I didn't know what forum this goes under but this seems as good as any.
After you have made your bubble bag hash, can you filter the accumulated water at the bottom of the bucket through a coffee filter for even purer stuff than the 20 micron stuff?
The hash that comes from the smallest micron bag will not be the best. The best will come from the next larger micron mesh bag. The smallest mesh will catch the broken trichome heads that have leaked their 'inner goodness' already so going even smaller will not get you anything better. If you used a coffee filter and caught leaked resin the coffee filter would tend to soak it up.
Brick Top Nailed it. You could filter your waste water but there isn't much point. All you would get would be trichrome stalks and contaminants. Anything smaller than 40 ru is usually pretty crappy hash.