With a ban on single-use plastic bags looming Farah Hancock takes a look at environmental impact of some of the alternative ways to get groceries from the supermarket to the kitchen

There’s little doubt single-use plastic bags can have a devastating effect on marine life but alternatives can come with their own set of environmental issues. 

Reducing the 750 million single-use plastic New Zealanders are estimated to use a year may not have an environmental impact on its own, but experts think raised awareness about single-use plastic could drive a behaviour change and result in a society that uses less and makes re-use a common practice.

It’s estimated plastic makes up around 80 percent of litter in the oceans and graphic footage of animals with plastic-filled stomachs have horrified consumers.

Without change some say the amount of plastic in the sea will out-weigh fish by 2050. Microplastics, the small particles which plastic breaks down into, also pose a growing problem with possible toxicity effects yet to be understood.

Results from 69 coastal clean-ups organised by Sustainable Coastlines show plastic makes up a significant amount of the rubbish volunteers cleared from beaches.

More than 300,000 plastic items of “unknown origin” were collected along with almost 150,000 plastic bags.

A Ministry for the Environment consultation document points out plastic bags are a starting point for greater change.

“Single-use plastic shopping bags are a small subset of all the sources of marine plastics. These bags have been chosen as a starting point to engage the community as they touch every consumer and many practical and affordable alternatives exist.”


Bans and levies on single-use plastic bags in other countries have shown a drop in consumption.

In Ireland, consumers using on average more than 300 bags a year dropped to 18 a year after the introduction of a levy. In Wales, average use went down from 115 per year to 24.

In England the number dropped from 140 to 25 after a levy was introduced. The levy also prompted change beyond plastic bags and increased awareness of the environmental impact of household plastic waste. 

Massey University senior lecturer Dr Trisia Farrelly said there’s an opportunity for a ban in New Zealand to have a flow-on effect to our attitudes to disposable items.

A well-implemented ban could help people better understand the “plastics crisis” and start to actively reduce their consumption of other single-use plastics such as drink bottles.

“A ban needs to be complemented with sound, convenient and cost-effective alternatives and awareness-raising to ensure everyone is on board,” she said.

“We also need reminders to bring our own bag or refillable containers (and places to refill water bottles, and other containers) since we have been so ‘programmed’ to live a disposable lifestyle since the mass production of single-use plastics in the 1950s and 60s.”

Farrelly said we need to move away from traditional plastics and was excited by some of the “bio and eco-benign alternatives” being developed.

The future

University of Auckland senior lecturer Dr Viji Sarojini works with biopolymers. Unlike synthetic polymers which plastic are made of, biopolymers are made from proteins and are degradable.

She said the biopolymers she works with can even break down in a consumer’s home compost heap.

The difficulty, according to Sarojini, is moving biopolymer research out of a science lab and into commercial reality.

Since synthetic polymers have been understood, the production of traditional plastic has become so cost-efficient it has become disposable. Biopolymers face a battle to compete with decades of production.

“It’s the scale of the operation. Doing something at the research level versus doing something at a commercial level and cheap enough cost so people can actually use it in our everyday lives, in our homes. I think that will be the biggest challenge.”

The current alternatives

Are the alternatives currently available to single-use plastic bags any better for the enviroment? The answer is it’s complicated.

Assessing what’s better or worse starts with identifying an objective. For marine life, a plastic bag poses a greater threat than a paper bag, which quickly breaks apart when wet.

For the environment though, a paper bag may require far more resources to be made and transported.

Assessing the overall impact of an item is often done using a life-cycle assessment.

These assessments look at a wide range of factors from the impacts of manufacturing, to transport and disposal. Water consumption, greenhouse gases, waste and energy consumption are then calculated so the overall environmental impact can be considered.

A recent Danish study has calculated how many times alternatives to the typical supermarket plastic bag need to be reused before being used as a bin liner and then discarded to have the same environmental impact as a plastic bag.

These assessments look at a wide range of factors from the impacts of manufacturing, to transport and disposal. Water consumption, greenhouse gases, waste and energy consumption are then calculated so the overall environmental impact can be considered.

The results, which relate to Denmark and may not be identical for New Zealand, show alternatives to single-use plastic bags need to be reused, sometimes thousands of times before their environmental impact compares to single-use plastic bags.

The Danish study follows previous United Kingdom life-cycle assessment study on the climate change impact of plastic bag alternatives. This study arrived at a lower number of reuse times than the Danish study but still pointed to an environmental price tag for plastic alternatives.

Paper bags

At first glance paper may seem like an obvious solution to plastic. It’s recyclable and biodegradable. However, the environmental impact of producing a paper bag is higher than the environmental impact of producing a single-use plastic bag.

A 2006 Scottish report said factors such as the pollution to waterways during manufacture, water usage, greenhouse gas emissions and solid waste are greater for paper bags than plastic. Even transporting paper bags comes with an impact. Paper bags are thicker than plastic, and the energy needed to transport them is higher.

Danish study results: Paper bags need to be used 43 times.

UK study results: Paper bags need to be used three times.

Cotton tote bags

Sturdier than paper, cotton bags can be reused a number of times. Producing cotton however comes with an environmental cost. Growing cotton can include the use of insecticides and fertilisers and uses large amounts of water.

Surprisingly, organic cotton has an even bigger footprint than non-organic cotton. Without the fertilisers and insecticides helping to increase the yield, much larger areas are needed to produce the same amount of cotton.

The Danish study suggests an organic cotton bag would need to be reused 20,000 times, a non-organic bag 7100 times to equal a plastic bag.

Danish study results: Organic cotton bags need to be used 20,000 times, non-organic cotton bags 7100 times.

UK study results: Cotton bags need to be used 131 times.

Polypropylene bags

Many of the reusable bags sold in supermarkets are made from woven or non-woven polypropylene. Stronger than single-use bags they often have a semi-rigid base and are designed to be used many times.

The Danish study suggests a bag needs to be used 52 times, and a woven-polypropylene bag 45 times to compare to a plastic bag.

Danish study results: Polypropylene bags need to be used 52 times, a woven-polypropylene bag 45 times.

UK study results: Polypropylene bags need to be used 11 times.

Biodegradable plastic bags

Included in the proposed ban of single-use plastic bags are biodegradable and compostable bags. Experts in New Zealand have warned of issues with them.

Confusion exists around the difference in the claims made – biodegradable is different to compostable. Claims of compostable can mean commercially compostable and not compostable in a backyard bin.

University of Auckland senior lecturer Dr Ivanhoe Leung said the challenge is separating biodegradable plastics from other waste products so they can be broken down in the best way.

“For example, undesirable substances like acids or methane gas can be produced from biodegradable plastics if they are broken down in places that lack oxygen.

“These could be landfill sites, or anaerobic marine habitats like salt marshes or brackish waters.”

Danish study results: Biopolymer bags need to be used 42 times.

Cause for scepticism

Massey University’s Farrelly said New Zealanders should take the Danish study results with “a large grain of salt”.

“The study is very misleading and all it does is feed into this anti-plastic bag ban movement you see in some places.

What might be true for Denmark does not necessarily relate to New Zealand. Production methods may differ, as can environmental impacts.

The Danish study focused on incineration as the final disposal method of bags. In New Zealand bags would be more likely to end up in landfill.

“One of the issues is the general public can misunderstand this as incorporating the full life-cycle of plastic bags.

“It should include long-term effects like fish stock depopulation. The physical properties of bags, the ingestion, the strangulation and the entanglement of marine fauna.”

New research into methane and ethane emissions was also unlikely to be included, as well as possible health effects of the leaching of toxins she said.

Farrelly also felt the impacts of behavioural change were missing from the study: “In fact, human beings seem to disappear out of the equation entirely which is highly problematic.”

Use less and reuse

Deakin University lecturer in hazardous materials management Dr Trevor Thornton has written on the results of plastic bag life-cycle assessments before. He has simple advice for consumers struggling to decide what bag is the best.

“Whatever you use, use as many times as possible. Even if you use a plastic shopping bag, use it as many times as you can.”

He also suggested common sense in deciding what bag is the best size, or material for what you plan to carry. Paper bags don’t make much sense for carrying home products which might be damp.

Supermarkets can play their part too, he said.

“There are simple things. Having large signs in the carparks, so when people park, there’s a big sign saying ‘Hey, have you remembered your bag?’. It’s not difficult stuff.”

Massey University’s Farrelly has the same message about selecting a bag you will likely reuse over and over.

Her advice to people unsure about the best plastic bag alternative is blunt.

“Ask your grandparents would be my answer. They all survived. Nobody died.”

Submissions to the consultation document close Friday 14 September 2018.

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