I also carry a kitchen sink (Sea to Summit) - same one for the last 7n or 8 years.  I find it very useful for carrying water and doing dishes. It weighs nothing and takes very little space. Mostly, though, I just like to be able to tell people that I bring everything including the kitchen sink.

Here is a product that may interest: Druide Ecotrail Multi Purpose Soap.  They claim it is biodegradable, phosphate free, made from 100% ingredients of natural origin and that it biodegrades immediately in water (it doesn't need microorganisms found in soil to biodegrade.)  It is highly concentrated so you only need to use a drop or two for body or dishes.  Because it is so concentrated you don't have to carry much even on a long trip.  Now whether or not it is all that it claims, I can't say.  But according to the folks at Mountain Equipment Co-op it's bone fide.
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