Why you don't want "old" sludge - how operating in endogenous respiration can hurt activated sludge effluent quality

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?

• 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