Consider cooking noodles as a comparison.
Colanders are the water filtration of noodle cooking.
A colander performs a rather passive but effective action. It just sits there as water is poured through it, catching the noodles you want separated out from the water. Another way to separate what you want from your boiling pot of noodles is to stick some tongs in and hunt for the noodles. That's almost how water purifiers work. They have components that contain chemicals which actively kill viruses and bacteria rather than straining them out.