How Ultraviolet Disinfection Works

Ever wonder how UV works? A UV lamp purifier should be installed to provide safe drinking water. A UV lamp is the only method of water purification that is acceptable to avoid boil orders.

Applications of UV water treatment include: residential, breweries, water stores, restaurants, municipalities, cooling towers, marine, hydroponics, aquaculture, etc.

Ultraviolet light (UV) is at the invisible, violet end of the light spectrum.The water treatment industry uses a high-powered form of UV light called UV-C or “germicidal UV” to disinfect water.

How does UV light work for water purification?

UV-C rays (photons) penetrate microorganisms and becomes absorbed by the DNA of the pathogen in the water being treated. The DNA is altered in such a way that the pathogen cannot reproduce and is essentially killed and cannot cause infection. This process of DNA modification is called inactivation.

UV-C rays will destroy a minimum of 99.99% of harmful microorganisms, including E. coli, Cryptosporidium and Giardia.

Unlike chemical disinfection the organisms are unable to develop any immune mechanism against UV light.

The degree of UV inactivation of pathogens is directly proportional to the UV dose applied to the water.

UV dose is the product of UV light intensity and exposure time and is expressed in mJ/cm2. NSF International has established a UV dose of 40 mJ/cm2 as the minimum UV dose required to ensure that all bacteria, viruses, Giardia and Cryptosporidium are killed or inactivated to a safe level.

The Advantages of UV Disinfection

  • More effective against viruses than chlorine
  • Environmentally and user friendly, no dangerous chemicals to handle or store, no risks of overdosing
  • Low initial capital cost as well as reduced operating expenses when compared with similar technologies such as ozone, chlorine, etc.
  • Immediate treatment process, no need for holding tanks, long retention times, etc.
  • Extremely economical, hundreds of gallons may be treated for each penny of operating cost
  • No chemicals added to the water supply – no byproducts, (i.e. chlorine + organic compounds = trihalomethanes)
  • No change in taste, odor, pH, conductivity, nor the general chemistry of the water
  • Automatic operation without special attention or measurement, operator friendly
  • Simplicity and ease of maintenance, periodic cleaning, (if applicable), and annual lamp replacement, no moving parts to wear out
  • No handling of toxic chemicals, no need for specialized storage requirements
  • Easy installation, only two water connections and a power connection
  • Compatible with all other water purification processes, (i.e. reverse osmosis, filtration, water conditioning and softening)

What size UV unit do I need?

To maximize the efficiency of the UV unit, pre-treatment is advisable. The most common pre-treatment methods are:

  • Sediment filters to remove silt that shelters microbes and scatters or absorbs the UV. Silt can drastically cut the effectiveness of a UV unit.
  • Carbon filtration to remove organic materials that absorb UV.
  • A water softening unit to remove minerals that coat the UV lamp sleeve and cut down transmitted light.

Deciding on the capacity of unit needed is fairly simple.

For point-of-entry (whole house) system estimate the flow rate needed according to these guidelines:

Small Home (1 – 2) occupants – 5 gallons per minute

Medium Home (2 – 4) occupants – 8 gallons per minute

Large Home (4 – 6) occupants or small restaurant – 12 gallons per minute

To estimate the flow rate of water into your home, follow these steps:

Make sure no water is running in the house.

Use the faucet closest to the water source (pump, tank or pipe)

Open the faucet completely and measure the time it takes to fill a one US gallon container

Divide 60 seconds by the time it took to fill the container flow rate

Example: if it takes 20 seconds to fill a 1 gallon container, the flow rate is 3 gallons per minute

Factors Affecting UV

The effectiveness of a UV system in eliminating microbiological contamination is directly dependent on the physical qualities and/or clarity of your water supply.

Suspended Solids

Particulate matter can cause shielding problems, in which a microbe may pass through the UV chamber without actually having any direct UV penetration.


Will cause staining on the quartz sleeve that houses the UV bulb, at levels as low as 0.3 PPM of iron and 0.05 PPM of manganese. In this case, we recommend the installation of a Water Softener and/or an automatic Iron Filter system, as a proper step towards pretreatment which will help eliminate this staining problem. Note: not all UV systems will require the use of a softener if there is iron or manganese present, contact us for details.


Hardness in your water will cause possible scale formation on the lamp protecting the quartz sleeve. Scaling problems will be especially magnified during low flow or no flow times when the calcium and magnesium ions tie up with carbonates and sulfates to form hard scale buildup inside the chamber and quartz sleeve.

Other Absorbing Compounds

Humic and fumic acids as well as tannins will reduce the amount of UV energy available to penetrate through the water to affect the genetic material or DNA of the molecule.

Temperature is a determining factor.

The optimal operating temperature of the UV lamp must be near 40 degrees C or 104 degrees F. UV levels will fluctuate with excessively high or low temperature levels.

UV energy required for 99.9% destruction of various microorganisms

This table shows the amount of ultraviolet energy required, (at 253.7nm wavelength), for 99.9% destruction of various microorganisms measured in microwatt seconds per centimeter squared.

Most lamps hould be replaced annually, but check your user manual for exact replacement frequency to ensure proper intensity output.

Microorganism Common Name Inactivation Level
Microorganism  Common Name  Inactivation Level


Bacillus anthracia  Anthrax Virus (not spores)  8.700 Bacillus anthracia  Anthrax Spore 40,000
 Agrobacterium tumefaciens  Crown Gall Disease (plants)  8.500  Bacillus Megatherium  Wet wood Disease  5,200
 Bacillus subtilis vegetative  11.000  Clostridium Tetani  Tetanus/Lockjaw  23,000
 Corynebacterium diphtheria  Diphtheria  6.500  Escherichia coli E. coli  7,000
 Legionella bozemanii Pontiac Fever  3.500  Legionella pneumophila  Legionnaires Disease  3,800
 Leptospira interrogans Infectious Jaundice & Leptospirosis 6,000  Mycobacterium tuberculosis  Pulmonary Tuberculosis  10,000
 Moraxella catarrhalis  Meningitis, Endocarditis, Pneumonia, Bronchitis, Otitis Media, Sinusitis, Bactoremia 8,500 Proteus vulgaris Urinary Tract Infection, Bacteremia, Pneumonia and Focal Lesions  6,600
Salmonella paratyphi  Para-Typhoid Fever, Enlargement of Spleen 6,100 Salmonella typhimurium  Gastroenteritis  15,200
 Salmonella typhose Typhoid fever, Enteric fever, Typhus Abdominales 6,000 Sacina lutea  Reproductive Problems  26,400
Shigella flexNeri  Dysentery 3,400  Shigella sonnei Enteric Infection  7,000
 Enterococcus faecalis  Urinary Tract Infection and Bacterial Endocarditis  10,000  Streptococcus hemolyticus  Various Infections  5,500


 Giardia Lamblia  Giardiasis (Beaver Fever) 5,000-10,000  Cryptosporidium Diarrheal Disease 5,000-10,000
 Vibrio cholera  Cholera  6,500

 Mold Spores

Mucor ramosissimus  Sinuses, Brain, Eyes, Lungs, & Skin Infections 35,200 Penicillium expansum Blue Mold  22,000
 Penicillium roqueforti  Fungi  26,400


 Chlorella vulgaris Green Algae 22,000


 Bacteriophage E. Coli / Bloody Diarrhea / Hemorrhagic Colitis 6,600  Hepatitis Virus  Hepatitis  8,000
 Influenza Virus  Influenza 6,600 Polio virus  Polio  21,000
Rota virus Rota Virus  24,000  Small Pox Virus  Small Pox  9,000


 Trichosporon Bakers Yeast 8,800  Brewers yeast Brewers Yeast 6,600
 Common yeast cake  Yeast Cake  13,200  Saccharomyces var. ellipsoideus Saccharomyces  13,200
 Saccharomycs  Saccharomyces  17,600