Throwaway plastic has found its way into almost every aspect of our lives: from the disposable coffee cup you pick up on the way to work or the straw in your smoothie, to the hidden fibres woven into wet wipes and tiny glittering fragments in make-up.
Of the 6.3 billion tonnes of plastic we’ve thrown away since we started mass-producing it in the 1950s, just 600 million tonnes has been recycled – and 4.9 billion tonnes has been sent to landfill or left in the natural environment.
While awareness of the detrimental impact plastic can have on the environment has exploded in recent years, environmentally friendly alternatives are only now picking up steam. As single-use plastics bans come in around the world – next year in the UK, and by 2021 in Canada – new materials are going to become ever more important. But are they all they’re cracked up to be?
Biodegradable plastics are one set of materials that are becoming a popular replacement as consumers demand green alternatives. Rather than remaining stable for hundreds of years – the quality for which we prized plastic when we first began using it – biodegradable plastics can be broken down by microbes, chewed up and turned into biomass, water and carbon dioxide (or in the absence of oxygen, methane rather than CO2).
A subset of them are compostable, which means that not only are they broken down by microbes, but they can be turned – alongside food and other organic waste – into compost. Only a minority of these plastics are home compostable, so, the label “compostable” most often means industrially compostable. That coffee cup with a Seedling logo you’re drinking from won’t decompose very quickly, if at all, on your home compost heap, but will break down inside the right kind of industrial equipment.
There’s a European standard for compostable packaging: EN 13432. It requires that the packaging break down under industrial-scale composting conditions within 12 weeks, leaving no more than 10% of the original material in pieces bigger than 2mm, and doing no harm to the soil itself through heavy metals or worsening its structure.
Most biodegradable and compostable plastics are bioplastics, made from plants rather than fossil fuels and depending on the application you need them for, there are plenty to choose from. Izabela Radecka, a professor of biotechnology at the University of Wolverhampton, and her colleagues are making a kind of bioplastic called polyhydroxyalkanoates (PHAs). Or rather, they’re getting microbes to do the production for them. “When they are under stress those microbes will produce granules inside the cells, and those granules are biopolymers,” she says. “When you extract them from the cell they present very good properties, similar to synthetic plastics, but they are fully biodegradable.”
She started off feeding waste cooking oil to the microbes to make PHAs, but in recent years has been investigating how waste plastics like polystyrene can be turned into new, biodegradable kinds of plastic. That’s preferable to using freshly grown crops as a source material, because it spares plants that could instead be used for food, at the same time as using up waste plastic.
At the moment, PHAs make up around 5% of biodegradable plastics worldwide. Around half of biodegradable plastics are starch-blends. Polylactic acid (PLA), typically used in compostable coffee cups and lids, makes up another quarter.But while most of these bioplastics require industrial composters to break them down after use, they are far from guaranteed to make it to one. Given humanity’s track record, it makes sense to ask what happens if they end up where they shouldn’t.
The problem with labels
To test how different kinds of plastic bag fare in different environments, Imogen Napper at the University of Plymouth collected carrier bags with various claims about biodegradability, and put them in three different natural environments over a period of three years: buried in soil, left in the sea, and hung up in the open air. She tested bags labelled as biodegradable, compostable, and oxo-biodegradable, as well as conventional high density polyethylene (HDPE) bags. (The European Commission has recently recommended a ban on oxo-biodegradable plastics, because of fears that they break down into microplastics.) In Napper’s experiment, the bag labelled “compostable” (which stated it adhered to standard EN 13432) disappeared entirely within three months when it was left in seawater. In soil it remained intact for two years, but disintegrated when the researchers loaded it with shopping. The rest of the bags – including the one labelled “biodegradable” – were still present in both soil and sea water after three years, and could even hold shopping. After nine months in the open air, all of the bags had disintegrated or were beginning to come apart, mostly breaking down into microplastics. That’s because sunlight helps break down plastics through a process called photo-oxidation, in which the plastic becomes weathered and brittle, eventually fragmenting rather than breaking down to its organic components. “That doesn't actually mean it's breaking down into its most natural counterparts of carbon and hydrogen, it just means they're becoming smaller pieces,” says Napper. “Which you could argue is more problematic because you can't clean up – it's like trying to pick up Smarties with chopsticks.”
Of course, even the compostable bag tested in Napper’s experiment is not designed to break down in the sea or in soil. But she says that the fact that these plastics have to be industrially composted is not adequately explained on the bags themselves, leaving consumers guessing about what the bags can and can’t do – and, crucially, what they should do with them once they’re finished. “People need to be aware that putting it in the recycling or trying to compost it, or putting in the general waste bin won't necessarily get them the results that they're being advertised,” says Napper. One company investigating how its own products break down in a marine environment is Novamont, producer of Mater-Bi – a starch-based plastic used in the compostable carrier bags launched by the Co-op this year. A report released by the company and conducted in partnership with Hydra, a German marine research institute, and the University of Siena, Italy, says that the product fully biodegrades in seawater on a timescale of between four months and a year, leaving no toxic residues.