Exam Study Guide: The MLE Process and Monochloramine Production

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Exam Study Guide: The MLE Process and Monochloramine Production

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Welcome back to TPO magazine's new and improved Exam Study Guide Series, which offers a pair of water/wastewater study questions each week with in-depth explanations of the answers. We covered a set of drinking water treatment questions last week on the topics of water chemistry and water softening. This week, you can test your knowledge about the MLE process for biological nitrogen removal, and the production and use of monochloramine. Take a look at the multiple-choice sample questions and answer explanations below.

Sample Question No. 1:

Regarding the Modified Ludzack-Ettinger (MLE) process for biological nitrogen removal in wastewater treatment, which statement below best describes the treatment plant flow scheme?​

A) The anoxic basin is situated after the main aeration basin to allow nitrification and phosphorus release using the RAS to enhance the nitrogen degasification.

B) The anaerobic (fermentation) basin is situated after the aeration basin and post anoxic basin to allow efficient phosphorus and nitrogen uptake. 

C) The aeration basin blowers or mechanical aerators are operated in an on-and-off cycle which allows both nitrification and denitrification simultaneously.

D) The anoxic basin is situated ahead of the aeration basin, and an internal recycle pump system sends nitrate-rich MLSS back to the anoxic basin for denitrification.

Answer: The answer is D, the anoxic basin is situated ahead of the aeration basin and an internal recycle pump system sends nitrate rich MLSS back to the anoxic basin for denitrification. The MLE process flow scheme is a popular treatment method for nitrogen removal and can be found as separate unit processes or as a part of the Carrousel/oxidation ditch process using pre-denitrification. Nitrification takes place in the aeration basins that follow the anoxic tanks, and nitrate-rich MLSS is pumped or otherwise diverted to the preceding anoxic basin. Denitrification of the nitrate takes place with the addition of influent wastewater containing carbon, the nitrate recycle stream and the return sludge from the clarifiers. The MLE process takes advantage of the carbon found in the influent waste stream to avoid the use of other carbon sources like methanol or acetate that must be purchased.

Sample Question No. 2:

Many utilities are modifying their disinfection method to eliminate the storage, use and handling of 100 percent gaseous chlorine. In order to prevent the formation of harmful disinfection byproducts, chloramination is gaining popularity, especially the production and use of monochloramine. What is the necessary ratio of chlorine to ammonia to form monochloramine? 

A) One part of ammonia to one part of chlorine 

B) One part of ammonia to three parts of chlorine

C) One part of chlorine to three parts of ammonia

D) One part of ammonia to ten parts of chlorine

Answer: The answer is B, one part of ammonia to three parts of chlorine. A ratio of three parts of chlorine and one part of ammonia will produce monochloramine, the most common chloramine used in water treatment. Monochloramine produces the least amount of chlorine-like taste and odor complaints, but may not be as powerful as dichloramine to kill pathogens. Nitrogen tri-chloride, or tri-chloramine, is produced at a higher chlorine to ammonia ratio at lower pH values.

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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