Exam Study Guide: Disinfection Contact Time; and Effluent Coloration

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 Quality; and Manganese Greensand Filters. This time, you can test your knowledge about disinfection contact time, and pink coloration in manganese greensand filter effluent.

Wastewater Treatment Sample Question

Contact time varies from state to state. Most states require either 15 or 30 minutes at design average flow rate. Why is a minimum contact time so important for disinfection?

A. Chlorine gas is hazardous and has to be regulated.

B. To allow excess chlorine gas to dissipate.

C. To allow the chlorine time to inactivate the microorganisms.

D. To allow the BOD time to bleed off before disinfection.

Answer: Disinfection using chlorine is based contact time, therefore the answer is C. It takes time for the chlorine to react with the components of the microorganism. There are several factors affecting the time needed to achieve disinfection. These are concentration, temperature, pH, mixing and level of disinfection required.

The contact time is based on the amount of flow moving through the facility and contact basin size. The operator cannot control either of these, but can control the dosage to achieve proper disinfection. This knowledge is important for the operator for many reasons, such as failing to meet chlorine residual limits or fecal coliform reporting. Understanding how to control the chlorine dosage is an important process control tool for successful disinfection.

Water Treatment Sample Question

What should an operator do if he or she notices a faint pink color in the manganese greensand filter effluent?

A. Increase the potassium permanganate dosage.

B. Decrease the flow of the water through the manganese greensand filter.

C. Stop the potassium permanganate feed for a short period and restart at a slightly lower dosage.

D. Be satisfied with the chemical feed rates.

Answer: The answer is C. Potassium permanganate is a strong oxidant used in groundwater treatment plants to oxidize iron and manganese. The oxidized iron and manganese is filtered out of solution as it passes through the manganese greensand filter media. An excessive potassium permanganate dose will cause the filtered effluent to have a pink color. To avoid a pink colored filter effluent water, simply perform a series of jar test to help determine the correct potassium permanganate feed rate.

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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