It is well known that bacteria and other micro-organisms contain enzymes that can repair UV-damaged DNA, a process known as 'reactivation'.

It is well known that bacteria and other micro-organisms contain enzymes that can repair UV-damaged DNA, a process known as 'reactivation'. Some of these enzymes need visible light to perform the repairs ('photo-reactivation'), while others can do so without light (known as 'dark repair'). Photo-reactivation is generally quicker than dark repair, but both of these phenomena pose obvious problems for operators of UV disinfection plant.

Recent research on the process, however, has produced interesting new developments.

When comparing photo-reactivation of E.coli DNA after exposure to UV wavelengths emitted by low and medium pressure lamps, independent researchers have shown that the DNA underwent extensive repair following exposure to UV from low pressure UV lamps, but virtually none following exposure to UV from medium pressure lamps.

The researchers concluded that it was the broad UV output of medium pressure lamps, between 185-400nm, that has this desirable effect.

By emitting UV over a wide range of the UV spectrum, medium pressure lamps appear to damage other intracellular molecules, such as enzymes, in addition to DNA.

It is this damage which seems to permanently inactivate the cell DNA repair mechanisms.

Low pressure UV lamps, on the other hand, produce only a single UV peak at 254nm which only affects DNA.

These preliminary results have to be considered seriously by operators of low pressure UV disinfection systems.

If, following exposure to low pressure UV lamps, micro-organisms are capable of recovering, users and operators need to seriously consider upgrading to medium pressure technology.

By providing permanent microbial deactivation, medium pressure UV offers peace of mind, something low pressure UV does not do.

Applications affected by these findings are wide-ranging and include municipal drinking water, bottled water, rinse or wash water in the food industries, wastewater and effluent, fisheries and swimming pools.