Filtering doesn't really do much - with those cell counts, filters clog so fast you'll go through hundreds of dollars in filters to get a few liters of concentrate. Commercial ops use a low gravity centrifuge or cream separator. For home use, putting a bottle of it in the fridge will reduce cell motility (if you're using motile cells). Once they've settled, pour off the supernatant (clear stuff) and keep the slurry at the bottom. If they are resistant to settling, you can add 50% v/vof freshwater to increase their relative specific gravity to help them drop.