Here is a question often debated in the UV world: which is the better UV lamp type for water disinfection: medium pressure or low pressure lamp? Like anything else in life, each has advantages and disadvantages that must be considered in light of operational requirements. In this post I will examine three of the main factors to determine which UV lamp is most suited for your operation.
As the title of this post suggests, there are two UV lamp-types which are most commonly used in UV systems: medium pressure and low pressure lamps. (Pressure relates to the mercury-gas pressure within the lamp).
Low pressure lamps are elongated lamps—about a meter in length—with low output per lamp (ranging between 30-600W). For disinfection purposes, low pressure lamps emit a monochromatic wavelength of 253.7nm (254nm) at high intensity.
Medium pressure lamps are considerably shorter than low pressure ones, with high output per lamp (normally ranging between 1-12kW). Medium pressure lamps emit a wide germicidal wavelength between 200-320nm at various intensities. Medium pressure lamps also emit 254nm wavelength, but not as intensive as low pressure lamps.
1. Disinfection Efficiency
254nm employed by low pressure lamps is indeed effective wavelength against microorganisms’ DNA. However, contrary to what is conventionally believed, the wide germicidal range of 200-320nm used by medium pressure lamps is more effective and achieves considerably better disinfection results at the same UV dose levels. How is that possible?
Low pressure lamps emit UV light near the peak of DNA and RNA absorbance to inactivate microorganisms. Medium Pressure lamps’ broad wavelengths affect the DNA and RNA plus other biological molecules such as proteins and enzymes, enabling greater inactivation impact. A wide germicidal wavelength attacks microorganisms on several fronts, inflicts damage to the microorganism’s essential components and inhibits microorganism’s mutation repair mechanism. For example: the absorbance spectra of proteins show a maximum peak at 280 nm, whereas the peptide bond in proteins displays a significant absorbance below 240 nm. Another example is Cryptosporidium and Bacillus subtilis spores which are inactivated most efficiently at 270-271nm, beyond the scope of low pressure lamps.
A couple of years ago, the US Food & Drug Administration (FDA) issued the Pasteurised Milk Ordinance (PMO) permitting the substitution of heat pasteurization of water with UV systems, if they comply with certain guidelines and conditions. One of the conditions is that the UV system must demonstrate a UV dose level: medium pressure systems must demonstrate 120mJ/cm2 (RED) while low pressure systems must demonstrate 186mJ/cm2 (RED). Why such a difference? the FDA relied on recent studies by various independent research institutions which demonstrated the higher disinfection effectiveness of medium pressure which, in order to achieve a certain disinfection level can use lower UV doses than low pressure lamps.
This is revolutionary, at least for the UV industry: it means that all the literature which exists about UV disinfection is correct about low pressure lamps, but is incorrect in relation to medium pressure lamps. It also means that the long-standing paradigm of low pressure lamps being the most effective UV lamp for disinfection has been unproven , with the result that now all major UV producers in the market offer medium pressure systems alongside their low pressure systems.
Bacteria recovery is another phenomenon with low pressure lamps, and is a source of continuing contamination in the fish tanks. UV light at 254nm damages the DNA, but microorganisms treated by low pressure lamps are often able to “repair” themselves and continue to replicate as if they were not affected by UV. On the other hand, bacteria treated with medium pressure lamps are less likely to repair themselves because of the severe damage to various essential parts inflicted by the wide germicidal range.
To summarize, medium pressure lamps have clear disinfection advantage over low pressure lamps, achieving higher and more sustainable disinfection levels as low pressure for the same UV dose level.
2. Power Efficiency
Low pressure conversion rate, that is the ratio between kW consumed by the lamp to germicidal UV light, is normally between 30-45%. Medium pressure conversion rate is about one third of this, ranging between 10-15%. This means that for each kW consumed, low pressure UV systems are usually about 3 times more power-efficient than medium pressure lamps for a given water volume to be treated. But that is not always the case:
Atlantium has engineered - a built-in amplification mechanism compensating for the power-disadvantage of medium pressure lamps: an optical amplification design which recycles and re-uses UV photons in the disinfection chamber, making them as power efficient as their low pressure UV systems counterparts.
To summarize, low pressure systems tend to be more power-efficient due to the better conversion rate of low pressure lamps. However, medium pressure systems can overcome this drawback by using an amplification mechanism that compensates for the lower conversion rate of the medium pressure lamp. As a client, always look at the overall power consumption of the UV system which is required to achieve the desired UV dose.
3. Lamp Life
Low pressure lamps are known to have a longer life span than medium pressure lamps, ranging between 8,000-16,000 hours while medium pressure range is 4,000-6000 hours. On paper the low pressure lamps look fantastic, but as always we must look at this data in light of the actual operation in the field. There are two aspect to this question:
1. Economical: on average, the above mentioned figures translate into one yearly lamp replacement for low pressure lamps and two yearly replacement for medium pressure lamps. Since medium pressure UV systems normally employ fewer lamps than low pressure, the overall yearly operational costs come out as roughly equal. Depending on the number of lamps, sometimes the scale is in favour of LP lamps and sometimes for MP. So economic analysis needs to be done for each project on an ad-hoc basis.
2. Operational: the purpose of the UV system is to deliver the correct UV dose which will ensure bio-security at all times. For this reason, lamps should be replaced according to their actual performance. There are many variables that may shorten or prolong the actual life span of a UV lamp including the amount of ignitions, water temperature and even the specific production batch of the UV lamp.You do not have to take my word on this point: simply read the small letters in each UV producer’s sheet regarding the lamp life warranty. It is all there. You do not want to take any risks on this matter and if a lamp is under performing it must be replaced even if it has not reached the stated warranty hours. The important point here is that the only way to be certain that the UV lamps are working properly at any given time is by having a dedicated UV sensor per lamp which gives clear indication about the performance of each individual lamp. The manufacturer’s stated hours of operation should be used only as a reference. In this respect, MP UV systems have a clear advantage since they use significantly fewer lamps which makes it much easier to control each lamp individually, as opposed to LP UV systems which can have dozens of lamps per system, making effective control and monitoring almost impossible. This specific point will be the topic of my next post, as it is one of the most crucial features of a UV system in order to ensure sustainable water bio-security.
To summarize, the UV lamp type is not a stand alone component in the UV system. Simply choosing one lamp type or another will not guarantee that the UV system will provide the required bio security. Lamp type should be examined in light of the overall design and construction of the UV system ensuring that it provides optimal conditions for the operation of the lamp. At Atlantium we are dedicated to the use of MP lamps. We have designed our system to fully optimize the disinfection superiority of MP lamps, implemented a sophisticated control and monitoring system for each and every lamp in the system, and engineered a unique amplification mechanism to compensate for the evident disadvantage in the conversion rate, making Atlantium systems as power-efficient as LP UV systems.
