Well that would explain why people are not doing it.  However, this is not going to be used stream side.  It's recirculating indoors.  I was kind of hoping Food grade Alum would be ok, since it's food grade and there are worse things going down the drain washing dishes BUT if you think the Feds would get thier thong in a knot I won't bother.  

Thanks for the insight!

as long as the recir water is discharged to a sanitary sewer system (WWTP) you should have no issues, as this is not an "Industrial Discharge".

Be mindful, any flocking agent could cause some drain/pipe issues unless you use a lot of dilution when discarding it.

I use a home made flock agent (w Al) and have had no issues with putting it down the drain to the WWTP.  Just as you say, "there are a lot worse things going down the drain"!  In this case, any toxicity or turbidity issue is handled easily by the Advanced Waste Water Treatment in the city/county WWTP.

Since your running it indoors, have you seen that little trick of taking your material, about a 1/3 to maximum 1/2 of a 5 gallon bucket at a time, and lots of water and a drill with a big paint stirrer on it you can pickup from Home Depot, Lowes or probably just about any hardware store.

Process it with the drill mixer, just like your mixing up a bucket of paint.  When its thoroughly busted up, most of the dirt and other junk will be in suspension.  If there is clay it will bust it up pretty good also.  Then pour the muddy water out of the bucket and run the washed material through the recirculation system.

This would have to be getting rid of over 99 percent of the crap for a small amount of water, versus running it and clogging your recirculation system causing lots more work and water loss cleaning it back up allot.  This could even be done on a recirculation system in the desert since you don't have to run any contaminants into the process.

 I haven't personally done this, but I have seen it done and it seems pretty solid on getting the most out of recirculation system by pre-cleaning.    

Yes, alum (aluminum sulfate) is used for drinking water treatment in USA.  It does require a certain amount of alkalinity in your water to be able to form the "floc" (which pulls stuff out of the water to make bigger pieces). The floc is removed by sedimentation (precipitation) or by filtration.  10 to 15 parts per million of alum into the water should be enough to see if it will work for you. That would be one teaspoon per gallon of water if my math is correct. 

The end product that most regulators are concerned with is Aluminum. You should not have enough aluminum in many tubs of water to be of concern if you are disposing to sewer. 

Likely if alum is not making a good floc for removal for you is because it does not have enough alkalinity in the water to react with. Try adding a bit of baking soda (sodium bicarbonate)  and see if that helps.  A bit = try a tablespoon per gallon to start. If it work at all,you can add more but should Not need 1 cup of baking soda per gallon of water.