TEXAS WASTEWATER CLASS C NEWEST ACTUAL EXAM COMPLETE 300 QESTIONS
AND CORRECT DETAILED ANSWERS (VERIFIED ANSWERS) |ALREADY GRADED A+
Question 1
When gas chlorine is added to water, which two compounds are primarily formed?
A) Sodium hypochlorite and water
B) Hypochlorous acid (HOCl) and hydrochloric acid (HCl)
C) Chloramines and nitrogen gas
D) Hydrogen sulfide and methane
E) Calcium hypochlorite and ozone
Correct Answer: B) Hypochlorous acid (HOCl) and hydrochloric acid (HCl)
Rationale: The text explicitly states that when gas chlorine is added to water, the chemical
reaction results in the formation of HOCl (hypochlorous acid) and HCl (hydrochloric acid).
Hypochlorous acid is the primary disinfecting agent in this reaction.
Question 2
At a pH of 7 and a temperature of 20°C, what is the approximate concentration of
Hypochlorous acid (HOCl)?
A) 10%
B) 25%
C) 50%
D) 80%
E) 100%
Correct Answer: D) 80%
Rationale: The effectiveness of chlorine is pH-dependent. The text specifies that at 20°C and a
pH of 7, the concentration of the effective disinfectant HOCl is approximately 80%. As pH rises,
HOCl dissociates into the less effective hypochlorite ion.
Question 3
What compound is formed when chlorine combines with ammonia present in wastewater?
A) Hydrochloric acid
B) Chloramines
C) Methane
, 2
D) Hydrogen sulfide
E) Sulfuric acid
Correct Answer: B) Chloramines
Rationale: Chlorine reacts with ammonia (
𝑁𝐻3
) in wastewater to form combined chlorine residuals known as chloramines. These are weaker
disinfectants than free chlorine but are longer-lasting.
Question 4
Calculate the chlorine residual if the dosage is 12 mg/L and the chlorine demand is 11.5
mg/L.
A) 0.5 mg/L
B) 1.5 mg/L
C) 12.0 mg/L
D) 23.5 mg/L
E) 0.0 mg/L
Correct Answer: A) 0.5 mg/L
Rationale: The formula for residual is Dosage - Demand = Residual.
12.0 mg/L − 11.5 mg/L = 0.5 mg/L
. The text notes this residual would likely be a monochloramine given the presence of ammonia.
Question 5
Which of the following is NOT a common use for chlorine in wastewater treatment?
A) Bulking control
B) Disinfection
C) Odor control
D) pH adjustment for digesters
E) Destruction of grease
, 3
Correct Answer: D) pH adjustment for digesters
Rationale: The text lists the uses of chlorine as bulking control, disinfection, odor control,
killing psychoda bacteria, and destroying grease. It is not listed as a chemical used to adjust the
pH of anaerobic digesters.
Question 6
According to TCEQ regulations, what is the minimum required chlorine residual and
contact time?
A) 0.5 mg/L after 15 minutes
B) 1.0 mg/L after 20 minutes
C) 2.0 mg/L after 20 minutes
D) 1.0 mg/L after 30 minutes
E) 0.2 mg/L after 10 minutes
Correct Answer: B) 1.0 mg/L after 20 minutes
Rationale: The Texas Commission on Environmental Quality (TCEQ) requires a minimum
residual of 1.0 mg/L after a detention time of 20 minutes to ensure adequate disinfection of the
effluent.
Question 7
Besides disinfection, chlorine can be used to control which specific operational problem in
trickling filters?
A) High dissolved oxygen
B) Ponding
C) High pH
D) Low BOD
E) Excessive flow
Correct Answer: B) Ponding
Rationale: The text states that chlorine acts as an aid in trickling filter operation specifically to
break up ponding problems, likely by reducing excessive biomass growth that clogs the filter
media.
, 4
Question 8
Chlorine is also effective in killing which specific type of bacteria often found in trickling
filters?
A) E. coli
B) Psychoda bacteria (filter flies)
C) Nitrifying bacteria
D) Methane formers
E) Acid formers
Correct Answer: B) Psychoda bacteria (filter flies)
Rationale: The text mentions that chlorine may be used to kill psychoda bacteria (the larvae of
which are known as filter flies) which can become a nuisance in trickling filter plants.
Question 9
A chlorine contact chamber must be designed to provide a detention time of at least 20
minutes at what flow rate?
A) Average daily flow
B) Minimum daily flow
C) Peak flow
D) 50% of design flow
E) Future expansion flow
Correct Answer: C) Peak flow
Rationale: To ensure public safety and compliance, the design standard requires that the 20-
minute contact time be maintained even during peak flow conditions, not just average flow.
Question 10
Gas chlorination facilities should be equipped with a bottle of which liquid to detect leaks?
A) Distilled water
B) Vinegar
C) Ammonium hydroxide
D) Sodium hypochlorite
E) Sulfuric acid
AND CORRECT DETAILED ANSWERS (VERIFIED ANSWERS) |ALREADY GRADED A+
Question 1
When gas chlorine is added to water, which two compounds are primarily formed?
A) Sodium hypochlorite and water
B) Hypochlorous acid (HOCl) and hydrochloric acid (HCl)
C) Chloramines and nitrogen gas
D) Hydrogen sulfide and methane
E) Calcium hypochlorite and ozone
Correct Answer: B) Hypochlorous acid (HOCl) and hydrochloric acid (HCl)
Rationale: The text explicitly states that when gas chlorine is added to water, the chemical
reaction results in the formation of HOCl (hypochlorous acid) and HCl (hydrochloric acid).
Hypochlorous acid is the primary disinfecting agent in this reaction.
Question 2
At a pH of 7 and a temperature of 20°C, what is the approximate concentration of
Hypochlorous acid (HOCl)?
A) 10%
B) 25%
C) 50%
D) 80%
E) 100%
Correct Answer: D) 80%
Rationale: The effectiveness of chlorine is pH-dependent. The text specifies that at 20°C and a
pH of 7, the concentration of the effective disinfectant HOCl is approximately 80%. As pH rises,
HOCl dissociates into the less effective hypochlorite ion.
Question 3
What compound is formed when chlorine combines with ammonia present in wastewater?
A) Hydrochloric acid
B) Chloramines
C) Methane
, 2
D) Hydrogen sulfide
E) Sulfuric acid
Correct Answer: B) Chloramines
Rationale: Chlorine reacts with ammonia (
𝑁𝐻3
) in wastewater to form combined chlorine residuals known as chloramines. These are weaker
disinfectants than free chlorine but are longer-lasting.
Question 4
Calculate the chlorine residual if the dosage is 12 mg/L and the chlorine demand is 11.5
mg/L.
A) 0.5 mg/L
B) 1.5 mg/L
C) 12.0 mg/L
D) 23.5 mg/L
E) 0.0 mg/L
Correct Answer: A) 0.5 mg/L
Rationale: The formula for residual is Dosage - Demand = Residual.
12.0 mg/L − 11.5 mg/L = 0.5 mg/L
. The text notes this residual would likely be a monochloramine given the presence of ammonia.
Question 5
Which of the following is NOT a common use for chlorine in wastewater treatment?
A) Bulking control
B) Disinfection
C) Odor control
D) pH adjustment for digesters
E) Destruction of grease
, 3
Correct Answer: D) pH adjustment for digesters
Rationale: The text lists the uses of chlorine as bulking control, disinfection, odor control,
killing psychoda bacteria, and destroying grease. It is not listed as a chemical used to adjust the
pH of anaerobic digesters.
Question 6
According to TCEQ regulations, what is the minimum required chlorine residual and
contact time?
A) 0.5 mg/L after 15 minutes
B) 1.0 mg/L after 20 minutes
C) 2.0 mg/L after 20 minutes
D) 1.0 mg/L after 30 minutes
E) 0.2 mg/L after 10 minutes
Correct Answer: B) 1.0 mg/L after 20 minutes
Rationale: The Texas Commission on Environmental Quality (TCEQ) requires a minimum
residual of 1.0 mg/L after a detention time of 20 minutes to ensure adequate disinfection of the
effluent.
Question 7
Besides disinfection, chlorine can be used to control which specific operational problem in
trickling filters?
A) High dissolved oxygen
B) Ponding
C) High pH
D) Low BOD
E) Excessive flow
Correct Answer: B) Ponding
Rationale: The text states that chlorine acts as an aid in trickling filter operation specifically to
break up ponding problems, likely by reducing excessive biomass growth that clogs the filter
media.
, 4
Question 8
Chlorine is also effective in killing which specific type of bacteria often found in trickling
filters?
A) E. coli
B) Psychoda bacteria (filter flies)
C) Nitrifying bacteria
D) Methane formers
E) Acid formers
Correct Answer: B) Psychoda bacteria (filter flies)
Rationale: The text mentions that chlorine may be used to kill psychoda bacteria (the larvae of
which are known as filter flies) which can become a nuisance in trickling filter plants.
Question 9
A chlorine contact chamber must be designed to provide a detention time of at least 20
minutes at what flow rate?
A) Average daily flow
B) Minimum daily flow
C) Peak flow
D) 50% of design flow
E) Future expansion flow
Correct Answer: C) Peak flow
Rationale: To ensure public safety and compliance, the design standard requires that the 20-
minute contact time be maintained even during peak flow conditions, not just average flow.
Question 10
Gas chlorination facilities should be equipped with a bottle of which liquid to detect leaks?
A) Distilled water
B) Vinegar
C) Ammonium hydroxide
D) Sodium hypochlorite
E) Sulfuric acid
