Search "yeast rinsing," this is how you separate the good yeast from non-yeast material. I collect my whole slurry in one half gal jar, rinse, and end up with around 4 oz of good slurry, about right for lager pitch rates. You might need to use all you collected to separate enough good stuff. But try it. Harvesting, rinsing and repitching (you can go many, many generations) is a great way to have good, healthy, well adapted yeast.