To precipitate silver out of fixer, just toss in some of aluminum foil. The aluminum will swap places with the silver which falls to the bottom of the container as a gray metallic sludge. After all the aluminum has disappeared (about 3 or 4 days), I siphon off the spent fixer and pour in another batch. I keep a one gallon wide mouth bottle just for this purpose; by now it probably has a couple of pounds of silver in the bottom.
No more heavy metal into the waste stream...
So if the aluminum swaps places with the silver what you are siphoning off contains dissolved aluminum. That doesn't sound like a good idea. Aluminum can be toxic as well. Haven't there been some studies linking aluminum and alzheimer's disease?