WTO - Water Treatment Operator Disinfection Processes Questions and Answers 1 — Questions and Answers
Question 1: A water treatment plant is using chlorine gas to disinfect water with a flow of 2.5 MGD. The operator needs to maintain a chlorine residual of 1.2 mg/L. If the chlorine demand of the water is 1.8 mg/L, what is the required chlorine dose in mg/L?
- 0.6 mg/L
- 1.2 mg/L
- 1.8 mg/L
- 3.0 mg/L (Correct answer)
Correct answer: 3.0 mg/L
The chlorine dose is calculated by adding the chlorine demand and the desired chlorine residual. In this scenario, Dose = Demand (1.8 mg/L) + Residual (1.2 mg/L), which equals 3.0 mg/L.
Question 2: Which of the following factors would decrease the effectiveness of chlorine disinfection?
- Increased water temperature
- Low turbidity
- High pH (Correct answer)
- Increased contact time
Correct answer: High pH
A higher pH reduces the effectiveness of chlorine disinfection. Chlorine is more effective at a lower pH because it favors the formation of hypochlorous acid (HOCl), which is a more potent disinfectant than the hypochlorite ion (OCl-) that dominates at higher pH levels.
Question 3: A water utility is considering switching from chlorine to chloramine for secondary disinfection in its distribution system. What is a primary advantage of using chloramines?
- They are a stronger and faster-acting disinfectant than chlorine.
- They produce a more stable, longer-lasting residual. (Correct answer)
- They are completely free from forming any disinfection byproducts.
- They are easier to remove from water for dialysis patients.
Correct answer: They produce a more stable, longer-lasting residual.
Chloramines are more stable than free chlorine, providing a longer-lasting disinfectant residual throughout the distribution system. This helps to control bacterial regrowth in the pipes. While they are a weaker disinfectant than chlorine, their stability is a key advantage for maintaining water quality over long distances.
Question 4: Disinfection byproducts (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs) are formed when:
- UV light reacts with inorganic matter in the water.
- Ozone is used as the primary disinfectant in low-alkalinity water.
- Chlorine reacts with natural organic matter (NOM) present in the water. (Correct answer)
- Ammonia is added to water that has already been chlorinated.
Correct answer: Chlorine reacts with natural organic matter (NOM) present in the water.
Disinfection byproducts like THMs and HAAs are formed from the chemical reaction between a disinfectant, most commonly chlorine, and naturally occurring organic matter (NOM) found in the source water.
Question 5: An operator is calculating the CT value to ensure adequate inactivation of Giardia. If the free chlorine residual (Concentration) is 1.5 mg/L and the contact time (Time) is 40 minutes, what is the calculated CT value in mg-min/L?
- 26.7
- 41.5
- 60.0 (Correct answer)
- 15.0
Correct answer: 60.0
The CT value is calculated by multiplying the disinfectant concentration (C) in mg/L by the contact time (T) in minutes. Therefore, CT = 1.5 mg/L * 40 min = 60 mg-min/L.
Question 6: Which of the following is a significant limitation of using Ultraviolet (UV) light for disinfection in a water treatment plant?
- It is ineffective against chlorine-resistant pathogens like Cryptosporidium.
- It significantly alters the taste and odor of the water.
- It does not provide a residual disinfectant to protect water in the distribution system. (Correct answer)
- It requires the handling of hazardous chemicals for operation.
Correct answer: It does not provide a residual disinfectant to protect water in the distribution system.
UV disinfection is a physical process that inactivates microorganisms as they pass through the UV reactor. It does not leave behind a chemical residual in the water to provide ongoing protection against recontamination in the distribution system.
WTO - Water Treatment Operator Disinfection Processes Questions and Answers 1
A water treatment plant is using chlorine gas to disinfect water with a flow of 2.5 MGD.
The operator needs to maintain a chlorine residual of 1.2 mg/L.
If the chlorine demand of the water is 1.8 mg/L, what is the required chlorine dose in mg/L?