Ultraviolet Disinfection of Drinking Water

Ultraviolet disinfection of water consists of a purely physical, chemical-free process. UV-C radiation in particular, with a wavelength in the 240 nm to 280 nanometers range, attacks the vital DNA of the bacteria directly. The radiation initiates a photochemical reaction that destroys the genetic information contained in the DNA. The bacteria lose their reproductive capability and are destroyed.

Even parasites such as Cryptosporidia or Giardia, which are extremely resistant to chemical disinfectants, are efficiently reduced. UV can also be used to remove chlorine and chloramine species from water ; this process is called photolysis, and requires a higher dose than normal disinfection. The sterilized microorganisms are not removed from the water. UV disinfection does not remove dissolved organics, inorganic compounds or particles in the water.[9] However, UV-oxidation processes can be used to simultaneously destroy trace chemical contaminants and provide high-level disinfection, such as the world’s largest indirect potable reuse plant in Orange County, California.

UV disinfection leaves no taint, chemicals or residues in the treated water. Disinfection using UV light is quick and clean and no bacteria, viruses or moulds are immune to UV.

What You Need to Know When Before Purchasing a UV Water Treatment Device


By Mike Mandell

Why U.V.? Ultraviolet Disinfection systems are popular among home owners with private wells or other sources of water. U.V. disinfection units bring a feeling of ease that you’re doing something to protect the water you and your family drinks and cooks with. And unlike chlorine, you can just plumb your U.V. unit into place, no chemical jugs to fill, no chlorine taste to deal with.

But not all U.V. units are created equal. You must choose the proper unit for your needs. As well, the concept that these units are set and forget is an incorrect thought. Just like anything else, these units need maintenance to keep running smoothly and effectively.

The purpose of this article is to give you an overview of what U.V. does, how it works, what to look for when purchasing a unit and why. As well, I’ll go over some of the most basic – and most important maintenance that these units require.

Who are You and How Do You Know All This? I work for the water department at a small municipality in Southern Ontario. The municipality operates two types of systems: Large Municipal Residential Systems, and Small Municipal Non-Residential Systems. The former are large water treatment plants with large distribution systems, feeding many houses and industries with water. The former are municipally owned systems on their own well supplies: Community centers and halls, Arenas, outlying service area offices and the like. These small centers have water consumption similar to a single family home, and it’s at these small centers that I have researched treatment, installed treatment including U.V. disinfection devices, and maintain and repair said equipment. At your own home, you are not under any regulations whatsoever. You could put state of the art treatment on your water, or you could put in nothing at all. That is not true for municipally operated systems. Especially here in Ontario, regulations are stiff and must be complied with at a minimum under penalty of a steep fine from the government.

The reasons for these regulations are simple: Everybody has the right to safe, clean drinking water. Whether it’s coming from the tap in a factory lunch room, municipally supplied to your home or coming from a well, anybody who enters a public facility should be assured that the water coming from the tap is safe and clean.

The water coming from your taps should be safe and clean as well. If a community center requires a certain level of protection for its water supply, then why is your home any different?

The Concept of Risk Anyone who lives in Ontario remembers the Walkerton incident. The City of Walkerton had a municipal water treatment system operated by two people. These people did not operate the system safely; they lied about water testing and did not provide proper water treatment. As a result, seven people died of an E.Coli outbreak and thousands of others got sick. Drastic changes were made to regulations and to government inspections to ensure that this never happened again.

What most people don’t know is this:

The Walkerton water treatment system was operated in exactly the same way for twenty years before people died because of it.

You don’t know when it’s coming. You could get dangerous microorganisms in your well water tomorrow, next month, next year or never. You just don’t know. And the only way to make SURE it never happens is to make sure that you have a quality barrier against potential problems.

When you select a U.V. system, you must do so with this risk assessment in mind. You aren’t trying just to disinfect what’s in the water now: You’re putting up a barrier for what COULD be in the water at a future date. Remember, risk events are similar to hurricanes. The conditions are many and have to be just right, but when and if they do come together, they are quick and can do a lot of damage. It’s better to be protected against something that may never come, than to not be protected when it does.

What Is U.V. Disinfection? Ultraviolet Light is radiation on the E.M. Spectrum just before visible light. At a certain range of wavelengths, U.V. light is able to destroy the DNA in microorganisms, rendering them unable to reproduce. If a microorganism cannot reproduce, it cannot harm the human body, and either dies or is destroyed by your immune system. The most effective wavelength for inactivating microorganisms is 254 nm (nano meters). U.V. lamps are designed to provide a 254 nm ultraviolet light radiation. If you look at a U.V. light, you’ll see it shines purple. That is not the U.V. light, it’s the visible light. You cannot actually see U.V. light as our eyes cannot pick up that wavelength. It is not advisable to look at U.V. bulbs directly, as the U.V. light can do damage to your eyes and skin.

Why Filter? Many people put filters on their treatment systems to maintain the aesthetic quality of their drinking water. Nobody wants to drink dirty or cloudy water. But a filter is a must with U.V. systems, to prevent a phenomenon called U.V. shadowing. U.V. light is just like your house hold light bulb, or the sun. If an opaque object is placed in front of it, it will cause a shadow to form behind it. There is no light in a shadow.

Similarly, if dirt, rust or sediment passes in front of a U.V. light, there will be a shadow behind it. If there are any micro organisms in the shadow, the U.V. light will not be able to reach them and inactivate them.

Making the water as free from dirt and sediment as possible will help ensure that there is no shadows created and your U.V. system is disinfecting all the water at all times.

Ultraviolet Dosage When you have a head ache, you take pain killers. You know your body and you know that a certain amount of pain killer is needed to do the job. If you need two pills, then taking only one pill won’t get rid of your head ache.

This is known as dosage. You need a certain, minimum dosage of medication to do what you need it to do, otherwise it will be ineffective.

Ultraviolet treatment systems also have an effective dosage. That is, you need a certain amount of ultraviolet energy to pass through the water column to ensure that you inactivate all the micro organisms that may be present. Different microorganisms can tolerate different amounts of U.V. dosage. To ensure that you inactivate all the microorganisms, you need to ensure that you have a minimum safe U.V. dosage at all times.

But how do you know what the minimum dosage is? You don’t, you can’t be expected to. That is why that much study has been done to determine the safe minimum dosage. Government organizations, third party labs and other organizations conduct tests using live organisms. They apply a known dosage of U.V. light and test to see what comes out the other side. From those tests, they determine a minimum safe dose with a margin of safety build in.

The most recognized of these third party organizations is NSF, or the National Sanitation Foundation. They have the highest, best documented and strictest methodology, which is why the minimum dosage they have prescribed has been adopted by almost all regulatory bodies as well as recommended minimum water treatment standards.

When looking for U.V. systems, ensure that the U.V. system can provide the minimum dosage rate so you can be sure your water is safe and free from pathogenic micro organisms.

The dosage is recorded as an energy per area of water. Generally, you’ll find the energy statement in uW, mW or mJ. nano watts (uW) is an energy reading one thousand times less than milliwatts (mW) or milli joules (mJ). Milli watts and milli joules represent the same amount of energy.

The safe minimum dosage your U.V. should provide is 40 mJ/cm2. This may also be stated as 40 mW/cm2 or 40,000 uw/cm2.

Ensuring Your Ultraviolet Dosage. Ensuring and maintaining a minimum safe U.V. dosage takes more than looking at the manufacturer’s specifications. The specifications may say 40 mJ/cm2, but that very much depends on the application. There are three factors that determine your U.V. dosage, and they are application dependent. The three factors are as follows:

  1. Flow rate
  2. UVT (ultraviolet transmittance, or how much U.V. light can penetrate water)
  3. Lamp energy output

The basic formula for U.V. dosage is lamp energy + UVT + flow rate = dosage.

The faster the flow rate, the less energy will be imparted from the lamps. The lower the UVT, the less energy your lamps will transmit into the water. And of course your lamps must be strong enough to impart the required energy at maximum flow rate and the lowest permissible UVT (unit dependent).

Read the manufacturers specifications closely. Any reputable manufacturer understands that dosage is not just how much raw energy the lamps put out. The specifications will (or should) state a maximum flow rate and a minimum UVT (ultraviolet transmittance).

You know that you want a minimum of 40 mJ/cm2 dosage, so make sure you know your maximum flow rate, and what your UVT is. You can get your UVT tested by a local laboratory listed in your yellow pages. Good U.V. manufacturers will have the facilities to test this for you.

But how does your lamp life come into play? U.V. lamps fade in intensity over time. Most manufacturers rate their U.V. lamps for 9,000 hours of continuous operation, which equals one year. At the end of that time, the U.V. output of the lamps will be at their weakest. The best U.V. manufacturers will have a dosage statement on their specifications similar to as follows:

Dosage: 40 mJ/cm2 at end of lamp life. The best units calculate U.V. dosage at the end of lamp life, at the lowest U.V.T and at maximum flow rate, to ensure that your minimum recommended dosage is just that, a minimum. Unlike other disinfecting agents like chlorine, you cannot ever have too much U.V. energy, only too little.

Selecting Your U.V. Unit When I was selecting the most appropriate U.V. unit for use in my municipality, I wanted to make sure that it complied with local regulations. But more than that, I wanted to make sure that it put out the appropriate U.V. dosage so I could be sure that I wouldn’t have to worry.

As stated above, lamp energy + UVT + flow rate = dosage. And I knew I wanted my dosage to be at least 40 mJ/cm2 at all times.

The first step for me was testing the UVT of each site. I had a portable UVT tester, but as they are expensive for a home owner you can easily and inexpensively get a laboratory to test if for you. After determining all my UVT readings, I went about calculating maximum flow rates. To make it easier for you, the average house hold will not need anything greater than 10 GPM. If you have a very large family and a very large house, you may need more but this is only if you have a very large and busy house hold.

The U.V. units I selected were perfect for my needs. The specifications stated the following:

Dosage at 40 mJ/cm2 at 13 GPM at a UVT of 75%, at end of lamp life.

I knew all my UVT tests came back at higher than 75%. I did not need anything more than 10 GPM. But more importantly, this unit came with a 13 GPM flow restrictor, so I would never exceed that flow rate. It also had sensors that measured the lamp output and the lamp output across the water column. These sensors could keep track of the dosage, and alert me if the U.V. dosage went lower than 40 mJ/cm2. As well, the unit came with an automatic shut off valve that cut the water supply if the dosage was ever too low. In short, I was either getting an appropriate dosage or I wasn’t, end of story.

To sum it up, you should look for a U.V. device that can maintain minimum appropriate dosage for:

  • Your maximum flow rate
  • The UVT of your water
  • And can do it at the end of its lamp life.

Frills and aesthetic considerations should come after that. The U.V disinfection system should be able to do its job first and foremost, and that is to properly and effectively disinfect your water.

Would You Like a Softener With That? Many water equipment retailers insist up and down that you MUST have a water softener if you install a U.V. unit. They will unequivocally insist that you cannot run a U.V. system without one.

This is all well and good if you want a water softener, but what if you don’t? After all, water softeners can cost a thousand dollars or more, and that’s a lot of money if you don’t want one.

Have you ever looked at the bottom of your kettle? It’s full of white scale. Sometimes this scale can even flake off, which is why many kettles come with mesh filters on the spigot. This is because water has a property called “hardness”. Simply put, hardness is dissolved calcium and other trace minerals. When you heat up water, like in your kettle, your water loses its ability to hold onto these minerals in solution, so they form a white scale.

All ultraviolet disinfection systems generate heat as a by product of operation. If water is flowing, or flows often, this is not a problem. But when water is allowed to stand in the U.V. system for long periods of time, you can get scale forming inside the unit. Typically, this hardness scale forms on a quartz sleeve that protects the U.V. lamp from getting wet while allowing the U.V. light to pass through it and disinfect the water. If too much scale forms on the unit, you will not get any or enough U.V. light to get your minimum safe U.V. dosage.

So, your water equipment salesman wants you to buy a softener for three possible reasons:

  1. The U.V. system he sells does not have any means to detect fouling (common to many units), so you must prevent it at all (and expensive) costs.
  2. He does not know any other way to prevent fouling
  3. He’s trying to make more money

So how do you prevent scale formation without the use of a softener? Very simple: You keep the temperature down.

The U.V. system I use has a piece of equipment called a temperature purge valve. Simply, this is a small solenoid valve with a temperature sensor placed on the outlet of the U.V. unit. When it detects the temperature reach 105 degrees F, it opens and allows approximately 1 liter of water through. When not being used, it will do this approximately once per hour. This is a simple and effective way to keep scale away from your U.V. unit, without going to the expense of purchasing a water softener.

Other Considerations You want to make sure that your unit is a good one. Make sure that there is a supply of spare parts that can be obtained locally. Ask the sales person for references, and make the calls. Do make sure that you have spare lamps on hand at all times.

You will need to replace your lamps about once a year. When doing so, make sure that you are very careful when removing the lamps. U.V. lamps contain mercury, so you don’t want that in your environment. Also, the quarts sleeves on the unit are fragile, and you don’t want to break them. Make sure you have a set of lamps on hand so you don’t have to go looking for them when you need them.

Be sure to check your unit once per month for scale build up. Even if you have a softener or a purge valve, it is possible for scale to form. You want to make sure your quartz sleeve is clean and does not restrict any of the U.V. light.

Ask your salesperson if there is any third party performance testing on the U.V. unit. A good and reputable manufacturer will have third party bio assay tests done on the unit to prove that it can indeed inactivate the appropriate amount of micro organisms. If the salesperson doesn’t know what you are talking about, is unable to provide proof of this or skates around the issue, walk out of the store. Remember, it’s your safety that’s at stake.

In Conclusion There are many good and affordable U.V. units in the market today. There is also much crap. Do your homework, take your time, and be careful. Don’t buy the cheapest unit available, but you also do not need the most expensive. Purchasing a U.V. unit is just like anything else, you get what you pay for. But unlike many other things, this device provides a barrier against harmful pathogens, so choose wisely. Know your maximum flow rate, find out what your UVT is, and make sure that your U.V. device can give you at least 40 mJ/cm2 based on that information. If you do, you can be sure that you’ll be protected, and have good, safe water to drink and cook with.

Mike Mandell works at a water treatment facility in Canada. As well, he runs a website providing free information and advice about drinking water, water treatment, pumps and equipment, and anything else to do with water. Visit him at http://www.truthofwater.com

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