You often hear that a biological treatment system is:
- Healthy
- In balance
- Stable
- Equilibrium
- Steady stateUnless your system receives a uniform influent at a constant flow in addition to other environmental variables not changing, a biological system is always changing and adapting to the system’s environmental conditions. So instead of an equilibrium or steady-state point, we actually have a range in which we try to operate. For fans of the growth curve, we call decline phase growth the ideal operating zone. Decline phase growth is the region where:
- Biofilm and floc size and density are optimized
- Water in settling tests is clear without pin floc
- Respiration rates (SOUR) are within normal range
- Nutrient removal by slow growing, ecological niche organisms are present in needed numbers
![Growth Curve for Aerobic Systems](https://asterbio.com/blog2/wp-content/uploads/2024/05/Growth-curve-with-system-details-1024x576.jpg)
Growth Curve for Aerobic Systems
With minor fluctuations in loadings or conditions, the system rapidly adjusts to the ideal point. But at times, the environment promotes a new undesirable equilibrium, or the system fails to stabilize in the decline phase growth zone. From an operator’s perspective, instability is seen as:
- Settling problems – pin floc, filamentous bulking, & Zoogleal (non-filamentous bulking)
- Foam – caused by log phase growth (easy to control), surfactants, & Nocardia forms
- Unexplained low D.O.
- Failure of nitrification or other parts of biological nutrient removal processes.
Restoring Equilibrium
I like to go through a rapid trouble shooting process to find out why a system had fallen out of target operating ranges – and the answer is never “Nothing has changed”. Here is a short checklist:
- Did the influent change – not just BOD/COD numbers – but did you receive new compounds organic or inorganic.
- Mechanical problems – aeration system issues, clarifier problems, down to even flow meter/pump issues
- Once you know the most likely cause of problems, you can move to control measures to restore a good biomass. Control tools available include antifoams, polymers, nutrients (some systems need macro & micronutrients to stay healthy), supplemental oxygen, and bioaugmentation.
Let us know if you have any questions!