Controlling the amount of microorganisms in the activated sludge process is critical to performance.  The best way to do this is to maintain a target Solids Retention Time (SRT).  The target for your plant can range from 5-30 days depending on many factors.  Constant MLSS is another common control strategy; targets commonly range from 2000-4000 mg/L.  SRT is the time (in days) the bugs are in the activated sludge process.  That time is directly related to the growth rate of the bugs you are trying to keep happy/healthy.  We waste (remove a portion of the bugs) to keep the SRT (or MLSS) in the right spot. How you waste (24/7 vs 1 week per month) is equally important as the overall amount you remove.

Wasting is fundamental practice that must be done to consistently meet permit.  It also affects the performance of my unit processes at your WWTP (solids and liquid streams).  Wasting a little each day is much better than wasting heavy, less frequently.

How do you make wasting decisions?