Exam Study Guide


Nitrosomonas and Nitrobacter bacteria perform what function in the activated sludge process?

A. They are responsible for denitrification.

B. They are required for restoring alkalinity to the process.

C. They convert the nitrate into nitrous oxide and nitrogen gas.

D. They convert ammonium into nitrite and nitrate.

Answer: D. Nitrosomonas and Nitrobacter are responsible for nitrification, the oxidation process where ammonium is first converted to nitrite, and then the nitrite is further oxidized to nitrate. Nitrification occurs mostly within an aerated basin like an aeration tank, but it can occur in other aerated locations like wetlands, rivers and streams, and within soils. We refer to these organisms as aerobic autotrophs. Answer choices A, B and C all refer to denitrification, where facultative heterotrophic bacteria reduce the nitrate to a final product of nitrogen gas. Other bacteria that convert ammonium to nitrate include Nitrosococcus and Nitrosospira. However, Nitrosomonas and Nitrobacter are the best known and most commonly referenced nitrifying organisms.


Which statement about treating colder water is accurate?

A. The colder the water, the longer it takes for particles to settle out.

B. Colder water requires less time for solids to float to the surface.

C. The colder the water, the less speed is required of the flash mixing equipment.

D. As water temperatures drop, less time is needed to accomplish settling.

Answer: A. Think about the change in density of water as it freezes in your kitchen freezer or ice maker. As the temperature falls, the activity of the water molecules slow down. The water becomes dense enough to change from a liquid state to solid. On the other hand, if we boil the same water on the stovetop, we can cause water to begin evaporating into steam, converting from liquid to vapor. In a settling basin, the settling velocity of particles is affected by the same changes in water density. Water molecules move slower as the water becomes denser, causing chemical reactions to become slower. The increased water density causes floc particles in the settling basin to settle more slowly, meaning it takes more detention time to achieve the target effluent turbidity and suspended solids goals. Most often, operators find that more energy is required to thoroughly mix the coagulants into the raw water, and increased flocculator speed is sometimes needed to ensure efficient particle collisions.

About the author

Ron Trygar, a certified environmental trainer, is the senior training specialist for water and wastewater programs at the University of Florida TREEO Center. He has worked in the wastewater industry for more than 30 years and holds Class A wastewater treatment operator and Class B drinking water operator licenses in Florida. 


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