Wastewater Operator Certification Disinfection Processes 2 — Questions and Answers
Question 1: At what approximate wavelength does UV radiation most effectively inactivate microorganisms?
- 185 nm
- 254 nm (Correct answer)
- 320 nm
- 400 nm
Correct answer: 254 nm
UV radiation at approximately 254 nm is most effective because it falls within the peak absorption range of DNA and RNA, maximizing nucleic acid damage in microorganisms.
Question 2: What is the primary mechanism by which UV disinfection inactivates microorganisms?
- Oxidizing and rupturing the cell membrane
- Damaging DNA and RNA, preventing reproduction (Correct answer)
- Denaturing key structural proteins
- Disrupting the cell wall synthesis
Correct answer: Damaging DNA and RNA, preventing reproduction
UV radiation causes the formation of thymine dimers and other photoproducts in DNA/RNA, preventing replication and rendering organisms unable to reproduce.
Question 3: Which effluent quality parameter most significantly limits UV disinfection performance?
- Dissolved oxygen concentration
- Water temperature
- Turbidity and UV transmittance (UVT) (Correct answer)
- Total dissolved solids
Correct answer: Turbidity and UV transmittance (UVT)
Turbidity and low UV transmittance shield microorganisms from UV exposure, directly reducing the delivered UV dose and disinfection efficacy.
Question 4: A major operational advantage of UV disinfection over chlorination is:
- Lower capital equipment costs
- No chemical residual or formation of harmful disinfection byproducts (Correct answer)
- Greater effectiveness against all protozoan cysts
- Simple monitoring with no specialized equipment
Correct answer: No chemical residual or formation of harmful disinfection byproducts
UV disinfection adds no chemicals to the effluent, eliminating concerns about chlorine toxicity to aquatic life and the formation of disinfection byproducts like trihalomethanes.
Question 5: Why must ozone (O₃) used for disinfection be generated on-site?
- It is prohibitively expensive to transport
- It is an unstable gas that decomposes rapidly and cannot be stored (Correct answer)
- Federal regulations prohibit off-site ozone production
- It reacts violently with water during transport
Correct answer: It is an unstable gas that decomposes rapidly and cannot be stored
Ozone has a very short half-life (approximately 20-30 minutes in water) and cannot be stored or transported, requiring on-site generation from oxygen or air.
Question 6: Against which pathogen does UV disinfection have a significant advantage over conventional chlorination?
- Escherichia coli
- Salmonella typhi
- Cryptosporidium parvum oocysts (Correct answer)
- Fecal streptococci
Correct answer: Cryptosporidium parvum oocysts
Cryptosporidium oocysts are highly resistant to chlorine at typical treatment doses but are effectively inactivated by UV irradiation at relatively low UV doses.
Question 7: What is the primary operational disadvantage of ozone disinfection compared to chlorination for wastewater reuse applications?
- Ozone is less effective at inactivating bacteria
- Ozone requires a higher pH to function
- Ozone provides no persistent residual disinfection after treatment (Correct answer)
- Ozone generates more regulated disinfection byproducts
Correct answer: Ozone provides no persistent residual disinfection after treatment
Ozone decomposes rapidly and provides no measurable residual in the treated effluent, offering no protection against recontamination during storage or distribution.
Wastewater Operator Certification Disinfection Processes 2
At what approximate wavelength does UV radiation most effectively inactivate microorganisms?