Exam Study Guide - October 2019

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WASTEWATER- By Rick Lallish

What is the expected or recommended time it should take for the contents in an oxidation ditch to travel the complete circuit of the ditch or go from rotor to rotor?

A. 30 seconds to 1.5 minutes

B. 2.0 minutes to 2.5 minutes

C. 3.0 minutes to 6.0 minutes

D. 7.5 minutes to 9.0 minutes

Answer: C. For proper treatment time and flow velocities, the textbooks say the velocity in the ditch should be maintained at 1.0 to 1.5 feet per second. This will prevent the organics from settling in the ditch. This should allow the treatment contents to flow around the ditch or rotor to rotor in 3.0 to 6.0 minutes. In an oxidation ditch, the treatment time is related to the flow of the mixed liquor and is typically controlled by an adjustable weir.

DRINKING WATER - By Drew Hoelscher

What simplified reaction represents monochloramine?

A. NHCl2 + HOCl à NCl3 + H2O

B. NH2Cl + HOCl à NHCl2 + H2O

C. NH3 + HOCl à NH2Cl + H2O

D. Cl2 + H2O à HOCl + HCl

Answer: C. Many reactions take place when chlorine is added to water for disinfection. Chlorine destruction by reducing agents dissolved in water such as hydrogen sulfide, ferrous ion or manganous ion is initially what occurs. If the water disinfected with chlorine contains naturally occurring ammonia, the hypochlorous acid formed from the addition of chlorine reacts with the nitrogen within the ammonia compound and forms chloramines. The chloramine residual is dependent on the pH and on the chlorine-to-ammonia ratio (by weight) of the water. The ideal ratio of chlorine to ammonia is somewhat specific to each system. Systems looking to boost the chloramine residual in the distribution system may prefer to have small amount of free ammonia available to react with additional chlorine added at a booster station.

About the authors

Rick Lallish is water pollution control program director and Drew Hoelscher is program director of drinking water operations at the Environmental Resources Training Center of Southern Illinois University Edwardsville.

