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UV - Water Cross Contamination
There is a commonly misunderstood or unknown component when an ultraviolet (UV) system is used for bacterial disinfection. Water cross contamination occurs when a water source containing bacteria is allowed to pass through the UV chamber and out into the water piping of an establishment or residence. This can occur through various different scenarios, all of which have the same risks and implications for water quality and safety.

UV Disinfection in Action
The UV Process 
A UV system disinfects water inside of a stainless-steel chamber by passing a UV ray that is emitted from a UV lamp (bulb) through a protective quartz sleeve (dome) and into the water, rendering the bacteria incapable of replication. 

Put in simple terms, this is a light passing through “glass” into the water and “scrambling” it’s DNA making it unable to cause harm.

Importance of UV Disinfection












UV Disinfection Does Not Remove Anything from Water


Understanding the Role of the Quartz Sleeve
Critical to the disinfection process is the UV rays ability to pass through the quartz sleeve into the water. Water quality can have a significant impact on the quartz sleeve and result in it becoming fouled by dirt, stains, scratches and surface coating. 

Coarse sediment can result in pitting or etching of the quartz sleeve’s exterior surface. Fouling and etching can refract the UV rays and prevent them from reaching the water in the proper strength to disinfect properly. In some cases, the sleeve is severely fouled by iron, hardness or sulphur and the rays do not reach the water flowing through the UV chamber at all.

Sulphur Coated UV SleeveWater Quality 
Elements present in ground or surface water can have an impact on the UV’s ability to “see” through the water or quartz sleeve with a high enough dosage to ensure disinfection.

Most elements result in the adherence to the exterior surface of the quartz sleeve and can accumulate over time completely “coating” it, causing bacteria to pass through the UV system. This sets off with no alarm or indication from the UV system. 

Other elements impede the UV ray from properly passing through the water within the UV chamber with no alarm or indication from the UV system as well.

IdeasHeavy Sulphur Coating
This is a photo of a sleeve heavily coated with sulphur
This sleeve resulted in no UV dosage at all. 



Water Quality Elements Affecting UV System Operation

ElementOperating ValueInformationImpact on UV System
Iron (Fe)<0.3 ppm / mg/LThree types: Ferric, Ferrous or BacterialCoats the outside of the sleeve with orange particulate or scaling
Hydrogen Sulphide Gas (H2S)<0.3 ppm / mg/LTwo types: Gas or BacteriaCoats the outside of the quartz sleeve with grey to black or slimy adherence
Water Hardness<7 GPG / ppm Dissolved Calcium & Magnesium in waterCoats the outside of the quartz sleeve with white scale deposits
Tannin / Lignin<0.1 ppm / mg/LYellow, Reddish or Brown coloured water from vegetation sourcesInterfere with UV's ability to penetrate into the water due to the tannins absorption capabilities
Manganese<0.5 ppm / mg/LMineral found naturally in the environmentCoats the outside of the quartz sleeve with a dark brown or black adherence
Turbidity<1 NTURefers to how clear the water is. Total suspended solids (TSS)Interfere with UV's ability to penetrate into the water due to water clarity 

Cross contamination can occur any time that the UV quartz sleeve is fouled or the water quality does not allow for proper disinfection rates. It is important to understand that as the quartz sleeve becomes fouled, the level of disinfection reduces and the risk for cross contamination increases. Water quality plays an important role in this process.


Pre-Treatment of the Water is Important


Power Outages
Rural areas experience power outages on a more frequent basis than their urban counterparts. Often people will turn on taps or flush toilets during an outage and inadvertently cross contaminate their water piping without ever knowing it. This action can result in water cross contamination with very little water passing through the UV.

As water passes through a non-operational UV, the bacteria does as well. When power is restored, most homeowners assume that everything is okay because the UV continues to function. This may not be the case, as bacteria have the opportunity to replicate on the other side of the operating UV light. Simply turning the UV system back on does not guarantee the water is bacteria free, as there is no residual disinfecting occurring in the pipes. Once cross contamination  has occurred, a proper pipe disinfection including the hot water tank should be performed.
Water Cross Contamination UV Systems
This scenario is plausible as the pressure tank associated with many rural water sources will provide 4 to 20 gallons of water after the power goes out and allow water to pass through the UV System untreated. While not as common in rural areas close to larger cities, it can be a common occurrence in rural areas with minimal development and large areas for power lines to travel before entering homes.

There is another scenario involving power which is seen many times in the field of water treatment. Many people simply turn on the UV system when they feel bacteria may be present in the water and turn it off again when they feel there is no bacteria. It is not possible to know without proper testing. Bacteria can be present in a water source through all of the seasons, 365 days a year.


When the Power Goes Out - Do Not Use the Water


Manual UV System BypassManual Plumbing Bypass
A plumbing bypass is often installed to allow water to “go around” the UV system. It is usually installed for emergency purposes, such as the UV system leaking. 

It is also installed to allow for uninterrupted water usage during routine maintenance of the UV system when service is required to replace the ultra violet lamp and quartz sleeve. 

The use of a bypass can lead to water cross contamination as bacteria makes its way into the plumbing system. Once power is restored and the bypass closed, there is the potential for bacteria to be present in the hot or cold water piping located after the UV system. 

CommentCaution when Using a Bypass Valve
Sometimes a bypass valve can be left in the open position resulting in water never entering the UV chamber at all despite the system being on and functioning properly.

Sediment Shadowing UV SystemsSediment Shadowing
All manufacturers recommend achieving 5 micron sediment filtration. This is to ensure that there is the proper path for the UV ray to reach bacteria flowing through the UV chamber.

When particles of sediment are larger than 5 microns in size they can act as a “shield” and prevent the UV Ray from coming into contact with microbes.

Once sediment is removed, the UV ray can “see” the bacteria in the water properly and disinfection can occur.

Comparing Micron Size







            A Sediment Filter is Required for All UV Systems






Check Box
If you are using a UV light for bacterial disinfection, it is important to understand how it operates and properly pre treat the incoming water source. If you are unsure about your water quality or have questions, consult a certified water treatment professional who can provide you with the necessary information to ensure that your UV system is capable of disinfecting the water in your home or cottage. 

Volume 5 - Issue 5 Wahl H2O - Water Awareness
Copyright 2021 Jeff Wahl – Wahl Water | All Rights Reserved
Contact Jeff via email [email protected]


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