Exam Study Guide: Removing Hydrogen Sulfide

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When faced with a licensing exam, don't struggle with test anxiety! Use the question below and others from our Exam Study Guide series to study and prepare for exam questions. Take a look at this sample water treatment licensing question along with an explanation of the correct answer.

Sample question:

What is the correct chlorine dosage when trying to eliminate a hydrogen sulfide problem if the H2SO4 is 14 mg/l?

A. Chlorine shouldn’t be used for hydrogen sulfide removal.
B. It is a 1:1 ratio, so if the concentration is 14 mg/l then 14 mg/l of 100 percent chlorine should be used.
C. The proper ratio is 6:1, so 6 X 14mg/l = 84mg/l of 100 percent pure chlorine would be needed.
D. It depends on water temperature because hydrogen sulfide is created in warm environments.


In water treatment, the source water should not contain hydrogen sulfide concentrations unless the bottom of a storage reservoir (raw water) has gone septic. However, in water treatment operations, it is not uncommon for customers to contact the utility regarding rotten-egg odors. Hydrogen sulfide is created when sulfates are detected in the treatment process but not removed, which is not required. When this parameter is left unchecked — in the hot water tank when the water is heated — hydrogen sulfide can form in dead end areas and particularly in homes with older hot water tanks where the sacrificial anode is in poor shape. Without the presence of a chlorine residual, hydrogen sulfide will almost always form because the conditions for the chemical reaction and compound creation are present.

So, the correct answer is C

About the author
Mike Smith is program coordinator and lead faculty of the Water Quality Management program at Red Rocks Community College in Lakewood, Colo.

He has been in charge of the multi-faceted training program since 1996.


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