Understanding different wasting control strategies helps operators respond effectively to changing conditions, maintain effluent quality, and manage costs. A deeper knowledge turns the routine task of wasting into a powerful tool for plant efficiency. There are three common control strategies for wasting:

1. Constant MLSS. Waste to keep constant the MLSS in the reactor
2. Food to Mass (F:M) Ratio. Waste to keep constant the ratio of influent BOD and MLVSS in the reactor
3. Solids Retention Time (SRT). Waste to keep constant the average time a mixed liquor particle is in the system. 

The table above summarizes some pros and cons of the three strategies.  When we have good wasting practices we keep the biology in a “sweet spot” that helps us get good oxygen demand, settleability, effluent quality, and whole plant performance.  Wasting with the SRT strategy is hands-down the best way to achieve this, but whatever you do, do it consistently and on a mass-basis.  I created a free tool to help calculate wasting needs using all three approaches.  Download it here.