Temps de lecture :4 minutes
Total recycling’ aims to make landfill history
BY 2011 all the broken kettles, potato peelings, smashed glass, holey socks, margarine pots, dirty tissues, light bulbs and pet litter thrown away by the 1.4 million people who live in Lancashire, UK, will be whisked away to two recycling plants near Preston.
While most waste-treatment plants consider this mess too wet, sticky and smelly to recycle, these new plants plan to reclaim between 25 and 80 per cent of the paper, glass, plastics, steel, aluminium and lead that misses the recycling bin. They will also make use of the organic-rich “goo” from food, gardening, wood and paper, which makes up about 50 per cent of household rubbish. As a result, just 30 per cent – and ultimately, as little as 15 per cent – of all household waste will become landfill, compared to around 50 per cent under conventional recycling.
The two Lancashire plants are in the early stages of preparation and will be run by the Australian company Global Renewables (GRL), whose first plant in Sydney was up and running in 2004. Companies like GRL are fast becoming the commercial face of a trend towards “zero waste” – a future in which every last gram of waste is reused and landfill is a thing of the past.
It is not a new idea. S0 called mechanical biological treatment plants (MBTs) aim to “mine” all waste. But they have struggled to convert the organic-rich mush into a marketable product. The problem with composting it, a seemingly obvious solution, has been contamination. “You get a really ugly compost – full of broken glass, plastic film and chemicals. It can be toxic and it’s difficult to market. There have been some spectacular failures,” says Jan Allen of engineering company CH2M Hill in Bellevue, Washington state.
Now, thanks to its patented process for decontaminating and separating waste, the Lancashire plants will transform this goo into high-grade compost. What’s more, burning the methane produced by the bacteria that feed on this waste will fuel the plant and return electricity to the national grid. “Third-generation MBTs like GRL’s are going to be essential for achieving zero waste,” says Matthew Warnken of Crucible Carbon in Sydney, Australia. “They don’t alleviate the need for minimising waste but they are essential for dealing with what is left over.”
Modern MBTs are driven by the realisation that landfill is little better than leaving garbage to rot in the street. Landfill can leak into waterways, for example, and the clean-up costs are huge when land is needed for something else. Landfill also produces methane, a greenhouse gas more than 20 times as potent as carbon dioxide. While modern landfills burn this off – producing CO2 – or use methane as biofuel, most are leaky, inefficient and destined to burp methane for years. Total recycling would also be a chance to reduce the greenhouse gas associated with extracting a material from the earth or making it from scratch – as well as recovering industrially important metals, which are abundant in household waste (New Scientist, 23 May, p 34).
To mine the waste for metals, plastics and paper, GRL’s first step is to pull out the solid components and stream them according to size, shape and density. This is done using spinning cylinders with different-sized slots on their curvy side that “sieve” the waste, plus vacuums of varying strengths known as wind sifters, and magnetic fields, which can be used to magnetise and then remove aluminium. To separate plastics further, strategically placed light sensors pick up the different frequencies of light reflected by a mixture of materials on a conveyor belt. A computer then calculates which material is where and airjets blow different materials off into bins.
Such advances have transformed even ordinary recycling plants. But GRL can also recycle the remaining organic fraction – partly because the organic waste starts out cleaner due to the high-tech separation process, and partly because of a decidedly low-tech procedure. As waste enters the plant, workers wearing protective gear sift through it by hand to remove most of the 3 per cent of waste deemed toxic, such as kidney dialysis tubing, paints, gas cylinders, asbestos, computers and car batteries. The remaining organic-rich matter is then piped to a percolator, which washes and aerates it, removing specks of glass, metal and plastics and dissolving some carbon.
The carbon-rich liquid is then fed to a digester, where anaerobic bacteria break it down to produce methane. In Lancashire, the methane will be used to generate 25,000 megawatt hours of electricity each year, which will run the plant, with any excess going to the national grid. The solid residue, meanwhile, is composted for a couple of months. Initially, GRL plans to use it to plant 100,000 trees a year to rehabilitate old industrial land. To go mainstream though, the compost will need to be sold widely, which could be hard.
“It’s not clear that you can generate any market value from these compost-like materials,” says Liz Goodwin, CEO of the Waste and Resource Action Programme, UK. Under existing UK law, waste-derived compost can’t be used on agricultural land, but by doing studies GRL hopes to convince the Department of Environment, Food and Rural Affairs that their compost is safe. There are also other solutions springing up to deal with the sludge, such as using it to make a soil improver that acts as a carbon sink (see “Carbon sinks without a trace“).
Even with extensive mining of waste, however, some bits will still be difficult to recycle. So putting pressure on manufacturers to take back used products and design them so they are inherently less wasteful will also be important. “There’s endless product design research on size, shape, colour, but virtually none on end-of-life impacts, and how to detoxify the waste stream,” says Warnken
© New Scientist, Reed Business Information
Total recycling’ aims to make landfill history Rachel NOWAK The New Scientist 20 octobre 2007 www.newscientist.com