WTO - Water Treatment Operator Disinfection Processes 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?

Accepted Answer

3.0 mg/L

Answer

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?

Accepted Answer

High pH

Answer

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?

Accepted Answer

They produce a more stable, longer-lasting residual.

Answer

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:

Accepted Answer

Chlorine reacts with natural organic matter (NOM) present in the water.

Answer

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?

Accepted Answer

60.0

Answer

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.

(WTO) Water Treatment Operator Certification Practice Test

(WTO) Water Treatment Operator Certification Practice Test

WTO - Water Treatment Operator Disinfection Processes Questions and Answers