Exam Study Guide: Corrosive Gases; and Calculating Brake Horsepower

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 Pumping Fundamentals; and Operating Pressure. This time, you can test your knowledge about corrosive gases, and calculating brake horsepower.

Wastewater Treatment Sample Question

What gas has the following characteristics: highly corrosive, very dangerous at low levels (NIOSH IDLH = 100 ppm), has no color, and smells like rotten eggs?

A. Carbon monoxide
B. Methane
C. Hydrogen sulfide
D. Carbon dioxide

Answer: The answer to this question is C. Hydrogen sulfide is a very common gas found in our industry, whether in the collections system or in various places around the treatment plant. Being able to identify this dangerous gas and knowing what to do is very important to the wastewater operator.

Hydrogen sulfide is very corrosive, it will deteriorate parts of collections systems such as electrical components of lift stations, and it can weaken concrete (holding ladders in manholes) and cause structural failures. It is hazardous to humans, as it has the potential of shutting down your olfactory abilities very quickly. It's harmful even at low concentrations. Just the slight rotten egg odor should tell an operator to allow the space-time to air out.

More information may be found on the OSHA website.

Water Treatment Sample Question

What is the brake horsepower (BHp) of a 75 percent efficient pump that is pumping water at a rate of 300 gpm against 150 feet of total dynamic head (TDH)?

A. 5
B. 10
C. 15
D. 20

Answer: The correct answer is C. Water weighs 8.34 pounds per gallon, so moving it requires a lot of energy and work. Work is expressed in terms of foot-pounds. One horsepower is equal to 33,000 foot-pounds per minute, so multiplying the weight of the water (300 gpm X 8.34 pounds = 2,502 pounds) by the distance (150 TDH) and dividing by 33,000 foot-pounds per minute will calculate the water horsepower (WHp). 

The WHp is the horsepower transferred to the water from the pump. To determine the BHp, divide the WHp by the percent efficiency of the pump (0.75). The BHp is the output horsepower from the motor to the shaft. Motors are also not 100 percent efficient and you can determine the required electrical horsepower (EHp) by dividing the BHp by the percent efficiency of the motor.


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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