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Science Spin July 2009
Bugs could stop plastic going to waste
By Sean Duke
Plastics - particularly those made from petrochemicals, called PET plastics - are very useful for all kinds of purposes. They are, however, also a major contributor to waste landfills. That is why the research of Dr Kevin O'Connor, UCD is so exciting. He has found three bacteria that can convert used PET into a more valuable form of plastic.
Bugs can be engineered to turn plastic waste, such as that pictured here, into something valuable. [Credit: Kevin O'Connor, UCD]
As a society, we are very dependent on plastics. They are everywhere, in the cars we drive, in the seats we sit on, and wrapping the convenience foods we eat. These products facilitate our fast-moving, modern lives, but they come with problems too.
What do we do once we have used them? Well, typically, they end up in landfill.
The vast majority of products based on PET plastics - the oil-based plastics - will end up in landfill after they have been used. As much as 80 per cent end up going there, something that is not desirable as landfill is the least-favoured waste disposal option.
PRE-TREATMENT
The idea of Dr O'Connor was to use bacteria - bugs - to convert the PET plastics back into usable format again, and, thus, avoid landfill. The problem he confronted at the start of his EPA-funded project is that there was no known micro-organism that could directly tackle used PET plastics, and break them down.
Thus, he came up first with the idea of pre-treating the plastics so that they would be converted into new formats that bacteria might then be able to deal with.
The pre-treating process involved heating the plastic in the absence of oxygen. This converted the plastic into gas and a material known as terephthalic acid, or simply TA. The gas was immediately useful, and it could be siphoned off to produce energy.
Some Pseudomonas bacteria are known to digest TA, converting it into a biodegradable plastic known as PHA, polyhydroxyalkanoate. PHA is a plastic used in medical devices, and it would have a lot more applications, such as food packaging, if a way could be found to make it in larger quantities.
The prospect of converting PET waste into something more valuable was attractive, so Dr O'Connor searched the world for suitable strains of the plastic eating bug.
As nothing suitable was turning up, he turned his attention to bacteria living around PET facilities in Ireland, reasoning that there might be enough terephthalic acid around to sustain a population.
SCREENING
After screening about 400 different strains, Dr O'Connor hit lucky. He found three that could devour the heat treated PET. The end product, a thermo-plastic elastomer, is a very useful temperature-resistant, highly stretchable plastic. This was a major step forward.
The next step was to trick the bugs into making as much of this useful plastic in the bio-reactor as possible. "It's nice that the bug gets fat, but we'd like it to get even fatter," commented Dr O'Connor.
This particular research project ends in 2010 and at that stage Dr O'Connor and his team will hope, at that stage, to have identified one of the three bugs that is the most suitable to be brought forward into a larger-scale study.
BENEFITS
Dr O'Connor's work potentially has great benefits to Ireland, by removing PET plastics from the waste stream and reducing the pressure on landfills. The work is also quite unique, as nowhere in the world is anyone using bacteria to break PET plastics down into a bio-degradable plastic that can be easily constituted into new products.
Although bio-plastics are being produced, they are being made from sugar and from corn. "Nobody is actually trying to convert PET plastic bottles into biodegradable plastics," said Dr O'Connor.
The UCD researcher is also looking at whether other waste types can be dealt with by the three bugs, to see how far their abilities can be pushed.
