Key Factors for UV Selection

• Pre-RO intermittent flow
• Post-RO storage tank recirculation
• Chilled or hot water systems
• Low pressure UV and medium pressure UV systems perform differently (disinfection performance, process fit, operational costs, capital costs, etc.)

General Comparison of Low Pressure UV and Medium Pressure UV: An advantage of MP lamps is higher intensity output. This means that one high pressure lamp may be able to replace 6 – 16 low pressure lamps resulting in lower costs. This allows the system to have a smaller physical footprint using fewer lamps, whereas a low pressure system may need multiple lamps to equal the same rate of disinfection of medium pressure. Medium pressure has much more power and therefore can inactivate more microorganisms and chemicals in the same amount of water more quickly. The spectrum of MP UV light is also broader in what it can inactivate and disinfect in a single pass versus low pressure. Medium pressure systems can be used in plants with higher flow rates. Medium pressure UV can kill cryptosporidium, giardia, and other dangerous diseases with ease. MP lamps require a higher flow rate to keep the system cool, and/or cooling is necessary, and if water is not constantly flowing, it can shorten the lifespan of the bulbs.A summary comparison of low pressure UV and medium pressure UV:

• Low pressure UV is better for
  • Intermittent flow
  • Lamp life
  • Energy usage
• Medium pressure UV is better for
  • Constant flow
  • Disinfection
  • Water quality
  • Maintenance needs
  • Footprint (smaller size of installation)
  • Consumables usage
  • Fluid temperature extremes

UV Dose

• UV fluence or dose is the energy required to inactivate a microorganism and is measured in mW sec/cm2 or mJ/cm2.
• Actual equations used by UV systems are more complex than this and vary from UV system to UV system to account for UV reactor design differences.
• While there is some regulation that drives the level of the dose required, many facilities look to UV manufacturers to make recommendations.
• Studies have been conducted to prove the dose required for most common bacteria, protozoa, molds & spores, and viruses.

UV Dose (mJ/cm2) for a given Log Reduction[log reduction: UV dose (mJ/cm2)]
Legionella pneumophila (ATCC 43660)1: 3.12: 53: 6.94: 9.4
Salmonella spp.1: <22: 23: 3.54: 75: 155: 29
Streptococcus faecalis )ATCC29212)1: 6.62: 8.83: 9.94: 11.2
Cryptosporidium hominis1: 32: 5.8
Giardia lamblia1: <22: <23: <4
Adenovirus (type 15)1: 402: 803: 1224: 1655: 210
Bacillus subtilis (ATCC6633)1: 362: 48.63: 614: 78

What is the right UV dose based on the organisms?

• Pseudomonas: 17mJ/cm2
• Brevundimonas: 12mJ/cm2 (1 log reduction)
• Sphingomonas: 45mJ/cm2 (3 log reduction)
• Bacillus: >80mJ/cm2 varies by species
• Micrococcus: >20mJ/cm2 (1 log reduction)
• Ralstonia: 12mJ/cm2 (1 log reduction)
• Stenotrophomonas: 10mJ/cm2 (2 log reduction)
• Burkholderia: 50mJ/cm2 (1 log reduction)

Understanding UV Dose

Many items can affect the ability of a UV system to perform. The main water quality items that are taken into consideration when sizing a UV system are:

• UV Transmittance
• Turbidity
• Mineral content
• Suspended solids

Dose or Intensity of UV

Intensity is just 1 variable in the equation. If all other variables are constant, monitoring intensity can make sense but the intensity readings need to be meaningful and actionable.

UV Quartz Sleeve Fouling

Visible fouling is considered major fouling for a UV system