Lighting the way to UV disinfection
Wolf Meese of Wedeco discusses the case for using low-pressure UV disinfection systems
As extensive tests in Germany and the US have proven, UV disinfection is now one of the most effective method of inactivating dangerous parasites in drinking water. Some health authorities now recommend the use of technologies such as UV disinfection, which generate no harmful byproducts. UV disinfection with low-pressure Spektrotherm UV lamps (primary emission at 254nm) impairs neither the taste nor the odour of potable water. The water retains its natural quality and there are no byproducts, such as those associated with chlorination and medium-pressure lamps. Neither does UV disinfection with Spektrotherm lamps increase the probability of recontamination. UV disinfection is based on the disruption of the DNA of micro-organisms by photochemical reactions.
This removes their ability to replicate and, therefore, their virulence. Such disinfection is usually carried out with low-pressure lamps at 254nm, close to the absorption maximum of the DNA. All commercially available UV lamps rely currently on a mercury discharge. Due to differences in the partial pressure of the mercury and the corresponding output spectrum (s.Emission spectra) they are differentiated in basically two lamp technologies:
- low-pressure lamp,
- medium-pressure lamp.
During the last few years, the market share of low-pressure high-intensity lamps has grown dramatically. Since the terminology low-pressure high-intensity is not defined, it requires a short explanation: The comparison basis is a well documented and widely used low-pressure lamp type G64T5. Its specific characteristics are an arc length of 58 inches and a UV output at 254nm at the beginning of lamp-life of 27W. This leads to a specific output of 0.46W/inch. Any low-pressure lamp, which has a higher specific output than 0.46W/inch, can be called a low-pressure high intensity lamp. Unmatched in efficiency and specific output, so far, are the low-pressure high intensity lamps produced by Wedeco UV Light under the brand name SpektrothermHP, the company claims. The SpektrothermHP lamps are so-called 'amalgam' lamps because the mercury is in contrast to conventional low-pressure lamps or medium-pressure lamps not introduced in liquid form but in a solid state (an amalgam in combination with indium).
The most powerful UV lamp, type Spektrotherm HP SLR32143HP, produces a UV output of 150W with an arc length of 56.3 inches, leading to a specific output of 2.66W/inch, almost six times more powerful than the G64T5. As the emission spectra figures show, the majority of wavelengths generated by medium-pressure lamps are wasted energy, not contributing to the disinfection performance at all (the UVC region from 200-280nm is relevant for disinfection, with the most effective wavelength at 260nm).
Hence, the overall energy costs of medium-pressure lamps compared to low-pressure lamps are usually higher. Besides the drastically increased operational costs, the costs for larger transformers, motor circuit breakers, power supply lines and power back-up devices (such as stand-by generators and/or UPS systems) need to be included in the investment costs. For more than ten years it has been known daylight can activate certain enzymes that can repair the damaged DNA in some bacteria and yeasts. Viruses themselves cannot be photoreactivated, thus potable water is stored and transported in dark vessels and pipes.
Photoreactivation can therefore only play a role as long as water is exposed to daylight at the tap connection. In any case, there is a significant drop in the photoreactivation capacity after 24h under dark conditions. In developing quality requirements for UV systems, therefore, the authorities responsible for potable water in Germany and Austria have looked carefully at the avoidance of photoreactivation. Res-earch at the universities of Bonn and Vienna has clearly demonstrated a fluence (UV dose) of 400J/m2 (= 40mJ/cm2) ensures 4-log (99.99%) inactivation of all pathogenic bacteria and viruses, also taking account of photoreactivation.
For this reason, regulations have been in force in Austria since 1996 (ÖNORM M5873-1) and Germany since 1997 (DVGW Arbeitsblatt W294), which explicitly stipulate a biodosimetrically verified fluence of 400J/m2 is required for UV systems. Low dosage due to insufficiently irradiated volume elements is therefore totally excluded. These regulations have been adopted also in Norway. The ÖNORM will become part of an European norm (CEN) in the near future. Research results from Japan, where UV disinfection of potable water is still relatively unfamiliar, refer to differences in photoreactivation behaviour with low and medium-pressure lamps. In fact, photoreactivation occured with both lamp technologies but the tests were carried out only at low fluence values of 10-70J/m2 (1-7mJ/cm2), which Wedeco believes are of no practical international relevance. Moreover, the radiation range from 200-240nm was not filtered out when the tests with medium-pressure lamps were carried out. Wavelengths in this range contribute to disinfection but may cause undesirable byproducts to be formed and therefore are not allowed to disinfect potable water. In the early 1990s, studies at the University of Bonn in the context of an international joint research project confirmed that, provided there is sufficient fluence (400J/m2), neither low-pressure nor medium-pressure lamps have shown relevant reactivation effects.
Claims photoreactivation only occurs when low-pressure lamps are used, but does not occur with medium-pressure lamps, are therefore highly misleading. Neither has any scientific proof, Wedeco believes, yet been provided to back-up claims the use of UV radiation above 240nm improves disinfection by damaging proteins. A biodosimetrically verified fluence of 400J/m2 ensures 4-log (99.99%) inactivation of all pathogenic bacteria and virus, also taking photoreactivation into account.
Thousands of installed low-pressure UV systems demonstrate this in daily practice in the provision of potable water. Large cities like Helsinki, Stockholm, Mühlheim and Bonn have installed Wedeco low-pressure systems with great success. More than 98% of all UV systems that are biodosimetrically certified to DVGW and ÖNORM are equipped with the economically operating low-pressure lamps.