WASTEWATER 

By Rick Lallish

What is the main cause of rising sludge in an activated sludge secondary clarifier?

A. Toxic shock load

B. Denitrification

C. Nitrification

D. 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, increasing the F/M ratio or decreasing sludge age. A quick and easy way to check rising sludge is to use a settleometer. Good sludge should remain settled for at least an hour before rising to the surface. More information may be found in the OWP, CSU-Sacramento textbook: Operation of Wastewater Treatment Plants, Seventh Edition, Volume 2, Chapter 11.

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DRINKING WATER

By Drew Hoelscher

At what temperature will an aqueous solution of 50% sodium hydroxide (by weight) freeze?

A. -12 degrees F

B. 0 degrees F

C. 32 degrees F

D. 58 degrees F

ANSWER: D. Aqueous sodium hydroxide is a common chemical used in water treatment. Also called caustic soda, sodium hydroxide is an extremely basic solution, so safety training is pertinent before handling. In addition to the safety concerns, proper feeding applications should be considered. Aqueous sodium hydroxide at 50% by weight will freeze at around 58 degrees F. This means the chemical storeroom and chemical feed point temperatures should be monitored. For example, sections of a chemical feed line can become clogged if the line is exposed to colder temperatures than the storage room. However, diluting to a 25% solution can eliminate freezing in most cases. Aqueous sodium hydroxide at 25% by weight will freeze at around 0 degrees F.

ABOUT THE AUTHOR

Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville.