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My wife recently replaced the soap dish in our kitchen (and the soap on it) with a liquid soap dispenser. Within about a week, we've used about half the bottle of liquid soap, whereas solid bars of soap would tend to last a month or two.

Clearly the liquid soap is going to be far more expensive (unless we find a very cheap supply of it), but I'm interested in comparing other whole-of-life impacts that the two options have.

Let's assume the liquid soap doesn't contain the common anti-bacterial agent "triclosan" (which has it's own set of problems).

Does one biodegrade better, or have a lower embodied energy, etc.

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    In my experience (with some liquid soaps) you have to be careful to not immediately wash them off again (unused). – Jan Doggen Apr 14 '17 at 12:06
  • I usually dilute it, about 10:1. This us strong enough soap for most of the time. – ctrl-alt-delor Apr 15 '17 at 10:46
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While I have no clue which one does biodegrade better I'd like to focus on some other life-cycle aspects of liquid soap vs solid bars. But of course it's certain that liquid soaps (need to) contain substances that are simply not required for a bar of soap - such as emulsifying agents.

Liquid soap contains added water and is therefore heavier. Transportation footprint is worse than solid bar - just to ship some water. (The same holds true for laundry detergents which are used in even greater amounts.)

Added packaging for the liquid soap compared to simple wrapping for the bar. This is an issue with respect to weight (again for shipping), resource consumption for making plastic bottles (use of non-renewable petrol) and also considering necessary recycling efforts or disposal measures (landfills, ocean dumps).

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Liquid soap can and should be diluted. As with toothpaste, manufacturers profit from consumers inadvertently using too much – often far too much. Dish soap in particular is usually sold explicitly as "concentrated", but the sustainability gain from less bulk is lost unless consumers take great care to use tiny quantities. Dilution is much easier.

Packaging can be reduced by opting for refill bags where available.

So, a couple of comparisons are possible:

  • between bar soap and bottled soap used carelessly
  • between bar soap and liquid soap bought in refill bags and diluted

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