There are three key biological design variables for the activated sludge process: Solids Retention Time (SRT), mixed Liquor Suspended Solids (MLSS) concentration, Food to Mass Ratio (F:M). Either can be used as the primary control variable for wasting, i.e. adjust wasting to maintain one variable within a desired target range.  However, they are all interrelated; you cannot change one without impacting the others.  

Most WWTPs use MLSS concentration as their main control variable, usually because it’s easy. Easy to grab a sample, easy to analyze, and easy to tell if the result is over or under the target value. But controlling the process via MLSS offers the least control over the biology because holding the MLSS constant while the raw influent loading changes (WWTPs do that right??) results in fluctuating SRT and fluctuating microbial behavior. In contrast, if you hold SRT steady, your MLSS will fluctuate based on raw influent loading but the microbial behavior will stabilize.  Stable bug behavior translates to consistent:

• Oxygen demand
• Settleability
• Foaming potential
• Treatment capacity
• Effluent quality
• Thickening performance
• Digestion performance

 All the things that matter. Are you experiencing variability with any of those items at your plant?  If so, consider taking a closer look at your wasting practices.  Specifically, how stable your operating SRT is.