Exam Study Guide: Falling Chlorine Residuals; and Total Trihalomethane Formation

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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 Hydraulic Loading Math; and RO Treatment Chemicals. This time, you can test your knowledge about falling chlorine residuals and total trihalomethane formation.

Wastewater Treatment Sample Question:

The plant effluent chlorine residual is falling, and inspection of the 150-pound cylinder and chlorine feed equipment shows ice forming on the chlorine cylinder, the regulator and feed lines.  What is the best method to fix the falling chlorine residual?

A) Switch disinfectant to sodium hypochlorite solution

B) Place a second chlorine cylinder online

C) Lay the cylinder on its side so liquid is feeding instead of gas

D) Place a 1,500-watt space heater next to the cylinder to thaw the ice

Answer: The answer is B, place a second chlorine cylinder online. Out of the choices given, answer B is the best choice. Since one 150-pound cylinder of chlorine can only deliver a constant feed rate of about 40 pounds of gas chlorine per day before chlorine icing or freezing occurs, a second cylinder manifolded into the disinfection piping could allow up to 80 pounds per day to be fed.

Many utilities have switched to sodium hypochlorite (bleach) as their primary disinfectant, but to do this in a situation like the one shown above could be too time consuming, especially if not all the necessary equipment is on site.

What about answers C and D? Never, ever lay a 150-pound chlorine cylinder on its side and attempt to withdraw the liquid rather than the gas. The feed equipment is only designed to handle gaseous chlorine and should never be used to feed liquefied chlorine. A leak of one part of liquefied chlorine can expand about 460 times to gas chlorine and cause a major chlorine leak emergency. Answer D is also a dangerous option and one that chlorine suppliers have warned operators about in the past. The high temperature of the space heater can be hot enough to melt the fusible plug in the valve causing a chlorine gas leak.

Water Treatment Sample Question: 

Which condition causes faster formation of total trihalomethane (TTHM)?

A) Low pH and warm water temperature

B) High pH and warm water temperature

C) Low water temperature and high pH

D) High hardness and low water temperature

Answer: The answer is B, high pH and warm water temperature. The formation of total trihalomethane is pH and temperature dependent, among other things. Trihalomethanes form in water treatment and water distribution systems when organic material containing humic and fulvic acids react with free chlorine residual. Free chlorine residual can be in the form of hypochlorous acid (HOCl) and/or the hypochlorite ion (OCl-). Higher pH levels, over 8.0 for example, tend to favor the formation of the hypochlorite ion over hypochlorous acid. Hypochlorite ion is very reactive with organic acids and form various combinations of trihalomethane. The four trihalomethane compounds of concern are chloroform, bromodichloromethane, dibromochloromethane and bromoform.

Warm water temperature will certainly allow faster and more thorough chemical reactions, thereby creating TTHMs faster than in colder water conditions.

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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