I don't think you understand the physics of the problem. Have you played connect 4, the game with the checkers that you drop down a slot?

Imagine the black checkers are dirt particles and the red checkers are microplastic. The game set with the slots is the filter the particles drop through. Play a game and then open the slider at the bottom to dump the checkers. Do the red checkers stay in the game set while only the black fall out? Of course not, because they are the same size.

There is no possibile way to filter the plastic because it is the same size as the dirt in all its different sizes. There are large and small dirt particles. There are large and small micro plastics. If you remove 1% of the microplastic you remove 1% of the dirt, so the remaining dirt is just as contaminated. You didn't filter it, you only removed an equal amount of dirt and plastic.

https://www.sciencedirect.com/science/article/pii/S2215016121003095