Exam Study Guide: Solids Retention Time; and Oxidizing Agents

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 Influent Organic Matter Removal; and Chemical Feed Rates. This time, you can test your knowledge about retention time, and oxidizing agents.

Wastewater Treatment Sample Question:

When operating a lagoon system, what is the difference between hydraulic retention time (HRT) and solids retention time (SRT)?

A. The SRT is significantly greater than the HRT
B. The HRT is significantly greater than the SRT
C. The HRT and the SRT are the same
D. The HRT and the SRT cannot be compared

Answer: The answer to this question is C. Lagoon treatment systems operate differently than fixed film or activated sludge systems. The operator must recognize that the solids, as they pass through a lagoon, settle at the bottom of the pond continuously and are not wasted (as they are in an activated sludge plant). They're not recycled and are limited to the microorganisms already in the lagoon and those introduced in the influent flow.

According to the WEF OM-9 manual, “Because the lagoon system is a pass-through system, the wastewater and microorganisms spend the same time in the process. The HRT and SRT are the same.”

Water Treatment Sample Question

What is a common chemical compound fed to oxidize iron and/or manganese? 

A. Ca(OH)2
B. Al2(SO4)3
C. KMnO4

Answer: The correct answer is C. The chemical symbol KMnO4 represents potassium permanganate. Potassium permanganate is a strong oxidizing agent that has the ability to oxidize soluble iron and manganese into insoluble iron oxide and manganese dioxide. To ensure proper feed rates, perform a series of Jar tests to determine the proper dose. Dosing the water with too much KMnO4 can cause the water to turn pink and dosing the water with too little KMnO4 will allow soluble iron and/or manganese to pass through the treatment plant causing discolored water complaints from customers.

About the authors: Rick Lallish is the Water Pollution Control program director at the Environmental Resources Training Center 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 also was 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 (ERTC) in Edwardsville, Illinois.


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