Extended Aeration Activated Sludge System
Extended aeration biological treatment systems operate at a lower F/M than the original activated sludge designs. Why did we move to extended aeration over other aerobic treatment systems?
While operating in extended aeration mode has a lot of upside, running a system at too low an F/M ratio can be counter productive and effluent quality can decline. As you move further into endogenous respiration (very low F/M) the following things happen:
- More resistance to hydraulic and shock/toxic loading events
- Lower solids yield (although you can argue that you are just moving the aerobic digester into the treatment basin)
- Enables nutrient removal - both N & P
- Better removal of xenobiotics - pharmaceuticals, pesticides, and other recalcitrant or insoluble organics
While operating in extended aeration mode has a lot of upside, running a system at too low an F/M ratio can be counter productive and effluent quality can decline. As you move further into endogenous respiration (very low F/M) the following things happen:
- Biological polymers holding floc together begin to degrade
- The living or active portion of the biomass decreases - meaning inert solids must be handled by clarifiers and pumps.
- Energy cost increase for maintaining aerobic conditions with high levels of biosolids
- Low F/M filaments can increase bulking
- Encapsulation (often called Zoogleal bulking)
- Turbidity and "fines" begin to carryover into the effluent with the MLSS settling very fast and not removing the small particles
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