How UV Light Purifies Water

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How UV Light Purifies Water

How UV Light Purifies Water

UV has been used for disinfection purposes for more than a century and is well proven to be effective against microorganisms, particularly the dangerous, chlorine-resistant parasites Cryptosporidium and Giardia. More than 400,000 people became very sick and more than 100 died in the 1993 Milwaukee Cryptosporidiosis (crypto) outbreak. UV disinfection inactivates pathogens, rendering them incapable of causing illness, and does so without the use of chemicals, avoiding the risk of unhealthy by-products being produced. Although a simple process, UV has only become truly effective through intensive research and technology development carried out by trusted organizations committed to providing safe, practical water treatment solutions. UV disinfection is now used in many large cities around the world, including New York City, Paris, and Rotterdam (ref: Trojan Technologies, Ontario, Canada). 

Why do I need to disinfect my water supply?

Disinfection is recommended for all water supplies that are not already proven to be disinfected. Unfortunately, the microbiological quality of your water supply can change due to environmental conditions. By providing your own disinfection, you are taking responsibility for ensuring the safety of your water supply for you and your family.

Is UV effective against the protozoan parasites Cryptosporidium (Crypto) and Giardia lamblia (beaver fever)? 

Yes, UV is known to be the best available technology to treat these protozoan cysts, which are highly resistant to chlorine disinfection. In addition, the UV dose level required to inactivate these cysts is relatively low, at 10mJ/cm2 for 99.9% reduction. Recommended flow rates for VIQUA UV disinfection systems are based on a UV dose level of 30mJ/cm2. (See ‘What is UV dose?’) 

Does UV inactivate waterborne E.coli?

Yes, E. coli requires a UV dose of 6 – 10mJ/cm2 to achieve 99.99% disinfection. As noted, VIQUA UV systems use a UV dose of 30mJ/cm2 (@ 95% UVT) as the basis for flow rate sizing. (See ‘What is UV dose?’) 

What is UV Dose?

UV dose or fluence is the amount of UV energy imparted to the water as it flows through the disinfection system. It’s related to UV lamp wattage but also how well the UV is transmitted through the water, which is measured as UVT. (See ‘What is UVT?’)

What is UVT?

UVT is the proportion (%) of the UV energy from the lamp that can penetrate water flowing through the UV system. Water quality varies and this affects UVT. Turbidity in the water reduces the transmission of light while contaminants that give rise to color (for example, in lake water) reduce UVT due to absorption of the light. Water drawn from a dug well or surface source (lake, river) may have UVT in the 55% – 80% range, while water drawn from a drilled well typically has UVT in the 85% – 97% range. Published flow rates for VIQUA systems are based on water with UVT of 95%, except for NSF-validated systems which use UVT of 70% as the basis for flow rate calculation. Different levels of UVT can be accommodated by sizing the UV disinfection system accordingly. 

Ultra Violet (UV) Technology

Using a low-pressure mercury source has become increasingly popular for point-of-use and/or point-of-entry drinking water disinfection. With enough energy, UV radiation at the 254-nm wavelength has the ability to disrupt DNA in pathogenic microorganisms so they cannot reproduce. For disinfection purposes, UV is typically applied as a deterrence device to potable water supplies that does not have a known contamination issue. UV can be part of an overall solution but is not considered to be an adequate stand-alone mitigation method for a water supply with established or suspected pathogens. 

Reduction of Total Oxidizable Carbons (TOC’s)

Reduction of Total Oxidizable Carbons (TOC’s) in water is achieved when 185 nm UV dissociates water molecules thereby creating hydroxyls (free OH- radicals). The hydroxyls are highly reactive and readily combine with other molecules such as the hydrocarbon molecules that make-up TOC’s. When hydroxyls combine with the TOC hydrocarbons they form water and carbon dioxide molecules; TOC’s are destroyed and the oxidation is complete. UV also removes TOC’s when the ultraviolet photons dissociate organic molecules directly. Ultraviolet energy will also ionize TOC’s which allows for further removal by a deionization system. 185 nm UV does not transmit as well through water as does 254 nm UV. Therefore, a very high UV dose is required for TOC reduction as compared to standard disinfection. A minimum UV dose of 120mJ/m2 is recommended for TOC reduction, approximately 10x the dose used for disinfection. A side benefit of using UV for TOC reduction is that the TOC lamps will generate significant levels of 254 nm output and consequently provide high levels of microbial reduction (disinfection), in addition to the TOC reduction. 

Ozone Destruction

 Ozone Destruction systems utilizing UV require more UV energy than inactivation of microorganisms (approx 90 mJ/cm2 versus 30 mJ/cm2). In order to ensure effective ozone destruction, 254 nm UV systems are often sized significantly larger than UV systems used for disinfection. A system designed for approximately 10 gpm disinfection flow rate might be sized to achieve 4 gpm for ozone destruction.

General guideline for ozone destruct:

254-nm at 1.4X rating to standard UV rating for disinfection.

Class A systems:

must deliver a 254 nm UV dose of 40 mJ/cm2 and should be used with well or surface water. They are designed to inactivate and/or re- move microorganisms including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts from contaminated water and are intended to be installed on visually clear water (not colored, cloudy or turbid). Class A systems must include a flow control and a UV sensor connected to an alarm which provides a visual and/or audible indication that the system is not performing, and/or terminates the discharge of treated water. The Standard includes a test to ensure that the UV sensor and alarm perform properly in low-dosage conditions.

Class B systems:

must deliver a 254 nm UV dose of 16 mJ/cm2 that is sufficient to inactivate nonpathogenic organisms and are designed for supplemental bactericidal treatment of water that has been tested and deemed acceptable for human consumption by the state or local health agency having jurisdiction.

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