WTO Disinfection Processes 2 — Questions and Answers
Question 1: What is the primary advantage of using chloramines over free chlorine for disinfection?
- Chloramines are stronger disinfectants
- Chloramines produce fewer disinfection byproducts (Correct answer)
- Chloramines work faster than free chlorine
- Chloramines are cheaper to produce
Correct answer: Chloramines produce fewer disinfection byproducts
Chloramines form fewer trihalomethanes and haloacetic acids compared to free chlorine, making them preferred in systems with high organic precursors.
Question 2: Which parameter most directly affects the disinfection efficiency of chlorine in water treatment?
- Water color
- pH (Correct answer)
- Water hardness
- Turbidity alone
Correct answer: pH
pH determines the ratio of hypochlorous acid (HOCl) to hypochlorite ion (OCl⁻); HOCl is the more effective disinfectant and dominates at lower pH.
Question 3: A water system switches from free chlorine to chloramine disinfection. What operational concern must be addressed?
- Increased formation of trihalomethanes
- Nitrification in the distribution system (Correct answer)
- Higher turbidity in treated water
- Reduced pH in finished water
Correct answer: Nitrification in the distribution system
Chloramine systems are susceptible to nitrification, where ammonia-oxidizing bacteria convert ammonia to nitrite, depleting the disinfectant residual.
Question 4: What does CT value represent in disinfection calculations?
- Contact time divided by chlorine dose
- Chlorine concentration multiplied by contact time (Correct answer)
- Temperature correction factor for chlorine
- Chlorine demand at a given turbidity
Correct answer: Chlorine concentration multiplied by contact time
CT is the product of disinfectant concentration (C, in mg/L) and contact time (T, in minutes), used to evaluate pathogen inactivation credit.
Question 5: Why is ultraviolet (UV) disinfection considered ineffective against Cryptosporidium in terms of CT-based comparison?
- UV does not inactivate Cryptosporidium at any dose
- CT tables do not apply to UV; UV efficacy is measured in mJ/cm² (Correct answer)
- UV requires very high temperatures to work on Cryptosporidium
- Cryptosporidium absorbs UV light and becomes stronger
Correct answer: CT tables do not apply to UV; UV efficacy is measured in mJ/cm²
UV disinfection efficacy is measured by UV dose (mJ/cm²), not CT; Cryptosporidium is actually highly susceptible to UV at standard doses.
Question 6: Which disinfectant is most effective for inactivating Cryptosporidium oocysts?
- Free chlorine at typical doses
- Chloramines
- Ozone or UV (Correct answer)
- Chlorine dioxide at low doses
Correct answer: Ozone or UV
Cryptosporidium oocysts are highly resistant to chlorine and chloramines but are effectively inactivated by ozone and UV at practical doses.
Question 7: A operator measures a chlorine residual of 0.0 mg/L in the distribution system. What is the immediate regulatory concern?
- Violation of the Lead and Copper Rule
- Potential violation of the Total Coliform Rule and surface water treatment requirements (Correct answer)
- Violation of the Arsenic Rule
- Only a concern if turbidity also exceeds 1 NTU
Correct answer: Potential violation of the Total Coliform Rule and surface water treatment requirements
Loss of detectable disinfectant residual in the distribution system can trigger coliform sampling requirements and may indicate a treatment technique violation.
WTO Disinfection Processes 2
What is the primary advantage of using chloramines over free chlorine for disinfection?