How long does it take for shopping bags to break down?

Friday, 24 November 2017

How long does it take for a plastic bag to break down - and what harm can it do?

Grab a bag, take home your shopping, chuck it in the bin. End of story? Not quite.

The humble plastic shopping bag might only get an average of 12 minutes of use, but the majority of them will hang around for over a century.

Victoria University lecturer Joanne Harvey recently conducted a review of scientific studies into how long the different materials your shopping carriers can be made of take to get rid of.

How long will it take for your carried bags to die?

She found that most bags you picked up in New Zealand shops would eventually break down. But some could become even more of a danger to the planet as they did so.

PLASTIC BAGS

Harvey said that, under optimal conditions, almost all types of plastic would eventually break down fully. However this required the right combination of sunlight, microbes, or composting.

Most of the conventional plastic bags we use in New Zealand would take over a century to vanish into nothing, and even then, it's not always simple.

'In reality, plastics are disposed of in many different ways, and usually they don't experience the correct conditions for full break down,' she said.

There are two types of break down: full break down, where the plastic is able to reenter the biological cycle, and fragmentation, which can still present a number of issues.

'Plastic that makes its way into the ocean can harm sea life and litter beaches, both as the whole bag and after fragmentation into smaller pieces.'

Plastics which are sent to landfill, meanwhile, may not experience enough sunlight or oxygen to start the break down process, and once buried can absorb other harmful toxins, leaching them over a long period into surrounding soil and waterways.

Massey University senior lecturer Trisia Farrelly said plastic actually became far more environmentally damaging when broken down into little pieces.

'This only makes it much easier to carry and leach toxins and be eaten by fish, and pollute soil and groundwater,' she said.

'Only a tiny percentage of bags are both biodegradable and environmentally benign.'

Victoria University lecturer Joanne Harvey said most plastics could break down to the point of reentering the biological cycle, but only under the right conditions.

BIODEGRADABLE/COMPOSTABLE BAGS

Compostable and biodegradable bags are rarely used by big retailers in New Zealand.

Escaped plastic shopping bags littering the ground at Christchurch's Bottle Lake landfill.

However, if you do get one, it is estimated to break down within two years, or in roughly half a year in compost or at sea.

Farrelly, however, warned that some 'biodegradable' bags still had 'toxic additives' in them.

'There are a growing number of bags that are degradable but terrible for the environment once 'degraded' into smaller pieces,' she said.

'When they biodegrade, they play havoc in various ecosystems when these toxins leach out of them.'

COTTON BAGS

There are two types of break down: full break down, and fragmentation. Fragmentation causes problems.

Cotton bags are composed mostly of cellulose, a natural polymer that is a major structural element of plants.

Harvey said these would break down more quickly than synthetics in soil because the chemical groups in cellulose were more readily digestible by bacteria and fungi.

'However, when we start to consider reusable bags, we should be considering factors other than degradability,' she warned.

'For example, cotton bags have a much greater global warming impact in their production than polyethylene bags because of the amount of carbon dioxide released.'

PAPER BAGS

Paper is made from natural fibres, and will be fully degraded within just two months.

However, according to a report by the Scottish Government, the process of making paper bags takes almost four times as much water compared to conventional plastic bags.