Exam Study Guide: Fusible Plugs; and Sodium Thiosulfate

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Exam Study Guide: Fusible Plugs; and Sodium Thiosulfate

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 Breakpoint Chlorination Method; and TTHM Removal. This time, you can test your knowledge about fusible plugs and sodium thiosulfate.

Wastewater Treatment Sample Question:

The fusible plug on a chlorine cylinder is designed to:

A) Melt between 158-165°F

B) Melt between 1200-1500°F

C) Melt when placed in the direct sunlight

D) Prevent a dangerous vacuum from forming in the cylinder

Answer: The answer is A, melt between 158-165°F. The fusible plugs found on chlorine cylinders, ton containers and larger containers are designed to melt and allow the pressure inside the container to decrease to safer levels. If the pressure is allowed to build within the cylinder, there is the possibility of a catastrophic release of the entire contents of that container. While liquid chlorine does not support a fire, it can react with other compounds to make fire. Also, the release of the liquid chlorine will expand to a gas about 460 times causing a very dangerous situation for first responders and emergency personnel.

The fusible plug is made of a metal alloy designed to melt. Once the plug melts away, the gaseous chlorine will spew out of the small hole. As the temperature of the released chlorine near the hole drops and chlorine icing slows the leak, the pressure inside the cylinder is also decreasing. This cycle will continue until all the chlorine is escaped or the leak is capped.

It is very important (and required) to display signage on the plant site that alerts first responders that chemicals like chlorine are on site. Wind socks allow the haz-mat teams to approach the fire and potentially leaking cylinders from a safer path and also indicate the wind direction for rapid evacuation decision making

Water Treatment Sample Question: 

Why is sodium thiosulfate added to sample bottles or sample bags used for total coliform testing?

A) Sodium thiosulfate increases chlorine efficiency in the absence of air

B) Sodium thiosulfate neutralizes only chloramine residual

C) Sodium thiosulfate is a method of pretreatment for coliform group bacteria

D) Sodium thiosulfate neutralizes all chlorine residual present

Answer: The answer is D, sodium thiosulfate neutralizes all chlorine residual present. The total coliform sample is collected as a grab sample from areas of the water distribution system that are representative of that system. This bacteriological sample represents the quality of the water as if it were consumed at that point in time. Sodium thiosulfate is a chemical that removes the available chlorine residual from the sample bottle, as if the water were consumed at that point in time. The chlorine is neutralized to zero residual by the dechlorinating action of the sodium thiosulfate, and must be confirmed as such by the lab analyst before the sample is used in the coliform test method.

The interior of the sample bottle including the dechlorinating agent within is sterilized before delivery to the water plant lab. Do not touch the inside of the bottle or the tablet of sodium thiosulfate or false readings may result.

The sodium thiosulfate is not a preservative; it is solely for the dechlorination of the sample contents. Placing the sealed container with the sample inside on ice in a cooler or in a lab refrigerator to 4oC is considered the preservation method.


About the author: Ron Trygar is the senior training specialist for water and wastewater programs at the University of Florida’s TREEO Center. Previously, he was the wastewater process control specialist at Hillsborough County Public Utilities in Tampa, Florida. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. Trygar became a Certified Environmental Trainer (CET) in 1998 and has since provided training for associations and regulatory agencies such as Florida Department of Environmental Protection (FDEP); Florida Water and Pollution Control Operators Association Short Schools; USABlueBook; Florida Water Environment Association sponsored training events; and local school environmental programs. Working alongside the FDEP Northeast District, Trygar helped begin the Florida Rural Water Association and FDEP joint operator certification review classes that are still given around the state today. He holds a Florida Class A wastewater treatment operator’s license and a Florida Class B drinking water operator’s license.



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