Hi Bsmith75,
The maximum percent solids that you can achieve will depnd on a number of factors including, the percent volatile matter in your sludge when it enters the digester and the detention time in the digester.
Typically an activated sludge will enter the digester at about 1% solids, (from a return sludge line), and with a volatile element of about 85%. The solids concentration that can be achieved will then depent on how much of the volatile fraction you can digest and how much decant you can remove.

We normally get 2-3% solids within aerobic digesters at 30-45 day detention time.