Wash synthetic clothes in a Guppyfriend-style microfibre bag, or fit a filter to your washing machine outlet. Wash cooler, fuller and less often, and skip the tumble dryer.
A 2021 PLOS One study measured synthetic microfibre loss at 9.6 to 1,240 mg per kg of fabric per wash. A 4 kg load of synthetics at a mid-range 300 mg per kg is about 1.2 g per wash; 100 washes a year is around 120 g of microplastic kept out of the drain. Acrylic sheds four to five times more than polyester.
The catch: filters and bags capture fibre but you then have to bin it, and they do nothing for shedding during wear; buying fewer synthetic clothes is the upstream fix.