I've been building an old Testor 1/72 scale B-2 and puttying all the seams, then wet sanding to keep dust from flying everywhere.
When other people wet sand, where do you dump your dirty waste water?
I live in an apartment, so not a lot of options for where to dump. And I don't want to dump it down the kitchen sink either.
I'm trying to think of the best way to dump the water since it will have toxic chemicals in it from the putty and/or tiny bits of microplastics.

I would first check with your local authorities to see if they consider the amounts to be harmful. Also, they may have toxic waste disposal facilities where you can take it to be properly handled. We have them here in Ontario, and to prevent people from dumping they are free.

You can always leave it on a balcony or sunny window and let the water evaporate, then dispose of the dust/mud in a plastic bag as just regularr trash

I just chuck mine out the shed door onto the grass.
You could put yours down the toilet.

Once the putty is cured the remaining material that you sand off is basically inert. The plastic sanding
dust is not the same as "microplastics" being far larger in size than those problematic materials. The
dust is also inert and acts like any other normal particulate in the water column and will settle out.
The small amount you generate will not cause any problems. 

If you want to be sure, build a filter. The water goes through the filter and can be disposed of down the drain, the solids stay behind. Over here, solidified paint goes in the normal household waste, I don't expect solidified putty to require handling as chemical waste.
Simplest option would be a coffee filter.

I emailed our  local council environmental health people .  Their answer was "We hadn't thought of this problem. The suggestion of filtering the liquid through a coffee filter, pouring away the water and binning the residue sounds reasonable.  I will get back to you  if  there is different advice after discussion"

This seems a reasonable and workable answer.

I will probably sound horribly old fashioned and environment unfriendly, but I do my sanding under a slow-running tap and the residue is just washed away through the drain....
I must admit that I never ever thought about it....

TomZ

You and me both Tom................

Quote from: PR19_Kit on February 02, 2023, 12:56:16 PMYou and me both Tom................

Must admit I tend to flush mine down the loo.

Quote from: Hobbes on February 01, 2023, 12:51:21 PMIf you want to be sure, build a filter. The water goes through the filter and can be disposed of down the drain, the solids stay behind. Over here, solidified paint goes in the normal household waste, I don't expect solidified putty to require handling as chemical waste.
Simplest option would be a coffee filter.

Ditto in the US.