An activated sludge plant has heavy Nocardia-type foam and has been running at a 22-day MCRT. Which process change is most effective for controlling it?
- Decrease wasting to raise the MLSS and thicken the mixed liquor
- Increase wasting to lower the MCRT, and remove the foam
- Raise the return sludge rate to move the foam into the clarifiers
- Raise the dissolved oxygen so the foam organisms are oxidized
Show the answer
B. Increase wasting to lower the MCRT, and remove the foam
These foam formers grow slowly and are favored by long sludge ages and by grease. Lowering the MCRT washes them out faster than they grow, and skimmed foam should be removed rather than recycled, because returning it reseeds the tank. Holding more solids lengthens the sludge age and makes the foam worse.
Reference: WEF Manual of Practice OM-9 Activated Sludge, foam control
The SVI at an activated sludge plant has risen to 230 mL/g. Microscopic examination shows many filaments, the aeration tank dissolved oxygen is averaging 0.4 mg/L, and loading and MCRT are normal. What should the operator do first?
- Increase the return sludge rate to thin the blanket
- Decrease wasting to build a heavier mixed liquor
- Add lime to the aeration tank to raise the pH to about 8
- Increase the air supply to raise the dissolved oxygen
Show the answer
D. Increase the air supply to raise the dissolved oxygen
Several filamentous organisms outcompete floc formers when dissolved oxygen is low for the loading. Restoring oxygen to about 2 mg/L removes their advantage and addresses the cause. Changing the return rate may move the blanket for a while but does nothing about why the filaments are growing.
Reference: WEF Manual of Practice OM-9 Activated Sludge, low dissolved oxygen filamentous bulking
Why would an operator feed a carefully measured dose of chlorine into the return activated sludge line?
- To disinfect the return sludge before it re-enters the aeration tank
- To oxidize the ammonia in the return sludge ahead of the aeration tank
- To damage filaments that extend out of the floc and so improve settling
- To raise the dissolved oxygen in the return sludge by chemical means
Show the answer
C. To damage filaments that extend out of the floc and so improve settling
Filaments stick out from the floc and are more exposed to an oxidant than the floc-forming bacteria inside it, so a controlled dose based on the pounds of solids can knock back a bulking episode. It is a stopgap that treats the symptom; the condition that favored the filaments still has to be found. Disinfection is not the aim, and an overdose harms the whole biomass.
Reference: WEF Manual of Practice OM-9 Activated Sludge, RAS chlorination for bulking control