Exam Study Guide: Biological Activity in Lagoons; and Calcium Hardness Calculation

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 Activated Sludge Conditions; and Well Screens.This time, you can test your knowledge about biological activity in lagoons, and calcium hardness calculation.

Wastewater Treatment Sample Question

For every 10-degree Celsius (18-degree Fahrenheit) drop in water temperature, what effect does this have on the biological process?  Hint: think lagoon operations.

A. No effect
B. Biological activity doubles
C. Biological activity halves
D. Biological activity halts

Answer: The answer to the question is C. Wastewater lagoons and ponds are sensitive to water temperatures. A 10-degree Celsius or 18-degree Fahrenheit drop in temperature will cause the biological active to decrease by 50%. The inverse is also true — the same increase in temperature will cause the biological activity to double.

This is a useful parameter to consider when operating lagoons or ponds in colder climates or during seasonal changes. It is important for all operators to understand the different modes of wastewater treatment and the equipment we use to perform the operations.

Water Treatment Sample Question

To report calcium hardness in mg/L as calcium carbonate (CaCO3) equivalent, ________ the calcium concentration in mg/L by ________.

A. Multiply, 2.5
B. Divide, 2.5
C. Multiply, 4.1
D. Divide, 4.1

Answer: The correct answer is A. The level of hardness is dependent on the concentration of divalent metallic cations, such as calcium and magnesium dissolved in the water. These dissolved ions are important and necessary to help protect the distribution system against corrosion. 

With hardness usually reported as CaCO3 equivalent, it is important for an operator to be familiar with how to calculate the equivalent weights of calcium and CaCO3. More information is available in the California State University, Sacramento training manual, Water Treatment Plant Operation Volume II – Softening, Chapter 14.

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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