Understanding how microbes use energy fosters a deeper understanding of microbial behavior and empowers operators to make smarter, more strategic decisions that optimize plant performance. In the activated sludge process, microbes use food to produce energy and they use this energy in a specific order:

• First, they use energy for cell maintenance (keeping existing things working well)
• Then, only after maintenance demands are met, they use remaining energy for growth (making new bugs)

Wasting controls how much food is available to each microbe by adjusting the total amount of bugs in the system. When there’s more food available, more energy is left over after maintenance, and so more new cell growth occurs.  Most operators learn that we waste in response to biomass growth. But at the microbial level, our wasting practices are a real and present force that causes growth.

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.