CWS Ultraviolet Disinfection Systems 3 — Questions and Answers
Question 1: What is 'UV fluence' (also called UV dose) and how is it expressed?
- Lamp wattage divided by flow rate, expressed in W/gpm
- The product of UV irradiance and exposure time, expressed in mJ/cm² (Correct answer)
- The total volume of water treated per lamp replacement cycle, in gallons
- Lamp emission spectrum width, expressed in nanometers
Correct answer: The product of UV irradiance and exposure time, expressed in mJ/cm²
UV fluence is the product of irradiance (mW/cm²) and contact time (seconds), yielding a dose in mJ/cm² that determines inactivation efficacy.
Question 2: Why is a quartz sleeve used around UV lamps in water systems rather than direct lamp-to-water contact?
- Quartz amplifies UV output by focusing light
- It isolates the lamp from water while transmitting UV-C, protecting the lamp and allowing safe replacement (Correct answer)
- It filters out harmful ozone-producing wavelengths below 200 nm
- It prevents UV light from damaging downstream plastic piping
Correct answer: It isolates the lamp from water while transmitting UV-C, protecting the lamp and allowing safe replacement
The quartz sleeve keeps the lamp dry and at optimal operating temperature while transmitting UV-C radiation through to the water with minimal attenuation.
Question 3: A UV system's 'validated dose' is established through what process?
- Manufacturer lamp bench testing only
- USEPA/DVGW-compliant bioassay challenge testing at a certified hydraulics test facility (Correct answer)
- Field monitoring of UV intensity sensors over 12 months
- Calculation using manufacturer-supplied irradiance maps
Correct answer: USEPA/DVGW-compliant bioassay challenge testing at a certified hydraulics test facility
Validated doses must be established by third-party bioassay testing at certified facilities following USEPA UV Disinfection Guidance Manual or equivalent protocols.
Question 4: Which water quality parameter, if elevated, most reduces UV system effectiveness by absorbing UV light before it reaches pathogens?
- Hardness (calcium and magnesium ions)
- Color and dissolved organic carbon (DOC) (Correct answer)
- Total dissolved solids (TDS)
- Alkalinity (bicarbonate/carbonate)
Correct answer: Color and dissolved organic carbon (DOC)
Dissolved organic carbon, tannins, and color compounds absorb UV at 254 nm, lowering UVT and reducing germicidal dose delivered to microorganisms.
Question 5: What operational advantage do medium-pressure (MP) UV lamps have over low-pressure (LP) lamps?
- Lower energy consumption per unit of UV output
- Polychromatic UV output that addresses photoreactivation more effectively and requires fewer lamps (Correct answer)
- Longer rated lamp life exceeding 16,000 hours
- Simpler ballast electronics requiring no power regulation
Correct answer: Polychromatic UV output that addresses photoreactivation more effectively and requires fewer lamps
MP lamps emit UV across a broad spectrum (200–400 nm), which can damage multiple DNA repair pathways and reduce photoreactivation, often allowing fewer lamps per installation.
Question 6: In a UV reactor, what is the role of the UV intensity sensor (radiometer)?
- To measure the flow rate through the reactor for dose calculation
- To continuously monitor in-situ UV irradiance and trigger alarms if dose delivery falls below setpoints (Correct answer)
- To detect iron fouling on the quartz sleeve by color change
- To regulate water temperature entering the reactor
Correct answer: To continuously monitor in-situ UV irradiance and trigger alarms if dose delivery falls below setpoints
The UV intensity sensor provides real-time irradiance data used with flow rate to calculate delivered dose, and alarms when irradiance drops below minimum validated levels.
Question 7: What is 'photoreactivation' in the context of UV disinfection?
- The re-ignition process of a UV lamp after a power interruption
- The repair of UV-induced DNA damage by microorganisms exposed to visible or near-UV light after treatment (Correct answer)
- The conversion of ozone back to oxygen in UV-ozone hybrid systems
- The restoration of UV lamp intensity after sleeve cleaning
Correct answer: The repair of UV-induced DNA damage by microorganisms exposed to visible or near-UV light after treatment
Photoreactivation occurs when UV-damaged microorganisms use light-dependent enzymes (photolyases) to repair thymine dimers, potentially restoring infectivity.
CWS Ultraviolet Disinfection Systems 3
What is 'UV fluence' (also called UV dose) and how is it expressed?