Exam Study Guide


By Rick Lallish

What piece of laboratory equipment is used to determine settleable solids?

A. Crucible

B. Settleometer

C. Imhoff cone

D. Filter

Answer: C. The settleable solids test is used to measure the volume of settleable solids in one liter of sample over a one-hour period. It is an indicator of how much solids will settle in a clarifier or pond. It is read as milliliters per liter. The Imhoff cone is made of plastic or glass and is the proper piece of equipment to perform this analysis. It should not be confused with a settleometer, which is used to measure settling of mixed liquors in an activated sludge process.  More information may be found in the OWP, CSU-Sacramento textbook: Operation of Wastewater Treatment Plants Vol 2 (Seventh Edition), Chapter 16.


By Drew Hoelscher

The NaOCl bulk tank concentration has weakened overtime, and the operator is tasked with increasing the concentration from 4% to 10%. How much 12% NaOCl should the operator add, if the bulk tank is currently holding 20 gallons of 4% NaOCl solution?

A. 6 gallons

B. 12 gallons

C. 24 gallons

D. 60 gallons

Answer: D. NaOCl is commonly used as a disinfectant in water treatment. The concertation of NaOCl should be monitored closely so that the correct dosage can be calculated. NaOCl loses disinfection power over time, especially if it is stored in a warm environment or exposed to sunlight. NaOCl will decompose into sodium chlorate and sodium chloride, which eventually results in salt water under certain conditions.

About the authors

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.  


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