First, we need to understand how Ultraviolet Systems work to kill the bacteria in your water. I have a great video here that explains the process but basically water passes across the UV light and while doing so the bacteria is killed. So potential reasons why the bacteria is not getting killed by the UV includes:
1) Either the lamp or the ballast (or both) is not working.
2) The lamp is over 12 months old and the strength of the UV light coming from the lamp is no longer strong enough to kill bacteria.
3) The light isn’t getting to the water because the Quartz Sleeve for the light has become so dirty the light can’t shine through this coating to kill the bacteria.
4) If water was used during a power failure, bacteria infected water could have re-infected the water downstream of the UV light and although the light is now on the disinfected water is constantly be re-infected after the UV.
5) The UV is too small for the flow rate being produced by your well pump. Check the flow rate specifications for your model UV and then do a flow test from your well pump using the procedure outlined in this video. If the flow rate coming from your well pump is more than what your UV is rated for, you are going to have a problem killing the bacteria.
6) The pre treatment requirements are not being met which is compromising the UV’s performance. Yes, UV’s have pre treatment requirements like hardness being below 7 gpg and iron being below .3 ppm to keep the sleeve clean enough to keep the quartz sleeve from becoming so opaque that the UV light cannot shine through with enough intensity to kill the bacteria. Also, if the Ultraviolet Transmittance (UVT %) is too low because the water is discoloured, again it won’t kill the bacteria. You definitely need a 5 micron pre filter before the light otherwise the bacteria will sneak past the light on the back of those particles of dirt.
7) Was the water downstream of the UV chemically disinfected when the UV was installed? The UV will kill the bacteria in your water as the water passes through it but any infected water down stream of the UV will not be treated. So unless your water is chemically disinfecting at the time of the UV installation, your water will constantly become re infected. Here is a link to a YouTube video of mine that will help you with the disinfection procedure.
8) Do you have plumbing dead ends? The water in plumbing dead ends cannot be chemically disinfected so they constantly keep re-infecting the treated water with bacteria. These dead ends must be removed during any UV installation. What ever you do, do not keep a hose connected to your water supply without turning off the water to that hose whenever not in use. That would be a very dirty source of bacteria for your household.