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 Falling Chlorine Residuals; and Total Trihalomethane Formation. This time, you can test your knowledge about the breakpoint chlorination method, and TTHM removal.
Wastewater Treatment Sample Question:
Using the breakpoint chlorination method of ammonia removal, approximately how many parts of chlorine are needed to remove one part of ammonia?
A) 2 parts chlorine to one part ammonia
B) 5 parts chlorine to one part ammonia
C) 10 parts chlorine to one part ammonia
D) 15 parts chlorine to one part ammonia
Answer: The answer is C, 10 parts chlorine to one part ammonia. To reach what is commonly referred to as breakpoint, approximately 10 parts of chlorine must be fed to wastewater effluent that contains ammonia. When chlorine and pure water combine, a process known as hydrolysis occurs, producing hypochlorous acid (HOCl), hypochlorite ion (OCl-) and hydrochloric acid (HCl). We call this new mixture a chlorine solution. Out of these three products, the HOCl and OCl- are the most effective as disinfectants. The water pH and temperature play a part in the ratio of these two compounds.
When the chlorine solution is applied to wastewater effluent, the available HOCl and OCl- are consumed by what is known as chlorine demand. Chlorine demand comes from organic and inorganic solids, soluble iron, soluble manganese, dissolved hydrogen sulfide and nitrite. Nitrite alone accounts for a 5:1 demand ratio – meaning that one part of nitrite can consume 5 parts of available HOCl and OCl-.
Once the chlorine demand is met, the chlorine reacts with any ammonia present. This reaction first produces compounds known as chloramines, then essentially destroys the chloramines as the chlorine to ammonia ratio increases. Once the chloramines have been destroyed, or oxidized by the HOCl and OCl- , the breakpoint is reached. The ammonia is oxidized to nitrogen gas and is released to the atmosphere, and free available chlorine residual is realized.
This entire reaction requires a dosage of about 10 parts of chlorine to convert one part of ammonia to nitrogen gas.
Water Treatment Sample Question:
Which of the following processes is effective in removing TTHMs after they have formed?
A) Slow sand filtration
B) Nanofiltration
C) Aeration
D) Coagulation, flocculation and sedimentation
Answer: The answer is C, aeration. Trihalomethanes and some other disinfection byproducts (DBPs) are known as volatile organic contaminants (VOCs), which means they can be easily become gases or vapors, and they contain carbon. Along with carbon, VOCs can also contain atoms of chlorine, hydrogen, bromine, sulfur, oxygen and/or nitrogen.
Since trihalomethane is a volatile compound and can be easily vaporized, it makes sense that aeration of the treated water can be an effective method of removing these DBPs. In fact, the sampling protocol for TTHM requires that no air be present in the sample bottle after collecting the sample, otherwise the VOCs can be lost from the water matrix.
The other answer choices shown may be effective at removing the TTHM precursors, the forms of organic acids present in the source water, but would not be effective at removing the dissolved gas forms of the established TTHMs.
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.
