Read this:

http://www.biofuelsforum.com/making_...glycerine.html

The thread's about acidifying glycerine with phosphoric acid, to partially refine the glycerine, separate out the FFA's and make potassium phosphate (the fertilizer bit).

Phosphoric acid is quite expensive and the proportion of potassium phosphate produced is small. It's a fair bit of work and expense, so I'll leave you to work out the economics. But if you are doing it on a large scale it might be viable.

By the way, you can use the resulting FFA's to make biodiesel in the acid/base process and the semi-refined glycerine might have an increased value (in bulk qty's).