Ultraviolet Water Purification

When it comes to eliminating microorganisms, ultraviolet (UV) light is one of your most effective water purification treatment options. UV systems that are set up optimally can attain removal efficiencies of more than 90% or even 99.99% for a number of viruses and bacteria.

Also, unlike other disinfectants, UV does not produce harmful byproducts. However, it is almost useless against other water contaminants such as VOCs, fluoride, heavy metals, and chlorine. That is why, when used in water purification systems, ultraviolet light is often deployed alongside a secondary disinfectant or a filtration system.

How ultraviolet light affects microorganisms


Certain microorganisms like viruses, bacteria, and protozoa, are sensitive to a narrow band of wavelengths that belong to the ultraviolet spectrum.

More accurately, the entire UV spectrum ranges from wavelengths of about 10 nm to about 400 nm. According to the EPA, the narrow UV band of 250 nm to 270 nm contains just the right wavelengths to inactivate microorganisms. From this point onward, every time we say ultraviolet light or UV, it is understood that we only mean this narrow band.

So what exactly happens to microorganisms when UV light hits them?

When UV strikes a microorganism, it damages the microbe’s DNA in such a way that the DNA can no longer replicate. This subsequently prevents the microorganism from growing and multiplying. In other words, if you allow ultraviolet light to pass through water, microbes found there will be rendered sterile and, henceforth, practically harmless.

You see, viruses and bacteria can only harm a human body if they can successfully colonize it. This is known as an infection and it entails massive reproduction of these parasites. If the parasites are too few, they can easily be overwhelmed by the body’s immune system. This can easily happen if the invading parasites are not able to multiply.

Besides, when the sterile microbes reach the end of their lifetime, there will be no ‘young’ ones to take their place. Thus, even if UV cannot directly destroy harmful microorganisms, it still serves its purpose in water purification.

Advantages of using ultraviolet in water purification


Perhaps the strongest argument supporting the use of ultraviolet light in water purification is that there are very few (if ever) microorganisms that are immune to it. Practically all moulds, yeasts, algae, bacteria, viruses, and protozoans can be easily incapacitated by UV.

Its effectiveness against viruses and bacteria, in particular, is very useful for preventing waterborne diseases like gastroenteritis, cholera, tuberculosis, and typhoid fever, which are known to cause major outbreaks.

Cryptosporidium, Giardia lamblian, E. coli, Hepatitis A virus, Influenza virus, and Polio virus, are just some of the many harmful microorganisms that can be easily inactivated by ultraviolet light.

Another reason why people like to use UV as a disinfectant for drinking water is that it does not alter the taste, color, odor, pH and other characteristics of the water. Also, unlike other water purification treatments such as distillation, it does not remove naturally occurring minerals, many of which are vital to certain bodily processes.

Lastly, ultraviolet light does not produce harmful byproducts. This is one of the reasons why it is often offered as an alternative to chlorine. Chlorine, undoubtedly the most widely used water disinfectant, can react with certain substances in the water to produce harmful byproducts like trihalomethanes and haloacetic acids. Some of these byproducts are feared to be carcinogenic.

Factors that affect ultraviolet light efficacy


The efficacy of UV in water purification treatment depends on certain factors. Here are some of them:

● Water flow rate - If water passes through the area subjected to UV too quickly, some of the microbes might not get enough exposure.
● Presence of other particles - Other particles present in the water can act as a shield and prevent UV from striking some of the microbes.
● UV lamp condition - In a typical water purification treatment system, ultraviolet light will come from a UV lamp. Just like any other lamp, it will eventually cease to function.

UV alone cannot guarantee safe drinking water

Despite its high efficacy in countering harmful microorganisms, ultraviolet light still has its limitations. In fact, an ideal water purification treatment facility or system does not rely solely on UV. Here’s why.

Pre-filtration needed before UV treatment

Although UV has just the right wavelength to affect microorganisms, it cannot penetrate other particles that may also be found in the water. So if the particles are in the way of the ultraviolet light or if the microorganisms are within the particles themselves, the microorganisms can survive the UV treatment.

For this reason, some water purifying systems have filtration systems placed before the UV lamp. These filters are responsible for removing the particles and clearing the water so that the UV rays can reach the microbes unimpeded.

Second disinfectant needed after UV treatment

Remember that not all microorganisms exposed to ultraviolet light actually die. They are just rendered sterile. But like we said earlier, harmful microorganisms like viruses and bacteria that aren’t able to multiply are practically harmless.

Still, there’s a problem. Some of these microorganisms can actually regain their reproductive capabilities through a process called photoreactivation, which can take place once they are exposed to visible light for a long period of time.

That is why large water systems that employ UV also apply a second disinfectant, usually chloramine. This second disinfectant, stays in the water after the UV treatment and suppresses photoreactivation or even prevents photoreactivation from taking place.



 
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