<p>I am pretty sure that sanding makes a lot of microplastics, so it would be nice to collect the dust, and melt it to got a blob of plastic again instead of millions of tiny particles. What is the best way for collecting it, do you use any dust extractor, or are there different techniques like sanding wet surfaces and filtering it from water?</p>

As an environmental thing, micro plastics are an urgent problem for ocean life, it’s getting into the entire food chain. With that in mind dumping it down the drain would be the worst, city filtration systems can’t get the tiny bits of plastic before it drains into the ocean. Sanding outside is kind of bad. I sand stuff over a lined waste basket outside, which maybe catches 75% of the dust. Inside (or outside) with a vacuum running would be good, I think best would be a vacuum with disposable bags. Yes it is still micro plastics, but it ends up at the dump, where the soil is already poisoned, and hopefully nowhere near the ocean/waterways, and likely will stay there while it breaks down. PLA takes ~80 years, but ABS takes 1000 years.
The idea of collecting the dust and fusing it sounds like it would be too much hassle to be feasible long term, it’s good, sustainable habits over years that add up.
As a side note, card scraping is a nice way to smooth 3D prints, that doesn’t make fine dust.