The university has switched up sorting by adding phosphors - luminescent materials that give strip lights their glow - to recyclable plastic packaging. But it has gone a step further, developing a set of invisible phosphor inks that can be coated on labels and detected by UV.
At the moment, plastics in the UK - the country goes through five million tonnes of plastic annually, recycling an estimated 5% - are sorted automatically on high speed conveyor belts. A beam of infrared radiation probes the plastic as it travels the sorting belt at three metres a second. The near-infrared spectrum which bounces back off each piece of plastic reveals a different fingerprint depending on its type of polymer.
The infrared is analysed by a spectrometer programmed to look for specific polymers such as PET.
Running four million chemical analyses a second, the beam pinpoints exactly where those pieces of plastic are, before jets of air eject them from the belt and into a collection bin.
"The existing system works extremely well, but has some big deficiencies," said the university's Dr Paul Harris.
"It is fine for transparent plastic but doesn't work for highly coloured plastic, which makes up a significant fraction of the waste stream. These are things such as shampoo bottles. Another problem is that near infrared spectroscopy can't tell what the container was used for, so it is impossible to separate pesticide containers from food containers, for instance."
The university's low cost technique - Plastic Recycling using Intelligent Separation Technologies for Materials (PRISM) - coats a transparent phosphor-loaded ink onto the product's existing label before use. Current sorting technology can detect the luminescence from the phosphors. And by using PRISM's coded mixtures of phosphors, it can identify both the plastic's material and what it was used for, even if it is heavily coloured.
Besides buy-in from manufacturers, all it needs is to add a UV source to existing near-infrared sorting systems which can be programmed to read the phosphor codes, allowing recyclers to retrofit PRISM at low cost.
Led by recycling consultants, Nextek and Brunel, PRISM is a European consortium and the first full-scale trials by member Tomra Sorting showed it collected 98% of labelled plastics with 95% accuracy.
"It will mean a larger proportion of waste will be identified and recycled, rather than going to landfill," Harris said.
"The system is working a treat and we have already had lots of international interest. The next phase is to get the costs down, but the process works, there's no doubt about it."
See the system in action here: