Open-channel or closed-pipe UV disinfection?

At a recent international conference on wastewater treatment technology, a discussion was included on why more and more operators of wastewater treatment facilities are now opting for closed-pipe UV disinfection systems instead of older, open-channel systems.

The reasons for this move are due partly to the drawbacks of open-channel systems, 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 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 as 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 changeover 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 all the wastewater receives the minimum required UV dose by passing close to the lamps.

Lamp technology

Along with closed-pipe UV systems, many engineers are also opting for medium pressure 'polychromatic' UV lamps. They produce UV over a broad wavelength and have been shown in independent tests to cause permanent inactivation of both pathogenic and non-pathogenic microorganisms such as E.coli and Cryptosporidium parvum (1, 2, 3). Low pressure 'monochromatic' lamps, on the other hand, produce a single peak of UV output. It has been shown that many microorganisms are able to repair themselves after exposure to UV from these low pressure lamps, especially if they are subsequently exposed to sunlight - as is often the case in wastewater treatment facilities. 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. Because of these factors, low pressure UV lamp technology should be avoided in wastewater applications.

Cleaning of quartz sleeves

A major factor to consider with UV wastewater treatment plant is fouling of the protective quartz sleeves surrounding the UV lamps. Suspended solids and minerals in the wastewater attach themselves to the sleeves and must be removed at regular intervals to ensure maximum UV output. This is something that happens to both low and medium pressure UV lamps, and in both open-channel and closed-pipe systems.

There are two main ways to control fouling: mechanical cleaning of the sleeves (with O-rings or brushes) or chemical cleaning with acids. Even when mechanical cleaning is used, the sleeves will still need to be chemically cleaned from time to time. As explained above, with open-channel UV systems the UV lamps must be physically lifted from the channel and transferred to an external chemical bath.

With closed-pipe UV systems, cleaning agents are simply added to the UV chamber and cleaning takes place internally.

To assist this process, Berson has developed the UltraWipe, a combined mechanical and chemical cleaning system. When the UV system is on, automatic wipers move up and down the quartz sleeves, removing any deposits. At the same time, a small volume of low concentration acid is applied directly to the sleeves. This 'direct dosing' means significantly fewer chemicals are required to keep the sleeves clean than with conventional chemical dosing. The chemicals used are not harmful in any way either to the environment or to the wastewater plant's pipeline infrastructure.

Conclusions

Closed-pipe UV wastewater treatment systems are increasing in popularity with operators of wastewater treatment plant. There are many reasons why these systems are now taking over from older, open-channel systems. Firstly, closed-pipe systems are safer. Secondly, cleaning the UV lamps' protective quartz sleeves is straightforward - either mechanically or chemically - without having to remove the lamps. Thirdly, the hydraulic design of closed-pipe systems ensures most of the wastewater receives the minimum required UV dose. Finally, closed-pipe systems, in conjunction with medium pressure UV lamps, ensure that microorganisms are permanently deactivated and cannot repair themselves.