Exam Study Guide: Flow Velocity; and Chlorine Reactions

Maintaining your education is important, especially in a career that demands licensing exams. Prove you’re an expert operator by answering these questions and others from our Exam Study Guide Series.

Welcome back to TPO magazine's Exam Study Guide Series, which offers a pair of water/wastewater study questions with in-depth explanations of the answers. Last time, we covered a set of wastewater and drinking water treatment questions on the topics of Sludge Blanket Level; and Chemical Feed Calculations. This time, you can test your knowledge about flow velocity, and chlorine reactions.

Wastewater Treatment Sample Question

What is the recommended flow velocity in a primary clarifier?

A. 0.03 feet per second
B. 1.0 feet per second
C. 1.3 feet per second
D. 2.0 feet per second

Answer: The answer to this question is A. The desired velocity in a primary clarifier is about 0.03 fps. This velocity allows the settleable solids to fall to the bottom of the tank and the lighter material to float to the surface. When designed properly, the detention time for a primary clarifier is about 1.5 to 2 hours, while secondary clarifiers are slightly longer. 

Knowing velocities in the many areas of your plant and collection system will allow an operator to understand how each system works independently, but also dependently on other parts of the system.

Water Treatment Sample Question

What simplified reaction represents monochloramine?

A. NHCl2 + HOCl → NCl3 + H2O
B. NH2Cl + HOCl → NHCl2 + H2O
C. NH3 + HOCl →NH2Cl + H2O
D. Cl2 + H2O → HOCl + HCl

Answer: The correct answer is C. Many reactions take place when chlorine is added to water for disinfection. Chlorine destroyed by reducing agents dissolved in water such as hydrogen sulfide, ferrous ion, and/or manganous ion is initially what occurs. If the water disinfected with chlorine contains naturally occurring ammonia, the hypochlorous acid formed from the addition of chlorine reacts with the nitrogen within the ammonia compound and forms chloramines. 

The chloramine residual is dependent on the pH and chlorine to ammonia ratio by weight of the water. The ideal ratio of chlorine to ammonia is somewhat specific to each system. Systems looking to boost the chloramine residual in the distribution system may prefer a small amount of free ammonia available to react with the additional chlorine added at a booster station.


About the authors: Rick Lallish is the Water Pollution Control program director at the Environmental Resources Training Center (ERTC) of Southern Illinois University Edwardsville. He provides training for entry-level operators in the wastewater field and operators throughout the state looking to further their education. Lallish was also named the 2017 Illinois Operator of the Year and 2018 president of the Illinois Association of Water Pollution Control Operators.

Drew Hoelscher is the program director of drinking water operations at the Environmental Resources Training Center in Edwardsville, Illinois.



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