Ground Water Canada

Water ed: Hydro and UV systems

June 23, 2017  By Jeff Wahl

The use of ultraviolet, or UV, systems has become commonplace for keeping bacteria out of rural water supplies. If you own a home in the country, it is very likely that an ultraviolet disinfection system (also known as blue light or UV sterilizer) is installed either at the kitchen sink or where the water enters the house.

The goal of this column is to raise awareness regarding the operation of these systems and the potential ramifications of power outages. When it comes to water, safety is of paramount concern and too often homeowners trust the water implicitly when a UV system is installed. Unfortunately, the consequences can lead to illness and be costly to fix.

The purpose of a UV system is to disinfect the water supply at the point of contact with the UV ray. This process has no residual disinfection of any type. For illustration purposes, any piping or fixture (sink, shower, toilet) after the UV is considered “upstream.” When applied correctly, the UV system will disinfect the water before it enters upstream areas of the home or cottage. This process can work exceptionally well when the system is applied correctly as per manufacturer’s recommendations and is operational. What happens to the UV system when there is no electricity and it is not operational? It is not as simple as you may think.


In November 2013, Manitoulin Island in Ontario experienced an extended power outage lasting over 14 hours. Having a known bacterial count in the water and knowing that a UV system is required to keep the water bacteria free, the homeowner did not use the water at all during the hydro outage. Believing they had done the right thing, the homeowner wanted to test the water just to be sure before consuming it again. The results from the local health unit were shocking: >80 for coliform and 3 for E. coli. A very high bacteria count. How is this possible when no water had been used and the water had been testing 0/0 before the hydro outage? Concerned, they hired an independent company to troubleshoot the UV system and determine the source of the problem. After a complete inspection of the entire water treatment system, including the UV, no problems were detected. Further investigation revealed a toilet that was leaking slowly to the drain. During the hydro outage, the pressure tank in the basement had enough pressure to have water pass through the UV system and flow to the toilet. The untreated water passing through the UV system when it was not on was deemed the source of the problem.

Most residential UV systems do not have a hydro outage protection device installed. This is a simple device that shuts the water off immediately when there is no power. It opens again when the hydro is restored. The device can plug into a standard 110-volt outlet or can be connected directly to some UV models, depending on the manufacturer. The installation of this type of device is a required component for regulated facilities that operate on rural water and supply water to the public. It is not required as part of a residential system.

Homeowners should inspect their home or cottage for leaking taps, shower fixture, toilets or garden hoses. These could potentially allow water to be used unnecessarily and have an impact on the water quality when the hydro is not present. A pressure gauge can identify water loss and be used to identify leaks in the plumbing system. The gauge at the pressure tank or pump should not move at all when there is no water running in the house or cottage. If it does, there is a leak somewhere in the plumbing system. A manual shut-off valve should be installed after the UV system for maintenance purposes; this can be closed during a hydro outage to prevent water flow. If the system is not readily accessible and no leaks are present, avoiding use of any water fixture during a power outage is recommended to help ensure no contamination of the plumbing occurs.

Jeff Wahl has 20 years of experience in the water treatment industry, is the CEO of Wahl Water and the creator of the program Wahl H20 – Educating Through Awareness. For more information, contact Jeff at

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