WASTEWATER
By Rick Lallish
What is the main cause of rising sludge in an activated sludge secondary clarifier?
A. Toxic shock loadB. DenitrificationC. NitrificationD. High F/M level
ANSWER: B. Rising sludge is common in secondary clarifiers and normally is caused by denitrification due to excessive detention time. When the sludge is allowed to settle and has used all the available oxygen, the sludge will begin to denitrify and release the nitrogen gas. Small particles of sludge will then be carried to the surface. This should not be confused with sludge bulking. Some measures to control the amount of rising sludge are increasing return rates,
