You can use ferricyanide in combination with potassium bromide to make a rehalogenating bleach and use that to bleach back your prints. With some care you may be able to get some of the edge fog to reduce while not attacking the image too much, especially if you overprint a little to compensate for the density loss. The advantage of the bromide bleach is that you can toss the print back into the developer (after a thorough rinse) if you've bleached too far.
Alternatively mix a little ferricyanide and a little fixer or thiosulfate (sodium or ammonium; doesn't matter) to make Farmer's reducer. This will also bleach back the print, but faster, and it's irreversible.
Can or should this stuff be put into a septic system?
Personally I rarely throw out any significant volumes. I mostly use it as a rehalogenating bleach, and in this form, it can be kept around almost indefinitely if you replenish it from time to time. A very weak working strength solution of Farmer's reducer I wouldn't have any qualms tossing down the drain; there's a tiny bit of ferricyanide in there anyway. The ferricyanide as such doesn't contain any heavy metals and will probably break down pretty quickly in the chemical soup of your septic tank. Since it's so little, the tiny bit of cyanide gas that may form over time is not going to do any harm. Used Farmer's reducer will have a little silver in it; much less so than used regular fixer. You could dispose of it together with your spent fixer if you prefer.
Larger amounts you best take to a waste disposal facility. You might also try to convert it into Prussian blue pigment, let that settle and dispose of just the sludge. But again, I've never run into a case where I had to dispose if significant amounts of ferricyanide, so I never really looked into disposal practices.
