The glycerol is soap? How interesting. What do you do with 4 or five gallons of soaf every fillup?
No, glycerine is byproduct. You get it whether you make soap or biodiesel. If you use water (or lye only) fat makes soap + glycerine, but if you use methoxide (which is lye + alcohol) then you get biodiesel + glycerine, but no soap. Soap is an incidental byproduct of biodiesel manufacture, when your grease is very wet, or has free fatty acids. Some biodiesel preps use sulfuric acid in the first step, which reduces the yield of soap and increases the yield of biodiesel, at the expense of using an additional step, and the hassle of dealing with sulfuric acid which is pretty nasty. But then you can use really rank grease and still get good biodiesel.
The glycerol is about 3/4 "soap" and 1/4 reclaimable methanol. I distill the methanol (for reuse) from the glycerin and give the de-methed soap by-product to shade-tree mechanic shops as degreasers, to the restaurants I get my raw oil from as soaps and degreasers, and I make bar and liquid soaps for my personal use once in a while, giving some of it away if my friends and family ask.
Also, the wastewater treatment plant uses some of the de-methed excess as a stimulant(?) food for the digesters, don't know exactly their use but they take it.
The de-methed glycerol makes good soap, works well as a leather conditioner provided no methanol remains, is a great degreaser, parts cleaner, auto wash, tire cleaner, pressure washer solution, weed killer, animal feed sticker, and several other uses ie. dust control for unpaved surfaces, and as a nutrient source for compost bins/piles. You know those little sponge things they sell to "polish" shoes? Glycerin on/in a sponge? This will do the same thing for free.
The uses are as varied as the need, but the biggest problem is matching the need with the by-product, but since it's "free" and I live in a rural agricultural setting, it's not too difficult to get rid of the bulk of it - properly.