If they are well ...

If they are well designed they can be a great and trouble-free process, but it is a big if.

I agree with Jan-Peter that transmissivity can be a big issue, particularly if something happens that can change the quality of the incoming water.  It only takes a small change in water composition to have a big effect on transmissivity.

One of the major issues we had disinfecting waste water was keeping the lamps clean and the scraper mechanism was not always reliable requiring regular removal and hand cleaning.

Sensors can be another area of concern both in the quality and reliability of the sensor and also the location so you can be sure that the transmissivity reading and lamp intensity reading are accurate for the entire flow.  How quickly the system can react to changes in flow rate is also an important design consdieration.

We also had reliability problems with the ballasts driving the lamps but that was an early generation so may not be a problem these days.

What I was most uncomfortable about is that there was no simple way to ensure disinfection was right.  Generally we know how much chlorine to add to get disinfection and it handles small variations well but with UV we have to balance transmissivity, lamp intensity and flow rate so you can never be 100% sure.

If you really put the effort in and do full due diligence and risk analysis during the design phase you should be ok.  Just remember each plant's installation is unique.

My final comment would be to ensure you have some significant performance guarantees with your supplier that cover not only the UV equipment but the UV equipment as installed in your plant.

Marcus