Proper wasting control helps operators respond effectively to changing conditions, and sometimes avoid a reaction altogether.  There are three common control strategies for wasting and by far the most common from my experience is Contant MLSS Control. Here are the basics:

• Concept: Waste to keep constant the MLSS concentration in the reactor
• Data needs: The target and actual MLSS concentration, Basin volume, RAS TSS
• Strengths: It’s quick and simple
• Weakness: Offers the least control over the process. Does not acknowledge load variability and its impact on the biology.

In most cases wasting requires calculating the mass of solids to waste in pounds and then converting that mass to a volume to pump out.  The figure above shows equations for calculating the pounds and volumes, where MLSS and RAS TSS are concentrations are in mg/L; Volume is in million gallons (MG), and mass is in pounds (lbs).

When we have good wasting practices we keep the biology in a “sweet spot” and that helps us in numerous ways across the whole plant, both in the liquid and solids streams.  I created a free tool to help calculate wasting needs using the Constant MLSS Control Strategy as well as the other two approaches (F:M and SRT).  Download it here.