Does a plastic bag charge save plastic?

The BBC reported today that, since the introduction of a 5p charge in England, plastic bag use had fallen very significantly. They estimated that 640 million bags had been issues in the last six months compared to 7.64 billion over the previous year.  Alan Sloman tweeted

“I wonder how the sales of plastic bags for pedal bins are doing? Wouldn’t mind betting no environmental saving.”

which is a very good point. If the carrier bag ban has merely resulted in displacement then, while it may be beneficial in reducing litter, it may not mean a significant saving in plastic use.  As a geek engineer, I always find such statements a challenge so I thought I would do some rough calculations and see what turned up (I haven’t done these in advance of writing this so at this stage I don’t know what the answer will be).

First, some data. I weighed a 30l bag for a pedal bin (20g) and a single use bag (15g). As the single use bag had holes in it, measuring its capacity by filling it with water is impractical but it looks as if it’s about 15l so, rather smaller than a custom bin bag. The annual reduction in the number of bags using the BBBC’s figures is 6.36 billion.

Because of the holes, you actually need to double up single-use bags for kitchen pedal bins. Otherwise, liquid drips through and makes a mess.

Now we know that not all bags are used in pedal bins but let’s assume 50% of them are. If these are doubled to avoid liquid leakage, then this means that the total capacity of the single use bags used as pedal bin liners is

25% * 6.36 billion * 15 litres = 23.85 billion litres

so, to match this with 30l bags, we need 795 million bags, which would weigh 15.9 million kg

However, if 6.36 billion fewer bags are used, this means that the total weight of these bags is 95.4 million kg so, even if my estimates are a good bit out there is still a pretty significant saving in plastic use. Against this, of course, there is the plastic used for ‘bag for life’ bags sold by the supermarkets but to work this into calculations, you need to know the average lifetime of such bags (which I don’t)

All in all then, there is likely to be a significant environmental saving from charging for plastic bags although this is likely to be a good bit less than it might appear. But for me, not having bags blowing around the countryside is a big win, whatever the environmental saving.

3 thoughts on “Does a plastic bag charge save plastic?

  • July 31, 2016 at 11:43 am
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    I love engineering approaches like this.
    However!

    I don’t know of many folk who doubled up their old carriers in their pedal bins. I didn’t. I merely rinsed the bin out when it was required. To my mind, that doubles your 15.9million kg to 31.8million kg

    Using the same approach let’s look at re-usable bags: My Waitrose bags weigh 47g each and have a capacity of 25 litres. This means you require (15/25)*6.36 billion reusable bags @ 47g each.
    This weighs 180 million kg.

    But let’s assume the bags last 20 trips to the shop. I have a sneaky feeling it might be half that figure but it’s good enough for now. That means we need 9 million kg of reusable bags.

    That’s all well and good. So far we now have used 31.8 + 9 = 40.8million kg.

    From the BBC article, prior to the banning of plastic give-aways, 7.64 billion bags were handed out. This equates to 7.64billion * 15g = 115 million tons.

    There is an 83% drop in use, so we still uses 0.17 * 115 million tons = 19.6 million tones of single use bags

    So plastic in use today = 31.8 + 9.0 + 19.6 = 60.4 million tons, compared to 115 million beforehand.

    That’s just over half of what used to be used. There has only been a 47% saving in plastic used, which is still admirable, but not nearly so bloody wonderful as the greenies would have us believe from the press release.

    🙂

    Thanks. Ian. I’ve enjoyed this.

    Reply
  • July 31, 2016 at 11:52 am
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    Whoops! Halfway through that calculation I slipped from kg to tons.

    So it should read
    So plastic in use today = 31.8 + 9.0 + 19.6 = 60.4 million kg, compared to 115 million kg beforehand.

    That’s just over half of what used to be used. There has only been a 47% saving in plastic used, which is still admirable, but not nearly so bloody wonderful as the greenies would have us believe from the press release.

    🙂

    Reply
  • July 31, 2016 at 5:04 pm
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    On the negative side, councils have now issued millions of biodegradable plastic bags for use in their recycling campaigns. The plastic used only biodegrades if it has full contact with oxygen and sunlight. If these bags are going into landfill then they are no better than the plastic bags they replaced.

    Reply

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