WTO Disinfection Processes 3 — Questions and Answers
Question 1: Chlorine dioxide (ClO₂) used for disinfection must be generated on-site because:
- It is too expensive to ship
- It is an unstable gas that cannot be safely stored or transported (Correct answer)
- It reacts with water during transport
- EPA prohibits its import from other states
Correct answer: It is an unstable gas that cannot be safely stored or transported
Chlorine dioxide is a highly reactive gas that decomposes rapidly and poses explosion risks, making on-site generation mandatory.
Question 2: What byproduct concern is unique to chlorine dioxide disinfection?
- Formation of trihalomethanes
- Formation of chlorite and chlorate ions (Correct answer)
- Formation of bromate
- Formation of NDMA
Correct answer: Formation of chlorite and chlorate ions
Chlorine dioxide produces chlorite (ClO₂⁻) and chlorate (ClO₃⁻) as byproducts, which are regulated under the DBPR.
Question 3: What is the purpose of a chlorine contact chamber (CCC) in a water treatment plant?
- To store excess chlorine gas safely
- To ensure sufficient contact time between chlorine and water for pathogen inactivation (Correct answer)
- To remove chlorine from water before distribution
- To measure chlorine residual continuously
Correct answer: To ensure sufficient contact time between chlorine and water for pathogen inactivation
The CCC is designed to maximize contact time between chlorine and water, achieving the required CT for inactivation credit under surface water treatment rules.
Question 4: An operator notices breakpoint chlorination requires more chlorine than expected. The most likely cause is:
- Low water temperature
- Elevated ammonia or organic nitrogen in the source water (Correct answer)
- Excessive UV exposure before chlorination
- Low turbidity in the raw water
Correct answer: Elevated ammonia or organic nitrogen in the source water
Elevated ammonia or organic nitrogen demands more chlorine to reach breakpoint because chloramines must first be formed and then destroyed before free chlorine residual appears.
Question 5: Which factor does NOT affect ozone disinfection effectiveness?
- Water temperature
- Water pH
- Water color (true color)
- Water fluoride concentration (Correct answer)
Correct answer: Water fluoride concentration
Fluoride concentration does not influence ozone disinfection; temperature, pH, and organic matter (contributing to color) all affect ozone demand and CT.
Question 6: Under the Surface Water Treatment Rule, what is the minimum log-inactivation credit required for Giardia lamblia?
- 1-log
- 2-log
- 3-log (Correct answer)
- 4-log
Correct answer: 3-log
The SWTR requires at least 3-log (99.9%) inactivation of Giardia lamblia through a combination of filtration removal and disinfection CT credit.
Question 7: What is the key difference between primary and secondary disinfection in a treatment plant?
- Primary uses UV; secondary always uses chlorine
- Primary achieves initial pathogen inactivation; secondary maintains residual through distribution (Correct answer)
- Primary treats groundwater; secondary treats surface water
- Primary disinfection is optional; secondary is required
Correct answer: Primary achieves initial pathogen inactivation; secondary maintains residual through distribution
Primary disinfection targets pathogen inactivation at the plant, while secondary disinfection (typically chlorine or chloramine) maintains a protective residual in the distribution system.
WTO Disinfection Processes 3
Chlorine dioxide (ClO₂) used for disinfection must be generated on-site because: