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As I ask myself the question and start thinking about it, I realise it might be obvious that it is worth it. But I thought I'd ask anyway as someone might have a nice analysis to share.

The other day, I cleaned a lightweight plastic bag (I don't usually do it, I put them in the bin when they are filthy) and thought it was a lot of water for a small single-use bag. It got me thinking: is the amount of water I used to clean that bag (I'd say about one litre) more than the water used for its manufacture and transport? Looking at both a recycled bag and one made from fossil raw materials would be great.

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    What about saving them for when you wash other stuff — then you use less water, and may have some washing up liquid in there too.
    – PJTraill
    Nov 1, 2017 at 22:54
  • That's a very good point actually. Show that I never really considered them to be just another kind of container.
    – stragu
    Nov 3, 2017 at 2:29
  • To wash more efficiently, turn inside out. Dry it inside out too.
    – RedSonja
    Nov 8, 2017 at 12:05
  • Are you using cold, warm, or hot water? This will make a substantial difference to the calculation.
    – Nic ♦
    Nov 8, 2019 at 0:52
  • As it is now part of our usual dishwashing process, I guess it is warm water I am using.
    – stragu
    Nov 9, 2019 at 13:31

3 Answers 3

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I've actually thought about this while washing bags. Something about it just felt dumb to me, so I looked into it.

First surprize: Plastic takes a lot of more water to produce than I expected. It takes roughly 22 gallons of water to make one pound of plastic. (180 liters to produce 1 kg)

Second surprize: My tap water gets pump lifted 3000 feet and travels 336 miles, which uses about 2 megawatt hours of energy per acre foot... and that power comes from a coal powered plant!

Your math will depend on your source of water and your source of plastic, but I was able to zero out my environmental impact by growing veggies with the water I use for washing plastic. Most of my plastic comes to me in the form of big bags that contain (literally) tons of coffee grounds. I rinse them off in a tub of rainwater that eventually goes to the plants.

When I use the plastic to prevent soil evaporation, my primary use for plastic, the water I save is greater than the water originally required to produce the plastic... by a factor of FIFTY! This goes to show that the destination can often be more significant than the source.

These next five paragraphs have been added for the benefit of those who disagree with the use of plastic in conjunction with plants. Firstly, I store custom mixed soils and soil ingredients in plastic bags until they are ready to be used. There's a reason why potting soil always comes in plastic. With the proper moisture level, such soils can remain alive and active, and can even increase in fertility. It sure beats having to hose it down periodically, or every day as would be the case where I live.

At some remote desert locations, plastic can be the most precious commodity for miles in every direction because it can produce and purify water through distillation, but when it's integrated into water collection and retention you can achieve stunning results. Even sandy soils in arid climates where nothing will grow can be restored with a little plastic if you form a bowl shaped bottom to your planting hole to make a deep pocket of moisture similar to a wicking planter or a mini cistern. This can create a toehold or springboard for diverse life and microbial generation in otherwise sterile environments allowing the establishment of drought resistant native plants.

Naturally, if you live in a wet climate with poor soil drainage this would be counterproductive, but I've seen scrap plastic being layered in a mound formation to shed water away from plants that are in danger of rotting from over-saturation. In the sweet zone between these two extremes you will be sure to find plenty of nearby organic material to stabilize your soil moisture which would be vastly superior to plastic, but you may also consider using old plastic bags to stave off a light freeze. They make a handy little tent or mini greenhouse over small individual plants and can be used year after year to extend your growing season.

Even after reusing my plastic in all these ways, I still end up with a surplus of small bags from the kitchen that I don't want to clean. I plant trees in them when I'm running short of small containers, and then pack them side to side in crates to make them easier to move. The bags prevent the roots from getting tangled until they are ready to be installed. If you do this, make sure to provide plenty of drainage holes or your trees may rot.

I realize that I'm getting a bit far afield from the original question about washing plastic bags, but the main point should be summarized in the statement that the ultimate destination of plastic can often redeem it from what many consider to be its inherent evil. A carefully washed plastic bag these days may end up in a landfill or a smokestack due to unscrupulous recycling programs, but when it's used creatively by you, the individual who considers and weighs its value against the value of water, it can be transformed from a waste product to a valuable resource that can help restore the planet, especially in regions where water is the greatest concern.

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  • I disagree with the use of plastic to prevent soil evaporation. Soil needs to breathe it is full of micro-organisms. Covered with plastic it will become sour and eventually sterile. It is better to use bark, straw, or even paper, but the best way of all to prevent evaporation is to use drought resistant planting, like sage.
    – Janet Rooke
    Nov 8, 2019 at 12:10
  • @JanetRooke I added the last five paragraphs to address your concerns.
    – Scott Tramposch
    Nov 13, 2019 at 17:30
  • Thank you Scott. A small but significant rider to the use of plastic. The ground beneath it would eventually dry out because it isn't getting any rain. BUT all the rain falling on it would evaporate much faster than it would, falling on soil! For this reason alone plastic is a no no.
    – Janet Rooke
    Nov 14, 2019 at 13:28
  • @JanetRooke In my experience, everything you've said is straight up not true. Plastics aren't some sort of anti soil demon. I literally see soil condensation on plastic every day. If the bottom of the soil being dry some how mattered to your specific plant, that's on you for not watering it enough. putting it in a metal, rock, clay, or potato skin container is not going to change that. Ideally however, you don't want the bottom of your plant's container to contain excess water, or the roots of your plant will suffocate and rot. You put holes in the bottom of pots to prevent over watering.
    – Krupip
    Nov 15, 2019 at 21:12
  • @JanetRooke Oh an Plastics aren't a sterilizer, period. I have about 10 plastic bottles that contain live fish and plants that have been thriving for over a year. It hasn't sterilized the bacteria (other wise everything would die because the nitrogen cycle would stop), or killed the plants. The best way to prevent evaporation was to just put something on top of the bottles (that didn't completely remove air circulation). There isn't any magic vodoo going on with this, you prevent water evaporating by reducing the area it can escape.
    – Krupip
    Nov 15, 2019 at 21:18
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Typical costs of water is 50 cents per 1000 gallons.

Typical low flow kitchen tap is 2 gpm. 20 seconds of spray -- under 1 gallon. Fraction of a cent.

More to the point is the value of your time. My usual problem is drying them.

Our rule here:

Breadbags come in, they get the crumbs shaken out and stored.

2nd use is usually breaking up bulk food bought from Costco. If it's a dry food, it goes into the stored bin again. If it's used for something wet, (meat, fish) it's discarded.

Ziplock get reused more agressively, but still are discarded after meat/fish use.

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    Apart from Sherwoods answer I think it's also worth looking at energy/material spent in making the water available, and in producing another bag. Both should be available, I guess. I don't have the time for an answer.
    – user2451
    Oct 25, 2017 at 15:34
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    For things like water and plastic bags, the energy used in production typically makes up the largest portion of the cost (compared to raw materials) -- so the cost serves as a fair proxy when comparing the energy (and thus, emissions) used in production.
    – LShaver ♦
    Oct 26, 2017 at 16:23
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    Thanks for your answer, Sherwood. But how does it compare to the water used during the production (extraction, manufacture, transport) of a plastic bag?
    – stragu
    Nov 3, 2017 at 3:16
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It is Always better to reuse things in the home, whatever they are. For a start you cut out the transport, and the manufacture. Your household water can, if you can afford it , be saved in barrels for use on the garden so that is not wasted. I.e. with a little extra care and 'elbow grease' you end up with zero waste which I would think compares favourably with all the other figures. You make me feel guilty because I recently threw away a plastic bag!

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