The UV Intensity (Dose) Needed for Purified Water

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Ultraviolet (UV) treatment is a highly effective disinfection process that deactivates harmful biological contaminants in drinking water supply systems.

Viruses, cysts, bacteria – depending on the type of pathogen, different UV intensities/doses are required for elimination.

So, here is our guide on UV intensity for purified water!


  • Class A UV-C systems emit 40 mJ /cm² (the minimum for pathogenic deactivation) and are rated to completely disinfect clear water.
  • Class B UV-C systems emit no more than 16mJ /cm² and are only rated to supplement disinfection in previously disinfected water; they are not rated to produce safe drinking water independently.
  • Cysts require 10mJ/cm² for disinfection of giardia and cryptosporidium.
  • Bacteria require 18 mJ/cm² for disinfection of E. coli.
  • Viruses require up to 22.5 mJ/cm² for disinfection of Echovirus 11.
  • Mold requires up to 240 mJ/cm² for disinfection of manure fungi.
  • Blue-green algae can require up to 1200 mJ/cm² for disinfection.

About UV Disinfection

UV radiation destroys the DNA of pathogens, rendering them inactive. After a microbe’s genetic code is destroyed, it cannot replicate. Although the germs themselves are not physically removed from water, they are made harmless by UV radiation.

A UV water disinfection system will consist of a cylindrical case housing the UV bulb or bulbs, which is encased in a quartz sleeve. Often using a flow regulator to ensure contact time, water flows parallel to and around the UV sleeve to ensure proper radiation of any microbes.

Because UV water treatment does not grant any residual disinfection after initial exposure, it should be the last step in a multi-barrier treatment process and should be installed as close to point-of-use as possible.

To ensure adequate deactivation, different microbes require different doses of UV energy per area. To make sure these energy levels are met, the contaminated water must maintain a certain contact time depending on the intensity of the chosen UV lamp.

Does UV Light Work in All Conditions?

UV is designed to disinfect clear water. Cloudy water with excessive turbidity and suspended solids will block radiation, rendering it ineffective against many pathogens.

It should also be noted that UV cannot reliably disinfect water that has a concentration of coliforms greater than 1,000/100ml. As such, depending on the source of the raw water, pre-treatment may be necessary (as well as supplementary disinfection).

For many groundwater applications, however, UV alone can be a sufficient method of disinfection. To know for certain whether your water needs pre-treatment, a water test should be conducted.

The UV Dose Required for Purified Water

UV intensity for purified water thumbnail

The UV spectrum spans from 100 nm to 400 nm. UV wavelengths are classified into three ranges: UV-A (315-400 nm), UV-B (280-315 nm) and UV-C (100-280 nm). Because most microbes are deactivated at about 260 nm, UV-C (254 nm) is the most effective wavelength at which to treat biological contamination in water.

The magnitude of pathogens deactivated depends on the UV intensity and the contact time of the system in question – the higher the intensity of the lamp, the faster pathogens can be broken down.

The UV dose(mJ/cm²) expresses the energy emitted on an area over time. Therefore, you’ll need to know the strength of your UV lamp and the deactivation points of pathogens to calculate the contact time necessary to disinfect your influent. The formula looks like this:

Energy Dose (mJ/cm²) = Energy Intensity (µW/cm²) x Contact Time (seconds)

Class A and B UV Water Purifiers

As per NSF/ANSI Standard 55, typical UV water systems are divided into classes A and B.[1]

  • A class A system makes sure it delivers 40 mJ/cm² to the influent water. These systems are designed to completely deactivate all harmful pathogens in clear water.
  • A class B system only delivers 16 mJ/cm² of ionizing energy to the influent water. Class B UV systems are only used to supplement disinfection in water already considered safe.

Required UV Intensity (Dose) for Cysts

Two of the most common pathogens in drinking water are Cryptosporidium and Giardia. Although numerous and dangerous, they are sensitive to UV treatment. Unlike other stubborn microbes, crypto and giardia are easily deactivated with only roughly 10 mJ/cm² of energy.

Required UV Intensity (Dose) for Bacteria

Generally, a 2-log reduction (99.00%) is adequate when dealing with bacterial contamination. Below are listed a few of the most dangerous and common types of bacteria in today’s water systems.

  • Escherichia coli (E. coli): 10.8 mJ/cm²
  • Dysentery bacilli: 4.5 mJ/cm²
  • Legionella pneumophila: 6 mJ/cm²
  • Bacillus anthracis: 9 mJ/cm²
  • Clostridium tetani: 9.9 mJ/cm²
  • Pseudomonas aeruginosa: 11.1 mJ/cm²
  • Salmonella paratyphi: 12.3 mJ/cm²
  • Mycobacterium tuberculosis: 12.3 mJ/cm²
  • Streptococcus haemolyticus: 13.5 mJ/cm²
  • Salmonella enteritidis: 15.3 mJ/cm²
  • Vibrio cholerae: 19.2 mJ/cm²
  • Staphylococcus albus: 36.9 mJ/cm²

Required UV Intensity (Dose) for Viruses

Viruses are notoriously difficult to get rid of – whether by filtration or disinfection. UV intensity needed to deactivate viruses can be high, but the safety it provides to your potable water source is invaluable.

According to the Canadian Drinking Water Standards Guideline, viruses must undergo a 4-log removal (at minimum) for water to be considered safe for human consumption.

Below are the UV deactivation doses for common waterborne viruses.

  • Coxsackie Virus A9: 2.4 mJ/cm²
  • Adenovirus 3: 3 mJ/cm²
  • Bacteriophages: 6 mJ/cm²
  • Influenza: 6.9 mJ/cm²
  • Rotavirus SA 11: 15.6 mJ/cm²
  • Echovirus 1: 21.9 mJ/cm²
  • Hepatitis B Virus: 21.9 mJ/cm²
  • Poliovirus 1: 24 mJ/cm²
  • Echovirus 11: 22.5 mJ/cm²

viruses in water animated

Alternatively, you can supplement UV disinfection with chlorine to finish off viruses not killed in a lower-rated UV system.

Required UV Intensity (Dose) for Molds

Although uncommon, mold spores are certainly possible in raw influent. Below are common intensity deactivation levels for the most common waterborne spores.

  • Mucor mucedo: 140.1 mJ/cm²
  • Oospora lactis: 9.9 mJ/cm²
  • Aspergillus amstelodami: 264 mJ/cm²
  • Penicillium digitatum: 26.1 mJ/cm²
  • Penicillium roqueforti: 87.9 mJ/cm²
  • Penicillium chrysogenum: 99.9 mJ/cm²
  • Aspergillus niger: 200 mJ/cm²
  • Manure Fungi: 240 mJ/cm²

Required UV Intensity (Dose) for Algae

Algae can be a big problem for UV systems. The exposure time necessary to deactivate algae cells can make UV disinfection unfeasible. The best way to mitigate algae contamination is to prevent it from entering your home plumbing to begin with. Check your source intake and disinfect your system if you detect algae contamination.

With that in mind, the following list contains the UV intensity needed to deactivate algae and the microbes that thrive on it.

  • Chlorella vulgaris: 27.9 mJ/cm²
  • Green Algae: 36.6 mJ/cm²
  • Nematode Eggs: 102 mJ/cm²
  • Protozoa: 201 mJ/cm²
  • Paramecium: 219 mJ/cm²
  • Blue-Green Algae: 1200 mJ/cm²

Household System Considerations

Prior to installation of a household UV water purifier, the plumbing should be flushed and chlorinated to ensure a contaminant-free distribution after the switch to radiation.

Treated water should subsequently be tested monthly for at least half a year following installation. If the presence of coliforms is noticed, the intensity of the UV will need to be adjusted.

Good domestic UV systems should contain a maintenance warning indicator to notify the operator of when the current UV intensity falls below germicidal levels. This ensures that the system will never fail to disinfect the influent.


UV radiation destroys the DNA of germs.

The required UV intensity/dose varies from germ type to germ type.

Cysts and bacteria require the lowest UV dosages of around 10 mJ/cm².

For most viruses, you need to apply a UV dose of 5 to 25 mJ/cm².

Mold and algae are much harder to kill (up to 240 mJ/cm²).

Further Reading


Meet Jason Hollow

Jason Hollow Jason is the founder of Water Masterz and head of content creation. After six years in the industry, he has tremendous knowledge and first-hand experience on all things related to water treatment.

His credo: Not a single American should have to drink unhealthy water at home.

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