Ultra violet light is quite commonly used as a disinfection strategy in health and aged care premises. It is a regular feature of dedicated warm water systems. There are a few things to consider before making it part of a Water Risk Management Plan. Lets have a look at how it works and it’s advantages and limitations.
A natural product
Ultra violet light (UV) is a form of radiation that bombards our planet constantly. We know it best as the cause of sunburn. It is also a powerful disinfectant. The UV that hits your clothes on the washing line probably does more disinfection than any ‘anti bacterial’ washing product you put in the washing machine. Many sewage treatment plants use the natural UV incidence as part of the disinfection process for treated water.
Large molecules, like DNA, in living things absorb UV light strongly. The energy from the light disrupts the molecules it hits and stops normal biological processes. The red swelling that comes from sunburn is your immune system responding to seriously damaged cells. So UV is a natural disinfectant. In high enough doses it is lethal to all living things including bacteria. It is especially effective against higher life forms like us – and protozoa.
The sun releases UV light that hits the earth. UV lamps are able to generate artificially and control the amount of UV required. Luckily for us it is not very penetrant at all. A thin layer of sunscreen or even a thin sheet of glass will block out UV effectively. For it to cause damage to cells it has to penetrate them.
UV is in common use as a disinfection process industrially. It is particularly useful for sterilising surfaces so long as they are relatively clean to start with. Of course, like any disinfection it needs a high enough dose for a long enough time period to be effective. With a long enough dose and time it will sterilise materials. Some medical supplies, devices and medical equipment and clean areas are routinely sterilised using UV lamps.
Source: Victorian Building Authority, Tech. Solution Sheet 6.11
UV is also used for disinfecting water. UV systems are a regular feature in potable warm water systems. Usually recirculating loops of water pass through a UV unit close to the water heater. Water flows through a sleeve surrounding a UV strip light. As the water passes through the UV kills microorganisms carried in the water. So in this way UV disinfects the water but it is not sterile. Like every other disinfectant UV has its’ advantages and disadvantages.
The Pros and Cons
There are some advantages to using UV. It is a clean technology in comparison with chemical alternatives. There is no residual effect on the water and no chemicals going into the environment. UV is more effective against protozoa than many of the chemical alternatives. For ‘once through’ applications such as hand washers and cold water supplies it can deliver high quality water consistently. Installation is relatively simple and requires routine maintenance and servicing.
Ironically its lack of residual effect is also one of its’ disadvantages. UV will only disinfect the water that passes through the sleeve. So it will not reach biofilm stuck on to surfaces in the system. It will also not reach deposits in other areas of the system like aerators. In re-circulating systems problems arise because biofilm in the water loop will seed into the system after the UV fitting and then distribute to outlets.
Because it is not very penetrant. UV needs clean water for the light to pass through. A filtration system is essential for UV to disinfect the water it contacts. It also needs a clean sleeve for the UV light to pass through. This needs to be either Teflon or quartz because, unlike glass, they don’t absorb UV.
Larger molecules and metals absorb UV light. This process can energise them and cause them to break down. UV light will actively destroy some disinfectants used for supply drinking water, these include chlorine and monochloramine. UV units need to be turned off or by-passed during decontamination either by pasteurisation or hyperchlorination. Some careful consideration needs to go into the location of a UV system during the planning and installation process to avoid removing the existing disinfectants from the water.
Like all disinfectants, UV has strength and weaknesses. It’s strength is its power at disinfecting surfaces and ‘clean’ water. This makes it very useful for ‘point of use’ applications. It’s weakness is the lack of a residual effect. It cannot be expected to control biofilm or remote areas of a water system. The literature suggests it is best not used on it’s own in potable water systems. This is chiefly because it can’t reach surfaces in remote places around the system. However, it can be an effective ‘add-on’ to other disinfection strategies.
Australian Drinking Water Quality Guidelines 6 2011 Version 3.3 Updated November 2016.
EnHealth (2015) Guidelines for Legionella Control.
Kim et al (2002) Literature Review – Efficacy of various disinfectants against Legionella in water systems. Water Research 36 (4433-4444).
US EPA Office of Water EPA 810-R-16-001 (2016) Technologies for Legionella Control in Premise Plumbing Systems: Scientific Literature Review