CWS Ultraviolet Disinfection Systems 2 — Questions and Answers
Question 1: What is the primary mechanism by which UV light inactivates microorganisms?
- Oxidizing the cell membrane
- Damaging DNA and RNA to prevent replication (Correct answer)
- Raising the temperature of the cell
- Disrupting ion transport channels
Correct answer: Damaging DNA and RNA to prevent replication
UV light at 254 nm is absorbed by nucleic acids, causing thymine dimers and other DNA/RNA damage that prevents microbial replication.
Question 2: Which UV wavelength range is considered most germicidal for water disinfection?
- 200–230 nm (UV-C far)
- 253–265 nm (UV-C peak) (Correct answer)
- 315–400 nm (UV-A)
- 280–315 nm (UV-B)
Correct answer: 253–265 nm (UV-C peak)
The 253–265 nm range within UV-C has the highest germicidal effectiveness because it coincides with peak DNA absorption.
Question 3: What does 'UV transmittance' (UVT) measure in a water system?
- The percentage of UV light absorbed by the lamp housing
- The fraction of UV light that passes through a 1 cm water sample at 254 nm (Correct answer)
- The total energy output of the UV lamp in milliwatts
- The reflectivity of the quartz sleeve surface
Correct answer: The fraction of UV light that passes through a 1 cm water sample at 254 nm
UVT is the percentage of 254 nm UV light transmitted through a 1 cm path length of water, indicating how easily UV penetrates the water.
Question 4: A UV system validated for a 3-log reduction of Cryptosporidium delivers what percentage inactivation?
- 99.9% (Correct answer)
- 99.99%
- 99%
- 90%
Correct answer: 99.9%
A 3-log (3 log10) reduction means 99.9% of the target organism is inactivated, leaving 0.1% viable.
Question 5: What is the purpose of a UV dose-response curve in system design?
- To measure lamp aging over time
- To determine the UV fluence required to achieve a specific log inactivation of a target organism (Correct answer)
- To calculate the electrical energy cost per gallon treated
- To set the alarm threshold for low flow events
Correct answer: To determine the UV fluence required to achieve a specific log inactivation of a target organism
Dose-response curves relate UV fluence (mJ/cm²) to log inactivation, allowing designers to specify the dose needed for regulatory compliance.
Question 6: In a low-pressure UV lamp, what gas is sealed inside the lamp to produce UV emission?
- Xenon
- Argon
- Mercury vapor at low pressure (Correct answer)
- Ozone
Correct answer: Mercury vapor at low pressure
Low-pressure UV lamps contain mercury vapor at low pressure, which emits predominantly at 254 nm when electrically excited.
Question 7: Which condition most directly causes UV lamp output to decrease without bulb failure?
- Increased flow rate
- Lamp aging and fouling of the quartz sleeve (Correct answer)
- Higher inlet water pressure
- Reduced electrical frequency
Correct answer: Lamp aging and fouling of the quartz sleeve
As UV lamps age they lose intensity, and mineral or biological fouling on the quartz sleeve further reduces UV transmission to the water.
CWS Ultraviolet Disinfection Systems 2
What is the primary mechanism by which UV light inactivates microorganisms?