Sustainable Manufacturing and Reuse/Recycling Technologies (SMART) at Loughborough University is hoping to change this untenable situation, following detailed research into how best to minimise 'end-of-life' waste in the footwear industry. In the past, centre staff studied the lifecycle of shoes, the materials contained within them and the potential for reuse. They concluded the best way to cut unnecessary waste is to embrace industrial-scale material recycling, and they set up their latest project to find ways to achieve this.
What do you do with shoes?
'We originally began working with the shoe industry ten years ago,' explains Shahin Rahimifard, Professor of Sustainable Engineering and director of SMART. 'European shoemakers wanted to compete better with Far East manufacturers and thought that producing more environmentally friendly shoes might provide a market advantage. However, their suggested approach was just to remove toxic chemicals and make shoes entirely biodegradable, so they would cause fewer problems as landfill.'
Centre staff soon discovered that this was far too simple a view. Not only does the volume of shoes make landfill unviable, so does the length of time they take to biodegrade. The world's oldest shoe, made of cowhide and discovered in Armenia earlier this year, survived being buried for 5,500 years.
The number and wide range of non-biodegradable materials used in shoes is also a problem. Modern shoes can be made of 40 different materials including leather, plastics, chemicals and metals. Some even contain electronic gadgets, such as LED lights and sensors aimed at monitoring the jogging performance of their wearer. Reusing shoes by sending them to developing world nations would appear to offer an alternative to landfill. However, although second-hand clothing industries provide livelihoods for people who wash, repair and redistribute goods, they can also undermine local textile production.
Another option is to consider shoes as an energy source. Some 60 per cent of shoes sold in the UK are primarily made of leather, which is highly calorific. Therefore, these shoes could potentially be burned to provide heat and electricity. At present, 2.8 million tonnes of municipal waste (9 per cent) and 100,000 tonnes of hazardous waste (3 per cent) are treated by incineration each year in the UK. In 2002, this process provided energy to 250,000 homes though 'combined heat and power' schemes. However, incineration causes harmful air emissions and is not well accepted by the pubic.
Gasification provides a cleaner alternative, as it converts any carbon-containing material into a combustible mix of carbon monoxide, hydrogen and methane. This can then be used as a fuel to generate heat and electricity. However, it is expensive and existing gasification units only accept raw materials from tanneries, rather than post-consumer shoe waste.
Professor Rahimifard and research associate Dr Mike Lee therefore concluded that material recycling would be the best solution for dealing with end-of-lifecycle shoe waste, and embarked on their latest project. They initially considered whether it would be possible to break down shoes into their component parts for re-use, but soon realised that it would be very difficult and not financially feasible to do so because of the sheer number of materials used. They realised it would be more realistic to grind down whole or parts of shoes and then use using material density differences to separate out the constituent materials once physically broken down. 'We believe that having some form of labelling would make this process easier,' explains Dr Lee.
By looking at existing equipment used in recycling other products, including cars, Lee built several prototype machines aimed at breaking down shoes into small particles of different materials. In a laboratory within the Wolfson School of Mechanical and Manufacturing Engineering at Loughborough University, he explains the process.
First, metal parts are manually removed, as these can break the machinery. He points out a jumble of large shoe pieces. Several have strips of metal sticking out from the sole, designed to give the shoe strength. Others have metal eyelets glued into fabric trims, and some have metal nails holding the sole to the heel.
Once the metal content has been removed, the remaining shoe parts are fed via a hopper into a granulator. This has a row of sharp metal teeth, which tear the material into small pieces. A filter ensures only parts smaller than a few millimetres across collect at the bottom. 'The manual removal of metals is labour-intensive and will add significant cost to the recycling operation, so we are exploring methods of automating the process using metallic separation machinery,' says Dr Lee.
Particles from the granulator primarily comprise small chunks of rubber, foam and leather, together with grey textile fluff. Dr Lee feeds this mix into a cascade air separator. Air blows into the bottom of a glass-fronted box, inside of which are angled dividers. This machine separates out particles according to their density and surface area; the dividers form obstacles that light particles blow over and heavier ones get trapped on.
It primarily separates the heavier particles of rubber, foam and leather from the fluff. The former fall to the bottom while the latter is sucked out of the machine. The larger particles are then fed onto a vibrating table, which further divides them according to gravity and the density of the particles. If trainers have been put through the process, for example, the vibrating table sorts the constituent rubber from the foam. 'Given correct labelling so that materials can be easily identified, the process could generate much purer end products, such as thermoplastic rubber granules,' says Dr Lee.
Assorted plastic bags of rubber, foam and fluff from Dr Lee's various machine experiments are lined up on a nearby bench. He explains that, once separated, the various materials may be suitable for uses within the construction industry. For example, the fluff could be used for insulation and sound-proofing, while rubber and foam might prove useful as springy flooring underlay.
EU landfill directive
In general, shoe manufacturers have been slow to adopt environmentally friendly practices. However, the introduction of the EU Landfill Directive will drive change in the industry. With landfill space becoming increasingly limited in Europe, this directive seeks to divert waste from landfills by promoting recycling. The landfill restrictions introduced by Article 5 dictate the amounts of biodegradable waste that can go into landfill, and bans burial for particular types of waste. The UK Landfill Allowances and Trading Scheme Regulations denote the percentages of certain waste types that are considered as biodegradable municipal waste, with footwear considered 50 per cent biodegradable.
One of the directive's targets for 2020 is to reduce the amount of biodegradable municipal waste sent to landfill to 35 per cent of the volume sent in 1995. This means that biodegradable materials frequently used by the shoe industry, such as leather and natural rubbers, will soon have to be recycled or reused instead of being buried at landfill sites.
At the Centre for SMART, Professor Rahimifard and Dr Lee have set up a Shoe Recycling Working Group to begin engaging industry manufacturers and retailers in the concept of recycling. Partners involved with the recent research into minimising end-of-life shoe waste include Nike, C and J Clark International, Pentland Group, Tesco, Next Retail, Terra Plana, Textile Recycling Association, The European Recycling Company, LMB and New Balance, suggesting that the shoe industry is keen to become more environmentally friendly. The plan is to charge a fee to members and use the money to conduct more research and scale up Dr Lee's recycling prototypes to industrial-scale recycling facilities. 'We will need better regulations, governmental input and investment if we are to achieve zero-waste to landfill in the coming decades,' says Professor Rahimifard.
He believes that the first stage will be to develop a labelling system similar to that used to aid the separation of plastics for recycling. This could include placing embedded information devices, such as barcodes, into shoes so they can be automatically sorted at the end of their life. Scaling up the recycling equipment to industrial scale will require initial investment, but if a certain level of throughput can be achieved, the costs could become much lower. However, a good collection system would need to be in place to ensure sufficient feedstock. Then there are design aspects to consider; if shoes can be made with fewer materials, recycling can be simplified and the outputs made more homogeneous. Taking these steps should enable the shoe industry to trim its environmental footprint to level that can be sustained.