Contra-Shear on open-channel or closed-pipe UV disinfection
At a recent international conference on wastewater treatment technology, a representative from UV disinfection specialist Berson described how more and more operators of wastewater treatment facilities are now opting for closed-pipe UV disinfection systems in favour of older, open-channel systems.
The reasons for this move are due in part to the drawbacks of open-channel technology, as well as the advantages of closed-pipe systems.
The disadvantages of open-channel disinfection include the danger of personnel being exposed to UV light; the growth of algae in the open channels; and the difficulty of cleaning the UV lamps, which have to be either cleaned manually, a laborious procedure, or physically lifted and moved to an acid bath. Also, the hydraulic movement of liquids through open channels is not particularly turbulent, so some sections of the wastewater may not pass close enough to the UV lamps to receive the minimum required UV dose.
Closed-pipe UV systems, on the other hand, present no hazard to staff because the UV lamps are enclosed in a stainless steel chamber; this also eliminates the problem of algal growth. Each lamp is fitted with a mechanical wiper on its protective quartz sleeve, which keeps it clean. Periodic chemical cleaning, if required, is simple and can be done in-line without removing the lamps.
In addition, lamp change-over is easy and can be done in minutes. The hydraulic design of closed-pipe systems also means the movement of wastewater through the treatment chamber is more turbulent than in open-channels, ensuring most of the wastewater passes close to the lamps and receives the minimum required UV dose.
Along with closed-pipe systems, many engineers are also opting for medium pressure, rather than low pressure, UV lamps. UV from medium pressure lamps causes permanent inactivation of microorganisms, while UV from low pressure lamps is not guaranteed to do so. In addition, only a few medium pressure lamps do the same job as many low pressure lamps, this makes medium pressure systems much easier to operate, monitor and maintain.