Exam Study Guide: Sludge Blanket Level; and Chemical Feed Calculations

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 Types of Liquid Waste; and Colloidal Particles in Water. This time, you can test your knowledge about sludge blanket level, and chemical feed calculation.

Wastewater Treatment Sample Question

What is the maximum sludge blanket level in a secondary (activated sludge) clarifier?

A. 5%
B. 10%
C. 25%
D. 50%

Answer: The answer to the question is C. Basic operation and knowledge of clarifier process controls are fundamentals that should be understood by most operators. In activated sludge processes, the sludge blanket should never exceed 25% of the sidewall depth of the secondary clarifier. In some texts, a 2-foot blanket is recommended. Whenever this depth is approaching or exceeding this depth, the operator should take precautions. This is usually an indicator of problems such as filamentous bulking or equipment malfunction. The blanket should be checked on a daily basis, and preferably at approximately the same time each day.

Water Treatment Sample Question

An operator is feeding a chemical with a specific gravity of 1.2 at a rate of 10 milliliters per minute. How many pounds of chemical did the operator feed in 24 hours? 

A. 31
B. 25
C. 38
D. 12

Answer: The correct answer is C. The first step in solving this problem requires converting milliliters per minute to gallons per day by multiplying 10 milliliters per minute by 1,440 minutes per day and then dividing by 3,785 milliliters per gallon. The second step requires converting the gallons per day of chemical to pounds per day by multiplying the gallons per day by 8.34 and the specific gravity of 1.2.

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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