I actually work in the wastewater industry and from what I'm reading, a properly functioning sewage treatment plant already captures a very high proportion of microplastics. This widely cited study noted above 98% removal efficiency at one plant.

We're already at approaching 2 log (99%) removal without actually trying to. It doesn't seem improbable to me that with a few relatively modest tweaks to the system we could get 3 log removal (99.9%). Getting to 4 or 5 log removal is likely where things will get really expensive and challenging. But for now, a 2-3 log removal is probably good enough to focus on other sources like tire fragments/dust that typically pass directly to receiving waters with no treatment at all.