SO YOU diligently separate your cardboard from your glass, rinse your tins and tear the staples off paper. You fill your various bins and put them out for the men in bright overalls to take away along with the remains of this week's meals and domestic debris. And then, safe in the knowledge that you've done your bit for the environment, you forget all about it.

In fact, the life story of your weekly garbage is just beginning. Your aluminium can, for example, could have a variety of fates. It might be crushed and sent back to the canning factory to be turned into new cans. Or it could end up in the nearest landfill, or get shipped to distant shores to be either recycled or dumped. The truth of the matter is, no one can be sure where an individual piece of trash will end up or how the junk in a landfill got there.

New Scientist and researchers at Massachusetts Institute of Technology have partnered to help pave the way to an ideal world of waste desposal, and its fascinating. In a pilot project in July and August, the MIT team tracked 60 pieces of trash as they made their way around Seattle. As the first results roll in (see "The Seattle experiment" and map), the next phase of the experiment kicks off. Next month, 1000 more pieces of garbage are being tagged and thrown away in New York, Seattle and London and tracked for two months. New Scientist is offering 10 readers the chance to join in and find out where their garbage goes to die (see competition details). "With the Trash Track project, the invisible waste system comes to life," says Assaf Biderman, associate director of MIT's SENSEable City Lab.

The experiment is more than just an attempt to satisfy our curiosity about where trash ends up. The idea is to help pave the way for an ideal world of waste disposal, where nearly everything gets recycled or reused and materials are sent to landfill no faster than the planet is able to produce them.

Right now, that green utopia is a distant dream. Part of the problem is that we don't know what we are dealing with. While a lot of effort has gone into creating green supply chains to bring products to consumers, almost nothing is known about what goes on in the "removal chain". Waste is monitored of course, but only to see how many tonnes of different kinds of garbage arrive at a sorting centre, landfill or incinerator, and how many leave. These are tallied up under vague categories such as electronic or household waste. "It's fairly well measured in terms of mass," says. Valerie Thomas, a sustainability expert at the Georgia Institute of Technology in Atlanta and a member of the US Environmental Protection Agency's science advisory board. "It is not well measured in terms of content."

And in terms of environmental impact, it is the content that matters. Within the harmless-sounding category of "household waste", for example, lurks everything from carrot peelings to used diapers to mercury-containing low-energy light bulbs, each of which throws up a very different set of environmental challenges. In our ideal world, each should be dealt with separately.

Before that can happen, though, we need to get a clearer picture of the life cycle of different kinds of waste, and that's where the "trash trackers" come in. The team, made up of engineers, designers and life cycle assessment researchers at MIT's SENSEable City Lab, have designed tags that can be fixed to all kinds of rubbish. These tags beam out their location every 15 minutes for up to two months. Each is built around a cellphone SIM card and battery, and a motion sensor (see diagram). A low-power microprocessor keeps track of the motion sensor and, when the sensor registers movement it switches on the SIM card, triggering a search for nearby cellphone towers. The SIM then sends an SMS containing this information to MIT, where custom software compares it with standard maps of signal strength fingerprints to determine the position of the tag.

The tags aren't precise to the metre, explains Lewis Girod, who designed them. In cities, where there is a dense network of cellphone towers, the team can locate an object to within 100 to 500 metres. In rural zones, that goes up to 1 kilometre or more. "We infer the object's location from correlations and rough final locations," Girod says. "For example, during a first test in Massachusetts, we inferred a tag that was last seen in Lawrence went to an incinerator in Haverhill, which is nearby."

To read more of this story visit New Scientist on the web.

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