UV disinfection

Ultraviolet (UV) is invisible light radiation with a wavelength between 200-300 nanometres.

It can be used to kill or damage organisms that cause disease in drinking water, including some that cannot be killed by chlorine such as Cryptosporidium.

UV lamps kill bacteria in your water, but won't change the taste.

It's vital that the UV radiation is able to pass through the water. (UV cannot pass through cloudy or coloured water.)

UV needs a reliable power supply because, if the power fails, the system will not work and your drinking water will not be disinfected.

Other substances in the water, such as iron and manganese, can also reduce the ability of the UV system to work.

If the quality of water to be disinfected isn't good enough, you can install pre-treatment. The extent and cost of this will depend on your water quality, but it usually consists of a series of cartridge filters.

As with any water treatment system, you should get advice from a suitably qualified professional.

Keeping your UV system clean

Although UV is an effective disinfectant, it leaves no lasting residual disinfectant in the water. So it's easy for a supply to become re-contaminated between the UV unit and the tap.

For this reason, it is important to make sure that all your pipes and tanks are in good condition, and nothing can get into, or grow within, the system.

From time to time, you may be need to flush pipes and tanks with a chemical disinfectant. You can install small point of use UV disinfection units (usually under sinks), to make sure your water remains safe at the point of consumption.

A UV system is relatively simple to maintain. If the quartz sleeve (the valve chamber) is dirty it won't be as efficient, so you need to clean it regularly - usually monthly - but it will depend on how clean the incoming water is.

UV lamps will also need to be changed (they typically last about a year). Even if they're working their efficiency will drop off after this time.

It's a good idea to have some sort of safeguard built into your system so that an alarm sounds if the UV lamp stops working, or the water quality deteriorates so that the UV is no longer able to pass through the water to kill pathogens.

Your UV system's size will depend on the volume of water produced by your water supply. UV disinfection units are rated for certain flows of water and it is important to keep within the stated ranges.

If the system isn't big enough, water can pass through too quickly and the UV light will not have enough time to disinfect the water properly.

Finding a contractor to install your UV system

A good place to find a contractor to install or work on your UV disinfection system is the Watersafe website.

Watersafe is a UK-wide approvals scheme that provides a searchable database of plumbers and contractors with the qualifications and experience to work safely on drinking water systems.

Your local council may have a list of contractors.

Changing a UV filter

Dirt can build up on the surface of the lamp and stop it from working effectively. This means disinfection will only be effective if you change the lamp regularly.

Most lamps need to be changed after a year. Some of them have a lamp-life countdown as a reminder. Check with your manufacturer to find out if yours has one.

You can clean the lamp yourself, or you can get advice from your local council about how to do this.

If you're going to do it yourself, you need a spare lamp, spare sleeves and disposable gloves.

• turn off the power and let the UV filter cool down so you can touch the bulb safely
• remove the safety cap at the top of the UV system by squeezing the tabs
• remove the lamp sleeve (if there is one) along the sleeve bolt from the chamber
• unscrew the UV lamp
• install and secure the new lamp into sleeve
• secure the lamp and sleeve assembly
• insert the safety cap and plug the system in again

These are generic instructions. You should follow your manufacturer's instructions if you have them.

Switching your UV filter off when you're not using the water

If you only switch your filter on just before using your water it will not have enough time to get rid of the bacteria.

Make sure you always keep your UV filter on, and have a plan for what you'll do if there's a power cut.

If your UV system isn't working

If there's a problem with your UV system, make sure no-one drinks the water unless it has been boiled until you're sure it's being disinfected.

You may need to take advice on an alternative way to disinfect your water supply until the UV is working.

As well as checking the quality of the incoming water and any pre-treatment, you should also replace faulty lamps and check if the:

• UV lamps are operating
• power supply's functioning
• UV transmittance (the lamp's intensity) is reduced
• quartz sleeves surrounding the lamps need cleaning or replacing
• UV sensor's working

If your UV system seems to be working but you're still getting microbiological failures from samples, there could be a number of reasons such as the:

• water supply's becoming re-contaminated after the UV system
• UV system needs more maintenance and cleaning
• incoming water isn't suitable quality for UV disinfection without pre-treatment
• water isn't getting enough contact time with the UV
• taps aren't clean

If your UV system makes your water warm.

Your water may be warm when you first draw it because it's just left the UV chamber, or if there's a low flow of water through the unit and it's the wrong size.

If this happens, try flushing water through the system until cooler water appears.