Ultraviolet disinfection is one of the simpler treatment installations: a chamber, a lamp, a power supply and two plumbing connections. It is also one of the easiest to compromise, because a UV unit that has been fitted in the wrong place or without the filtration it needs runs happily, shows a green light and disinfects very little.
Where it must go in the system
Last, after every filtration stage, and as close as practical to the point where water is distributed. Everything upstream exists to give the lamp clear water to work on: sediment removal to a fine rating, plus iron, manganese and hardness treatment where those are present, since all of them absorb or block the wavelength. Anything downstream of the lamp is unprotected, so storage tanks, long dead legs and any point where contamination could enter after the unit defeat the whole arrangement.
Sizing and the plumbing details
Rate the unit for the household's peak simultaneous flow rather than its average, since the dose delivered falls as flow rises and an undersized unit under doses precisely when the house is using most water. Fit isolation valves either side and enough straight clearance at one end to withdraw the lamp and the quartz sleeve, which are the length of the chamber. Support the chamber properly and orient it as the manufacturer specifies, since some designs must be horizontal and others vertical.
The electrical side
The ballast needs a permanent supply that will not be switched off casually, because a UV unit that has been off for an evening is not protecting anything when the taps are used. Fit it where a failure will not soak the electronics, and consider a solenoid valve that shuts off the supply if the lamp fails, which is the only arrangement that guarantees untreated water never reaches the house. A UV intensity monitor is worth the cost, since a lamp that still glows may be well below useful output.
Commissioning and ongoing service
Sanitize the plumbing downstream of the unit at commissioning, or the system is disinfecting water on its way into contaminated pipework. Then test, and test again after a week. In service, the lamp is replaced annually on the calendar regardless of whether it lights, the quartz sleeve is cleaned on schedule because mineral film blocks the light, and O rings are renewed with every sleeve service. Our water filtration guide covers what belongs upstream to make the UV honest.
Questions people ask about water uv systems
Where in the system does a UV unit go?
Last, after all filtration and close to distribution. Nothing downstream of the lamp is protected, including storage and dead legs.
What clearance does the installation need?
Enough straight run at one end to withdraw the lamp and quartz sleeve, which are as long as the chamber itself.
Should the power be switched?
No. It needs a permanent supply. A unit switched off for an evening is not protecting the water that is used afterwards.
What does commissioning involve?
Sanitizing the plumbing downstream, then testing at start up and again a week later, since the unit cannot fix contaminated pipework.