Thursday, November 13, 2008

One plastic bottle of water is equivalent to driving roughly half a mile.

Many people feel strongly about bottled water. They feel that bottled water is terrible for the environment, much too expensive, and perhaps unhealthy due to chemicals in the plastic.

The other camp believes that bottled water is convenient and much healthier than soft drinks. And bottled water sure is popular.

I think it's important to compare the environmental impact of bottled water to other human activities. For me, it's most intuitive to compare the impact with driving around in a car. So, let's suppose I drive with a few friends from Mountain View to Lake Tahoe on a weekend and back (450 miles) while consuming 20 plastic water bottles. It turns out that very roughly, the environmental impact of the water bottles makes up about 2% of the trip.

Here is my rough calculation:
  • The Pacific Institute says that "the total amount of energy embedded in our use of bottled water can be as high as the equivalent of filling a plastic bottle one quarter full with oil".
  • These water bottles are usually 500 ml (16.9 FL OZ), so that's 125 ml of oil per bottle.
  • Wikipedia says that "on average, about 19.5 US gallons (16.2 imp gal/74 L) of gasoline are available from a 42-US-gallon (35 imp gal/160 L) barrel of crude oil (about 46% by volume), varying due to quality of crude and grade of gasoline". This means that the bottle is equivalent to roughly 58 ml of gasoline, or 0.0153 US gallons.
  • My car gets about 30 miles per gallon on lengthy trips, so that's 0.460 miles of driving for the 0.0153 gallons of gasoline.
  • It's around 450 miles to Tahoe and back, so the 20 bottles are equivalent to an additional 9.2 miles of driving, which is 2% of 450+9.2.
This calculation of course is very rough and has multiple weak points; among them:
  • The estimate by the Pacific Institute may be inaccurate.
  • There is more environmental impact from the car than just burning the gasoline. Relatively speaking this would lower the relative impact of the water bottles.
  • The conversion from oil to gasoline may be more or less efficient. That is, one may need to invest additional energy to convert from oil to gas - this would lower the relative impact of the water bottles.
  • Perhaps we should compare with a diesel engine (since diesel fuel is closer to oil) or more efficient cars - this would make the relative impact of the water bottles greater.
  • The trip might involve other activities with environmental impact (e.g. sleeping at a hotel, eating ribs, etc.). This would lower the relative impact of the water bottles.
Despite all this I still think it's a valuable ballpark estimate for the order of magnitude of the environmental impact of plastic water bottles. Another, simpler comparison to make is that according to the Pacific Institute, roughly 17 million barrels of oil each year are used to make bottled water; and according to the US government, the US consumes 20.68 million barrels per day. So, very roughly, eliminating all use of water bottles with something that has no environmental impact is equivalent to the US consuming no oil for one day each year.

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