New kinds of plant-based plastic are beginning to permeate our lives. Supermarkets stock biodegradable bags to carry our groceries home, cafes serve coffee in compostable cups, and this year London Marathon runners could refuel with seaweed-based edible bubbles filled with sports drink. But if we just replace one kind of throwaway packaging with another, are we really solving the bigger problem?

There’s no denying that we have a plastic crisis on our hands. A 2017 study found that, of the 8.3bn tonnes of plastic produced since 1950, 6.3bn tonnes has been thrown away. And whether it ends up in landfill or in the sea, that plastic – thanks to the stability and durability that it is prized for – is not going anywhere fast.

Next year, a ban on single-use plastics including plastic straws, drink stirrers and cotton buds will come into effect in the UK. Canada recently announced it will ban “harmful” single-use plastics by 2021, becoming the latest in a long list of countries and states to limit the use of disposable plastics. This year Glastonbury banned the sale of single-use plastic bottles on site by replacing them with aluminium cans and offering water refills to festivalgoers.

Biodegradable carrier bags will still hold shopping after being buried in soil or left in the sea for three years

But ditching single-use plastics by replacing them with biodegradable or compostable versions could be causing more problems than it’s fixing. “What worries me is that this isn’t really a solution, it’s just swapping one polymer for another,” says Mark Miodownik, professor of materials and society at University College London. ”If they end up in the environment, then it really does depend on the conditions that they find themselves in as to whether they biodegrade in any reasonable time.”

“If you’re putting your nappies and your wipes in [your home compost bin], you’re going to be disappointed.”

Deciphering the many labels applied to these plastics is no easy task. Many products described as as “compostable” actually only break down in industrial composters that maintain high temperatures for weeks at a time. Some are certified for home composting, but how long that takes will depend on the conditions of your individual compost heap. Plastics that are labelled “biodegradable” typically can’t be composted, but can be broken down by microorganisms – though the exact method and timescale for this process varies. A recent study found that biodegradable carrier bags could still hold shopping after having been buried in soil or left in the sea for three years.

Biodegradable plastics can be made from fossil fuels like traditional plastics, but in recent years the focus has been on those made from plants or other renewable resources, collectively called bioplastics. Not all bioplastics are biodegradable – Coca-Cola’s PlantBottle is partially made from plants but otherwise acts like a typical PET (polyethylene terephthalate ie regular plastic) bottle. Blends of plant-based starches account for nearly half of bioplastics, but other starting materials include fungi, milk and even leftover lobster shells.

While making plastics out of renewable materials is undoubtedly a step in the right direction, it won’t stop plastic piling up in landfill sites and in our oceans. The problem, says Miodownik, is that there’s no such thing as a sustainable material – only a sustainable system. And in the UK we don’t have a sustainable system to deal with the increasing number of compostable plastics finding their way into our daily lives.

Facebook Twitter Pinterest In it for the long haul: a plastic fishing net off Hawaii being used as a habitat by goose barnacles and fish. Photograph: Bryce Groark/Alamy

Take the compostable coffee cup made by companies like Edinburgh-based Vegware. They are lined with a bioplastic called polylactic acid – or PLA – made from corn starch. Their lids are made with CPLA, crystallised polylactic acid, designed to withstand high heat. And they are certified with the Seedling logo, showing they meet a European standard for compostability known as EN 13432. This means the packaging will break down within 12 weeks to water, CO2 and biomass, leaving no more than 10% of the original material in pieces bigger than 2mm – but only under industrial-scale composting conditions.

PLA makes up around a quarter of all compostable plastics sold worldwide, and is used for transparent cold drinks cups, clear windows on food packaging like sandwich boxes, drinking straws and more. A home compost heap won’t reach the high temperatures needed to break it down. You might be tempted to put it in the recycling but there, PLA is often indistinguishable from traditional PET plastic bottles and can contaminate the recycling process. Council waste collections vary across the UK but few if any accept PLA in anything other than general waste. Most people will be left with only one option: the bin.

Vegware argues that when they are sent to landfill, its products are still the better choice because they’re made from plants and use lower carbon, recyclable or renewable sources. “These sustainability benefits still apply no matter what happens to them after use,” says Lucy Frankel, the company’s communications director.

With food waste, the bag may be separated and sent to landfill or incinerated before your leftovers are composted

But if it ends up in the ocean, PLA is bad news. “PLA is basically nondegradable in sea water,” says Dr Frederik Wurm of the Max Planck Institute for Polymer Research in Mainz, Germany, which means if it goes into the sea, “it’s not better than any other plastic”.

In fact, Vegware’s website points out that compostables are not a solution to marine plastic pollution – but in the rush to replace plastic with more environmentally friendly packaging, it’s a caveat that seems to have got lost.

Plastics that are certified for home composting fare better. Those include the potato-based wrapping for the Saturday Guardian, and a starch-based material called Mater-Bi made by bioplastics company Novamont, which the Co-op uses for its compostable carrier bags.

For people who don’t have a compost bin, some councils that collect food waste will accept compostable bags. But when food waste is processed, the bag may be separated out and sent to landfill or incinerated before your leftovers are composted. That’s because, as things stand, compostable plastics can’t just be sent to any industrial composter. “In-vessel” composters are best suited to breaking down these plastics – but lots of food waste is sent to anaerobic digesters, most of which can’t process compostable plastics at all.

If they are discarded in the environment, research has shown that compostable bags can survive for two years in soil. In sea water they could disappear in as little as three months, although exactly what products they release into the marine environment is not yet clear.

There are some new materials that do claim to totally break down in the environment. Pods made from seaweed by London-based Skipping Rocks Lab are not just biodegradable or compostable, but edible. As well as using the bubbles to deliver hits of Lucozade Sport to marathon runners this year, the company has been trialling edible sauce sachets with takeaways from 10 restaurants in a partnership with Just Eat. If you choose not to eat the packaging after you’re done with the sauce inside, the company says it breaks down in six weeks without needing any special treatment.

“There probably are niche applications for those where it’s the right thing to do,” says Miodownik, but he doesn’t see the edible pods as a big part of the plastics solution. A cofounder of the company has said they are focusing on sports events, festivals and takeaways.

In proposals published earlier this year, Defra said that it was aware of the trend for compostable packaging but that “appropriate treatment infrastructure” needed to exist before it could add compostables to a core list of materials that every council must collect for recycling.

Facebook Twitter Pinterest Good enough to eat: packaging developed by Skipping Rocks Lab using seaweed. Photograph: notpla.com

Frankel says Vegware would like to see compostables added to that list. In the meantime, it is working with the waste sector to set up composting collections for its clients, and says its products are now accepted by 27 industrial composting facilities around the UK covering 38% of postcodes. This summer, it’s also starting to offer collection by courier costing £10 for small cafes and individuals using Vegware at parties, in partnership with recycling company First Mile.

To make compostable collections available for everyone, though, as well as making in-vessel composters that can break down these plastics more widespread, helping the machines that sort our rubbish identify what is compostable plastic and what is not will be key. “We’re looking into the possibility of putting some sort of marker into biodegradable plastic that you could then have a detector for,” says Miodownik.

We could all carry our own coffee cups and cutlery – and take containers to stock up on pasta and cereal at supermarkets

In the short term, it would also be helpful to make the labels we use for plastics clearer. Wurm says that the label “biodegradable” should come with a time limit and information on what environment the plastic will break down in, to help bust the myth that biodegradable littering is no big deal.

He’d also like to see manufacturers think about the life cycle of their products before they decide what to make them from – including how long they’re likely to be used for, and what would happen if they were discarded. If you think it’s likely your drinking straws will end up in the sea, for example, you should not make them out of PLA. “It’s something that in many cases is not considered,” says Wurm.

A more radical solution would be for companies to rethink products so that they don’t require plastic packaging in the first place. “What most companies are doing is saying, we don’t want to redesign our system, we’re going to swap out one material for another, and then it’s someone else’s problem to deal with this down the line,” says Miodownik. “And that’s a disaster.”

As consumers, we could go all-in on reusables, carrying around our own coffee cups and cutlery, and even taking containers to stock up on pasta and cereal at the supermarket. But there are some situations where demand for throwaway packaging will remain. “Even with a major shift to reusable containers, some disposables will always be needed, especially for serving food,” says Frankel.

In that case, unless your packaging is part of a closed loop system with a dedicated compostables collection – as exists at the Houses of Parliament, for example, which switched to compostable packaging last year – recyclable bioplastics could be the more sustainable choice.

“For millions of people in their everyday lives, the best thing is to have plastics that don’t biodegrade, that have long lives – that’s the whole brilliant point of them – and that get collected and recycled back into new things,” says Miodownik.

Until compostables are widely collected, encouraging companies to make more packaging recyclable, as well as reducing and reusing where you can, is the best way to make an impact on the future of plastics, he says: “Every time you choose not to buy something because the thing it’s wrapped in is not recyclable, you’ve made a difference.”