Exam Study Guide: Changing Chlorine Cylinders; and Calculating Chemical Feed

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 Bacteria in Anoxic Zones; and Naturally Occurring Ammonia. This time, you can test your knowledge about changing chlorine gas cylinders, and calculating chemical feed.

Wastewater Treatment Sample Question

When changing a 150-pound chlorine gas cylinder, how many times may you reuse the lead washer?

A. As often as possible
B. Twice 
C. One time
D. Never 

Answer: The answer to this question is D. Safety should be your prime concern when working around chlorine gas. It is very hazardous and can be fatal if proper precautions are not taken. Lead gaskets should never be reused under any circumstances. They are flattened and deformed when properly sealed, and they should never be manipulated to be placed back on the cylinder. In almost every case, a proper seal will not be attained and this will allow for a chlorine gas release. This not only can cause health issues, but it can cause corrosion in your chlorine feed building. 

Water Treatment Sample Question

A chemical feed pump is delivering 12.0 mL/min of 10% NaOCl to the clearwell. What is the chlorine demand if the flow rate through the clearwell is 320,000 gallons per day and the chlorine residual in the clearwell effluent is 1.1 mg/L?

A. 0.00 mg/L
B. 0.33 mg/L
C. 1.10 mg/L
D. 1.43 mg/L

Answer: The correct answer is B. There are many variables taken into consideration when calculating the proper chlorine dosage for any given water. However, it is of upmost importance to dose the water with enough chlorine to satisfy the chlorine demand and to provide an adequate chlorine residual throughout the entire distribution system. In other words, chlorine dosage is equal to chlorine demand plus chlorine residual, and chlorine demand is equal to chlorine dosage minus chlorine residual. See the calculations below:

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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