Here’s how many times you actually need to reuse your shopping bags



File 20180806 41344 wus0pb.jpg?ixlib=rb 1.1

Jeremy Piehler/Flickr, CC BY-NC

Trevor Thornton, Deakin University

The plastic bag ban by the major supermarkets (and Coles’ pivot away from its ban after backlash, then pivot back to the ban after a backlash to the backlash) has left plenty of people scratching their heads.




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What are the best replacements for single-use plastic bags? Given that reusable bags are much sturdier, how many times must we use them to compensate for their larger environmental impact?

The simple answer is that there is no simple answer. However, a kind of research called “life cycle assessment” can help us work out the impact of common types of reusable bags.

Life cycle assessments

I am not aware of any Australian studies of plastic-bag substitutes. Research conducted overseas can offer a basic guide.

Life cycle assessments consider a wide range of factors, including raw materials, manufacturing, transport, and eventual disposal.

Looking at all of these elements, researchers calculate greenhouse gas emissions, waste disposal, water and energy consumption and a variety of other impacts.

To complicate the decision further, if you choose a plastic bag, is it made from virgin resin or from recycled plastic? Even if the bag is recycled, transport is an issue – where was it made? Printing on the bag also adds to the environmental burden.

Finally, what happens to the bags when they can no longer meet their purpose? Are they recycled, reused as bin liners, or thrown away immediately?




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How many times must a bag be reused?

Once all of this information has been distilled, scientists can usually offer a fairly straightforward guide: the number of times a given bag should be reused when compared to the standard supermarket plastic bag.

A 2018 Danish study, looking at the number of times a bag should be reused before being used as a bin liner and then discarded, found that:

  • polypropylene bags (most of the green reusable bags found at supermarkets) should be used 37 times
  • paper bags should be used 43 times
  • cotton bags should be used 7,100 times.

Another UK study, which only considered the climate change impact, found that to have lower global warming potential than single-use plastic bags:

  • paper bags should be used three times
  • low-density polyethylene bags (the thicker plastic bags commonly used in supermarkets) should be used four times
  • non-woven polypropylene bags should be used 11 times
  • cotton bags should be used 131 times.

Note, however, that if a plastic bag is reused (even as a bin liner) the number of times an alternative needs to be used increases.

It’s worth noting that, according to the 2018 Danish study, using organic cotton has a greater environmental impact than non-organic due to higher production costs. Our assumptions about what is environmentally friendly don’t always stand up to scrutiny.

A 2014 study in the United States found that reusable LDPE and polypropylene bags do have a lower environmental impact than the usual plastic bags found in supermarkets – but only if they are reused enough times. This study found that about 40% of shoppers forgot to bring their reusable bags and therefore end up using the plastic bags. This then adds to the environmental burden of shopping.

One final consideration is how many bags you need. The Danish researchers equalised the volume of the bags so that evaluations were made on the same volume of space (this meant that for some assessments it was necessary to consider the impact of two bags).

As with all matters environmental, it’s essential that we have the right knowledge to make informed decisions. After looking at all this data, here are the things I’d like you to remember:

  1. whatever bag type you use, use it as many times as possible

  2. choose bags made from recyclable materials

  3. avoid bags that have printing or decorations – these alone can add significantly to the environmental burden of the bag

  4. never allow a bag to become litter – recycle, reuse and repurpose your bags.


The Conversation


Read more:
Why can’t all plastic waste be recycled?


Trevor Thornton, Lecturer, School of Life and Environmental Sciences, Deakin University

This article was originally published on The Conversation. Read the original article.

Three reasons why it’s a bad idea to ramp up Adelaide’s desalination plant


Sarah Ann Wheeler, University of Adelaide

Drought-affected farmers in New South Wales have called for South Australia to increase the use of its desalination plant to enable an increase in water allocations for other users along the Murray River.

The farmers’ argument is that if Adelaide in particular draws less water from the river more will be available for agriculture in NSW and Victoria.

The logic may sound appealing, but there are three good reasons why it’s not a good idea. Not only is desalination an incredibly expensive project, there are other strategies – like water pricing – that can more effectively reduce water demand.




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It’s also important to remember that the water flowing to South Australia is not “wasted” if it’s not diverted for industry. Stream flows are vital for keeping many ecosystems alive, and there are already serious concerns about current levels.

Adelaide doesn’t use that much water

First, Adelaide uses a very small amount of water from the River Murray. Over the past two decades, average diversions for metropolitan Adelaide and associated country areas have been just over 100 gigalitres (GL). This represents an average 1.25% of the water diversions in the Murray-Darling Basin.

SA in general (including irrigator use) has used an average 11% of water diversions over the past two decades. NSW has diverted 52% of surface water in the Murray-Darling Basin over the same period. Hence, the reality is that ramping up SA’s desal plant will have very little actual impact on NSW irrigators’ water allocations.

South Australia (the grey line) receives much less water from the Murray River than other states.
MDBA Water Audit Monitoring reports, CC BY

Desalination is expensive

Second, increasing desalination has heavy financial and environmental costs. The financial cost is why the plant has been run at only about 10% capacity (the minimum needed to maintain its working condition).

Previous economic analysis by consultants has suggested the desalination plant should only be used to increase water allocations to SA irrigators when temporary water market prices are above A$510 per megalitre (ML).

Given that temporary water prices are now trading around A$300/ML (albeit increasing due to increased water scarcity), we’re still a long way from a financial argument for turning to the desal plant, let alone considering the cost of its negative environmental impacts.




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Farmers experiencing drought-related stress need targeted support


Water is always variable

Third, we need to take into account the variable nature of Australia’s irrigation sector.

Farmers in NSW are complaining that they are on zero percentages of water allocations at the start of the water year (August 1). This sounds like a big problem. But it doesn’t mean the farmers will get no water from the river at all.

At the start of the season, water managers allot water in districts to irrigators every two weeks. Depending on the type of water entitlement owned, and given the current state of water conditions (e.g. storage, inflows, rainfall predictions), water managers will allocate a percentage of water to high security entitlements first, then general security, then low security. Only in high rainfall years will the low security group get any water. Most years they get none.

Since water was separated from land in the Murray-Darling Basin, this water is not tied to any particular use. Once allocated, irrigators can use their seasonal water for their crops, store it, sell it on the water market, or even choose to let it flow down the river.

It is not unusual for irrigators with low or general security entitlements to start the water year with low or no water allocated, especially in times of water scarcity. One reason is that these rights are traditionally associated with districts where farmers typically grow annual crops like rice and cotton. They have larger farms and more water rights – but also more flexibility about the amount they plant every year.

Low water allocations at the start of the water year act as a signal to these producers to think carefully about the amount of acreage to plant in the coming months. In addition, many NSW and Victorian farmers have access to carry-over water (unused stored water from the previous year).

The other option is the water market; irrigators can enter the water market to buy water. Current temporary water prices are still historically a lot lower than prices in the Millennium Drought, when temporary prices hit over A$1000/ML in many regions.

Finally, it is important to emphasise that water destined for urban use in SA is not “wasted water” for NSW and Victoria. It is still providing surface-water flows in the basin as it makes it way to SA.

Economic studies show that a healthy and sustainable river is worth millions of dollars in tourism and recreation, plus providing important cultural values, on which many small regional town economies are heavily dependent. And there is ongoing evidence that the Murray-Darling Basin is still far from being sustainable.




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Unfortunately, many of the people who support more sustainable water reallocation for the environment are widely dispersed. They often do not have the lobbying power or the resources to engage successfully in policy debate.

The ConversationOf course, farmers need support. But the calls for more water to be allocated to irrigators will sound loudly as the drought continues, and it’s important to remember that there are other, less costly, options that also protect our environment.

Sarah Ann Wheeler, Professor in Water Economics, University of Adelaide

This article was originally published on The Conversation. Read the original article.