Guilty as charged - I did look right at my own post and missed that I did specifically mention "sand and clay" twice!... Time to get my brain tuned

I guess I was looking at what I wrote but saw what I ment to say. I talked to a guy who was manually setting moveable font (mostly ancient technology now) and he told me that just for that reason of thinking instead of just reading they get much higher accuracy setting a book in unknown foreign language then in English.
Well, I am sure you understand that I ment stuff that was extracted was put back - most of it sand and clay and whatever else that was in the river to start with. Also article I saw mentioned only that some alum was introduced - but I already knew that it was natural (at least occuring in nature - the one they used could have well been synthesised) non-toxic and almost insoluble in water. As far as I know, it is a catalyst and used in miniscule quantities without being consumed. It may actually help keep the river clean when it ends up there because it will continue to act and cause suspended dirt to fall to the bottom as well.
I know abour alum because it's the same stuff that I use to treat nicks after shaving.
Note - I shave with a straight razor which allows me not to pollute environment with disposable razor blades or dull electric razor heads. Let me hear some kudos here 
Chlorine and lyme could not be that bad because we drink water with them and fill pools with it and our acquarium fish lives in it just fine. I hope potassium permanganate is not too bad. Anyway, clorine and potassium permanganate go into the drinking water that gets separated most likely after filtering - not into the sluge that gets filtered out.
And while lyme probably reacts and forms sediment that does go into sluge, it must be some kind of stuff that occures naturally too - as water (which is really a weak acid due to CO2 in it) and its dissolved minerals constantly react with soil, stone etc. in the course of it's flow.
I bet that wet sludge
free of toxic stuff that is alowed to accumulate for two weeks sprouts the whole variety of swamp life which stinks to high haven - but that is not the same category of "toxic waste" as, say PCB, radioactive stuff, ets.
miko